U.S. patent application number 15/291314 was filed with the patent office on 2017-02-09 for lithium secondary batteries and nonaqueous electrolyte for use in the same.
This patent application is currently assigned to MITSUBISHI CHEMICAL CORPORATION. The applicant listed for this patent is Ryoichi KATO, Minoru KOTATO, Hitoshi MATSUMOTO, Hidekazu MIYAGI, Takayuki NAKAJIMA, Hiroyuki OSHIMA, Tomohiro SATOU, Hitoshi SUZUKI, Hiroyuki UONO, Masakazu YOKOMIZO. Invention is credited to Ryoichi KATO, Minoru KOTATO, Hitoshi MATSUMOTO, Hidekazu MIYAGI, Takayuki NAKAJIMA, Hiroyuki OSHIMA, Tomohiro SATOU, Hitoshi SUZUKI, Hiroyuki UONO, Masakazu YOKOMIZO.
Application Number | 20170040593 15/291314 |
Document ID | / |
Family ID | 38023094 |
Filed Date | 2017-02-09 |
United States Patent
Application |
20170040593 |
Kind Code |
A1 |
MIYAGI; Hidekazu ; et
al. |
February 9, 2017 |
LITHIUM SECONDARY BATTERIES AND NONAQUEOUS ELECTROLYTE FOR USE IN
THE SAME
Abstract
Provided is a lithium secondary battery having a nonaqueous
electrolyte which contains at least one compound selected from the
group consisting of monofluorophosphoric acid salts; and
difluorophosphoric acid salts, in an amount of 10 ppm or more of
the whole nonaqueous electrolyte, and a negative electrode selected
from [1], [2], [3] and [6]: [1]: a negative electrode containing
two or more carbonaceous substances differing in crystallinity;
[2]: a negative electrode containing an amorphous carbonaceous
substance which, when examined by wide-angle X-ray diffractometry,
has an interplanar spacing (d002) for the (002) planes of 0.337 nm
or larger and a crystallite size (Lc) of 80 nm or smaller and
which, in an examination by argon ion laser Raman spectroscopy, has
a Raman. R value of 0.2 or higher defined as the ratio of the peak
intensity at 1,360 cm.sup.-1 to the peak intensity at 1,580
cm.sup.-1; [3]: a negative-electrode active material a
titanium-containing metal oxide capable of occluding and releasing
lithium; [6]: as a negative-electrode active material, graphitic
carbon particles which have a roundness of 0.85 or higher and an
interplanar spacing (d002) for the (002) planes of smaller than
0.337 nm as determined by wide-angle X-ray diffractometry and
which, in an examination by argon ion laser Raman spectroscopy,
have a Raman R value of from 0.12 to 0.8 defined as the ratio of
the peak intensity at 1,360 cm.sup.-1 to the peak intensity at
1,580 cm.sup.-1.
Inventors: |
MIYAGI; Hidekazu;
(Inashiki-gun, JP) ; KATO; Ryoichi; (Inashiki-gun,
JP) ; YOKOMIZO; Masakazu; (Inashiki-gun, JP) ;
UONO; Hiroyuki; (Inashiki-gun, JP) ; MATSUMOTO;
Hitoshi; (Inashiki-gun, JP) ; SATOU; Tomohiro;
(Inashiki-gun, JP) ; KOTATO; Minoru;
(Inashiki-gun, JP) ; NAKAJIMA; Takayuki;
(Inashiki-gun, JP) ; SUZUKI; Hitoshi;
(Inashiki-gun, JP) ; OSHIMA; Hiroyuki;
(Inashiki-gun, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MIYAGI; Hidekazu
KATO; Ryoichi
YOKOMIZO; Masakazu
UONO; Hiroyuki
MATSUMOTO; Hitoshi
SATOU; Tomohiro
KOTATO; Minoru
NAKAJIMA; Takayuki
SUZUKI; Hitoshi
OSHIMA; Hiroyuki |
Inashiki-gun
Inashiki-gun
Inashiki-gun
Inashiki-gun
Inashiki-gun
Inashiki-gun
Inashiki-gun
Inashiki-gun
Inashiki-gun
Inashiki-gun |
|
JP
JP
JP
JP
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
MITSUBISHI CHEMICAL
CORPORATION
Chiyoda-ku
JP
|
Family ID: |
38023094 |
Appl. No.: |
15/291314 |
Filed: |
October 12, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14660517 |
Mar 17, 2015 |
|
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15291314 |
|
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13614215 |
Sep 13, 2012 |
9029022 |
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14660517 |
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12090655 |
Jun 5, 2009 |
9112236 |
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PCT/JP2006/320882 |
Oct 19, 2006 |
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13614215 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/625 20150401;
H01M 10/0525 20130101; H01M 4/134 20130101; H01M 2300/0025
20130101; Y02E 60/10 20130101; H01M 4/364 20130101; H01M 4/583
20130101; H01M 2004/027 20130101; H01M 2/0262 20130101; H01M 4/133
20130101; H01M 2/08 20130101; H01M 4/525 20130101; H01M 4/0471
20130101; H01M 4/131 20130101; H01M 10/0568 20130101; H01M 4/1391
20130101; H01M 4/505 20130101; H01M 4/587 20130101; H01M 10/0569
20130101; H01M 2/1673 20130101; H01M 4/0428 20130101; H01M 2220/20
20130101; H01M 10/0567 20130101; H01M 10/613 20150401; H01M 4/1393
20130101; H01M 10/052 20130101; H01M 10/0587 20130101; H01M
2300/004 20130101; Y02T 10/70 20130101; H01M 2004/021 20130101;
Y02E 60/122 20130101; B60L 50/64 20190201; H01M 4/485 20130101;
H01M 2/0287 20130101; Y02T 10/7011 20130101 |
International
Class: |
H01M 4/131 20060101
H01M004/131; H01M 10/0568 20060101 H01M010/0568; H01M 4/133
20060101 H01M004/133; H01M 4/04 20060101 H01M004/04; H01M 4/1393
20060101 H01M004/1393; H01M 4/525 20060101 H01M004/525; H01M 4/485
20060101 H01M004/485; H01M 4/587 20060101 H01M004/587; H01M 10/0525
20060101 H01M010/0525; H01M 4/1391 20060101 H01M004/1391 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 20, 2005 |
JP |
2005-305300 |
Oct 20, 2005 |
JP |
2005-305368 |
Oct 28, 2005 |
JP |
2005-314043 |
Oct 28, 2005 |
JP |
2005-314260 |
Nov 16, 2005 |
JP |
2005-331255 |
Nov 16, 2005 |
JP |
2005-331362 |
Nov 16, 2005 |
JP |
2005-331391 |
Nov 16, 2005 |
JP |
2005-331477 |
Nov 16, 2005 |
JP |
2005-331513 |
Nov 16, 2005 |
JP |
2005-331585 |
Nov 16, 2005 |
JP |
2005-332173 |
Nov 25, 2005 |
JP |
2005-339794 |
Nov 29, 2005 |
JP |
2005-343629 |
Nov 29, 2005 |
JP |
2005-344732 |
Dec 2, 2005 |
JP |
2005-349052 |
Dec 7, 2005 |
JP |
2005-353005 |
Dec 13, 2005 |
JP |
2005-359061 |
Dec 21, 2005 |
JP |
2005-367747 |
Dec 22, 2005 |
JP |
2005-369824 |
Dec 22, 2005 |
JP |
2005-370024 |
Dec 28, 2005 |
JP |
2005-377366 |
Jan 13, 2006 |
JP |
2006-005622 |
Jan 23, 2006 |
JP |
2006-013664 |
Jan 27, 2006 |
JP |
2006-019863 |
Jan 27, 2006 |
JP |
2006-019879 |
Claims
1. (canceled)
2. A lithium secondary battery comprising: an electrode group
comprising: a positive electrode, a negative electrode, a
microporous film separator interposed between the electrodes; and a
nonaqueous electrolyte comprising a nonaqueous solvent and a
lithium salt, wherein the electrode group and the nonaqueous
electrolyte are held in a battery case, and the positive electrode
and the negative electrode each comprising a current collector and,
formed thereon, an active-material layer containing an active
material capable of occluding and releasing a lithium ion, wherein
the nonaqueous electrolyte is a nonaqueous electrolyte which
contains at least one compound selected from the group consisting
of monofluorophosphoric acid salts, and difluorophosphoric acid
salts, in an amount of 10 ppm or more of the whole nonaqueous
electrolyte, and the negative electrode is any negative electrode
selected from the group consisting of the following negative
electrodes [1], [2], [3] and [6]: negative electrode [1]: a
negative electrode containing as a negative-electrode active
material two or more carbonaceous substances differing in
crystallinity; negative electrode [2]: a negative electrode
containing as a negative-electrode active material an amorphous
carbonaceous substance which, when examined by wide-angle X-ray
diffractometry, has an interplanar spacing (d002) for the (002)
planes of 0.337 nm or larger and a crystallite size (Lc) of 80 nm
or smaller and which, in an examination by argon ion laser Raman
spectroscopy, has a Raman. R value of 0.2 or higher defined as the
ratio of the peak intensity at 1,360 cm.sup.-1 to the peak
intensity at 1,580 cm.sup.-1, negative electrode [3]: a negative
electrode containing as a negative-electrode active material a
titanium-containing metal oxide capable of occluding and releasing
lithium; negative electrode [6]: a negative electrode containing,
as a negative-electrode active material, graphitic carbon particles
which have a roundness of 0.85 or higher and an interplanar spacing
(d002) for the (002) planes of smaller than 0.337 nm as determined
by wide-angle X-ray diffractometry and which, in an examination by
argon ion laser Raman spectroscopy, have a Raman R value of from
0.12 to 0.8 defined as the ratio of the peak intensity at 1,360
cm.sup.-1 to the peak intensity at 1,580 cm.sup.-1.
3. The lithium secondary battery according to claim 2, wherein the
nonaqueous electrolyte comprises the at least one compound selected
from the group consisting of monofluorophosphoric acid salts and
difluorophosphoric acid salts, in an amount of from 0.01% by mass
to 5% by mass.
4. The lithium secondary battery according to claim 2, wherein the
negative electrode [1] contains as a negative-electrode active
material a composite carbonaceous material comprising two or more
carbonaceous substances differing in crystallinity.
5. The lithium secondary battery according to claim 4, wherein the
composite carbonaceous material comprises a particulate
carbonaceous substance and has an interface where the particulate
carbonaceous substance is covered with and/or bonded to a
carbonaceous substance differing in crystallinity from the
particulate carbonaceous substance, the interface changing in
crystallinity discontinuously and/or continuously.
6. The lithium secondary battery according to claim 5, wherein the
particulate carbonaceous substance is a graphitic carbonaceous
substance comprising a natural graphite and/or an artificial
graphite.
7. The lithium secondary battery according to claim 5, wherein the
particulate carbonaceous substance comprises at least one member
selected from the group consisting of the following (a), (b), and
(c): (a) a product of organic-substance pyrolysis selected from the
group consisting of coal coke, petroleum coke, furnace black,
acetylene black, and pitch-derived carbon fibers; (b) a product of
the carbonization of the gas of an organic substance; and (c) a
carbonaceous substance obtained by partly or wholly graphitizing
(a) or (b).
8. The lithium secondary battery according to claim 5, wherein the
carbonaceous substance differing in crystallinity from the
particulate carbonaceous substance is a lowly crystalline
carbonaceous substance having lower crystallinity than the
particulate carbonaceous substance.
9. The lithium secondary battery according to claim 5, wherein the
carbonaceous substance differing in crystallinity from the
particulate carbonaceous substance is a product of the
carbonization of the following (d) or (e): (d) an organic substance
capable of carbonization which is selected from the group
consisting of coal-derived heavy oil, straight-run heavy oil,
petroleum-derived cracked heavy oil, aromatic hydrocarbons,
nitrogen-ring compounds, sulfur-ring compounds, polyphenylenes,
organic synthetic polymers, natural polymers, thermoplastic resins,
and thermosetting resins; and (e) a solution obtained by dissolving
the organic substance capable of carbonization (d) in a
low-molecular organic solvent.
10. The lithium secondary battery according to claim 4, wherein the
composite carbonaceous material comprises a graphitic carbonaceous
substance and a lowly crystalline carbonaceous substance, and the
content of the graphitic carbonaceous substance is 50% by mass or
higher based on the whole composite carbonaceous material.
11. The lithium secondary battery according to claim 4, wherein the
composite carbonaceous material has a roundness of 0.85 or
higher.
12. The lithium secondary battery according to claim 4, wherein the
composite carbonaceous material is obtained by a method comprising
mixing a particulate carbonaceous substance with an organic
substance capable of carbonization and heat-treating the resultant
mixture at 400.degree. C.-3,200.degree. C. one or more times.
13. The lithium secondary battery according to claim 12, wherein
the organic substance capable of carbonization is heat-treated via
a liquid-phase state.
14. The lithium secondary battery according to claim 2, wherein the
amorphous carbonaceous substance contained as a negative-electrode
active material in the negative electrode [2] has a true density of
2.22 g/cm.sup.3 or lower.
15. The lithium secondary battery according to claim 2, wherein the
amorphous carbonaceous substance contained as a negative-electrode
active material in the negative electrode [2] has an atomic amount
ratio O/C value of from 0 to 0.15.
16. The lithium secondary battery according to claim 2, wherein the
amorphous carbonaceous substance contained as a negative-electrode
active material in the negative electrode [2] has an atomic amount
ratio H/C value of from 0.01 to 0.3.
17. The lithium secondary battery according to claim 2, wherein the
amorphous carbonaceous substance contained as a negative-electrode
active material in the negative electrode [2] has a tap density of
0.1 g/cm.sup.3 or higher.
18. The lithium secondary battery according to claim 2, wherein the
amorphous carbonaceous substance contained as a negative-electrode
active material in the negative electrode [2] has a BET specific
surface area of from 0.1 m.sup.2/g to 100 m.sup.2/g.
19. The lithium secondary battery according to claim 2, wherein the
amorphous carbonaceous substance contained as a negative-electrode
active material in the negative electrode [2] has a volume-average
particle diameter of from 1 .mu.m to 50 .mu.m.
20. The lithium secondary battery according to claim 2, wherein the
amorphous carbonaceous substance contained as a negative-electrode
active material in the negative electrode [2] has a pore volume, in
terms of the volume of particle pores corresponding to the diameter
range of from 0.01 .mu.m to 1 .mu.m as determined by mercury
porosimetry, of 0.01 mL/g or larger.
21. The lithium secondary battery according to claim 2, wherein the
amorphous carbonaceous substance contained as a negative-electrode
active material in the negative electrode [2] has an ash content of
from 1 ppm to 1% by mass.
22. The lithium secondary battery according to claim 2, wherein at
least one of the amorphous carbonaceous substances contained as a
negative-electrode active material in the negative electrode [2] is
an amorphous carbonaceous substance selected from the group
consisting of (1) to (4): (1) one obtained by further heat-treating
a carbonization product selected from the group consisting of coal
coke, petroleum coke, furnace black, acetylene black, and
pitch-derived carbon fibers, (2) an organic substance selected from
the group consisting of pitch materials, aromatic hydrocarbons,
nitrogen-ring compounds, sulfur-ring compounds, polyphenylenes,
organic synthetic polymers, natural polymers, thermoplastic resins,
and thermosetting resins and/or a product of the pyrolysis of the
organic substance and/or one obtained by further heat-treating the
pyrolysis product; (3) a product of the pyrolysis of a solution
obtained by dissolving the organic substance (2) in a low-molecular
organic solvent and/or one obtained by further heat-treating the
pyrolysis product; and (4) a product of the carbonization of a gas
containing an organic substance.
23. The lithium secondary battery according to claim 2, wherein the
amorphous carbonaceous substance contained as a negative-electrode
active material in the negative electrode [2] is one which has
undergone a heat treatment at from 600.degree. C. to 2,500.degree.
C. one or more times.
24. The lithium secondary battery according to claim 23, wherein
the amorphous carbonaceous substance underwent a pulverization
treatment before and/or after the heat treatment.
25. The lithium secondary battery according to claim 2, wherein the
metal oxide contained as a negative-electrode active material in
the negative electrode [3] is a lithium composite oxide.
26. The lithium secondary battery according to claim 2, wherein the
metal oxide contained as a negative-electrode active material in
the negative electrode [3] is one having a spinel structure.
27. The lithium secondary battery according to claim 2, wherein the
metal oxide contained as a negative-electrode active material in
the negative electrode [3] is a lithium-titanium composite oxide of
composition formula (1): Li.sub.xTi.sub.yM.sub.zO.sub.4 composition
formula (1) wherein M is at least one element selected from the
group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and
Nb], and 0.7.ltoreq.x.ltoreq.1.5, 1.5.ltoreq.y.ltoreq.2.3, and
0.ltoreq.z.ltoreq.1.6.
28. The lithium secondary battery according to claim 27, wherein
1.2.ltoreq.x.ltoreq.1.4, 1.5.ltoreq.y.ltoreq.1.7, and z=0.
29. The lithium secondary battery according to claim 27, wherein
0.9.ltoreq.x.ltoreq.1.1, 1.9.ltoreq.y.ltoreq.2.1, and z=0.
30. The lithium secondary battery according to claim 27, wherein
0.7.ltoreq.x.ltoreq.0.9, 2.1.ltoreq.y.ltoreq.2.3, and z=0.
31. The lithium secondary battery according to claim 2, wherein the
metal oxide contained as a negative-electrode active material in
the negative electrode [3] has a BET specific surface area of from
0.5 m.sup.2/g to 200 m.sup.2/g.
32. The lithium secondary battery according to claim 2, wherein the
metal oxide contained as a negative-electrode active material in
the negative electrode [3] has a volume-average particle diameter
of from 0.1 .mu.m to 50 .mu.m.
33. The lithium secondary battery according to claim 2, wherein the
metal oxide contained as a negative-electrode active material in
the negative electrode [3] has an average primary-particle diameter
of from 0.01 .mu.m to 2 .mu.m.
34. The lithium secondary battery according to claim 2, wherein the
current collector of the lithium secondary battery is a metal foil
film comprising Cu and/or Al.
35. The lithium secondary battery according to claim 2, wherein the
graphitic carbon particles contained as a negative-electrode active
material in the negative electrode [6] have a tap density of 0.55
or higher.
36. The lithium secondary battery according to claim 2, wherein the
graphitic carbon particles contained as a negative-electrode active
material in the negative electrode [6] have a BET specific area of
from 0.1 m.sup.2/g to 100 m.sup.2/g.
37. The lithium secondary battery according to claim 2, wherein the
graphitic carbon particles contained as a negative-electrode active
material in the negative electrode [6] have a volume-average
particle diameter of from 1 .mu.m to 50 .mu.m.
38. The lithium secondary battery according to claim 2, wherein the
graphitic carbon particles contained as a negative-electrode active
material in the negative electrode [6] have a pore volume, in terms
of the volume of pores in the range of from 0.01 .mu.m to 1 .mu.m
as determined by mercury porosimetry, of 0.01 mL/g or larger.
39. The lithium secondary battery according to claim 2, wherein the
graphitic carbon particles contained as a negative-electrode active
material in the negative electrode [6] have an ash content of from
1 ppm to 1% by mass.
40. The lithium secondary battery according to claim 2, wherein the
graphitic carbon particles contained as a negative-electrode active
material in the negative electrode [6] are ones comprising a
natural graphite.
41. The lithium secondary battery according to claim 2, wherein the
negative-electrode active material contained in the negative
electrode [6] is graphitic carbon particles obtained by subjecting
carbonaceous particles to one or more treatments comprising a
mechanical energy treatment, the mechanical energy treatment being
one which reduces the particle diameter so that the ratio of the
volume-average particle diameter after the treatment to that before
the treatment is less than 1 and which heightens the tap density
and increases the Raman R value, i.e., the ratio of the peak
intensity at 1,360 cm.sup.-1 to the peak intensity at 1,580
cm.sup.-1 in an argon ion laser Raman spectrum, at least
1.1-fold.
42. The lithium secondary battery according to claim 41, wherein
the carbonaceous particles comprise a natural graphite.
43. The lithium secondary battery according to claim 41, wherein
the mechanical energy treatment is conducted in an apparatus
comprising a casing and disposed therein a rotor equipped with
blades, the treatment being conducted by rotating the rotor at a
high speed.
Description
TECHNICAL FIELD
[0001] The present invention relates to lithium secondary batteries
and a nonaqueous electrolyte for use therein. More particularly,
the invention relates to: lithium secondary batteries which
comprise a nonaqueous electrolyte for lithium secondary batteries
which contains a specific ingredient and a positive electrode and a
negative electrode each having a specific composition and specific
properties and capable of occluding/releasing lithium, and which
are excellent especially in low-temperature discharge
characteristics and have a high capacity, long life, and high
output; and the nonaqueous electrolyte for use in these
batteries.
BACKGROUND ART
[0002] With the recent trend toward size reduction in electronic
appliances, secondary batteries are increasingly required to have a
higher capacity. Attention is hence focused on lithium secondary
batteries, which have a higher energy density than nickel-cadmium
batteries and nickel-hydrogen batteries.
[0003] Lithium secondary batteries are used in various applications
because they are high-capacity secondary batteries. In these
applications, however, the lithium secondary batteries are mainly
used as batteries of relatively small sizes as in, e.g., portable
telephones. In the future, the batteries are expected to be used in
a wider range of applications as large batteries for motor
vehicles, etc. Although output is especially required of large
batteries, mere enlargement of conventional small batteries results
in insufficient performances. Various improvements in battery
materials for an output improvement have been proposed (see, for
example, patent documents 1 to 25 and non-patent document 1).
However, a sufficient output has not been obtained so far, and a
further improvement is desired. [0004] Patent Document 1:
JP-A-2005-071749 [0005] Patent Document 2: JP-A-2005-123180 [0006]
Patent Document 3: JP-A-2001-206722 [0007] Patent Document 4:
JP-A-2003-267732 [0008] Patent Document 5: JP-A-2001-015108 [0009]
Patent Document 6: WO 2003/34518 [0010] Patent Document 7:
JP-A-11-067270 [0011] Patent Document 8: JP-A-61-168512 [0012]
Patent Document 9: JP-A-6-275263 [0013] Patent Document 10:
JP-A-2000-340232 [0014] Patent Document 11: JP-A-2005-235397 [0015]
Patent Document 12: JP-A-11-031509 [0016] Patent Document 13:
JP-A-3-055770 [0017] Patent Document 14: JP-A-2004-071458 [0018]
Patent Document 15: JP-A-2004-087459 [0019] Patent Document 16:
JP-A-10-270074 [0020] Patent Document 17: JP-A-2002-075440 [0021]
Patent Document 18: JP-A-10-270075 [0022] Patent Document 19:
JP-A-8-045545 [0023] Patent Document 20: JP-A-2001-006729 [0024]
Patent Document 21: JP-A-10-050342 [0025] Patent Document 22:
JP-A-9-106835 [0026] Patent Document 23: JP-A-2000-058116 [0027]
Patent Document 24: JP-A-2001-015158 [0028] Patent Document 25:
JP-A-2005-306619 [0029] Non-Patent Document 1: J. Electrochem.
Soc., 145, L1 (1998)
DISCLOSURE OF THE INVENTION
Problems that the Invention is to Solve
[0030] The invention has been achieved in view of such
background-art techniques. An object of the invention is to provide
lithium secondary batteries which, even when fabricated so as to
have a larger size, have a high capacity, long life, and high
output.
Means for Solving the Problems
[0031] The present inventors made intensive investigations in view
of the problems described above. As a result, they have found that
a lithium secondary battery having a high capacity, long life, and
high output is obtained by using a positive electrode and a
negative electrode each having a specific composition and specific
properties and further using a nonaqueous electrolyte containing a
compound selected from a specific group. The invention has been
thus completed.
[0032] The invention provides a lithium secondary battery at least
comprising: an electrode group comprising a positive electrode, a
negative electrode, and a microporous separator interposed between
the electrodes; and a nonaqueous electrolyte comprising a
nonaqueous solvent and a lithium salt contained therein, the
electrode group and the nonaqueous electrolyte being held in a
battery case, the positive electrode and the negative electrode
each comprising a current collector and, formed thereon, an
active-material layer containing an active material capable of
occluding/releasing a lithium ion,
[0033] wherein the nonaqueous electrolyte is a nonaqueous
electrolyte which contains at least one compound selected from the
group consisting of cyclic siloxane compounds represented by the
following general formula (1):
##STR00001##
[wherein R.sup.1 and R.sup.2 may be the same or different and
represent an organic group which has 1-12 carbon atoms and n
represents an integer of 3-10], fluorosilane compounds represented
by the following general formula (2):
[Ka-2]
[0034] SiF.sub.xR.sup.3.sub.pR.sup.4.sub.qR.sup.5.sub.r (2)
[wherein R.sup.3 to R.sup.5 may be the same or different and
represent an organic group which has 1-12 carbon atoms; x
represents an integer of 1-3; and p, q, and r each represents an
integer of 0-3, provided that 1.ltoreq.p+q+r.ltoreq.3], compounds
represented by the following general formula (3):
##STR00002##
[wherein R.sup.6 to R.sup.8 may be the same or different and
represent an organic group which has 1-12 carbon atoms and symbol A
represents a group constituted of H, C, N, O, F, S, Si, and/or P],
compounds having an S--F bond in the molecule, nitric acid salts,
nitrous acid salts, monofluorophosphoric acid salts,
difluorophosphoric acid salts, acetic acid salts, and propionic
acid salts in an amount of 10 ppm or more of the whole nonaqueous
electrolyte, and the positive electrode is any positive electrode
selected from the group consisting of the following positive
electrode [1] to positive electrode [5]:
[0035] positive electrode [1]: a positive electrode containing a
positive-electrode active material containing manganese;
[0036] positive electrode [2]: a positive electrode containing a
positive-electrode active material having a composition represented
by the following composition formula (4):
Li.sub.xNi.sub.(1-y-z)Co.sub.yM.sub.zO.sub.2 composition formula
(4)
[wherein M represents at least one element selected from the group
consisting of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x
represents a number satisfying 0<x.ltoreq.1.2; y represents a
number satisfying 0.05.ltoreq.y.ltoreq.0.5; and z represents a
number satisfying 0.01.ltoreq.z.ltoreq.0.5];
[0037] positive electrode [3]: a positive electrode containing a
positive-electrode active material selected from the group
consisting of the following (a) to (d):
[0038] (a) a positive-electrode active material having a BET
specific surface area of from 0.4 m.sup.2/g to 2 m.sup.2/g,
[0039] (b) a positive-electrode active material having an average
primary-particle diameter of from 0.1 .mu.m to 2 .mu.m,
[0040] (c) a positive-electrode active material having a median
diameter d.sub.50 of from 1 .mu.m to 20 .mu.m,
[0041] (d) a positive-electrode active material having a tap
density of from 1.3 g/cm.sup.3 to 2.7 g/cm.sup.3;
[0042] positive electrode [4]: a positive electrode satisfying any
requirement selected from the group consisting of the following (e)
to (f):
[0043] (e) is a positive electrode produced by forming a
positive-electrode active-material layer comprising a
positive-electrode active material, a conductive material, and a
binder on a current collector, wherein the content of the
conductive material in the positive-electrode active-material layer
is in the range of from 6% by mass to 20% by mass,
[0044] (f) is a positive electrode produced by forming a
positive-electrode active-material layer comprising a
positive-electrode active material and a binder on a current
collector, wherein the positive-electrode active-material layer has
a density in the range of from 1.7 g/cm.sup.3 to 3.5
g/cm.sup.3,
[0045] (g) is a positive electrode produced by forming a
positive-electrode active-material layer comprising a
positive-electrode active material and a binder on a current
collector, wherein the ratio of the thickness of the
positive-electrode active-material layer to the thickness of the
current collector, i.e., the value of (thickness of the
active-material layer on one side just before impregnation with the
nonaqueous electrolyte)/(thickness of the current collector), is in
the range of from 1.6 to 20; and
[0046] positive electrode [5]: a positive electrode containing two
or more positive-electrode active materials differing in
composition.
[0047] The invention further provides a lithium secondary battery
at least comprising: an electrode group comprising a positive
electrode, a negative electrode, and a microporous separator
interposed between the electrodes; and a nonaqueous electrolyte
comprising a nonaqueous solvent and a lithium salt contained
therein, the electrode group and the nonaqueous electrolyte being
held in a battery case, and the positive electrode and the negative
electrode each comprising a current collector and, formed thereon,
an active-material layer containing an active material capable of
occluding/releasing a lithium ion,
[0048] wherein the nonaqueous electrolyte is a nonaqueous
electrolyte which contains at least one compound selected from the
group consisting of cyclic siloxane compounds represented by
general formula (1) given above, fluorosilane compounds represented
by general formula (2) given above, compounds represented by
general formula (3) given above, compounds having an S--F bond in
the molecule, nitric acid salts, nitrous acid salts,
monofluorophosphoric acid salts, difluorophosphoric acid salts,
acetic acid salts, and propionic acid salts in an amount of 10 ppm
or more of the whole nonaqueous electrolyte, and
the negative electrode is any negative electrode selected from the
group consisting of the following negative electrode [1] to
negative electrode [10]:
[0049] negative electrode [1]: a negative electrode containing as a
negative-electrode active material two or more carbonaceous
substances differing in crystallinity;
[0050] negative electrode [2]: a negative electrode containing as a
negative-electrode active material an amorphous carbonaceous
substance which, when examined by wide-angle X-ray diffractometry,
has an interplanar spacing (d002) for the (002) planes of 0.337 nm
or larger and a crystallite size (Lc) of 80 nm or smaller and
which, in an examination by argon ion laser Raman spectroscopy, has
a Raman R value of 0.2 or higher defined as the ratio of the peak
intensity at 1,360 cm.sup.-1 to the peak intensity at 1,580
cm.sup.-1;
[0051] negative electrode [3]: a negative electrode containing as a
negative-electrode active material a titanium-containing metal
oxide capable of occluding and releasing lithium;
[0052] negative electrode [4]: a negative electrode containing as a
negative-electrode active material a carbonaceous substance having
a roundness of 0.85 or higher and a surface functional-group amount
O/C value of from 0 to 0.01;
[0053] negative electrode [5]: a negative electrode containing as a
negative-electrode active material an orientation-differing-carbon
composite comprising two or more carbonaceous substances differing
in orientation;
[0054] negative electrode [6]: a negative electrode containing, as
a negative-electrode active material, graphitic carbon particles
which have a roundness of 0.85 or higher and an interplanar spacing
(d002) for the (002) planes of smaller than 0.337 nm as determined
by wide-angle X-ray diffractometry and which, in an examination by
argon ion laser Raman spectroscopy, have a Raman R value of from
0.12 to 0.8 defined as the ratio of the peak intensity at 1,360
cm.sup.-1 to the peak intensity at 1,580 cm.sup.-1;
[0055] negative electrode [7]: a negative electrode containing as a
negative-electrode active material a multielement-containing
negative-electrode active material (C) which contains at least one
of a lithium-occluding metal (A) selected from the group consisting
of Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, and
Sb and/or a lithium-occluding alloy (B) and further contains C
and/or N as element Z;
[0056] negative electrode [8]: a negative electrode containing as a
negative-electrode active material two or more negative-electrode
active materials differing in property;
[0057] negative electrode [9]: a negative electrode containing a
negative-electrode active material which has a tap density of 0.1
g/cm.sup.3 or higher and has a pore volume, in terms of the volume
of particle pores corresponding to the diameter range of from 0.01
.mu.m to 1 .mu.m as determined by mercury porosimetry, of 0.01 mL/g
or larger; and
[0058] negative electrode [10]: a negative electrode which, when
charged to 60% of a nominal capacity, has a reaction resistance in
a negative-electrode opposing cell of 500.OMEGA. or lower.
[0059] The invention furthermore provides a lithium secondary
battery at least comprising: an electrode group comprising a
positive electrode, a negative electrode, and a microporous
separator interposed between the electrodes; and a nonaqueous
electrolyte comprising a nonaqueous solvent and a lithium salt
contained therein, the electrode group and the nonaqueous
electrolyte being held in a battery case, and the positive
electrode and the negative electrode each comprising a current
collector and, formed thereon, an active-material layer containing
an active material capable of occluding/releasing a lithium
ion,
[0060] wherein the nonaqueous electrolyte is a nonaqueous
electrolyte which contains at least one compound selected from the
group consisting of cyclic siloxane compounds represented by
general formula (1) given above, fluorosilane compounds represented
by general formula (2) given above, compounds represented by
general formula (3) given above, compounds having an S--F bond in
the molecule, nitric acid salts, nitrous acid salts,
monofluorophosphoric acid salts, difluorophosphoric acid salts,
acetic acid salts, and propionic acid salts in an amount of 10 ppm
or more of the whole nonaqueous electrolyte, and
the nonaqueous electrolyte satisfies any requirement selected from
the group consisting of the following electrolyte [1] to
electrolyte [9]:
[0061] electrolyte [1]: the nonaqueous solvent as a component of
the electrolyte is a mixed solvent at least containing ethylene
carbonate and the proportion of the ethylene carbonate to the whole
nonaqueous solvent is from 1% by volume to 25% by volume;
[0062] electrolyte [2]: the nonaqueous solvent as a component of
the electrolyte contains at least one asymmetric chain carbonate
and the proportion of the asymmetric chain carbonate to the whole
nonaqueous solvent is from 5% by volume to 90% by volume;
[0063] electrolyte [3]: the nonaqueous solvent as a component of
the electrolyte contains at least one chain carboxylic acid
ester;
[0064] electrolyte [4]: the nonaqueous solvent as a component of
the electrolyte contains at least one solvent having a flash point
of 70.degree. C. or higher in an amount of 60% by volume or larger
based on the whole nonaqueous solvent;
[0065] electrolyte [5]: the lithium salt as a component of the
electrolyte comprise LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein
n is an integer of 1-4) and/or lithium bis(oxalato)borate;
[0066] electrolyte [6]: the lithium salt as a component of the
electrolyte comprise a fluorine-containing lithium salt and the
nonaqueous electrolyte contains hydrogen fluoride (HF) in an amount
of from 10 ppm to 300 ppm of the whole electrolyte;
[0067] electrolyte [7]: the electrolyte contains vinylene carbonate
and the content of the vinylene carbonate is in the range of from
0.001% by mass to 3% by mass based on the whole electrolyte;
[0068] electrolyte [8]: the electrolyte further contains at least
one compound selected from the group consisting of compounds
represented by general formula (4), heterocyclic compounds
containing nitrogen and/or sulfur, cyclic carboxylic acid esters,
and fluorine-containing cyclic carbonates in an amount of from
0.001% by mass to 5% by mass based on the whole nonaqueous
electrolyte:
##STR00003##
[wherein R.sup.9 to R.sup.12 may be the same or different and each
represent a group constituted of one or more elements selected from
the group consisting of H, C, N, O, F, S, and P]; and electrolyte
[9]: the electrolyte further contains an overcharge inhibitor.
[0069] The secondary batteries described above each especially
preferably are one which has any property selected from the group
consisting of the following (1) to (3):
(1) the overall electrode area of the positive electrode is at
least 20 times the surface area of the case of the secondary
battery; (2) the secondary battery has a direct-current resistance
component of 20 milliohms (m.OMEGA.) or less; and (3) the battery
elements held in each battery case of the secondary battery have an
electric capacity of 3 ampere-hours (Ah) or higher.
[0070] The invention still further provides a lithium secondary
battery at least comprising: an electrode group comprising a
positive electrode, a negative electrode, and a microporous
separator interposed between the electrodes; and a nonaqueous
electrolyte comprising a nonaqueous solvent and a lithium salt
contained therein, the electrode group and the nonaqueous
electrolyte being held in a battery case, and the positive
electrode and the negative electrode each comprising a current
collector and, formed thereon, an active-material layer containing
an active material capable of occluding/releasing a lithium
ion,
[0071] wherein the nonaqueous electrolyte is a nonaqueous
electrolyte which contains at least one compound selected from the
group consisting of cyclic siloxane compounds represented by
general formula (1) given above, fluorosilane compounds represented
by general formula (2) given above, compounds represented by
general formula (3) given above, compounds having an S--F bond in
the molecule, nitric acid salts, nitrous acid salts,
monofluorophosphoric acid salts, difluorophosphoric acid salts,
acetic acid salts, and propionic acid salts in an amount of 10 ppm
or more of the whole nonaqueous electrolyte,
[0072] wherein the secondary battery has any property selected from
the group consisting of the following structure [1] to structure
[6]:
[0073] structure [1]: the overall electrode area of the positive
electrode is at least 20 times the surface area of the case of the
secondary battery;
[0074] structure [2]: the secondary battery has a direct-current
resistance component of 20 milliohms (m.OMEGA.) or less;
[0075] structure [3]: the battery elements held in each battery
case of the secondary battery have an electric capacity of 3
ampere-hours (Ah) or higher;
[0076] structure [4]: the current collectors of the positive
electrode and negative electrode of the secondary battery each are
made of a metallic material, and the metallic material of each
current collector has been bonded to a conductor for outwardly
discharging current by any of spot welding, high-frequency welding,
and ultrasonic welding;
[0077] structure [5]: the case of the secondary battery is made of
aluminum or an aluminum alloy; and
[0078] structure [6]: the battery case of the secondary battery is
made of a casing material in which at least part of the battery
inner side comprises a sheet formed from a thermoplastic resin and
with which the electrode group can be enclosed by placing the
electrode group in the casing material and heat sealing layers of
the thermoplastic resin to each other.
[0079] The invention still further provides a nonaqueous
electrolyte for secondary battery which at least comprises a
nonaqueous solvent and a lithium salt, wherein
[0080] the nonaqueous electrolyte contains at least one compound
selected from the group consisting of cyclic siloxane compounds
represented by general formula (1) given above, fluorosilane
compounds represented by general formula (2) given above, compounds
represented by general formula (3) given above, compounds having an
S--F bond in the molecule, nitric acid salts, nitrous acid salts,
monofluorophosphoric acid salts, difluorophosphoric acid salts,
acetic acid salts, and propionic acid salts in an amount of 10 ppm
or more of the whole nonaqueous electrolyte, and
[0081] the nonaqueous electrolyte satisfies any requirement
selected from the group consisting of the following electrolyte [1]
to electrolyte [9]:
[0082] electrolyte [1]: the nonaqueous solvent as a component of
the electrolyte is a mixed solvent at least containing ethylene
carbonate and the proportion of the ethylene carbonate to the whole
nonaqueous solvent is from 1% by volume to 25% by volume;
[0083] electrolyte [2]: the nonaqueous solvent as a component of
the electrolyte contains at least one asymmetric chain carbonate
and the proportion of the asymmetric chain carbonate to the whole
nonaqueous solvent is from 5% by volume to 90% by volume;
[0084] electrolyte [3]: the nonaqueous solvent as a component of
the electrolyte contains at least one chain carboxylic acid
ester;
[0085] electrolyte [4]: the nonaqueous solvent as a component of
the electrolyte contains at least one solvent having a flash point
of 70.degree. C. or higher in an amount of 60% by volume or larger
based on the whole nonaqueous solvent;
[0086] electrolyte [5]: the lithium salt as a component of the
electrolyte comprise LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein
n is an integer of 1-4) and/or lithium bis(oxalato)borate;
[0087] electrolyte [6]: the lithium salt as a component of the
electrolyte comprise a fluorine-containing lithium salt and the
nonaqueous electrolyte contains hydrogen fluoride (HF) in an amount
of from 10 ppm to 300 ppm of the whole electrolyte;
[0088] electrolyte [7]: the electrolyte contains vinylene carbonate
and the content of the vinylene carbonate is in the range of from
0.001% by mass to 3% by mass based on the whole electrolyte;
[0089] electrolyte [8]: the electrolyte further contains at least
one compound selected from the group consisting of compounds
represented by general formula (4) given above, heterocyclic
compounds containing nitrogen and/or sulfur, cyclic carboxylic acid
esters, and fluorine-containing cyclic carbonates in an amount of
from 0.001% by mass to 5% by mass based on the whole nonaqueous
electrolyte; and
[0090] electrolyte [9]: the electrolyte further contains an
overcharge inhibitor.
Advantages of the Invention
[0091] According to the invention, lithium secondary batteries
especially having excellent low-temperature discharge
characteristics can be provided. The embodiments of the invention
which will be described later produce, for example, the following
effects.
Positive Electrode [1]:
[0092] According to the invention, a lithium secondary battery
especially suitable for use as a large battery in, e.g.,
applications such as motor vehicles can be provided because a long
life and a high output are obtained with less expensive
materials.
Positive Electrode [2]:
[0093] According to the invention, a lithium secondary battery
having a high capacity, long life, and high output can be provided,
and a lithium secondary battery especially suitable for use as a
large battery in, e.g., applications such as motor vehicles can be
provided.
Positive Electrode [3]:
[0094] According to the invention, a lithium secondary battery
having a long life and a high output is obtained and, hence, a
lithium secondary battery especially suitable for use as a large
battery in, e.g., applications such as motor vehicles can be
provided.
Positive Electrode [4]:
[0095] According to the invention, a lithium secondary battery
having a high output is obtained and, hence, a lithium secondary
battery especially suitable for use as a large battery in, e.g.,
applications such as motor vehicles can be provided.
Positive Electrode [5]:
[0096] According to the invention, a lithium secondary battery
which has a high capacity, long life, and high output and is
inhibited from decreasing in battery capacity or output with
repetitions of charge/discharge (has excellent suitability for
repetitions of charge/discharge (cycle characteristics)) is
obtained. Consequently, a lithium secondary battery especially
suitable for use as a large battery in, e.g., applications such as
motor vehicles can be provided.
Negative Electrode [1]:
[0097] According to the invention, a lithium secondary battery can
be provided which can realize high-output characteristics from an
initial to a final stage in a cycle while retaining improved
suitability for cycling and which, even after having deteriorated
through charge/discharge cycling, retains high-output
characteristics. This battery is suitable especially for use as a
large battery.
Negative Electrode [2]:
[0098] According to the invention, a lithium secondary battery
having satisfactory suitability for short-time high-current-density
charge/discharge can be provided.
Negative Electrode [3]:
[0099] According to the invention, a lithium secondary battery
having a low output resistance and enabling the effective
utilization of energy can be provided.
Negative Electrode [4]:
[0100] According to the invention, a lithium secondary battery
improved in high-temperature storability at a low state of charge
can be provided.
Negative Electrode [5]:
[0101] According to the invention, a lithium secondary battery can
be provided which can retain satisfactory suitability for long-term
repetitions of charge/discharge at a low state of charge.
Negative Electrode [6]:
[0102] According to the invention, a lithium secondary battery
showing a quick output recovery from a low-output state at low
temperatures can be provided.
Negative Electrode [7]:
[0103] According to the invention, a lithium secondary battery can
be provided which, even when fabricated so as to have a larger
size, has a high capacity and is satisfactorily chargeable.
Negative Electrode [8]:
[0104] According to the invention, a lithium secondary battery
having both satisfactory cycle characteristics and a satisfactory
low-temperature output can be provided.
Negative Electrodes [9] and [10]:
[0105] According to the invention, a lithium secondary battery can
be provided which, even when fabricated so as to have a larger
size, has a high capacity retention through cycling and can attain
a satisfactory battery life, and which can attain a high output
even after a charge/discharge cycle test. This battery has both of
these properties.
Electrolytic Solution [1]:
[0106] According to the invention, the low-temperature
characteristics of a nonaqueous-electrolyte secondary battery can
be greatly improved. More specifically, the low-temperature
characteristics can be improved without impairing cycle
characteristics and storability.
Electrolytic Solution [2]:
[0107] According to the invention, a nonaqueous electrolyte for
secondary battery greatly improved in both cycle characteristics
and low-temperature characteristics and a secondary battery which
employs this nonaqueous electrolyte and is excellent in these
performances can be provided.
Electrolytic Solution [3]:
[0108] According to the invention, a nonaqueous electrolyte for
secondary battery greatly improved in low-temperature output
characteristics and a secondary battery having such performances
can be provided.
Electrolytic Solution [4]:
[0109] According to the invention, a nonaqueous electrolyte for
secondary battery can be provided which has an electrolyte
composition less apt to pose problems associated with the
incorporation of a large amount of a low-viscosity solvent, such as
the problem that salt precipitation, a decrease in flash point, and
an increase in internal pressure at high temperatures are apt to
occur due to solvent vaporization, and which, despite this, enables
high low-temperature characteristics and high output
characteristics to be retained. A secondary battery having such
performances can also be provided.
Electrolytic Solution [5]:
[0110] According to the invention, a nonaqueous electrolyte for
secondary battery which greatly improves output characteristics and
brings about excellent high-temperature storability and excellent
cycle characteristics and a secondary battery having such
performances can be provided.
Electrolytic Solution [6]:
[0111] According to the invention, a nonaqueous electrolyte for
secondary battery excellent in high-temperature storability and
cycle characteristics and greatly improved in output
characteristics can be provided.
Electrolytic Solution [7]:
[0112] According to the invention, a secondary battery greatly
improved in both cycle characteristics and low-temperature
characteristics can be provided.
Electrolytic Solution [8]:
[0113] According to the invention, a nonaqueous electrolyte for
secondary battery greatly improved in low-temperature discharge
characteristics and excellent in high-temperature storability and
cycle characteristics and a secondary battery having such
performances can be provide.
Electrolytic Solution [9]:
[0114] According to the invention, a secondary battery satisfying
both of high rate characteristics and safety in overcharge can be
provided.
Structures [1] to [5]:
[0115] According to the invention, a lithium secondary battery
which has a high capacity, long life, and high output and is highly
safe in overcharge is obtained. Consequently, a lithium secondary
battery especially suitable for use as a large battery in
applications such as, e.g., motor vehicles can be provided.
Structure [6]:
[0116] According to the invention, a lithium secondary battery
which has a high capacity, long life, and high output, is reduced
in gas evolution, and is highly safe in overcharge is obtained.
Consequently, a lithium secondary battery especially suitable for
use as a large battery in applications such as, e.g., motor
vehicles can be provided.
BEST MODE FOR CARRYING OUT THE INVENTION
[0117] Embodiments of the invention will be explained below in
detail. The following explanations on constituent elements are for
embodiments (typical embodiments) of the invention, and should not
be construed as limiting the invention unless they depart from the
spirit of the invention.
[0118] The lithium secondary batteries of the invention each are a
lithium secondary battery which comprises an electrode group
constituted at least of a positive electrode, a negative electrode,
and a microporous separator interposed between the electrodes and a
nonaqueous electrolyte comprising a nonaqueous solvent and one or
more lithium salts contained therein, the electrode group and the
nonaqueous electrolyte being held in a battery case, and in
which
the positive electrode and the negative electrode each comprise a
current collector and, formed thereon, an active-material layer
containing an active material capable of occluding/releasing
lithium ions, the nonaqueous electrolyte is a nonaqueous
electrolyte which contains at least one compound selected from the
group consisting of cyclic siloxane compounds represented by
general formula (1) given above, fluorosilane compounds represented
by general formula (2) given above, compounds represented by
general formula (3) given above, compounds having an S--F bond in
the molecule, nitric acid salts, nitrous acid salts,
monofluorophosphoric acid salts, difluorophosphoric acid salts,
acetic acid salts, and propionic acid salts (hereinafter, these
compounds are often abbreviated to "specific compounds") in an
amount of 10 ppm or more of the whole nonaqueous electrolyte, and
the positive electrode, negative electrode, electrolyte, or battery
constitution satisfies a specific requirement.
[0119] The lithium secondary batteries of the invention will be
explained below in detail.
<Positive Electrode>
[0120] The positive electrode for use in the lithium secondary
batteries of the invention is explained below.
[0121] The positive electrode for use in the invention is not
particularly limited as long as it has an active-material layer
containing an active material capable of occluding and releasing
lithium ions. However, it preferably is any positive electrode
selected from the group consisting of the following positive
electrode [1] to positive electrode [5]:
positive electrode [1]: a positive electrode containing a
positive-electrode active material containing manganese; positive
electrode [2]: a positive electrode containing a positive-electrode
active material having a composition represented by the following
composition formula (4):
Li.sub.xNi.sub.(1-y-z)Co.sub.yM.sub.zO.sub.2 composition formula
(4)
[wherein M represents at least one element selected from the group
consisting of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x
represents a number satisfying 0<x.ltoreq.1.2; y represents a
number satisfying 0.05.ltoreq.y.ltoreq.0.5; and z represents a
number satisfying 0.01.ltoreq.z.ltoreq.0.5]; positive electrode
[3]: a positive electrode containing a positive-electrode active
material selected from the group consisting of the following (a) to
(d): (a) a positive-electrode active material having a BET specific
surface area of from 0.4 m.sup.2/g to 2 m.sup.2/g, (b) a
positive-electrode active material having an average
primary-particle diameter of from 0.1 .mu.m to 2 .mu.m, (c) a
positive-electrode active material having a median diameter
d.sub.50 of from 1 .mu.m to 20 .mu.m, (d) a positive-electrode
active material having a tap density of from 1.3 g/cm.sup.3 to 2.7
g/cm.sup.3; positive electrode [4]: a positive electrode satisfying
any requirement selected from the group consisting of the following
(e) to (f): (e) it is a positive electrode produced by forming a
positive-electrode active-material layer comprising a
positive-electrode active material, a conductive material, and a
binder on a current collector, wherein the content of the
conductive material in the positive-electrode active-material layer
is in the range of from 6% by mass to 20% by mass, (f) it is a
positive electrode produced by forming a positive-electrode
active-material layer comprising a positive-electrode active
material and a binder on a current collector, wherein the
positive-electrode active-material layer has a density in the range
of from 1.7 g/cm.sup.3 to 3.5 g/cm.sup.3, (g) it is a positive
electrode produced by forming a positive-electrode active-material
layer comprising a positive-electrode active material and a binder
on a current collector, wherein the ratio of the thickness of the
positive-electrode active-material layer to the thickness of the
current collector, i.e., the value of (thickness of the
active-material layer on one side just before impregnation with the
nonaqueous electrolyte)/(thickness of the current collector), is in
the range of from 1.6 to 20; and positive electrode [5]: a positive
electrode containing two or more positive-electrode active
materials differing in composition.
[0122] Positive electrodes which are usually employed in the
lithium secondary batteries of the invention will be first
explained below.
[Positive-Electrode Active Material]
[0123] The positive-electrode active material for general use in
the positive electrodes will be described below.
[[Composition]]
[0124] The positive-electrode active material is not particularly
limited as long as it is capable of electrochemically
occluding/releasing lithium ions. It preferably is a substance
containing lithium and at least one transition metal. Examples
thereof include lithium-transition metal composite oxides and
lithium-containing transition metal/phosphoric acid compounds.
[0125] The transition metal in the lithium-transition metal
composite oxides preferably is V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or
the like. Specific examples of the composite oxides include
lithium-cobalt composite oxides such as LiCoO.sub.2, lithium-nickel
composite oxides such as LiNiO.sub.2, lithium-manganese composite
oxides such as LiMnO.sub.2, LiMn.sub.2O.sub.4, and
Li.sub.2MnO.sub.3, and ones formed by partly replacing the
transition metal atom(s) as a main component of these
lithium-transition metal composite oxides by one or more other
metals, e.g., Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga,
Zr, Si, etc. Examples of such compounds formed by replacement
include LiNi.sub.0.5Mn.sub.0.5O.sub.2,
LiNi.sub.0.85Co.sub.0.10Al.sub.0.05O.sub.2,
LiNi.sub.0.33Co.sub.0.33Mn.sub.0.33O.sub.2,
LiMn.sub.1.8Al.sub.0.2O.sub.4, and
LiMn.sub.1.5Ni.sub.0.5O.sub.4.
[0126] The transition metal in the lithium-containing transition
metal/phosphoric acid compounds preferably is V, Ti, Cr, Mn, Fe,
Co, Ni, Cu, or the like. Specific examples of the compounds include
iron phosphate compounds such as LiFePO.sub.4,
Li.sub.3Fe.sub.2(PO.sub.4).sub.3, and LiFeP.sub.2O.sub.7, cobalt
phosphate compounds such as LiCoPO.sub.4, and ones formed by partly
replacing the transition metal atom(s) as a main component of these
lithium-transition metal/phosphoric acid compounds by one or more
other metals, e.g., Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg,
Ga, Zr, Nb, Si, etc.
[[Surface Coating]]
[0127] It is preferred to use a material comprising any of those
positive-electrode active materials and, adherent to the surface
thereof, a substance having a composition different from that of
the core positive-electrode active material. Examples of the
substance to be adhered to the surface (hereinafter abbreviated to
"surface-adherent substance") include oxides such as aluminum
oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium
oxide, calcium oxide, boron oxide, antimony oxide, and bismuth
oxide, sulfates such as lithium sulfate, sodium sulfate, potassium
sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate,
and carbonates such as lithium carbonate, calcium carbonate, and
magnesium carbonate.
[0128] Those surface-adherent substances each can be adhered to the
surface of a positive-electrode active material, for example, by: a
method in which the substance is dissolved or suspended in a
solvent and this solution or suspension is infiltrated into a
positive-electrode active material and dried; a method in which a
precursor for the surface-adherent substance is dissolved or
suspended in a solvent and this solution or suspension is
infiltrated into a positive-electrode active material and then
heated or otherwise treated to react the precursor; or a method in
which the substance is added to a precursor for a
positive-electrode active material and heat-treated together with
the precursor.
[0129] The amount of the surface-adherent substance to be used may
be as follows. The lower limit thereof is preferably 0.1 ppm or
larger, more preferably 1 ppm or larger, even more preferably 10
ppm or larger, in terms of mass ppm of the positive-electrode
active material. The upper limit thereof is preferably 20% or
smaller, more preferably 10% or smaller, even more preferably 5% or
smaller, in terms of % by mass based on the positive-electrode
active material. The surface-adherent substance serves to inhibit
the nonaqueous electrolyte from undergoing an oxidation reaction on
the surface of the positive-electrode active material, whereby the
battery life can be improved. However, in case where the amount of
the substance adhered is too small, that effect is not sufficiently
produced. On the other hand, too large amounts thereof may result
in an increase in resistance because the surface-adherent substance
inhibits the occlusion/release of lithium ions.
[[Shape]]
[0130] The particle shape of the positive-electrode active material
to be used in the invention may be any of massive, polyhedral,
spherical, ellipsoidal, platy, acicular, columnar, and other shapes
such as those in common use. Preferred of these is one in which the
primary particles have aggregated to form secondary particles and
these secondary particles have a spherical or ellipsoidal shape. In
electrochemical elements, the active material in each electrode
usually expands/contracts with the charge/discharge of the element
and, hence, deterioration is apt to occur, such as active-material
breakage and conduction path breakage, due to the stress caused by
the expansion/contraction. Because of this, a positive-electrode
active material in which the primary particles have aggregated to
form secondary particles is preferable to an active material
composed of primary particles only since the particles in the
former active material relieve the stress caused by
expansion/contraction to prevent deterioration. Furthermore,
particles of a spherical or ellipsoidal shape are preferable to
particles showing axial orientation, e.g., platy ones, because the
former particles are less apt to orient during electrode molding
and hence this electrode is reduced in expansion/contraction during
charge/discharge, and because these particles are apt to be evenly
mixed with a conductive material in electrode production.
[[Tap Density]]
[0131] The tap density of the positive-electrode active material is
generally 1.3 g/cm.sup.3 or higher, preferably 1.5 g/cm.sup.3 or
higher, more preferably 1.6 g/cm.sup.3 or higher, most preferably
1.7 g/cm.sup.3 or higher. When the tap density of the
positive-electrode active material is lower than the lower limit,
there are cases where it is necessary to use a larger amount of a
dispersion medium and larger amounts of a conductive material and a
binder in forming a positive-electrode active-material layer and
this results in a limited loading of the positive-electrode active
material in the positive-electrode active-material layer and a
limited battery capacity. By employing a composite oxide powder
having a high tap density, a positive-electrode active-material
layer having a high density can be formed. The higher the tap
density, the more the positive-electrode active material is
generally preferred. Although there is no particular upper limit
thereon, too high tap densities may result in cases where the
diffusion of lithium ions in the positive-electrode active-material
layer through the nonaqueous electrolyte as a medium becomes a
rate-determining stage and this is apt to reduce load
characteristics. Consequently, the tap density of the
positive-electrode active material is generally 2.9 g/cm.sup.3 or
lower, preferably 2.7 g/cm.sup.3 or lower, more preferably 2.5
g/cm.sup.3 or lower.
[0132] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the capacity of
the cell with the sample, subsequently conducting tapping
operations 1,000 times over a stroke length of 10 mm using a powder
densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise
Co., Ltd.), and determining a density from the resultant volume of
the sample and the weight thereof. This density is defined as the
tap density.
[[Median Diameter d.sub.50]]
[0133] The median diameter d.sub.50 (secondary-particle diameter in
the case where the primary particles have aggregated to form
secondary particles) of the particles of the positive-electrode
active material is generally 0.1 .mu.m or larger, preferably 0.5
.mu.m or larger, more preferably 1 .mu.m or larger, most preferably
3 .mu.m or larger. The upper limit thereof is generally 20 .mu.m or
smaller, preferably 18 .mu.m or smaller, more preferably 16 .mu.m
or smaller, most preferably 15 .mu.m or smaller. When the median
diameter thereof is smaller than the lower limit, there are cases
where a product having a high tap density cannot be obtained. When
the median diameter thereof exceeds the upper limit, lithium
diffusion in the individual particles requires a longer time and
this results in a decrease in battery performance. In addition,
there are cases where such positive-electrode active-material
particles, when used in producing a positive electrode for
batteries, i.e., when the active material and other ingredients
including a conductive material and a binder are slurried with a
solvent and this slurry is applied in a thin-film form, pose a
problem, for example, that streak lines generate. It is possible to
further improve loading in positive-electrode production by mixing
two or more positive-electrode active materials differing in median
diameter d.sub.50.
[0134] Median diameter d.sub.50 in the invention is determined with
a known laser diffraction/scattering type particle size
distribution analyzer. In the case where LA-920, manufactured by
HORIBA, Ltd., is used as a particle size distribution analyzer, a
0.1% by mass aqueous solution of sodium hexametaphosphate is used
as a dispersion medium for the examination and a five-minute
ultrasonic dispersing treatment is conducted before the particles
are examined at a measuring refractive index set at 1.24.
[[Average Primary-Particle Diameter]]
[0135] In the case where the primary particles have aggregated to
form secondary particles, the average primary-particle diameter of
this positive-electrode active material is generally 0.01 .mu.m or
larger, preferably 0.05 .mu.m or larger, more preferably 0.08 .mu.m
or larger, most preferably 0.1 .mu.m or larger, and is generally 3
.mu.m or smaller, preferably 2 .mu.m or smaller, more preferably 1
.mu.m or smaller, most preferably 0.6 .mu.m or smaller. In case
where the average primary-particle diameter thereof exceeds the
upper limit, spherical secondary particles are difficult to form
and this adversely influences powder loading or results in a
considerably reduced specific surface area. There may hence be a
high possibility that battery performances such as output
characteristics might decrease. Conversely, when the average
primary-particle diameter thereof is smaller than the lower limit,
crystal growth is usually insufficient and, hence, there are cases
where this positive-electrode active material poses a problem, for
example, that charge/discharge reversibility is poor.
[0136] Primary-particle diameter is determined through an
examination with a scanning electron microscope (SEM).
Specifically, arbitrarily selected 50 primary-particle images in a
photograph having a magnification of 10,000 diameters each are
examined for the length of the longest segment of a horizontal line
which extends across the primary-particle image from one side to
the other side of the boundary. These measured lengths are
averaged, whereby the average value can be determined.
[[BET Specific Surface Area]]
[0137] The BET specific surface area of the positive-electrode
active material to be used in the lithium secondary batteries of
the invention is generally 0.2 m.sup.2/g or larger, preferably 0.3
m.sup.2/g or larger, more preferably 0.4 m.sup.2/g or larger. The
upper limit thereof is generally 4.0 m.sup.2/g or smaller,
preferably 2.5 m.sup.2/g or smaller, more preferably 1.5 m.sup.2/g
or smaller. In case where the BET specific surface area thereof is
smaller than the lower limit of that range, battery performances
are apt to decrease. In case where the BET specific surface area
thereof exceeds the upper limit, a high tap density is difficult to
obtain and this may pose a problem concerning applicability in
forming a positive-electrode active material.
[0138] BET specific surface area is measured with a surface area
meter (e.g., a fully automatic surface area measuring apparatus
manufactured by Ohukura Riken Co., Ltd.). The specific surface area
is defined as a value obtained by preliminarily drying a sample at
150.degree. C. for 30 minutes in a nitrogen stream and then
measuring the specific surface area thereof by the gas-flowing
nitrogen adsorption BET one-point method using a nitrogen/helium
mixture gas precisely regulated so as to have a nitrogen pressure
of 0.3 relative to atmosphere pressure.
[[Production Processes]]
[0139] For producing the positive-electrode active material,
processes in general use for producing inorganic compounds may be
employed. Especially for producing spherical or ellipsoidal active
materials, various techniques are usable. Examples thereof include:
a method which comprises dissolving or pulverizing/dispersing a
transition metal source, e.g., a transition metal nitrate or
transition metal sulfate, optionally together with a source of
other element(s) in a solvent, e.g., water, regulating the pH of
the solution or dispersion with stirring to produce a spherical
precursor, recovering and optionally drying the precursor,
subsequently adding thereto an Li source, e.g., LiOH,
Li.sub.2CO.sub.3, or LiNO.sub.3, and heat-treating the mixture at a
high temperature to obtain the active material; a method which
comprises dissolving or pulverizing/dispersing a transition metal
source, e.g., a transition metal nitrate, transition metal sulfate,
transition metal hydroxide, or transition metal oxide, optionally
together with a source of other element(s) in a solvent, e.g.,
water, drying and forming the solution or dispersion with a spray
dryer or the like to obtain a spherical or ellipsoidal precursor,
adding thereto an Li source, e.g., LiOH, Li.sub.2CO.sub.3, or
LiNO.sub.3, and heat-treating the mixture at a high temperature to
obtain the active material; and a method which comprises dissolving
or pulverizing/dispersing a transition metal source, e.g., a
transition metal nitrate, transition metal sulfate, transition
metal hydroxide, or transition metal oxide, together with an Li
source, e.g., LiOH, Li.sub.2CO.sub.3, or LiNO.sub.3, and optionally
with a source of other element(s) in a solvent, e.g., water, drying
and forming the solution or dispersion with a spray dryer or the
like to obtain a spherical or ellipsoidal precursor, and
heat-treating the precursor at a high temperature to obtain the
active material.
[Constitution of Positive Electrode]
[0140] The constitution of the positive electrode to be used in the
invention will be described below.
[[Electrode Structure and Production Process]]
[0141] The positive electrode to be used in the lithium secondary
batteries of the invention is produced by forming a
positive-electrode active-material layer comprising a
positive-electrode active material and a binder on a current
collector. The production of the positive electrode with a
positive-electrode active material can be conducted in an ordinary
manner. Namely, a positive-electrode active material and a binder
are mixed together by a dry process optionally together with a
conductive material, thickener, etc. and this mixture is formed
into a sheet and press-bonded to a positive-electrode current
collector. Alternatively, those materials are dissolved or
dispersed in a liquid medium to obtain a slurry and this slurry is
applied to a positive-electrode current collector and dried. Thus,
a positive-electrode active-material layer is formed on the current
collector, whereby the positive electrode can be obtained.
[0142] The content of the positive-electrode active material in the
positive-electrode active-material layer in the invention is
generally 10% by mass or higher, preferably 30% by mass or higher,
especially preferably 50% by mass or higher. The upper limit
thereof is generally 99.9% by mass or lower, preferably 99% by mass
or lower. When the content of the positive-electrode
active-material particles in the positive-electrode active-material
layer is too low, there are cases where an insufficient electrical
capacity results. Conversely, when the content thereof is too high,
there are cases where the positive electrode has an insufficient
strength. One positive-electrode active material may be used alone,
or any desired combination of two or more positive-electrode active
materials differing in composition or powder properties may be used
in any desired proportion.
[[Compaction]]
[0143] It is preferred that the positive-electrode active-material
layer obtained by coating fluid application and drying should be
compacted with a handpress, roller press, or the like in order to
heighten the loading density of the positive-electrode active
material. The lower limit of the density of the positive-electrode
active-material layer is preferably 1 g/cm.sup.3 or higher, more
preferably 1.5 g/cm.sup.3 or higher, even more preferably 2
g/cm.sup.3 or higher. The upper limit thereof is preferably 4
g/cm.sup.3 or lower, more preferably 3.5 g/cm.sup.3 or lower, even
more preferably 3 g/cm.sup.3 or lower. When the density thereof
exceeds the upper limit of that range, the infiltration of a
nonaqueous electrolyte into around the current collector/active
material interface becomes insufficient and there are cases where
charge/discharge characteristics especially at a high current
density decrease. When the density thereof is lower than the lower
limit, there are cases where electrical conductivity among the
active-material particles decreases to increase battery
resistance.
[[Conductive Material]]
[0144] As the conductive material, a known conductive material can
be used at will. Examples thereof include metallic materials such
as copper and nickel; graphites such as natural graphites and
artificial graphites; carbon blacks such as acetylene black; and
carbon materials such as amorphous carbon, e.g., needle coke. One
of these materials may be used alone, or any desired combination of
two or more thereof in any desired proportion may be used.
[0145] The conductive material may be used so that it is
incorporated in the positive-electrode active-material layer in an
amount of generally 0.01% by mass or larger, preferably 0.1% by
mass or larger, more preferably 1% by mass or larger, and that the
upper limit of the content thereof is generally 50% by mass or
lower, preferably 30% by mass or lower, more preferably 15% by mass
or lower. When the content thereof is lower than the lower limit of
that range, there are cases where electrical conductivity becomes
insufficient. Conversely, when the content thereof exceeds the
upper limit of that range, there are cases where battery capacity
decreases.
[[Binder]]
[0146] The binder to be used for producing the positive-electrode
active-material layer is not particularly limited. In the case
where the layer is to be formed through coating fluid application,
any binder may be used as long as it is a material which is soluble
or dispersible in the liquid medium for use in electrode
production. Examples thereof include resinous polymers such as
polyethylene, polypropylene, poly(ethylene terephthalate),
poly(methyl methacrylate), aromatic polyamides, cellulose, and
nitrocellulose; rubbery polymers such as SBR (styrene/butadiene
rubbers), NBR (acrylonitrile/butadiene rubbers), fluororubbers,
isoprene rubbers, butadiene rubbers, and ethylene/propylene
rubbers; thermoplastic elastomeric polymers such as
styrene/butadiene/styrene block copolymers or products of
hydrogenation thereof, EPDM (ethylene/propylene/diene terpolymers),
styrene/ethylene/butadiene/ethylene copolymers, and
styrene/isoprene/styrene block copolymers or products of
hydrogenation thereof; flexible resinous polymers such as
syndiotactic 1,2-polybutadiene, poly(vinyl acetate), ethylene/vinyl
acetate copolymers, and propylene/.alpha.-olefin copolymers;
fluorochemical polymers such as poly(vinylidene fluoride) (PVdF),
polytetrafluoroethylene, fluorinated poly(vinylidene fluoride), and
polytetrafluoroethylene/ethylene copolymers; and polymer
compositions having the property of conducting alkali metal ions
(especially lithium ions). One of these substances may be used
alone, or any desired combination of two or more thereof in any
desired proportion may be used.
[0147] The proportion of the binder in the positive-electrode
active-material layer is generally 0.1% by mass or higher,
preferably 1% by mass or higher, more preferably 3% by mass or
higher. The upper limit thereof is generally 80% by mass or lower,
preferably 60% by mass or lower, more preferably 40% by mass or
lower, most preferably 10% by mass or lower. When the proportion of
the binder is too low, there are cases where the positive-electrode
active material cannot be sufficiently held and the positive
electrode has an insufficient mechanical strength to impair battery
performances such as cycle characteristics. On the other hand, too
high proportions thereof may lead to a decrease in battery capacity
or conductivity.
[[Liquid Medium]]
[0148] The kind of the liquid medium to be used for forming a
slurry is not particularly limited as long as it is a solvent in
which the positive-electrode active material, conductive material,
and binder and a thickener, which is used according to need, can be
dissolved or dispersed. Either an aqueous solvent or an organic
solvent may be used.
[0149] Examples of the aqueous medium include water and mixed
solvents comprising an alcohol and water. Examples of the organic
medium include aliphatic hydrocarbons such as hexane; aromatic
hydrocarbons such as benzene, toluene, xylene, and
methylnaphthalene; heterocyclic compounds such as quinoline and
pyridine; ketones such as acetone, methyl ethyl ketone, and
cyclohexanone; esters such as methyl acetate and methyl acrylate;
amines such as diethylenetriamine and N,N-dimethylaminopropylamine;
ethers such as diethyl ether, propylene oxide, and tetrahydrofuran
(THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide,
and dimethylacetamide; and aprotic polar solvents such as
hexamethylphosphoramide and dimethyl sulfoxide.
[0150] Especially when an aqueous medium is used, it is preferred
to use a thickener and a latex of, e.g., a styrene/butadiene rubber
(SBR) to prepare a slurry. A thickener is used generally for the
purpose of regulating the slurry viscosity. The thickener is not
particularly limited. Examples thereof include carboxymethyl
cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl
cellulose, poly(vinyl alcohol), oxidized starch, phosphorylated
starch, casein, and salts of these. One of these thickeners may be
used alone, or any desired combination of two or more thereof in
any desired proportion may be used. In the case where such a
thickener is further added, the proportion of the thickener to the
active material is generally 0.1% by mass or higher, preferably
0.5% by mass or higher, more preferably 0.6% by mass or higher. The
upper limit thereof is generally 5% by mass or lower, preferably 3%
by mass or lower, more preferably 2% by mass or lower. When the
proportion thereof is lower than the lower limit of that range,
there are cases where applicability decreases considerably.
Proportions thereof exceeding the upper limit result in a reduced
proportion of the active material in the positive-electrode
active-material layer, and this may pose a problem that battery
capacity decreases and a problem that resistance among the
particles of the positive-electrode active material increases.
[[Current Collector]]
[0151] The material of the positive-electrode current collector is
not particularly limited, and a known one can be used at will.
Examples thereof include metallic materials such as aluminum,
stainless steel, nickel-plated materials, titanium, and tantalum;
and carbon materials such as carbon cloths and carbon papers. Of
these, metallic materials are preferred. Especially preferred is
aluminum.
[0152] In the case of a metallic material, examples of the shape of
the current collector include metal foils, metal cylinders, metal
coils, metal plates, thin metal films, expanded metals, punching
metals, and metal foams. In the case of a carbon material, examples
of the collector shape include carbon plates, thin carbon films,
and carbon cylinders. Of these, a thin metal film is preferred. The
thin film may be in a suitable mesh form. Although the thin film
may have any desired thickness, the thickness thereof is generally
1 .mu.m or larger, preferably 3 .mu.m or larger, more preferably 5
.mu.m or larger, and is generally 1 mm or smaller, preferably 100
.mu.m or smaller, more preferably 50 .mu.m or smaller. When the
thin film is thinner than the lower limit of that range, there are
cases where this film is deficient in strength required of a
current collector. Conversely, when the thin film is thicker than
the upper limit of that range, there are cases where this film has
impaired handleability.
[0153] The thickness ratio between the current collector and the
positive-electrode active-material layer is not particularly
limited. However, the value of (thickness of the active-material
layer on one side just before impregnation with the nonaqueous
electrolyte)/(thickness of the current collector) is preferably 150
or smaller, more preferably 20 or smaller, most preferably 10 or
smaller. The lower limit thereof is preferably 0.1 or larger, more
preferably 0.4 or larger, most preferably 1 or larger. When that
ratio exceeds the upper limit of that range, there are cases where
this current collector heats up due to Joule's heat during
high-current-density charge/discharge. When that ratio decreases
beyond the lower limit of that range, the proportion by volume of
the current collector to the positive-electrode active material
increases and this may reduce the capacity of the battery.
[[Electrode Area]]
[0154] When the nonaqueous electrolyte of the invention is used, it
is preferred that the area of the positive-electrode
active-material layer relative to the outer surface area of the
battery case should be large from the standpoint of attaining a
high output and enhanced high-temperature stability. Specifically,
the overall electrode area of the positive electrode is preferably
at least 15 times, more preferably at least 40 times, the surface
area of the case of the secondary battery. When the battery has a
bottomed polygonal shape, the term "outer surface area of the case"
means a total area calculated from the length, width, and thickness
dimensions of that case part packed with power-generating elements
which excludes the terminal projections. When the battery has a
bottomed cylindrical shape, that term means a geometric surface
area obtained through the approximation to a cylinder of that case
part packed with power-generating elements which excludes the
terminal projections. The term "overall electrode area of the
positive electrode" means the geometric surface area of the
positive-electrode mix layer facing the mix layer containing a
negative-electrode active material. In a structure in which a
positive-electrode mix layer has been formed on each side of a
current collector foil, that term means the sum of the areas
separately calculated respectively for both sides.
[[Discharge Capacity]]
[0155] When the nonaqueous electrolyte for secondary battery of the
invention is used, it is preferred that the battery elements held
in each battery case of the secondary battery should have an
electric capacity (electric capacity measured when the battery in a
fully charged state is discharged to a discharged state) of 3
ampere-hours (Ah) or higher. This is because such electric capacity
is highly effective in improving low-temperature discharge
characteristics.
[0156] For attaining that, a positive-electrode plate is designed
to have a discharge capacity as measured in discharge from a fully
charged state of preferably from 3 ampere-hours (Ah) to less than
20 Ah, more preferably from 4 Ah to less than 10 Ah. When the
discharge capacity thereof is lower than 3 Ah, there are cases
where high-current discharge results in a large decrease in voltage
due to electrode reaction resistance and hence in a poor power
efficiency. When the discharge capacity thereof is 20 Ah or higher,
electrode reaction resistance is low and a satisfactory power
efficiency is obtained. In this case, however, the battery has a
wide temperature distribution during pulse charge/discharge due to
internal heat generation and has poor durability in repetitions of
charge/discharge. In addition, the efficiency of heat dissipation
in the case of abrupt heat generation in an abnormality, such as
overcharge or internal short-circuiting, is also poor. There are
hence cases where the probability that the poor heat dissipation
efficiency leads to a phenomenon in which the internal pressure
increases and the gas release valve works (valve working) or a
phenomenon in which the battery contents are vigorously ejected
outside (rupture) increases.
[[Thickness of Positive-Electrode Plate]]
[0157] The thickness of the positive-electrode plate is not
particularly limited. However, from the standpoints of high
capacity and high output, the thickness of the mix layer, excluding
the thickness of the core metal foil, on one side of the current
collector is such that the lower limit thereof is preferably 10
.mu.m or larger, more preferably 20 .mu.m or larger, and the upper
limit thereof is preferably 200 .mu.m or smaller, more preferably
100 .mu.m or smaller.
<Positive Electrode [1]>
[0158] Positive electrode [1], i.e., "a positive electrode
containing a positive-electrode active material containing
manganese", for use in a lithium secondary battery of the invention
will be explained below.
[Positive Electrode [1]: Positive-Electrode Active Material]
[0159] The positive-electrode active material for use in positive
electrode [1] is described below.
[[Composition]]
[0160] The positive-electrode active material is a substance which
contains one or more transition metals and is capable of
electrochemically occluding/releasing lithium ions. One containing
manganese as at least part of the transition metals is used in this
invention. It is preferred that lithium should be further
contained. More preferred is a composite oxide containing lithium
and manganese.
[0161] The manganese-containing positive-electrode active material
is not particularly limited. However, it preferably is one having a
composition represented by the following composition formula
(5):
Li.sub.xMn.sub.(1-y)M.sup.1.sub.yO.sub.2 composition formula
(5)
[wherein M.sup.1 represents at least one element selected from the
group consisting of Ni, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb;
x represents a number satisfying 0<x.ltoreq.1.2; and y
represents a number satisfying 0.05.ltoreq.y.ltoreq.0.8].
[0162] In composition formula (5), M.sup.1 especially preferably is
Ni, Co, or Fe; x especially preferably satisfies
0.2.ltoreq.x.ltoreq.1.15; and y especially preferably satisfies
0.1.ltoreq.y.ltoreq.0.7. Examples of the positive-electrode active
material having a composition represented by composition formula
(5) include LiMn.sub.0.5Ni.sub.0.5O.sub.2.
[0163] Especially preferred is one in which M.sup.1 is Ni and which
has a composition represented by the following composition formula
(6):
Li.sub.xMn.sub.(1-y-z)Ni.sub.yM.sup.2.sub.zO.sub.2 composition
formula (6)
[wherein M.sup.2 represents at least one element selected from the
group consisting of Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x
represents a number satisfying 0<x.ltoreq.1.2; y represents a
number satisfying 0.05.ltoreq.y.ltoreq.0.8; and z represents a
number satisfying 0.01.ltoreq.z.ltoreq.0.5].
[0164] In composition formula (6), M.sup.2 especially preferably is
Co, Al, Fe, or Mg; x especially preferably satisfies
0.2.ltoreq.x.ltoreq.1.15; y especially preferably satisfies
0.1.ltoreq.y.ltoreq.0.7; z especially preferably satisfies
0.1.ltoreq.z.ltoreq.0.7; and y+z especially preferably satisfies
0.2.ltoreq.y+z.ltoreq.0.7. Examples of the positive-electrode
active material having a composition represented by composition
formula (6) include LiMn.sub.0.33Ni.sub.0.33Co.sub.0.33O.sub.2.
[0165] Also preferred as the manganese-containing
positive-electrode active material is one having a composition
represented by the following composition formula (7):
Li.sub.xMn.sub.(2-y)M.sup.3.sub.yO.sub.4 composition formula
(7)
[wherein M.sup.3 represents at least one element selected from the
group consisting of Ni, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb;
x represents a number satisfying 0<x.ltoreq.1.2; and y
represents a number satisfying 0.05.ltoreq.y.ltoreq.0.8].
[0166] In composition formula (7), M.sup.3 especially preferably is
Ni, Co, Al, or Mg; x especially preferably satisfies
0.05.ltoreq.x.ltoreq.1.15; and y especially preferably satisfies
0.1.ltoreq.y.ltoreq.0.7. Examples of the positive-electrode active
material having a, composition represented by composition formula
(7) include LiMn.sub.1.8Al.sub.0.2O.sub.4 and
LiMn.sub.1.5Ni.sub.0.5O.sub.4.
[[Surface Coating]]
[0167] It is preferred to use an active material which comprises
the manganese-containing positive-electrode active material and,
adherent to the surface thereof, a substance having a composition
different from that of the positive-electrode active material. The
kind of the surface-adherent substance, method of adhesion, amount
of the substance to be adherend, etc. are the same as those
described above.
[[Shape]]
[0168] The shape of the particles of this positive-electrode active
material is the same as described above.
[[Tap Density]]
[0169] The tap density of this positive-electrode active material
is the same as described above.
[[Median Diameter d.sub.50]]
[0170] The median diameter d.sub.50 (secondary-particle diameter in
the case where the primary particles have aggregated to form
secondary particles) of the particles of this positive-electrode
active material is the same as described above.
[[Average Primary-Particle Diameter]]
[0171] In the case where the primary particles have aggregated to
form secondary particles, the average primary-particle diameter of
this positive-electrode active material is the same as described
above.
[[BET Specific Surface Area]]
[0172] The BET specific surface area of this positive-electrode
active material is the same as described above.
[[Production Processes]]
[0173] For producing this positive-electrode active material,
processes in general use tor producing inorganic compounds may be
employed. Especially for producing the active material which is
spherical or ellipsoidal, various techniques are usable. Examples
thereof include: a method which comprises dissolving or
pulverizing/dispersing a manganese source, e.g., manganese nitrate
or manganese sulfate, optionally together with a source of other
element(s) in a solvent, e.g., water, regulating the pH of the
solution or dispersion with stirring to produce a spherical
precursor, recovering and optionally drying the precursor,
subsequently adding thereto an Li source, e.g., LiOH,
Li.sub.2CO.sub.3, or LiNO.sub.3, and heat-treating the mixture at a
high temperature to obtain the active material; a method which
comprises dissolving or pulverizing/dispersing a manganese source,
e.g., manganese nitrate, manganese sulfate, manganese oxide, or
manganese oxyhydroxide, optionally together with a source of other
element(s) in a solvent, e.g., water, drying and forming the
solution or dispersion with a spray dryer or the like to obtain a
spherical or ellipsoidal precursor, adding thereto an Li source,
e.g., LiOH, Li.sub.2CO.sub.3, or LiNO.sub.3, and heat-treating the
mixture at a high temperature to obtain the active material; and a
method which comprises dissolving or pulverizing/dispersing a
manganese source, e.g., manganese nitrate, manganese sulfate,
manganese oxide, or manganese oxyhydroxide, together with an Li
source, e.g., LiOH, Li.sub.2CO.sub.3, or LiNO.sub.3, and optionally
with a source of other element(s) in a solvent, e.g., water, drying
and forming the solution or dispersion with a spray dryer or the
like to obtain a spherical or ellipsoidal precursor, and
heat-treating the precursor at a high temperature to obtain the
active material.
[0174] In the invention, the manganese-containing
positive-electrode active material may be used alone as the only
positive-electrode active material, or a combination of the
manganese-containing positive-electrode active material with any
desired one or more positive-electrode active materials differing
in composition or powder properties from the manganese-containing
positive-electrode active material may be used in any desired
proportion. In this case, preferred examples of the combination
include: a combination of the manganese-containing
positive-electrode active material with LiNiO.sub.2 or an active
material formed by replacing part of the Ni by another transition
metal; and a combination of the manganese-containing
positive-electrode active material with LiCoO.sub.2 or an active
material formed by replacing part of the Co by another transition
metal. Especially preferred examples of the combination include: a
combination of the positive-electrode active material represented
by any of composition formulae (5) to (7) with LiNiO.sub.2 or an
active material formed by replacing part of the Ni by another
transition metal; and a combination of the positive-electrode
active material represented by any of composition formulae (5) to
(7) with LiCoO.sub.2 or an active material formed by replacing part
of the Co by another transition metal. The content of the
manganese-containing positive-electrode active material, in
particular the positive-electrode active material represented by
any of composition formulae (5) to (7), is preferably 30% by mass
or higher, more preferably 50% by mass or higher, based on all
positive-electrode active materials. When the positive-electrode
active material represented by any of composition formulae (5) to
(7) is used in too small a proportion, there are cases where the
cost of the positive electrode is not reduced.
[Positive Electrode [1]: Constitution of Positive Electrode]
[0175] The constitution of the positive electrode to be used as
positive electrode [1] is described below.
[0176] The electrode structure, process for electrode production,
compaction of a positive-electrode active-material layer,
conductive material, binder for use in producing a
positive-electrode active-material layer, liquid medium for slurry
formation, current collector, electrode area, discharge capacity,
thickness of the positive-electrode plate, etc. in positive
electrode [1] are the same as described above.
<Positive Electrode [2]>
[0177] Positive electrode [2], i.e., "a positive electrode
containing a positive-electrode active material having a specific
composition represented by composition formula (4)", for use in a
lithium secondary battery of the invention will be explained
below.
[Positive Electrode [2]: Positive-Electrode Active Material]
[0178] The positive-electrode active material for use in positive
electrode [2] is described below.
[[Composition]]
[0179] As the positive-electrode active material is used a
substance which contains one or more transition metals and is
capable of electrochemically occluding/releasing lithium ions and
which has a composition represented by the following composition
formula (4) (hereinafter abbreviated to "this positive-electrode
active material"):
Li.sub.xNi.sub.(1-y-z)Co.sub.yM.sub.zO.sub.2 composition formula
(4)
[wherein M represents at least one element selected from the group
consisting of Mn, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb; x
represents a number satisfying 0<x.ltoreq.1.2; y represents a
number satisfying 0.05.ltoreq.y.ltoreq.0.5; and z represents a
number satisfying 0.01.ltoreq.z.ltoreq.0.5].
[0180] In composition formula (4), M preferably is Mn, Al, or Mg,
and x preferably satisfies 0.2.ltoreq.x.ltoreq.1.15. Symbol y
preferably satisfies 0.08.ltoreq.y.ltoreq.0.4 and especially
preferably satisfies 0.1.ltoreq.y.ltoreq.0.3. Furthermore, z
preferably satisfies 0.02.ltoreq.z.ltoreq.0.4 and especially
preferably satisfies 0.03.ltoreq.z<0.3.
[[Surface Coating]]
[0181] It is preferred to use an active material which comprises
this positive-electrode active material and, adherent to the
surface thereof, a substance having a composition different from
that of the positive-electrode active material. The kind of the
surface-adherent substance, method of adhesion, amount of the
substance to be adherend, etc. are the same as those described
above.
[[Shape]]
[0182] The shape of the particles of this positive-electrode active
material is the same as described above.
[[Tap Density]]
[0183] The tap density of this positive-electrode active material
is the same as described above.
[[Median Diameter d.sub.50]]
[0184] The median diameter d.sub.50 (secondary-particle diameter in
the case where the primary particles have aggregated to form
secondary particles) of the particles of this positive-electrode
active material is the same as described above.
[[Average Primary-Particle Diameter]]
[0185] In the case where the primary particles have aggregated to
form secondary particles, the average primary-particle diameter of
this positive-electrode active material is the same as described
above.
[[BET Specific Surface Area]]
[0186] The BET specific surface area of this positive-electrode
active material is the same as described above.
[[Production Processes]]
[0187] For producing this positive-electrode active material,
processes in general use for producing inorganic compounds may be
employed. Especially for producing the active material which is
spherical or ellipsoidal, various techniques are usable. Examples
thereof include: a method which comprises dissolving or
pulverizing/dispersing a nickel source, e.g., nickel nitrate or
nickel sulfate, a cobalt source, e.g., cobalt nitrate or cobalt
sulfate, and a source of M in composition formula (4) in a solvent,
e.g., water, regulating the pH of the solution or dispersion with
stirring to produce a spherical precursor, recovering and
optionally drying the precursor, subsequently adding thereto an Li
source, e.g., LiOH, Li.sub.2CO.sub.3, or LiNO.sub.3, and
heat-treating the mixture at a high temperature to obtain the
active material; a method which comprises dissolving or
pulverizing/dispersing a nickel source, e.g., nickel nitrate,
nickel sulfate, nickel oxide, nickel hydroxide, or nickel
oxyhydroxide, a cobalt source, e.g., cobalt nitrate, cobalt
sulfate, cobalt oxide, cobalt hydroxide, or cobalt oxyhydroxide,
and a source of M in composition formula (4) in a solvent, e.g.,
water, drying and forming the solution or dispersion with a spray
dryer or the like to obtain a spherical or ellipsoidal precursor,
adding thereto an Li source, e.g., LiOH, Li.sub.2CO.sub.3, or
LiNO.sub.3, and heat-treating the mixture at a high temperature to
obtain the active material; and a method which comprises dissolving
or pulverizing/dispersing a nickel source, e.g., nickel nitrate,
nickel sulfate, nickel oxide, nickel hydroxide, or nickel
oxyhydroxide, a cobalt source, e.g., cobalt nitrate, cobalt
sulfate, cobalt oxide, cobalt hydroxide, or cobalt oxyhydroxide, a
source of M in composition formula (4), and an Li source, e.g.,
LiOH, Li.sub.2CO.sub.3, or LiNO.sub.3 in a solvent, e.g., water,
drying and forming the solution or dispersion with a spray dryer or
the like to obtain a spherical or ellipsoidal precursor, and
heat-treating the precursor at a high temperature to obtain the
active material.
[0188] For producing the positive electrode in this invention, this
positive-electrode active material (the positive-electrode active
material represented by composition formula (4) and/or the
positive-electrode active material represented by composition
formula (4) covered with the surface-adherent substance) may be
used alone, or a combination of this positive-electrode active
material with any desired one or more positive-electrode active
materials differing in composition from that positive-electrode
active material may be used in any desired proportion. In this
case, preferred examples of the combination include: a combination
of this positive-electrode active material with LiMn.sub.2O.sub.4
or an active material formed by replacing part of the Mn by another
transition metal, etc.; and a combination of this
positive-electrode active material with LiCoO.sub.2 or an active
material formed by replacing part of the Co with another transition
metal, etc.
[0189] Here, the content of this positive-electrode active material
is preferably 30% by mass or higher, more preferably 50% by mass or
higher, based on all positive-electrode active materials. When this
positive-electrode active material is used in too small a
proportion, there are cases where the battery capacity decreases.
Incidentally, "this positive-electrode active material" and
"positive-electrode active materials other than this
positive-electrode active material" are inclusively referred to as
"positive-electrode active materials".
[Positive Electrode [2]: Constitution of Positive Electrode]
[0190] The constitution of the positive electrode to be used as
positive electrode [2] is described below.
[0191] The electrode structure, process for electrode production,
compaction of a positive-electrode active-material layer,
conductive material, binder for use in producing a
positive-electrode active-material layer, liquid medium for slurry
formation, current collector, electrode area, discharge capacity,
thickness of the positive-electrode plate, etc. in positive
electrode [2] are the same as described above.
<Positive Electrode [3]>
[0192] An explanation will be given below on positive electrode [3]
for use in a lithium secondary battery of the invention, i.e., "a
positive electrode containing a positive-electrode active material
selected from the group consisting of the following (a) to (d):
(a) a positive-electrode active material having a BET specific
surface area of from 0.4 m.sup.2/g to 2 m.sup.2/g, (b) a
positive-electrode active material having an average
primary-particle diameter of from 0.1 .mu.m to 2 .mu.m, (c) a
positive-electrode active material having a median diameter
d.sub.50 of from 1 .mu.m to 20 .mu.m, (d) a positive-electrode
active material having a tap density of from 1.3 g/cm.sup.3 to 2.7
g/cm.sup.3".
[Positive Electrode [3]: Positive-Electrode Active Material]
[0193] The positive-electrode active material for use in positive
electrode [3] is described below.
[[Composition]]
[0194] The positive-electrode active material is not particularly
limited as long as it is a substance capable of electrochemically
occluding/releasing lithium ions. It preferably is a substance
containing lithium and at least one transition metal. Examples
thereof include lithium-transition metal composite oxides and
lithium-containing transition metal/phosphoric acid compounds.
Specifically, such a compound having the same composition as
described above may be used.
[[Surface Coating]]
[0195] It is also preferred that the positive-electrode active
material should comprise a core positive-electrode active material
and, adherent to the surface thereof, a substance having a
composition different from that of the core positive-electrode
active material. The kind of the surface-adherent substance, method
of adhesion, amount of the substance to be adhered, etc. are the
same as those described above.
[[Shape]]
[0196] The shape of the particles of this positive-electrode active
material is the same as described above.
[[BET Specific Surface Area]]
[0197] The BET specific surface area of this positive-electrode
active material is preferably 0.4 m.sup.2/g or larger, more
preferably 0.5 m.sup.2/g or larger, even more preferably 0.6
m.sup.2/g or larger. The upper limit thereof is preferably 2
m.sup.2/g or smaller, more preferably 1.8 m.sup.2/g or smaller,
even more preferably 1.5 m.sup.2/g or smaller. In case where the
BET specific surface area thereof is smaller than the lower limit
of that range, battery performances are apt to decrease. In case
where the BET specific surface area thereof exceeds the upper
limit, a high tap density is difficult to obtain and this may pose
a problem concerning applicability in forming a positive-electrode
active material.
[0198] BET specific surface area is measured with a surface area
meter (e.g., a fully automatic surface area measuring apparatus
manufactured by Ohukura Riken Co., Ltd.). The specific surface area
is defined as a value obtained by preliminarily drying a sample at
150.degree. C. for 30 minutes in a nitrogen stream and then
measuring the specific surface area thereof by the gas-flowing
nitrogen adsorption BET one-point method using a nitrogen/helium
mixture gas precisely regulated so as to have a nitrogen pressure
of 0.3 relative to atmosphere pressure.
[[Average Primary-Particle Diameter]]
[0199] In the case where the primary particles have aggregated to
form secondary particles, the average primary-particle diameter of
this positive-electrode active material is preferably 0.1 .mu.m or
larger, more preferably 0.2 .mu.m or larger, even more preferably
0.3 .mu.m or larger, most preferably 0.4 .mu.m or larger. The upper
limit thereof is preferably 2 .mu.m or smaller, more preferably 1.6
.mu.m or smaller, even more preferably 1.3 .mu.m or smaller, most
preferably 1 .mu.m or smaller. In case where the average
primary-particle diameter thereof exceeds the upper limit,
spherical secondary particles are difficult to form and this
adversely influences powder loading or results in a considerably
reduced specific surface area. There may hence be a high
possibility that battery performances such as output
characteristics might decrease. Conversely, when the average
primary-particle diameter thereof is smaller than the lower limit,
crystal growth is usually insufficient and, hence, there are cases
where this positive-electrode active material poses a problem, for
example, that charge/discharge reversibility is poor.
[0200] Primary-particle diameter is determined through an
examination with a scanning electron microscope (SEM).
Specifically, arbitrarily selected 50 primary-particle images in a
photograph having a magnification of 10,000 diameters each are
examined for the length of the longest segment of a horizontal line
which extends across the primary-particle image from one side to
the other side of the boundary. These measured lengths are
averaged, whereby the average value can be determined.
[[Median Diameter d.sub.50]]
[0201] The median diameter d.sub.50 (secondary-particle diameter in
the case where the primary particles have aggregated to form
secondary particles) of the particles of the positive-electrode
active material is preferably 1 .mu.m or larger, more preferably
1.2 .mu.m or larger, even more preferably 1.5 .mu.m or larger, most
preferably 2 .mu.m or larger. The upper limit thereof is preferably
20 .mu.m or smaller, more preferably 18 .mu.m or smaller, even more
preferably 16 .mu.m or smaller, most preferably 15 .mu.m or
smaller. When the median diameter thereof is smaller than the lower
limit, there are cases where a product having a high tap density
cannot be obtained. When the median diameter thereof exceeds the
upper limit, lithium diffusion in the individual particles requires
a longer time and this results in a decrease in battery
performance. In addition, there are cases where such
positive-electrode active-material particles, when used in
producing a positive electrode for batteries, i.e., when the active
material and other ingredients including a conductive material and
a binder are slurried with a solvent and this slurry is applied in
a thin-film form, pose a problem, for example, that streak lines
generate. The positive-electrode active material for use in this
invention preferably is a mixture of two or more positive-electrode
active materials differing in median diameter, because this mixture
of positive-electrode active materials is effective in further
improving loading in positive-electrode production.
[0202] Median diameter d.sub.50 in the invention is determined with
a known laser diffraction/scattering type particle size
distribution analyzer. In the case where LA-920, manufactured by
HORIBA, Ltd., is used as a particle size distribution analyzer, a
0.1% by mass aqueous solution of sodium hexametaphosphate is used
as a dispersion medium for the examination and a five-minute
ultrasonic dispersing treatment is conducted before the particles
are examined at a measuring refractive index set at 1.24.
[[Tap Density]]
[0203] The tap density of this positive-electrode active material
is preferably 1.3 g/cm.sup.3 or higher, more preferably 1.5
g/cm.sup.3 or higher, even more preferably 1.6 g/cm.sup.3 or
higher, most preferably 1.7 g/cm.sup.3 or higher. When the tap
density of the positive-electrode active material is lower than the
lower limit, there are cases where it is necessary to use a larger
amount of a dispersion medium and larger amounts of a conductive
material and a binder in forming a positive-electrode
active-material layer and this results in a limited loading of the
positive-electrode active material in the positive-electrode
active-material layer and a limited battery capacity. By employing
a composite oxide powder having a high tap density, a
positive-electrode active-material layer having a high density can
be formed. The higher the tap density, the more the
positive-electrode active material is generally preferred. Although
there is no particular upper limit thereon, too high tap densities
may result in cases where the diffusion of lithium ions in the
positive-electrode active-material layer through the electrolyte as
a medium becomes a rate-determining stage and this is apt to reduce
load characteristics. Consequently, the upper limit of the tap
density thereof is preferably 2.7 g/cm.sup.3 or lower, more
preferably 2.5 g/cm.sup.3 or lower.
[0204] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the capacity of
the cell with the sample, subsequently conducting tapping
operations 1,000 times over a stroke length of 10 mm using a powder
densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise
Co., Ltd.), and determining the resultant bulk density. This bulk
density is defined as the tap density.
[0205] From the standpoints of long life and high output, this
positive-electrode active material for use in a lithium secondary
battery of the invention especially preferably is one which
simultaneously satisfies two or more of the properties described
above, i.e., BET specific surface area, average primary-particle
diameter, median diameter d.sub.50, and tap density.
[[Production Processes]]
[0206] Processes usable for producing this positive-electrode
active material are the same as described above.
[0207] This positive-electrode active material for use in a lithium
secondary battery of the invention may be a single
positive-electrode active material alone, or any desired
combination of two or more positive-electrode active materials
differing in composition or powder properties may be used in any
desired proportion as that positive-electrode active material.
Furthermore, from the standpoint of life improvement, this positive
electrode in the invention preferably contains a positive-electrode
active material having any one of the properties described above
(this positive-electrode active material) and a positive-electrode
active material differing in composition from that
positive-electrode active material.
[Positive Electrode [3]: Constitution of Positive Electrode]
[0208] The constitution of the positive electrode to be used as
positive electrode [3] is described below.
[0209] The electrode structure, process for electrode production,
compaction of a positive-electrode active-material layer,
conductive material, binder for use in producing a
positive-electrode active-material layer, liquid medium for slurry
formation, current collector, electrode area, discharge capacity,
thickness of the positive-electrode plate, etc. in positive
electrode [3] are the same as described above.
<Positive Electrode [4]>
[0210] An explanation will be given below on positive electrode [4]
for use in a lithium secondary battery of the invention, i.e., "a
positive electrode satisfying any requirement selected from the
group consisting of the following (e) to (f):
(e) it is a positive electrode produced by forming a
positive-electrode active-material layer comprising a
positive-electrode active material, a conductive material, and a
binder on a current collector, wherein the content of the
conductive material in the positive-electrode active-material layer
is in the range of from 6% by mass to 20% by mass (embodiment 1),
(f) it is a positive electrode produced by forming a
positive-electrode active-material layer comprising a
positive-electrode active material and a binder on a current
collector, wherein the positive-electrode active-material layer has
a density in the range of from 1.7 g/cm.sup.3 to 3.5 g/cm.sup.3
(embodiment 2), (g) it is a positive electrode produced by forming
a positive-electrode active-material layer comprising a
positive-electrode active material and a binder on a current
collector, wherein the ratio of the thickness of the
positive-electrode active-material layer to the thickness of the
current collector, i.e., the value of (thickness of the
active-material layer on one side just before impregnation with the
nonaqueous electrolyte)/(thickness of the current collector), is in
the range of from 1.6 to 20 (embodiment 3)".
[Positive Electrode [4]: Positive-Electrode Active Material]
[0211] The positive-electrode active material tor use in positive
electrode [4] is described below.
[0212] This positive-electrode active material is not particularly
limited as long as it is a substance capable of electrochemically
occluding/releasing lithium ions. It preferably is a substance
containing lithium and at least one transition metal. Examples
thereof include lithium-transition metal composite oxides and
lithium-containing transition metal/phosphoric acid compounds.
Specifically, such a compound having the same composition as
described above may be used.
[0213] The surface coating, shape, tap density, median diameter
d.sub.50, average primary-particle diameter, and BET specific
surface area of this positive-electrode active material, processes
for producing the active material, etc. are the same as described
above.
[Positive Electrode [4]: Constitution of Positive Electrode]
[0214] The constitution of the positive electrode to be used as
positive electrode [4] is described below.
[[Electrode Structure and Production Process]]
[0215] Positive electrode [4] in a lithium secondary battery of the
invention is produced by forming a positive-electrode
active-material layer comprising a positive-electrode active
material and a binder on a current collector. The production of
this positive electrode with a positive-electrode active material
can be conducted in an ordinary manner. Namely, at least a
positive-electrode active material and a binder are mixed together
by a dry process optionally together with a conductive material
(essential ingredient in embodiment 1) and optionally further with
a thickener, etc. and this mixture is formed into a sheet and
press-bonded to a positive-electrode current collector.
Alternatively, those materials are dissolved or dispersed in a
liquid medium to obtain a slurry and this slurry is applied to a
positive-electrode current collector and dried. Thus, a
positive-electrode active-material layer is formed on the current
collector, whereby the positive electrode can be obtained.
[0216] The content of the positive-electrode active material in the
positive-electrode active-material layer is preferably 80% by mass
or higher, more preferably 82% by mass or higher, especially
preferably 84% by mass or higher. The upper limit thereof is
preferably 95% by mass or lower, more preferably 93% by mass or
lower. When the content of the positive-electrode active material
in the positive-electrode active-material layer is too low, there
are cases where an insufficient electric capacity results.
Conversely, when the content thereof is too high, there are cases
where the positive electrode has an insufficient strength.
[[Conductive Material]]
[0217] As the conductive material, a known conductive material can
be used at will. Examples thereof include the same conductive
materials as shown above.
[0218] In this positive electrode for use in a lithium secondary
battery of the invention, the content of the conductive material in
the positive-electrode active-material layer in embodiment 1 should
be 6% by mass or higher and is preferably 7% by mass or higher,
especially preferably 8% by mass or higher, more preferably 9% by
mass or higher. In embodiment 2 and embodiment 3, there are no
particular limitations thereon. However, the content of the
conductive material in embodiments 2 and 3 is preferably 6% by mass
or higher, especially preferably 7% by mass or higher, more
preferably 8% by mass or higher. When the content of the conductive
material in the positive-electrode active-material layer is too
low, there are cases where this positive-electrode active-material
layer has insufficient electrical conductivity and a high output
cannot be obtained.
[0219] The upper limit of the content of the conductive material in
the positive-electrode active-material layer is as follows. In
embodiment 1, the content thereof should be 20% by mass or lower
and is preferably 18% by mass or lower, especially preferably 15%
by mass or lower. In embodiment 2 and embodiment 3, the upper limit
thereof is not particularly limited. However, the content of the
conductive material in embodiments 2 and 3 is preferably 20% by
mass or lower, especially preferably 18% by mass or lower, more
preferably 15% by mass or lower. When the content thereof is too
high, there are cases where battery capacity decreases and a high
output cannot be obtained.
[[Binder]]
[0220] The binder to be used for producing the positive-electrode
active-material layer is not particularly limited. In the case
where the layer is to be formed through coating fluid application,
any binder may be used as long as it is a material which is soluble
or dispersible in the liquid medium for use in electrode
production. Examples thereof include the same binders as shown
above.
[0221] The proportion of the binder in the positive-electrode
active-material layer is generally 0.1% by mass or higher,
preferably 1% by mass or higher, especially preferably 3% by mass
or higher. The upper limit thereof is generally 80% by mass or
lower, preferably 60% by mass or lower, especially preferably 40%
by mass or lower, more preferably 10% by mass or lower. When the
proportion of the binder is too low, there are cases where the
positive-electrode active material cannot be sufficiently held and
the positive electrode has an insufficient mechanical strength to
impair battery performances such as cycle characteristics. On the
other hand, too high proportions thereof may lead to a decrease in
battery capacity or conductivity.
[[Liquid Medium for Slurry Formation]]
[0222] The kind of the liquid medium to be used for forming a
slurry is not particularly limited as long as it is a solvent in
which the positive-electrode active material, conductive material,
and binder and a thickener, which is used according to need, can be
dissolved or dispersed. Either an aqueous solvent or an organic
solvent may be used. Examples thereof include the same liquid media
as shown above. The kind and addition amount of the thickener also
are the same as described above.
[[Compaction]]
[0223] It is preferred that the positive-electrode active-material
layer obtained by coating fluid application and drying should be
compacted with a handpress, roller press, or the like in order to
heighten the loading density of the positive-electrode active
material. The density of the positive-electrode active-material
layer is as follows. In embodiment 2, the lower limit thereof
should be 1.7 g/cm.sup.3 or higher and is preferably 2.0 g/cm.sup.3
or higher, especially preferably 2.2 g/cm.sup.3 or higher. In
embodiment 1 and embodiment 3, there are no particular limitations.
However, the lower limit thereof in embodiments 1 and 3 is
preferably 1.7 g/cm.sup.3 or higher, especially preferably 2.0
g/cm.sup.3 or higher, more preferably 2.2 g/cm.sup.3 or higher.
When the density of the layer is lower than the lower limit, there
are cases where conductivity among the active-material particles
decreases to increase battery resistance and a high output cannot
be obtained.
[0224] The upper limit of the density of the positive-electrode
active-material layer, in embodiment 2, should be 3.5 g/cm.sup.3 or
lower and is preferably 3.0 g/cm.sup.3 or lower, especially
preferably 2.8 g/cm.sup.3 or lower. In embodiment 1 and embodiment
3, the upper limit thereof is not particularly limited. However,
the upper limit thereof in embodiments 1 and 3 is preferably 3.5
g/cm.sup.3 or lower, especially preferably 3.0 g/cm.sup.3 or lower,
more preferably 2.8 g/cm.sup.3 or lower. When the density of the
positive-electrode active-material layer exceeds the upper limit of
that range, the infiltration of a nonaqueous electrolyte into
around the current collector/active material interface decreases
and there are cases where charge/discharge characteristics
especially at a high current density decrease and a high output
cannot be obtained.
[[Current Collector]]
[0225] The material of the positive-electrode current collector is
not particularly limited and is the same as described above. The
shape of the current collector and the thickness of the thin film
thereof also are the same as described above.
[0226] With respect to the ratio of the thickness of the
positive-electrode active-material layer to that of the current
collector, the value of (thickness of the active-material layer on
one side just before impregnation with the nonaqueous
electrolyte)/(thickness of the current collector) in embodiment 3
should be 20 or smaller and is preferably 15 or smaller, especially
preferably 10 or smaller. In embodiment 1 and embodiment 2, that
value is preferably 20 or smaller, especially preferably 15 or
smaller, more preferably 10 or smaller, although there are no
particular limitations thereon. When that ratio exceeds the upper
limit of that range, there are cases where the current collector
heats up due to Joule's heat during high-current-density
charge/discharge to damage the positive electrode.
[0227] The lower limit of the value of (thickness of the
active-material layer on one side just before impregnation with the
nonaqueous electrolyte)/(thickness of the current collector) in
embodiment 1 should be 1.6 or larger and is preferably 1.8 or
larger, especially preferably 2 or larger. In embodiment 1 and
embodiment 2, the lower limit thereof is preferably 1.6 or larger,
especially preferably 1.8 or larger, more preferably 2 or larger,
although there are no particular limitations thereon. When that
ratio decreases beyond the lower limit, the proportion by volume of
the current collector to the positive-electrode active material
increases and there are hence cases where battery capacity
decreases and a high output cannot be obtained.
[0228] When the preferred ranges of "the content of the conductive
material in the positive-electrode active-material layer", "the
density of the positive-electrode active-material layer", and "the
ratio of the thickness of the positive-electrode active-material
layer to that of the current collector, i.e., the value of
(thickness of the active-material layer on one side just before
impregnation with the nonaqueous electrolyte)/(thickness of the
current collector)" are combined, then a more preferred high-output
lithium secondary battery is obtained.
[[Electrode Area]]
[0229] When the nonaqueous electrolyte of the invention is used, it
is preferred that the area of the positive-electrode
active-material layer relative to the outer surface area of the
battery case should be large from the standpoint of attaining a
high output and enhanced high-temperature stability. The electrode
area is the same as described above.
[[Discharge Capacity]]
[0230] When the nonaqueous electrolyte for secondary battery of the
invention is used, it is preferred that the battery elements held
in each battery case of the secondary battery should have an
electric capacity (electric capacity measured when the battery in a
fully charged state is discharged to a discharged state) of 3
ampere-hours (Ah) or higher. This is because such electric capacity
is highly effective in improving output characteristics.
[0231] The design of a positive-electrode plate, etc. for attaining
that electric capacity are the same as described above.
[[Thickness of Positive-Electrode Plate]]
[0232] The thickness of the positive-electrode plate is not
particularly limited, and is the same as described above.
<Positive Electrode [5]>
[0233] Positive electrode [5] for use in a lithium secondary
battery of the invention will be explained below.
[Positive Electrode [5]: Positive-Electrode Active Materials]
[0234] The positive-electrode active materials for use in positive
electrode [5] are described below.
[[Composition]]
[0235] The positive-electrode active materials are not particularly
limited as long as these are substances capable of
electrochemically occluding/releasing lithium ions. They preferably
are substances containing lithium and at least one transition
metal. Examples thereof include lithium-transition metal composite
oxides and lithium-containing transition metal/phosphoric acid
compounds.
[0236] The transition metal in the lithium-transition metal
composite oxides preferably is V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or
the like. Specific examples of the composite oxides include
lithium-cobalt composite oxides such as LiCoO.sub.2, lithium-nickel
composite oxides such as LiNiO.sub.2, lithium-manganese composite
oxides such as LiMnO.sub.2, LiMn.sub.2O.sub.4, and
Li.sub.2MnO.sub.3, and ones formed by partly replacing the
transition metal atom(s) as a main component of these
lithium-transition metal composite oxides by one or more other
metals, e.g., Al, Ti, V. Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga,
Zr, Si, etc.
[0237] The lithium-cobalt composite oxides are not particularly
limited. However, one having a composition represented by the
following composition formula (8) is preferred.
Li.sub.xCo.sub.(1-y)M.sup.1.sub.yO.sub.2 composition formula
(8)
[In composition formula (8), represents at least one element
selected from the group consisting of Ni, Mn, Al, Fe, Ti, Mg, Cr,
Ga, Cu, Zn, and Nb; x represents a number satisfying
0<x.ltoreq.1.2; and y represents a number satisfying
0.05.ltoreq.y.ltoreq.0.8.]
[0238] In composition formula (8), M.sup.1 especially preferably is
Ni, Mn, Al, or Fe; x especially preferably satisfies
0.2.ltoreq.x.ltoreq.1.15; and y especially preferably satisfies
0.1.ltoreq.y.ltoreq.0.5.
[0239] The lithium-nickel composite oxides are not particularly
limited. However, one having a composition represented by the
following composition formula (9) is preferred.
Li.sub.xNi.sub.(1-y)M.sup.2.sub.yO.sub.2 composition formula
(9)
[In composition formula (9), M.sup.2 represents at least one
element selected from the group consisting of Co, Mn, Al, Fe, Ti,
Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying
0<x.ltoreq.1.2; and y represents a number satisfying
0.05.ltoreq.y.ltoreq.0.8.]
[0240] In composition formula (9), M.sup.2 especially preferably is
Co, Mn, Al, or Fe; x especially preferably satisfies
0.2.ltoreq.x.ltoreq.1.15; and y especially preferably satisfies
0.1.ltoreq.y.ltoreq.0.5.
[0241] The lithium-manganese composite oxides are not particularly
limited. However, one having a composition represented by the
following composition formula (10) is preferred.
Li.sub.xMn.sub.(1-y)M.sup.3.sub.yO.sub.2 composition formula
(10)
[In composition formula (10), M.sup.3 represents at least one
element selected from the group consisting of Ni, Co, Al, Fe, Ti,
Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying
0<x.ltoreq.1.2; and y represents a number satisfying
0.05.ltoreq.y.ltoreq.0.8.]
[0242] In composition formula (10), M.sup.3 especially preferably
is Ni, Co, or Fe; x especially preferably satisfies
0.2.ltoreq.x.ltoreq.1.15; and y especially preferably satisfies
0.1.ltoreq.y.ltoreq.0.7.
[0243] Another preferred example of the lithium-manganese composite
oxides is one having a composition represented by the following
composition formula (11).
Li.sub.xMn.sub.(2-y)M.sup.4.sub.yO.sub.4 composition formula
(11)
[In composition formula (11), M.sup.4 represents at least one
element selected from the group consisting of Ni, Co, Al, Fe, Ti,
Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying
0<x.ltoreq.1.2; and y represents a number satisfying
0.05.ltoreq.y.ltoreq.0.8.]
[0244] In composition formula (11), M.sup.4 especially preferably
is Ni, Co, Al, or Mg; x especially preferably satisfies
0.05.ltoreq.x.ltoreq.1.15; and y especially preferably satisfies
0.1.ltoreq.y.ltoreq.0.7.
[0245] Still another preferred example of the lithium-manganese
composite oxides is one having a composition represented by the
following composition formula (12).
Li.sub.xMn.sub.(1-y)M.sup.5.sub.yO.sub.3 composition formula
(12)
[In composition formula (12), M.sup.5 represents at least one
element selected from the group consisting of Ni, Co, Al, Fe, Ti,
Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying
0<x.ltoreq.2.4; and y represents a number satisfying
0.05.ltoreq.y.ltoreq.0.8.]
[0246] In composition formula (12), M.sup.5 especially preferably
is Ni, Co, Al, or Mg; x especially preferably satisfies
0.1.ltoreq.x.ltoreq.2.3; and y especially preferably satisfies
0.1.ltoreq.y.ltoreq.0.5.
[0247] The transition metal in the lithium-containing transition
metal/phosphoric acid compounds preferably is V, Ti, Cr, Mn, Fe,
Co, Ni, Cu, or the like. Specific examples of the compounds include
iron phosphate compounds such as LiFePO.sub.4, Li.sub.3Fe.sub.2
(PO.sub.4).sub.3, and LiFeP.sub.2O.sub.7, cobalt phosphate
compounds such as LiCoPO.sub.4, and ones formed by partly replacing
the transition metal atom(s) as a main component of these
lithium-transition metal/phosphoric acid compounds by one or more
other metals, e.g., Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg,
Ga, Zr, Nb, Si, etc.
[0248] The iron phosphate compounds are not particularly limited.
However, one having a composition represented by the following
composition formula (13) is preferred.
Li.sub.xFe.sub.(1-y)M.sub.y.sup.6PO.sub.4 composition formula
(13)
[In composition formula (13), M.sup.6 represents at least one
element selected from the group consisting of Ni, Co, Mn, Al, Ti,
Mg, Cr, Ga, Cu, Zn, and Nb; x represents a number satisfying
0<x.ltoreq.1.2; and y represents a number satisfying
0.05.ltoreq.y.ltoreq.0.8.]
[0249] In composition formula (13), M.sup.6 especially preferably
is Ni, Co, Mn, or Al; x especially preferably satisfies
0.2.ltoreq.x.ltoreq.1.15; and y especially preferably satisfies
0.1.ltoreq.y.ltoreq.0.5.
[0250] It is preferred in this invention that any desired
combination of two or more positive-electrode active materials
differing in composition selected from the positive-electrode
active materials described above should be used in any desired
proportion. Those positive-electrode active materials differ from
each other in merit performance and, hence, it is preferred to
combine necessary performances according to the intended use of the
battery. It is generally thought that a combination results in
arithmetic-mean performances. With respect to life, however, there
are cases where effects of the positive-electrode active material
having a longer life are more produced unexpectedly. By using a
positive-electrode active material attaining a relatively
satisfactory life in combination with a positive-electrode active
material which is relatively poor in life but is satisfactory in
other performances, the lithium secondary battery of the invention
having a high output, high capacity, and long life could be
achieved.
[0251] Examples of such preferred combinations include:
[0252] a positive-electrode active material represented by
composition formula (8) and a positive-electrode active material
represented by composition formula (10);
[0253] a positive-electrode active material represented by
composition formula (8) and a positive-electrode active material
represented by composition formula (11);
[0254] a positive-electrode active material represented by
composition formula (8) and a positive-electrode active material
represented by composition formula (12);
[0255] a positive-electrode active material represented by
composition formula (8) and a positive-electrode active material
represented by composition formula (13);
[0256] a positive-electrode active material represented by
composition formula (9) and a positive-electrode active material
represented by composition formula (10);
[0257] a positive-electrode active material represented by
composition formula (9) and a positive-electrode active material
represented by composition formula (11);
[0258] a positive-electrode active material represented by
composition formula (9) and a positive-electrode active material
represented by composition formula (12);
[0259] a positive-electrode active material represented by
composition formula (9) and a positive-electrode active material
represented by composition formula (13);
[0260] a positive-electrode active material represented by
composition formula (10) and a positive-electrode active material
represented by composition formula (11);
[0261] a positive-electrode active material represented by
composition formula (10) and a positive-electrode active material
represented by composition formula (12); and
[0262] a positive-electrode active material represented by
composition formula (10) and a positive-electrode active material
represented by composition formula (13).
[0263] Especially preferred are:
[0264] a positive-electrode active material represented by
composition formula (9) and a positive-electrode active material
represented by composition formula (11); and
[0265] a positive-electrode active material represented by
composition formula (10) and a positive-electrode active material
represented by composition formula (11).
[0266] The proportion in each combination is not particularly
limited. However, it is preferably from 10:90 to 90:10, more
preferably from 20:80 to 80:20.
[0267] Although two or more positive-electrode active materials
differing in composition are contained in the positive electrode
for use in the lithium secondary battery of this invention, it is
preferred that at least one of these positive-electrode active
materials should be one whose BET specific surface area, average
primary-particle diameter, median diameter d.sub.50, and/or tap
density (hereinafter abbreviated to "following properties") is
within the following specific range. The two or more
positive-electrode active materials differing in composition may
include a positive-electrode active material in which any property
is not within the range shown below. It is, however, preferred that
all of the two or more positive-electrode active materials should
be ones in each of which any property is within the range shown
below. The proportion of "all positive-electrode active materials
in each of which any of the following properties is within the
specific range shown below" to "all positive-electrode active
materials contained in the positive electrode" depends on that
property and is not particularly limited. However, that proportion
is preferably 30% by mass or higher, more preferably 50% by mass or
higher. Especially preferably, the former positive-electrode active
materials account for the whole of the latter. It is preferred that
each of the two or more positive-electrode active materials
contained in the positive electrode should be one in which any one
or more of the following properties are within the specific
range(s) shown below. However, more preferably any two or more of,
especially preferably any three or more of, most preferably all of
the following properties of each positive-electrode active material
are within the specific ranges shown below.
[[BET Specific Surface Area]]
[0268] The BET specific surface area of at least one of the
positive-electrode active materials is preferably 0.4 m.sup.2/g or
larger, more preferably 0.5 m.sup.2/g or larger, even more
preferably 0.6 m.sup.2/g or larger. The upper limit thereof is 2
m.sup.2/g or smaller, preferably 1.8 m.sup.2/g or smaller, more
preferably 1.5 m.sup.2/g or smaller. In case where the BET specific
surface area thereof is smaller than the lower limit of that range,
battery performances are apt to decrease. In case where the BET
specific surface area thereof exceeds the upper limit, a high tap
density is difficult to obtain and this may pose a problem
concerning applicability in forming a positive-electrode active
material.
[0269] BET specific surface area is measured with a surface area
meter (e.g., a fully automatic surface area measuring apparatus
manufactured by Ohukura Riken Co., Ltd.). The specific surface area
is defined as a value obtained by preliminarily drying a sample at
150.degree. C. for 30 minutes in a nitrogen stream and then
measuring the specific surface area thereof by the gas-flowing
nitrogen adsorption BET one-point method using a nitrogen/helium
mixture gas precisely regulated so as to have a nitrogen pressure
of 0.3 relative to atmosphere pressure.
[[Average Primary-Particle Diameter]]
[0270] The average primary-particle diameter of at least one of the
positive-electrode active materials is preferably 0.1 .mu.m or
larger, more preferably 0.2 .mu.m or larger, even more preferably
0.3 .mu.m or larger, most preferably 0.4 .mu.M or larger. The upper
limit thereof is preferably 2 .mu.m or smaller, more preferably 1.6
.mu.m or smaller, even more preferably 1.3 .mu.m or smaller, most
preferably 1 .mu.m or smaller. In case where the average
primary-particle diameter thereof exceeds the upper limit,
spherical secondary particles are difficult to form and this
adversely influences powder loading or results in a considerably
reduced specific surface area. There may hence be a high
possibility that battery performances such as output
characteristics might decrease. Conversely, when the average
primary-particle diameter thereof is smaller than the lower limit,
crystal growth is usually insufficient and, hence, there are cases
where this positive-electrode active material poses a problem, for
example, that charge/discharge reversibility is poor.
[0271] Primary-particle diameter is determined through an
examination with a scanning electron microscope (SEM).
Specifically, arbitrarily selected 50 primary-particle images in a
photograph having a magnification of 10,000 diameters each are
examined for the length of the longest segment of a horizontal line
which extends across the primary-particle image from one side to
the other side of the boundary. These measured lengths are
averaged, whereby the average value can be determined. There are
cases where the primary particles have aggregated to form secondary
particles. In such cases, however, the measurement is made only on
primary particles.
[[Median Diameter d.sub.50]]
[0272] The median diameter d.sub.50 (secondary-particle diameter in
the case where the primary particles have aggregated to form
secondary particles) of the particles of at least one of the
positive-electrode active materials is preferably 1 .mu.m or
larger, more preferably 1.2 .mu.m or larger, even more preferably
1.5 .mu.m or larger, most preferably 2 .mu.m or larger. The upper
limit thereof is preferably 20 .mu.m or smaller, more preferably 18
.mu.m or smaller, even more preferably 16 .mu.m or smaller, most
preferably 15 .mu.m or smaller. When the median diameter thereof is
smaller than the lower limit, there are cases where a product
having a high tap density cannot be obtained. When the median
diameter thereof exceeds the upper limit, lithium diffusion in the
individual particles requires a longer time and this results in a
decrease in battery performance. In addition, there are cases where
such positive-electrode active-material particles, when used in
producing a positive electrode for batteries, i.e., when the active
material and other ingredients including a conductive material and
a binder are slurried with a solvent and this slurry is applied in
a thin-film form, pose a problem, for example, that streak lines
generate. It is possible to further improve loading in
positive-electrode production by mixing two or more
positive-electrode active materials differing in median diameter
d.sub.50.
[0273] Median diameter d.sub.50 in the invention is determined with
a known laser diffraction/scattering type particle size
distribution analyzer. In the case where LA-920, manufactured by
HORIBA, Ltd., is used as a particle size distribution analyzer, a
0.1% by mass aqueous solution of sodium hexametaphosphate is used
as a dispersion medium for the examination and a five-minute
ultrasonic dispersing treatment is conducted before the particles
are examined at a measuring refractive index set at 1.24.
[[[Tap Density]]]
[0274] The tap density of at least one of the positive-electrode
active materials is preferably 1.3 g/cm.sup.3 or higher, more
preferably 1.5 g/cm.sup.3 or higher, even more preferably 1.6
g/cm.sup.3 or higher, most preferably 1.7 g/cm.sup.3 or higher.
When the tap density of the positive-electrode active material is
lower than the lower limit, there are cases where it is necessary
to use a larger amount of a dispersion medium and larger amounts of
a conductive material and a binder in forming a positive-electrode
active-material layer and this results in a limited loading of the
positive-electrode active material in the positive-electrode
active-material layer and a limited battery capacity. By employing
a composite oxide powder having a high tap density, a
positive-electrode active-material layer having a high density can
be formed. The higher the tap density, the more the
positive-electrode active material is generally preferred. Although
there is no particular upper limit thereon, too high tap densities
may result in cases where the diffusion of lithium ions in the
positive-electrode active-material layer through the nonaqueous
electrolyte as a medium becomes a rate-determining stage and this
is apt to reduce load characteristics. Consequently, the upper
limit of the tap density thereof is preferably 2.7 g/cm.sup.3 or
lower, more preferably 2.5 g/cm.sup.3 or lower.
[0275] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the capacity of
the cell with the sample, subsequently conducting tapping
operations 1,000 times over a stroke length of 10 mm using a powder
densimeter (e.g., Tap Denser, manufactured by Seishin Enterprise
Co., Ltd.), and determining the resultant bulk density. This bulk
density is defined as the tap density.
[[Surface Coating]]
[0276] It is preferred that at least one of the positive-electrode
active materials should be one which has, adherent to the surface
thereof, a substance having a composition different from that of
the main positive-electrode active material or core
positive-electrode active material (hereinafter, the substance
adherent to the surface is abbreviated to "surface-adherent
substance"). The kind of the surface-adherent substance, method of
adhesion, amount of the substance to be adhered, etc. are the same
as those described above.
[[Shape]]
[0277] The shape of the particles of at least one of the
positive-electrode active materials is the same as any of those of
positive-electrode active-material particles heretofore in use.
[[Production Processes]]
[0278] For producing the positive-electrode active materials, the
same general processes for producing inorganic compounds as
described above may be used.
[Positive Electrode [5]: Constitution of Positive Electrode]
[0279] The constitution of positive electrode [5] for use in this
invention is described below.
[0280] The electrode structure, process for electrode production,
compaction of a positive-electrode active-material layer,
conductive material, binder for use in producing a
positive-electrode active-material layer, liquid medium for slurry
formation, current collector, electrode area, discharge capacity,
thickness of the positive-electrode plate, etc. in this invention
are the same as described above. In producing the positive
electrode, the two or more positive-electrode active materials may
be mixed beforehand or may be mixed by simultaneously adding these
during positive-electrode production.
<Negative Electrode>
[0281] The negative electrode for use in the lithium secondary
batteries of the invention is not particularly limited as long as
it comprises a current collector and formed thereon an
active-material layer containing an active material capable of
occluding/releasing lithium ions. However, the negative electrode
preferably is any negative electrode selected from the group
consisting of the following negative electrode [1] to negative
electrode [10]:
negative electrode [1]: a negative electrode containing as a
negative-electrode active material two or more carbonaceous
substances differing in crystallinity; negative electrode [2]: a
negative electrode containing as a negative-electrode active
material at least one amorphous carbonaceous substance which, when
examined by wide-angle X-ray diffractometry, has an interplanar
spacing (d002) for the (002) planes of 0.337 nm or larger and a
crystallite size (Lc) of 80 nm or smaller and which, in an
examination by argon ion laser Raman spectroscopy, has a Raman R
value defined as the ratio of the peak intensity at 1,360 cm.sup.-1
to the peak intensity at 1,580 cm.sup.-1 of 0.2 or higher; negative
electrode [3]: a negative electrode containing as a
negative-electrode active material a titanium-containing metal
oxide capable of occluding and releasing lithium; negative
electrode [4]: a negative electrode containing as a
negative-electrode active material a carbonaceous substance having
a roundness of 0.85 or higher and a surface functional-group amount
O/C value of from 0 to 0.01; negative electrode [5]: a negative
electrode containing as a negative-electrode active material an
orientation-differing-carbon composite comprising two or more
carbonaceous substances differing in orientation; negative
electrode [6]: a negative electrode containing, as a
negative-electrode active material, graphitic carbon particles
which have a roundness of 0.85 or higher and an interplanar spacing
(d002) for the (002) planes as determined by wide-angle X-ray
diffractometry of smaller than 0.337 nm and which, in an
examination by argon ion laser Raman spectroscopy, have a Raman R
value defined as the ratio of the peak intensity at 1,360 cm.sup.-1
to the peak intensity at 1,580 cm.sup.-1 of from 0.12 to 0.8;
negative electrode [7]: a negative electrode containing as a
negative-electrode active material a multielement-containing
negative-electrode active material (C) which contains at least one
of a lithium-occluding metal (A) selected from the group consisting
of Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, and
Sb and/or a lithium-occluding alloy (B) and further contains C
and/or N as element Z; negative electrode [8]: a negative electrode
containing as a negative-electrode active material two or more
negative-electrode active materials differing in property; negative
electrode [9]: a negative electrode containing a negative-electrode
active material which has a tap density of 0.1 g/cm.sup.3 or higher
and has a pore volume, in terms of the volume of particle pores
corresponding to the diameter range of from 0.01 .mu.m to 1 .mu.m
as determined by mercury porosimetry, of 0.01 mL/g or larger; and
negative electrode [10]: a negative electrode which, when charged
to 60% of a nominal capacity, has a reaction resistance in a
negative-electrode opposing cell of 500.OMEGA. or lower.
[0282] Negative electrodes which are usually employed in the
lithium secondary batteries of the invention will be first
explained below.
[Negative-Electrode Active Material]
[0283] The negative-electrode active material for general use in
the negative electrodes will be explained below.
[[Composition]]
[0284] The negative-electrode active material is not particularly
limited as long as it is capable of electrochemically
occluding/releasing lithium ions. Examples thereof include a
carbonaceous material, a metal oxide such as tin oxide or silicon
oxide, a metal composite oxide, elemental lithium, a lithium alloy
such as a lithium-aluminum alloy, and a metal capable of alloying
with lithium, such as Sn or Si. One of these may be used alone, or
any desired two or more thereof may be used in combination in any
desired proportion. From the standpoint of safety, it is preferred
to use a carbonaceous material or a lithium composite oxide among
those.
[0285] The metal composite oxide is not particularly limited as
long as it is capable of occluding/releasing lithium. However, from
the standpoint of high-current-density charge/discharge
characteristics, it is preferred that the composite oxide should
contain titanium and/or lithium as a component.
[0286] The carbonaceous material preferably is one selected
from:
(1) natural graphites; (2) artificial carbonaceous substances and
artificial graphitic substances; and carbon materials obtained by
subjecting carbonaceous substances [e.g., natural graphites, coal
coke, petroleum coke, coal pitch, petroleum pitch, carbonaceous
substances obtained by oxidizing these pitches, needle coke, pitch
coke, carbon materials obtained by partly graphitizing these cokes,
products of the pyrolysis of organic substances, such as furnace
black, acetylene black, and pitch-derived carbon fibers, organic
substances capable of carbonization (e.g., coal tar pitches ranging
from soft pitch to hard pitch, coal-derived heavy oil such as dry
distillation/liquefaction oil, straight-run heavy oil such as
topping residues and vacuum distillation residues, heavy oils
resulting from petroleum cracking, such as ethylene tar as a
by-product of the thermal cracking of crude oil, naphtha, etc.,
aromatic hydrocarbons such as acenaphthylene, decacyclene,
anthracene, and phenanthrene, nitrogen-ring compounds such as
phenazine and acridine, sulfur-ring compounds such as thiophene and
bithiophene, polyphenylenes such as biphenyl and terphenyl,
poly(vinyl chloride), poly(vinyl alcohol), poly(vinyl butyral),
substances obtained by insolubilizing these compounds,
nitrogen-containing organic polymers such as polyacrylonitrile and
polypyrrole, sulfur-containing organic polymers such as
polythiophene, organic polymers such as polystyrene, natural
polymers such as polysaccharides represented by cellulose, lignin,
mannan, poly(galacturonic acid), chitosan, and saccharose,
thermoplastic resins such as poly(phenylene sulfide) and
poly(phenylene oxide), and thermosetting resins such as furfuryl
alcohol resins, phenol-formaldehyde resins, and imide resins) and
products of the carbonization thereof, or solutions obtained by
dissolving any of such organic substances capable of carbonization
in a low-molecular organic solvent, e.g., benzene, toluene, xylene,
quinoline, or n-hexane, and products of the carbonization of these
solutions] to a heat treatment at a temperature in the range of
from 400.degree. C. to 3,200.degree. C. one or more times; (3)
carbon materials constituting a negative-electrode active-material
layer which comprise at least two carbonaceous substances differing
in crystallinity and/or have an interface where at least two
carbonaceous substances differing in crystallinity are in contact
with each other; and (4) carbon materials constituting a
negative-electrode active-material layer which comprise at least
two carbonaceous substances differing in orientation and/or have an
interface where at least two carbonaceous substances differing in
orientation are in contact with each other. This is because this
carbonaceous material brings about a satisfactory balance between
initial irreversible capacity and high-current-density
charge/discharge characteristics.
[Constitution of Negative Electrode, Properties, and Method of
Preparation]
[0287] With respect to the properties of the carbonaceous material,
negative electrode containing the carbonaceous material, method of
electrode formation, current collector, and lithium secondary
battery, it is desirable that any one of the following (1) to (19)
should be satisfied or two or more thereof be simultaneously
satisfied.
(1) X-Ray Parameter
[0288] The carbonaceous material preferably has a value of d
(interplanar spacing) for the lattice planes (002), as determined
by X-ray diffractometry in accordance with the method of the Japan
Society for Promotion of Scientific Research, of 0.335 nm or
larger. It is desirable that the upper limit of the value of d
should be generally 0.36 nm or smaller, preferably 0.35 nm or
smaller, more preferably 0.345 nm or smaller. The crystallite size
(Lc) of the carbonaceous material, as determined by X-ray
diffractometry in accordance with the method of the Japan Society
for Promotion of Scientific Research, is preferably 1 nm or larger,
more preferably 1.5 nm or larger.
(2) Ash Content
[0289] The ash content of the carbonaceous material may be 1% by
mass or lower and is especially 0.5% by mass or lower, in
particular 0.1% by mass or lower, based on the whole carbonaceous
material. The lower limit of the ash content thereof is preferably
at least 1 ppm by mass of the whole carbonaceous material. When the
ash content thereof exceeds the upper limit of that range, there
are cases where battery performance deterioration caused by
reaction with the nonaqueous electrolyte during charge/discharge
becomes not negligible. When the ash content thereof is lower than
the lower limit of that range, there are cases where the production
of this carbonaceous material necessitates much time and energy and
an apparatus for pollution prevention, resulting in an increase in
cost.
(3) Volume-Average Particle Diameter
[0290] With respect to the volume-average particle diameter of the
carbonaceous material, the volume-average particle diameter (median
diameter) thereof as determined by the laser diffraction/scattering
method is generally 1 .mu.m or larger, preferably 3 .mu.m or
larger, more preferably 5 .mu.m or larger, even more preferably 7
.mu.m or larger. The upper limit thereof is generally 100 .mu.m or
smaller, preferably 50 .mu.m or smaller, more preferably 40 .mu.m
or smaller, even more preferably 30 .mu.m or smaller, especially
preferably 25 .mu.m or smaller. When the volume-average particle
diameter thereof is smaller than the lower limit of that range,
there are cases where irreversible capacity increases, leading to a
loss in initial battery capacity. When the volume-average particle
diameter thereof exceeds the upper limit of that range, there are
cases where such a carbonaceous material is undesirable from the
standpoint of battery production because an uneven coating surface
is apt to result when an electrode is produced through coating
fluid application.
[0291] Volume-average particle diameter in the invention is defined
as the median diameter determined by dispersing the carbon powder
in a 0.2% by mass aqueous solution (about 10 mL) of
poly(oxyethylene(20)) sorbitan monolaurate as a surfactant and
examining the dispersion with a laser diffraction/scattering type
particle size distribution analyzer (e.g., LA-700, manufactured by
HORIBA, Ltd.).
(4) Raman R Value, Raman Half-Value Width
[0292] The R value of the carbonaceous material as determined by
the argon ion laser Raman spectroscopy is generally 0.01 or higher,
preferably 0.03 or higher, more preferably 0.1 or higher. The upper
limit thereof may be 1.5 or lower and is preferably 1.2 or lower,
more preferably 1.0 or lower, even more preferably 0.5 or lower.
When the R value thereof is lower than the lower limit of that
range, the surface of such particles has too high crystallinity and
there are cases where the number of intercalation sites into which
lithium comes with charge/discharge decreases. Namely, there are
cases where suitability for charge decreases. In addition, when a
coating fluid containing such a carbonaceous material is applied to
a current collector and the resultant coating is pressed to
heighten the density of the negative electrode, then the crystals
are apt to orient in directions parallel to the electrode plate and
this may lead to a decrease in load characteristics. On the other
hand, when the R value thereof exceeds the upper limit of that
range, the surface of such particles has reduced crystallinity and
enhanced reactivity with the nonaqueous electrolyte and this may
lead to a decrease in efficiency and enhanced gas evolution.
[0293] The Raman half-value width around 1,580 cm.sup.-1 of the
carbonaceous material is not particularly limited. However, the
half-value width thereof is generally 10 cm.sup.-1 or larger,
preferably 15 cm.sup.-1 or larger. The upper limit thereof is
generally 100 cm.sup.-1 or smaller, preferably 80 cm.sup.-1 or
smaller, more preferably 60 cm.sup.-1 or smaller, even more
preferably 40 cm.sup.-1 or smaller. When the Raman half-value width
thereof is smaller than the lower limit of that range, the surface
of such particles has too high crystallinity and there are cases
where the number of intercalation sites into which lithium comes
with charge/discharge decreases. Namely, there are cases where
suitability for charge decreases. In addition, when a coating fluid
containing such a carbonaceous material is applied to a current
collector and the resultant coating is pressed to heighten the
density of the negative electrode, then the crystals are apt to
orient in directions parallel to the electrode plate and this may
lead to a decrease in load characteristics. On the other hand, when
the Raman half-value width thereof exceeds the upper limit of that
range, the surface of such particles has reduced crystallinity and
enhanced reactivity with the nonaqueous electrolyte and this may
lead to a decrease in efficiency and enhanced gas evolution.
[0294] The examination for a Raman spectrum is made with a Raman
spectrometer (e.g., a Raman spectrometer manufactured by Japan
Spectroscopic Co., Ltd.). In the examination, a sample is charged
into a measuring cell by causing the sample to fall naturally into
the cell and the surface of the sample in the cell is irradiated
with argon ion laser light while rotating the cell in a plane
perpendicular to the laser light. The Raman spectrum obtained is
examined for the intensity I.sub.A of a peak P.sub.A around 1,580
cm.sup.-1 and the intensity I.sub.B of a peak P.sub.B around 1,360
cm.sup.-1. The ratio between these intensities R
(R=I.sub.B/I.sub.A) is calculated and this ratio is defined as the
Raman R value of the carbonaceous material. Furthermore, the
half-value width of the peak P.sub.A around 1,580 cm.sup.-1 in the
Raman spectrum obtained is measured, and this value is defined as
the Raman half-value width of the carbonaceous material.
[0295] Conditions for this Raman spectroscopy are as follows.
[0296] Wavelength of argon ion laser: 514.5 nm [0297] Laser power
on sample: 15-25 mW [0298] Resolution: 10-20 cm.sup.-1 [0299]
Examination range: 1,100 cm.sup.-1 to 1,730 cm.sup.-1 [0300]
Analysis for R value and half-value width: background processing
[0301] Smoothing: simple average; convolution, 5 points
(5) BET Specific Surface Area
[0302] The specific surface area of the carbonaceous material for
use in the invention, as determined by the BET method, is generally
0.1 m.sup.2/g or larger, preferably 0.7 m.sup.2/g or larger, more
preferably 1.0 m.sup.2/g or larger, even more preferably 1.5
m.sup.2/g or larger. The upper limit thereof is generally 100
m.sup.2/g or smaller, preferably 25 m.sup.2/g or smaller, more
preferably 15 m.sup.2/g or smaller, even more preferably 10
m.sup.2/g or smaller. When the specific surface area thereof is
smaller than the lower limit of that range, there are cases where
use of this carbonaceous material as a negative-electrode material
is apt to result in poor lithium acceptance during charge and in
lithium deposition on the electrode surface. On the other hand,
when the specific surface area thereof exceeds the upper limit of
that range, there are cases where use of this carbonaceous material
as a negative-electrode material is apt to result in enhanced
reactivity with the nonaqueous electrolyte and enhanced gas
evolution and a preferred battery is difficult to obtain.
[0303] The specific surface area as determined by the BET method is
a value measured with a surface area meter (e.g., a fully automatic
surface area measuring apparatus manufactured by Ohukura Riken Co.,
Ltd.) by preliminarily drying a sample at 350.degree. C. for 15
minutes in a nitrogen stream and then measuring the specific
surface area thereof by the gas-flowing nitrogen adsorption BET
one-point method using a nitrogen/helium mixture gas precisely
regulated so as to have a nitrogen pressure of 0.3 relative to
atmosphere pressure.
(6) Pore Diameter Distribution
[0304] With respect to the pore diameter distribution of the
carbonaceous material for use in the invention, the amount of
interstices corresponding to a pore diameter of from 0.01 .mu.m to
1 .mu.m which include pores in the particles, particle surface
irregularities formed by steps, and contact surfaces among the
particles, as determined by mercury porosiometry (mercury intrusion
method), may be 0.01 mL/g or larger and is preferably 0.05 mL/g or
larger, more preferably 0.1 mL/g or larger. The upper limit thereof
may be 0.6 mL/g or smaller and is preferably 0.4 mL/g or smaller,
more preferably 0.3 mL/g or smaller. When the amount of interstices
therein is larger than the upper limit of that range, there are
cases where a large amount of a binder is necessary in electrode
plate formation. When the amount thereof is smaller than the lower
limit, there are cases where high-current-density charge/discharge
characteristics decrease and the effect of diminishing electrode
expansion/contraction during charge/discharge is not obtained.
[0305] The total volume of pores corresponding to the pore diameter
range of from 0.01 .mu.m to 100 .mu.m is preferably 0.1 mL/g or
larger, more preferably 0.25 mL/g or larger, even more preferably
0.4 mL/g or larger. The upper limit thereof may be 10 mL/g or
smaller and is preferably 5 mL/g or smaller, more preferably 2 mL/g
or smaller. When the total volume thereof exceeds the upper limit
of that range, there are cases where a large amount of a binder is
necessary in electrode plate formation. When the total volume
thereof is smaller than the lower limit, there are cases where the
dispersing effect of a thickener or binder in electrode plate
formation is not obtained.
[0306] The average pore diameter of the carbonaceous material is
preferably 0.05 .mu.m or larger, more preferably 0.1 .mu.m or
larger, even more preferably 0.5 .mu.m or larger. The upper limit
thereof may be 50 .mu.m or smaller and is preferably 20 .mu.m or
smaller, more preferably 10 .mu.m or smaller. When the average pore
diameter thereof exceeds the upper limit of that range, there are
cases where a large amount of a binder is necessary. When the
average pore diameter thereof is smaller than the lower limit,
there are cases where high-current-density charge/discharge
characteristics decrease.
[0307] As an apparatus for the mercury porosimetry is used a
mercury porosimeter (Autopore 9520, manufactured by Micromeritics
Corp.). About 0.2 g of a sample is placed in a powder cell and this
cell is closed. The sample in the cell is pretreated by degassing
it at room temperature under vacuum (50 .mu.mHg or lower) for 10
minutes. Subsequently, the pressure in the cell is reduced to 4
psia (about 28 kPa) and mercury is introduced thereinto. The
internal pressure is stepwise elevated from 4 psia (about 28 kPa)
to 40,000 psia (about 280 MPa) and then lowered to 25 psia (about
170 kPa). The number of steps in the pressure elevation is 80 or
larger. In each step, the amount of mercury intruded is measured
after an equilibrium time of 10 seconds. A pore diameter
distribution is calculated from the mercury intrusion curve thus
obtained, using the Washburn equation. Incidentally, the surface
tension (.gamma.) and contact angle (.psi.) of mercury are taken as
485 dyne/cm and 140.degree., respectively. The average pore
diameter used is the pore diameter corresponding to a cumulative
pore volume of 50%.
(7) Roundness
[0308] Roundness is used as an index to the degree of sphericity of
the carbonaceous material. The roundness of the particles having a
particle diameter in the range of 3-40 .mu.m is preferably 0.1 or
higher, especially preferably 0.5 or higher, more preferably 0.8 or
higher, even more preferably 0.85 or higher, most preferably 0.9 or
higher. High roundness values are preferred because they are
effective in improving high-current-density charge/discharge
characteristics.
[0309] Roundness is defined by the following equation. When a
particle has a roundness of 1, this particle theoretically is a
true sphere.
Roundness=(length of periphery of equivalent circle having the same
area as projected particle shape)/(actual length of periphery of
projected particle shape)
[0310] The value of roundness is determined, for example, with a
flow type particle image analyzer (e.g., FPIA, manufactured by
Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a
0.2% by mass aqueous solution (about 50 mL) of poly(oxyethylene
(20)) sorbitan monolaurate as a surfactant, propagating an
ultrasonic wave of 28 kHz to the dispersion for 1 minute at an
output of 60 W, and then examining particles having a particle
diameter in the range of 3-40 .mu.m with the analyzer having a
detection range set at 0.6-400 .mu.m.
[0311] Methods for improving roundness are not particularly
limited. However, a carbonaceous material in which the particles
have been rounded by a rounding treatment (mechanical energy
treatment) is preferred because it gives an electrode in which the
interstices among particles are uniform in shape. Examples of the
rounding treatment include: a method in which a shear force or
compressive force is applied to thereby mechanically make the shape
of the particles close to sphere; and a method of
mechanical/physical treatment in which fine particles are
aggregated into particles by means of the bonding force of a binder
or of the fine particles themselves.
(8) True Density
[0312] The true density of the carbonaceous material is generally
1.4 g/cm.sup.3 or higher, preferably 1.6 g/cm.sup.3 or higher, more
preferably 1.8 g/cm.sup.3 or higher, even more preferably 2.0
g/cm.sup.3 or higher. The upper limit thereof may be 2.26
g/cm.sup.3 or lower. This upper limit is a theoretical value for
graphites. When the true density of the carbonaceous material is
lower than the lower limit of that range, there are cases where
this carbon has too low crystallinity, resulting in an increase in
initial irreversible capacity. In the invention, true density is
defined as one determined by the liquid-phase displacement method
(pycnometer method) using butanol.
(9) Tap Density
[0313] The tap density of the carbonaceous material desirably is
generally 0.1 g/cm.sup.3 or higher, preferably 0.5 g/cm.sup.3 or
higher, more preferably 0.7 g/cm.sup.3 or higher, especially
preferably 1.0 g/cm.sup.3 or higher. The upper limit thereof is
preferably 2 g/cm.sup.3 or lower, more preferably 1.8 g/cm.sup.3 or
lower, especially preferably 1.6 g/cm.sup.3 or lower. When the tap
density thereof is lower than the lower limit of that range, there
are cases where this carbonaceous material, when used in a negative
electrode, is less apt to have a high loading density and cannot
give a battery having a high capacity. On the other hand, when the
tap density thereof exceeds the upper limit of that range, the
amount of interparticle interstices in the electrode is too small
and it is difficult to secure electrical conductivity among the
particles. There are hence cases where preferred battery
performances are difficult to obtain. Tap density is determined by
the same method as described above with regard to the positive
electrode and is defined as described therein.
(10) Orientation Ratio
[0314] The orientation ratio of the carbonaceous material is
generally 0.005 or higher, preferably 0.01 or higher, more
preferably 0.015 or higher. The upper limit thereof is
theoretically 0.67 or lower. When the orientation ratio thereof is
lower than the lower limit of that range, there are cases where
high-density charge/discharge characteristics decrease.
[0315] Orientation ratio is determined by X-ray dittractometry
after a sample is molded by compaction. A molding obtained by
packing 0.47 g of a sample into a molding machine having a diameter
of 17 mm and compacting the sample at 600 kgf/cm.sup.2 is set with
clay on a sample holder for examination so as to be flush with the
holder. This sample molding is examined for X-ray diffraction. From
the intensities of the resultant (110) diffraction peak and (004)
diffraction peak for the carbon, the ratio represented by (110)
diffraction peak intensity/(004) diffraction peak intensity is
calculated. This ratio is defined as the orientation ratio of the
active material.
[0316] Conditions for this X-ray diffractometry are as follows.
Incidentally, "2.theta." represents diffraction angle. [0317]
Target: Cu(K.alpha. line) graphite monochromator [0318] Slit:
[0319] Divergence slit=0.5 degrees [0320] Receiving slit=0.15 mm
[0321] Scattering slit=0.5 degrees [0322] Examination range and
step angle/measuring time: [0323] (110) plane:
75.degree..ltoreq.2.theta..ltoreq.80.degree. 1.degree./60 sec
[0324] (004) plane: 52.degree..ltoreq.2.theta..ltoreq.57.degree.
1.degree./60 sec
(11) Aspect Ratio (Powder)
[0325] The aspect ratio is theoretically 1 or higher. The upper
limit thereof may be 10 or lower and is preferably 8 or lower, more
preferably 5 or lower. When the aspect ratio exceeds the upper
limit, there are cases where this carbonaceous material causes
streak lines in electrode plate formation and an even coating
surface cannot be obtained, resulting in a decrease in
high-current-density charge/discharge characteristics.
[0326] The aspect ratio of a carbonaceous-material particle is
represented by A/B, wherein A is the length of the longest axis of
the particle in a three-dimensional examination and B is the length
of the shortest axis perpendicular to that axis. The examination of
carbon particles is made with a scanning electron microscope
capable of enlargement examination. Fifty are arbitrarily selected
from graphite particles fixed to an edge face of a metal sheet
having a thickness of 50 .mu.m or smaller, and each particle is
examined for A and B while rotating or inclining the stage to which
the sample is fixed. The average of the A/B values is
determined.
(12) Minor-Material Mixing
[0327] "Minor-material mixing" means that the negative electrode
and/or the negative-electrode active material contains two or more
carbonaceous materials differing in property. The term "property"
herein means one or more properties selected from X-ray diffraction
parameter, median diameter, aspect ratio, BET specific surface
area, orientation ratio, Raman R value, tap density, true density,
pore distribution, roundness, and ash content.
[0328] Especially preferred embodiments include: one in which the
volume-based particle size distribution is not symmetrical about
the median diameter; one in which two or more carbonaceous
materials differing in Raman R value are contained; and one in
which carbonaceous materials differing in X-ray parameter are
contained.
[0329] One example of the effects thereof is that the incorporation
of a carbonaceous material, such as a graphite, e.g., a natural
graphite or artificial graphite, or an amorphous carbon, e.g., a
carbon black such as acetylene black or needle coke, as a
conductive material serves to reduce electrical resistance. One of
these carbonaceous materials may be used alone, or any desired two
or more thereof may be used in combination in any desired
proportion. In the case where such a carbonaceous material is added
as a conductive material, the amount thereof may be 0.1% by mass or
larger and is preferably 0.5% by mass or larger, more preferably
0.6% by mass or larger. The upper limit thereof may be 45% by mass
or smaller and is preferably 40% by mass. Amounts thereof smaller
than the lower limit of that range may result in difficulties in
obtaining the effect of improving conductivity. Amounts thereot
exceeding the upper limit may lead to an increase in initial
irreversible capacity.
(13) Electrode Production
[0330] Electrode production may be conducted by an ordinary method.
For example, a binder and a solvent are added to a
negative-electrode active material optionally together with a
thickener, conductive material, filler, etc. to obtain a slurry and
this slurry is applied to a current collector and dried.
Thereafter, the coated current collector is pressed, whereby an
electrode can be formed. The thickness of the negative-electrode
active-material layer per one side in the stage just before the
step of injecting a nonaqueous electrolyte in battery fabrication
is generally 15 .mu.m or larger, preferably 20 .mu.m or larger,
more preferably 30 .mu.m or larger. The upper Limit thereof may be
150 .mu.m or smaller and is preferably 120 .mu.m or smaller, more
preferably 100 .mu.m or smaller. When the thickness thereof is
larger than the upper limit of that range, a nonaqueous electrolyte
is less apt to infiltrate into around the interface of the current
collector and, hence, there are cases where high-current-density
charge/discharge characteristics decrease. When the thickness
thereof is smaller than the lower limit of that range, the
proportion by volume of the current collector to the
negative-electrode active material increases and there are cases
where battery capacity decreases. The negative-electrode active
material may be roller pressed to obtain a sheet electrode. It is
also possible to subject the negative-electrode active material to
compression molding to obtain a pellet electrode.
(14) Current Collector
[0331] As the current collector, a known one can be used at will.
Examples of the current collector for the negative electrode
include metallic materials such as copper, nickel, stainless steel,
and nickel-plated steel. Copper is especially preferred of these
from the standpoints of processability and cost. In the case where
the current collector is a metallic material, examples of the shape
of the current collector include metal foils, metal cylinders,
metal coils, metal plates, thin metal films, expanded metals,
punching metals, and metal foams. Preferred of these are thin metal
films. More preferred are copper foils. Even more preferred are a
rolled copper foil, which is produced by the rolling process, and
an electrolytic copper foil, which is produced by the electrolytic
process. Either of these can be used as a current collector. In the
case of a copper foil having a thickness smaller than 25 .mu.m, use
can be made of a copper alloy (e.g., phosphor bronze,
titanium-copper, Corson alloy, or Cu--Cr--Zr alloy) having a higher
strength than pure copper. Furthermore, an aluminum toil can be
advantageously used because it has a low specific gravity and,
hence, use of the foil as a current collector can reduce the weight
of the battery.
[0332] The current collector comprising a copper foil produced by
the rolling process is less apt to crack even when the negative
electrode is rolled tightly or rolled at an acute angle, because
the copper crystals are oriented in the rolling direction. This
current collector can be advantageously used in small cylindrical
batteries. The electrolytic copper foil is obtained by immersing a
metallic drum in a nonaqueous electrolyte containing copper ions
dissolved therein, causing current to flow through the system while
rotating the drum to thereby deposit copper on the drum surface,
and peeling the copper deposit from the drum. Copper may be
deposited on a surface of the rolled copper foil by the
electrolytic process. One or each side of such a copper foil may
have undergone a surface-roughening treatment or a surface
treatment (e.g., a chromate treatment in a thickness of from
several nanometers to about 1 .mu.m or a priming treatment with
titanium).
[0333] The current collector base is desired to further have the
following properties.
(1) Average Surface Roughness (Ra) The average surface roughness
(Ra) of that side of the current collector base on which a thin
active-material film is to be formed, as determined by the method
provided for in JIS B0601-1994, is not particularly limited.
However, the average surface roughness thereof is generally 0.05
.mu.m or higher, preferably 0.1 .mu.m or higher, especially
preferably 0.15 .mu.m or higher. The upper limit thereof is
generally 1.5 .mu.m or lower, preferably 1.3 .mu.m or lower,
especially preferably 1.0 .mu.m or lower. By regulating the average
surface roughness (Ra) of the current collector base so as to be in
the range of from the lower limit to upper limit, satisfactory
charge/discharge cycle characteristics can be expected. By
regulating the average surface roughness thereof to a value not
lower than the lower limit, the area of the interface between the
base and a thin active-material film is increased and adhesion to
the thin active-material film is improved. The upper limit of the
average surface roughness (Ra) thereof is not particularly limited.
However, it is preferably 1.5 .mu.m or lower because a foil having
a practical thickness for batteries and having an average surface
roughness (Ra) exceeding 1.5 .mu.m is generally difficult to
procure.
(2) Tensile Strength
[0334] The tensile strength of the current collector base is not
particularly limited. However, it is generally 100 N/mm.sup.2 or
higher, preferably 250 N/mm.sup.2 or higher, more preferably 400
N/mm.sup.3 or higher, especially preferably 500 N/mm.sup.3 or
higher. Tensile strength is a quotient obtained by dividing the
maximum tensile force required before test piece breakage by the
sectional area of the test piece. In the invention, the tensile
strength is determined through a measurement conducted with the
same apparatus and by the same method as for elongation. A current
collector base having a high tensile strength can be inhibited from
cracking with the expansion/contraction of the thin active-material
film which occur upon charge/discharge. With this current collector
base, satisfactory cycle characteristics can be obtained.
(3) 0.2% Proof Stress
[0335] The 0.2% proof stress of the current collector base is not
particularly limited. However, it is generally 30 N/mm.sup.2 or
higher, preferably 150 N/mm.sup.2 or higher, especially preferably
300 N/mm.sup.2 or higher. The term 0.2% proof stress means the
degree of load necessary for imparting a plastic (permanent)
deformation of 0.2%. Namely, it means that application of that
degree of load and subsequent removal thereof result in a 0.2%
deformation. In the invention, the 0.2% proof stress is determined
through a measurement conducted with the same apparatus and by the
same method as for elongation. A current collector base having a
high 0.2% proof stress can be inhibited from plastically deforming
with the expansion/contraction of the thin active-material film
which occur upon charge/discharge. With this current collector
base, satisfactory cycle characteristics can be obtained.
[0336] The thin metal film may have any desired thickness. However,
the thickness thereof is generally 1 .mu.m or larger, preferably 3
.mu.m or larger, more preferably 5 .mu.m or larger. The upper limit
thereof is generally 1 mm or smaller, preferably 100 .mu.m or
smaller, more preferably 30 .mu.m or smaller. In case where the
thin metal film is thinner than 1 .mu.m, this film has a reduced
strength and there are hence cases where coating is difficult. When
the thin metal film is thicker than 100 .mu.m, there are cases
where this film deforms an electrode shape, e.g., a rolled form.
The thin metal film may be in a mesh form.
(15) Thickness Ratio Between Current Collector and Active-Material
Layer
[0337] The thickness ratio between the current collector and the
active-material layer is not particularly limited. However, the
value of (thickness of the active-material layer on one side just
before impregnation with the nonaqueous electrolyte)/(thickness of
the current collector) is preferably 150 or smaller, especially
preferably 20 or smaller, more preferably 10 or smaller. The lower
limit thereof is preferably 0.1 or larger, especially preferably
0.4 or larger, more preferably 1 or larger. When that ratio exceeds
the upper limit of that range, there are cases where this current
collector heats up due to Joule's heat during high-current-density
charge/discharge. When that ratio decreases beyond the lower limit
of that range, the proportion by volume of the current collector to
the negative-electrode active material increases and this may
reduce the capacity of the battery.
(16) Electrode Density
[0338] When the negative-electrode active material is used to form
an electrode, the electrode structure is not particularly limited.
However, the density of the active material present on the current
collector is preferably 1.0 g/cm.sup.3 or higher, more preferably
1.2 g/cm.sup.3 or higher, even more preferably 1.3 g/cm.sup.3 or
higher. The upper limit thereof is generally 2.0 g/cm.sup.3 or
lower, preferably 1.9 g/cm.sup.3 or lower, more preferably 1.8
g/cm.sup.3 or lower, even more preferably 1.7 g/cm.sup.3 or lower.
When the density thereof exceeds the upper limit of that range,
there are cases where the active-material particles are broken and
this increases the initial irreversible capacity and reduces the
infiltration of a nonaqueous electrolyte into around the current
collector/active material interface. As a result,
high-current-density charge/discharge characteristics may decrease.
When the density thereof is lower than the lower limit, there are
cases where electrical conductivity among the active-material
particles decreases and this increases battery resistance,
resulting in a reduced capacity per unit volume.
(17) Binder
[0339] The binder for binding the active material is not
particularly limited as long as it is stable to the nonaqueous
electrolyte and the solvent to be used for electrode production.
Examples thereof include resinous polymers such as polyethylene,
polypropylene, poly(ethylene terephthalate), poly(methyl
methacrylate), aromatic polyamides, cellulose, and nitrocellulose;
rubbery polymers such as SBR (styrene/butadiene rubbers), isoprene
rubbers, butadiene rubbers, fluororubbers, NBR
(acrylonitrile/butadiene rubbers), and ethylene/propylene rubbers;
styrene/butadiene/styrene block copolymers or products of
hydrogenation thereof; thermoplastic elastomeric polymers such as
EPDM (ethylene/propylene/diene terpolymers),
styrene/ethylene/butadiene/styrene copolymers, and
styrene/isoprene/styrene block copolymers or products of
hydrogenation thereof; flexible resinous polymers such as
syndiotactic 1,2-polybutadiene, poly(vinyl acetate), ethylene/vinyl
acetate copolymers, and propylene/.alpha.-olefin copolymers;
fluorochemical polymers such as poly(vinylidene fluoride),
polytetrafluoroethylene, fluorinated poly(vinylidene fluoride), and
polytetrafluoroethylene/ethylene copolymers; and polymer
compositions having the property of conducting alkali metal ions
(especially lithium ions). One of these may be used alone, or any
desired combination of two or more thereof in any desired
proportion may be used.
[0340] The kind of the solvent to be used for forming a slurry is
not particularly limited as long as it is a solvent in which the
active material and binder and the thickener and conductive
material which are optionally used according to need can be
dissolved or dispersed. Either an aqueous solvent or an organic
solvent may be used. Examples of the aqueous solvent include water
and alcohols. Examples of the organic solvent include
N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide,
methyl ethyl ketone, cyclohexanone, methyl acetate, methyl
acrylate, diethyltriamine, N,N-dimethylaminopropylamine, propylene
oxide, tetrahydrofuran (THF), toluene, acetone, diethyl ether,
dimethylacetamide, hexamethylphosphoramide, dimethyl sulfoxide,
benzene, xylene, quinoline, pyridine, methylnaphthalene, and
hexane. Especially when an aqueous solvent is used, it is preferred
to add a dispersant or the like in combination with the thickener
described above and prepare a slurry using a latex of, e.g., SBR.
One of such ingredients may be used alone, or any desired
combination of two or more thereof in any desired proportion may be
used.
[0341] The proportion of the binder to the active material is
preferably 0.1% by mass or higher, especially preferably 0.5% by
mass or higher, more preferably 0.6% by mass or higher. The upper
limit thereof is generally 20% by mass or lower, preferably 15% by
mass or lower, more preferably 10% by mass or lower, even more
preferably 8% by mass or lower. In case where the binder amount
exceeds the upper limit of that range, the proportion of the binder
which does not contribute to battery capacity increases and this
may lead to a decrease in battery capacity. When the binder amount
is small than the lower limit, there are cases where the negative
electrode has a reduced strength. Especially when the binder
comprises a rubbery polymer represented by SBR as the main
component, the proportion of this binder to the active material is
generally 0.1% by mass or higher, preferably 0.5% by mass or
higher, more preferably 0.6% by mass or higher. The upper limit
thereof is generally 5% by mass or lower, preferably 3% by mass or
lower, more preferably 2% by mass or lower. In the case where the
binder comprises a fluorochemical polymer represented by
poly(vinylidene fluoride) as the main component, the proportion of
this binder to the active material is generally 1% by mass or
higher, preferably 2% by mass or higher, more preferably 3% by mass
or higher. The upper limit thereof is generally 15% by mass or
lower, preferably 10% by mass or lower, more preferably 8% by mass
or lower.
[0342] A thickener is used generally for the purpose of regulating
the slurry viscosity. The thickener is not particularly limited.
Examples thereof include carboxymethyl cellulose, methyl cellulose,
hydroxymethyl cellulose, ethyl cellulose, poly(vinyl alcohol),
oxidized starch, phosphorylated starch, casein, and salts of these.
One of these thickeners may be used alone, or any desired
combination of two or more thereof in any desired proportion may be
used. In the case where such a thickener is further added, the
proportion of the thickener to the active material is generally
0.1% by mass or higher, preferably 0.5% by mass or higher, more
preferably 0.6% by mass or higher. The upper limit thereof is
generally 5% by mass or lower, preferably 3% by mass or lower, more
preferably 2% by mass or lower. When the proportion thereof is
lower than the lower limit of that range, there are cases where
applicability decreases considerably. Proportions thereof exceeding
the upper limit result in a reduced proportion of the active
material in the negative-electrode active-material layer, and this
may pose a problem that battery capacity decreases and a problem
that resistance among the particles of the negative-electrode
active material increases.
(18) Orientation Ratio in Electrode Plate
[0343] The orientation ratio in the electrode plate is preferably
0.001 or higher, more preferably 0.005 or higher, especially
preferably 0.01 or higher. The upper limit thereof is up to 0.67,
which is a theoretical value. When the orientation ratio therein is
lower than the lower limit of that range, there are cases where
high-density charge/discharge characteristics decrease.
[0344] An examination for determining the orientation ratio in the
electrode plate is as follows. The negative electrode which has
been pressed to a target density is examined by X-ray
diffractometry to determine the orientation ratio of the active
material in this electrode. Although specific techniques therefor
are not particularly limited, a standard method is as follows. The
peaks attributable to the (110) diffraction and (004) diffraction
of the carbon obtained by X-ray diffractometry are subjected to
peak separation by fitting with asymmetric Pearson VII as a profile
function. Thus, the integrated intensities of the (110) diffraction
and (004) diffraction peaks are calculated. From the integrated
intensities obtained, the ratio represented by (integrated
intensity of (110) diffraction)/(integrated intensity of (004)
diffraction) is calculated. The active-material orientation ratio
for the electrode thus calculated is defined as the orientation
ratio in the electrode plate.
[0345] Conditions for this X-ray diffractometry are as follows.
Incidentally, "2.theta." represents diffraction angle. [0346]
Target: Cu(K.alpha. line) graphite monochromator [0347] Slit:
[0348] Divergence slit=1 degree [0349] Receiving slit=0.1 mm [0350]
Scattering slit=1 degree [0351] Examination range and step
angle/measuring time: [0352] (110) plane:
76.5.degree..ltoreq.2.theta..ltoreq.78.5.degree. 0.01.degree./3 sec
[0353] (004) plane:
53.5.degree..ltoreq.2.theta..ltoreq.56.0.degree. 0.01.degree./3 sec
[0354] Sample preparation:
[0355] The electrode is fixed to a glass plate with a double-faced
pressure-sensitive adhesive tape having a thickness of 0.1 mm.
(19) Impedance
[0356] At the time when the battery in a discharged state has been
charged to 60% of its nominal capacity, the resistance of the
negative electrode is preferably 100.OMEGA. or lower, more
preferably 50.OMEGA. or lower, especially preferably 20.OMEGA. or
lower and/or the double-layer capacity of the negative electrode is
preferably 1.times.10.sup.-6 F or higher, more preferably
1.times.10.sup.-5 F, especially preferably 1.times.10.sup.-4 F.
This range is preferred because satisfactory output characteristics
are obtained.
[0357] The resistance and double-layer capacity of the negative
electrode are determined by the following procedure. The lithium
secondary battery to be examined is one which, when charged at a
current at which the nominal capacity can be charged over 5 hours,
subsequently kept for 20 minutes in the state of being neither
charged nor discharged, and then discharged at a current at which
the nominal capacity can be discharged over 1 hour, has a capacity
of at least 80% of the nominal capacity. This lithium secondary
battery in a discharged state is charged to 60% of the nominal
capacity at a current at which the nominal capacity can be charged
over 5 hours. Immediately thereafter, the lithium secondary battery
is transferred to a gloved box filled with an argon atmosphere. In
this gloved box, the lithium secondary battery is rapidly
disassembled and taken out while preventing the negative electrode
from suffering discharge or short-circuiting. In the case where
this negative electrode is a double-sided electrode, the electrode
active material on one side is stripped off without marring the
electrode active material on the other side. Two disks of 12.5
mm.phi. are punched out of this negative electrode, and are
superposed through a separator so that the active-material sides
rightly face each other. Sixty microliters of the nonaqueous
electrolyte which has been used in the battery is dropped onto the
separator and between the two negative electrodes, and the negative
electrodes and separator are brought into tight contact with each
other. While keeping this assemblage isolated from the surrounding
air, the current collectors of the respective negative electrodes
are electrically connected to each other and the assemblage is
examined by the alternating-current impedance method. In the
examination, the complex impedance is measured at a temperature of
25.degree. C. in a frequency region of 10.sup.-2-10.sup.5 Hz to
determine a Cole-Cole plot. The circular arc for the
negative-electrode resistance component in the plot is approximated
to a semicircle to determine the surface resistance (R) and the
double-layer capacity (Cdl).
(20) Area and Thickness of Negative-Electrode Plate
[0358] The area of the negative-electrode plate is not particularly
limited. However, the negative-electrode plate is designed to be
slightly larger than the positive-electrode plate facing it so that
the positive-electrode plate does not protrude outward from the
negative-electrode plate. From the standpoints of cycle life in
repetitions of charge/discharge and inhibition of deterioration
caused by high-temperature storage, it is preferred to regulate the
area of the negative electrode so as to be as close as possible to
the area of the positive electrode. This is because such
negative-electrode area heightens the proportion of electrodes
which function more evenly and effectively and thereby improves
properties. Especially when the battery is to be used at a high
current, this electrode area design is important.
[0359] The thickness of the negative-electrode plate is designed so
as to be suitable for the positive-electrode plate to be used, and
is not particularly limited. However, the thickness of the mix
layer, excluding the thickness of the core metal foil, is generally
15 .mu.m or larger, preferably 20 .mu.m or larger, more preferably
30 .mu.m or larger. The upper limit thereof is generally 150 .mu.m
or smaller, preferably 120 .mu.m or smaller, more preferably 100
.mu.m or smaller.
<Negative Electrode [1]>
[0360] Negative electrode [1], i.e., "a negative electrode
containing as a negative-electrode active material two or more
carbonaceous substances differing in crystallinity", for use in a
lithium secondary battery of the invention will be explained
below.
[Negative Electrode [1]: Negative-Electrode Active Material]
[0361] The negative-electrode active material for use in negative
electrode [1] is described below.
[[Constitution]]
[0362] The negative-electrode active material for use in negative
electrode [1] in a lithium secondary battery of the invention is
characterized by comprising two or more carbonaceous substances
differing in crystallinity. The term "comprising two or more
carbonaceous substances differing in crystallinity" herein means
that carbonaceous substances differing in crystallinity coexist in
the negative electrode. The state in which these carbonaceous
substances coexist is not limited. Namely, particles of one
carbonaceous substance and particles of another may be present as a
mixture of these, or the carbonaceous substances may be contained
in one secondary particle. Furthermore, a mixture of both is
possible. The negative-electrode active material preferably is one
which comprises a composite carbonaceous material comprising two or
more carbonaceous substances differing in crystallinity. Also
preferred is one which comprises the composite carbonaceous
material and, as a minor ingredient, one or more carbonaceous
substances (carbonaceous materials) differing in property from the
carbonaceous substances constituting the composite carbonaceous
material.
[0363] The term "contained in one secondary particle" means, for
example, the state in which the carbonaceous substances differing
in crystallinity are restrained by bonding, the state in which the
substances are physically restrained, or the state in which the
substances retain a shape by means of electrostatic restraint. The
term "physically restrained" herein means a state such as that in
which one of the carbonaceous substances differing in crystallinity
is surrounded by the other or the state in which one is caught by
the other. The term "electrostatic restraint" means the state in
which one of the carbonaceous substances differing in crystallinity
is adherent to the other by means of electrostatic energy.
Furthermore, the term "the state in which the substances are
restrained by bonding" means chemical bonding by hydrogen bonds,
covalent bonds, ionic bonds, etc.
[0364] Of those, the state in which the surface of a core
carbonaceous substance at least partly has an interface where a
coating layer differing in crystallinity is bonded to the core
carbonaceous substance is advantageous because resistance in
lithium movement between the carbonaceous substances differing in
crystallinity is low. The coating layer is not limited as to
whether it is one formed by the bonding of an externally supplied
material and/or of a product of alteration thereof or one formed by
the alteration of a surface-part material of a carbonaceous
substance. The term "coating" herein means that the interface
between the coating layer and the surface of the carbonaceous
substance at least partly has chemical bonds and the coating layer
is (1) in the state of covering the whole surface, (2) in the state
of locally covering the carbonaceous particles, (3) in the state of
selectively covering part of the surface, or (4) in the state of
being present in ultrafine regions including chemical bonds. At the
interface, crystallinity may change continuously or
discontinuously.
[0365] It is preferred that the composite carbonaceous material
should be one which includes a particulate carbonaceous substance
and has an interface where the particulate carbonaceous substance
is covered with and/or bonded to a carbonaceous substance differing
in crystallinity from the particulate carbonaceous substance, the
interface changing in crystallinity discontinuously and/or
continuously. There are no limitations on which of the "particulate
carbonaceous substance" and the "carbonaceous substance differing
in crystallinity from the particulate carbonaceous substance" has
higher crystallinity. However, from the standpoint of producing the
effects of the invention, it is preferred that the particulate
carbonaceous substance should have higher crystallinity.
[0366] A difference in crystallinity herein is judged based on a
difference in interplanar spacing (d002) for the (002) planes,
difference in Lc, or difference in La, each determined by
wide-angle X-ray diffractometry. From the standpoint of producing
the effects of the invention, the difference in crystallinity is
preferably 0.0002 nm or larger in terms of (d002), or 1 nm or
larger in terms of La, or 1 nm or larger in terms of Lc. The
difference in (d002), among those ranges, is preferably 0.0005 nm
or larger, more preferably 0.001 nm or larger, even more preferably
0.003 nm or larger. The upper limit thereof is generally 0.03 nm or
smaller, preferably 0.02 nm or smaller. When the difference therein
is smaller than the lower limit of that range, there are cases
where the effect of the difference in crystallinity becomes low. On
the other hand, in case where the difference therein exceeds the
upper limit of that range, the part having lower crystallinity
tends to be too low in crystallinity and there is a possibility
that the reduced crystallinity might increase irreversible
capacity. The difference in La or Lc, among those ranges, is
preferably 2 nm or larger, more preferably 5 nm or larger, even
more preferably 10 nm or larger. In graphites, values larger than
100 nm cannot be specified because definition is generally
impossible. When the difference in La or Lc is smaller than the
lower limit of that range, there are cases where the effect of the
difference in crystallinity becomes low.
[0367] As stated above, the composite carbonaceous material may
comprise a particulate carbonaceous substance and a carbonaceous
substance which differs in crystallinity from the particulate
carbonaceous substance and which covers and/or is bonded to the
particulate carbonaceous substance. In this material, either of the
"particulate carbonaceous substance" and the "carbonaceous
substance differing in crystallinity from the particulate
carbonaceous substance" may be a graphitic carbonaceous substance
and the other may be a lowly crystalline carbonaceous substance. It
is preferred that the "particulate carbonaceous substance" should
be a graphitic carbonaceous substance and the "carbonaceous
substance differing in crystallinity from the particulate
carbonaceous substance" be a lowly crystalline carbonaceous
substance.
[[[Particulate Carbonaceous Substance]]]
[0368] The particulate carbonaceous substance preferably is a
graphitic carbonaceous substance comprising a natural graphite
and/or an artificial graphite or is one comprising at least one
member selected from the group consisting of the following (a),
(b), and (c), which have slightly lower crystallinity than those
graphites:
(a) a product of organic-substance pyrolysis selected from the
group consisting of coal coke, petroleum coke, furnace black,
acetylene black, and pitch-derived carbon fibers; (b) a product of
the carbonization of the gas of an organic substance; and (c) a
carbonaceous substance obtained by partly or wholly graphitizing
(a) or (b).
[[[[Graphitic Carbonaceous Substance]]]]
[0369] The particulate carbonaceous substance preferably is a
graphitic carbonaceous substance comprising a natural graphite
and/or an artificial graphite. The term graphitic carbonaceous
substance means any of various carbonaceous substances which have
such high crystallinity that the interplanar spacing (d002) for the
(002) planes thereof as determined by wide-angle X-ray
diffractometry is smaller than 0.340 nm.
[0370] Preferred examples of the graphitic carbonaceous substance
include powders selected from natural graphites, artificial
graphites, products obtained by the mechanical pulverization or
re-heat treatment of these graphites, products of the re-heat
treatment of an expanded graphite, and highly purified products
obtained from those graphites. Preferred examples of the artificial
graphites are ones obtained by graphitizing one or more organic
substances selected from coal tar pitch, coal-derived heavy oil,
topping residues, petroleum-derived heavy oil, aromatic
hydrocarbons, nitrogen-containing cyclic compounds,
sulfur-containing cyclic compounds, polyphenylenes, poly(vinyl
chloride), poly(vinyl alcohol), polyacrylonitrile, poly(vinyl
butyral), natural polymers, poly(phenylene sulfide), poly(phenylene
oxide), furfuryl alcohol resins, phenol-formaldehyde resins, imide
resins, and the like at a heat-treating temperature of generally
about from 2,500.degree. C. to 3,200.degree. C. and powdering the
graphitized substances by an appropriate pulverizer.
(Properties of Graphitic Carbonaceous Substance)
[0371] It is desirable that the graphitic carbonaceous substance
should satisfy any one of the following properties (1) to (11) or
simultaneously satisfy two or more thereof.
(1) X-Ray Parameter
[0372] The graphitic carbonaceous substance preferably has a value
of d (interplanar spacing) for the lattice planes (002), as
determined by X-ray diffractometry in accordance with the method of
the Japan Society for Promotion of Scientific Research, of 0.335 nm
or larger. The lower limit of the value of d is smaller than 0.340
nm according to the definition, and is preferably 0.337 nm or
smaller. When the value of d is too large, there are cases where
crystallinity decreases to increase initial irreversible capacity.
On the other hand, 0.335 is a theoretical value for graphites.
Furthermore, the crystallite size (Lc) of this carbon material, as
determined by X-ray diffractometry in accordance with the method of
the Japan Society for Promotion of Scientific Research, is
generally 30 nm or larger, preferably 50 nm or larger, more
preferably 100 nm or larger. In case where the Lc thereof is
smaller than the lower limit of that range, there is a possibility
that crystallinity might decrease to increase initial irreversible
capacity.
(2) Ash Content
[0373] The ash content of the graphitic carbonaceous substance is
preferably 1% by mass or lower, especially 0.5% by mass or lower,
in particular 0.1% by mass or lower, based on the whole graphitic
carbonaceous substance. The lower limit of the ash content thereof
is preferably at least 1 ppm by mass of the whole carbonaceous
substance. When the ash content thereof exceeds the upper limit of
that range, there are cases where battery performance deterioration
caused by reaction with the electrolyte during charge/discharge
becomes not negligible. When the ash content thereof is lower than
the lower limit of that range, there are cases where the production
of this carbonaceous material necessitates much time and energy and
an apparatus for pollution prevention, resulting in an increase in
cost.
(3) Volume-Average Particle Diameter
[0374] With respect to the volume-average particle diameter of the
graphitic carbonaceous substance, the volume-average particle
diameter (median diameter) thereof as determined by the laser
diffraction/scattering method is generally 1 .mu.m or larger,
preferably 3 .mu.m or larger, more preferably 5 .mu.m or larger,
even more preferably 7 .mu.m or larger. The upper limit thereof is
generally 100 .mu.m or smaller, preferably 50 .mu.m or smaller,
more preferably 40 .mu.m or smaller, even more preferably 30 .mu.m
or smaller, especially preferably 25 .mu.m or smaller. When the
volume-average particle diameter thereof is smaller than the lower
limit of that range, there are cases where irreversible capacity
increases, leading to a loss in initial battery capacity. When the
volume-average particle diameter thereof exceeds the upper limit of
that range, there are cases where such a carbonaceous substance is
undesirable from the standpoint of battery production because an
uneven coating surface is apt to result when an electrode plate is
produced.
[0375] Volume-average particle diameter in the invention is defined
as the median diameter determined by dispersing the carbon powder
in a 0.2% by mass aqueous solution (about 1 mL) of
poly(oxyethylene(20)) sorbitan monolaurate as a surfactant and
examining the dispersion with a laser diffraction type particle
size distribution analyzer (e.g., LA-700, manufactured by HORIBA,
Ltd.).
(4) Raman R Value, Raman Half-Value Width
[0376] The R value of the graphitic carbonaceous substance as
determined by the argon ion laser Raman spectroscopy is generally
0.01 or higher, preferably 0.03 or higher, more preferably 0.10 or
higher. The upper limit thereof is generally 0.60 or lower,
preferably 0.40 or lower. When the R value thereof is lower than
the lower limit of that range, the surface of such particles has
too high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. On the other hand, when the R value thereof exceeds the
upper limit of that range, the surface of such particles has
reduced crystallinity and enhanced reactivity with the electrolyte
and this may lead to a decrease in efficiency and enhanced gas
evolution.
[0377] The Raman half-value width around 1,580 cm.sup.-1 of the
graphitic carbonaceous substance is not particularly limited.
However, the half-value width thereof is generally 10 cm.sup.-1 or
larger, preferably 15 cm.sup.-1 or larger. The upper limit thereof
is generally 60 cm.sup.-1 or smaller, preferably 45 cm.sup.-1 or
smaller, more preferably 40 cm.sup.-1 or smaller. When the Raman
half-value width thereof is smaller than the lower limit of that
range, the surface of such particles has too high crystallinity and
there are cases where the number of intercalation sites into which
lithium comes with charge/discharge decreases. Namely, there are
cases where suitability for charge decreases. On the other hand,
when the Raman half-value width thereof exceeds the upper limit of
that range, the surface of such particles has reduced crystallinity
and enhanced reactivity with the electrolyte and this may lead to a
decrease in efficiency and enhanced gas evolution.
[0378] The examination for a Raman spectrum is made with a Raman
spectrometer (e.g., a Raman spectrometer manufactured by Japan
Spectroscopic Co., Ltd.). In the examination, a sample is charged
into a measuring cell by causing the sample to fall naturally into
the cell and the surface of the sample in the cell is irradiated
with argon ion laser light while rotating the cell in a plane
perpendicular to the laser light. The Raman spectrum obtained is
examined for the intensity I.sub.A of a peak P.sub.A around 1,580
cm.sup.-1 and the intensity I.sub.B of a peak P.sub.8 around 1,360
cm.sup.-1. The ratio between these intensities R
(R=I.sub.B/I.sub.A) is calculated and this ratio is defined as the
Raman R value of the carbon material. Furthermore, the half-value
width of the peak P.sub.A around 1,580 cm.sup.-1 in the Raman
spectrum obtained is measured, and this value is defined as the
Raman half-value width of the carbon material.
[0379] Conditions for this Raman spectroscopy are as follows.
[0380] Wavelength of argon ion laser: 514.5 nm [0381] Laser power
on sample: 15-25 mW [0382] Resolution: 10-20 cm.sup.-1 [0383]
Examination range: 1,100-1,730 cm.sup.-1 [0384] Analysis for R
value and half-value width: background processing [0385] Smoothing:
simple average; convolution, 5 points
(5) BET Specific Surface Area
[0386] The specific surface area of the graphitic carbonaceous
substance, as determined by the BET method, is generally 0.1
m.sup.2/g or larger, preferably 0.7 m.sup.2/g or larger, more
preferably 1 m.sup.2/g or larger, even more preferably 1.5
m.sup.2/g or larger. The upper limit thereof is generally 100
m.sup.2/g or smaller, preferably 25 m.sup.2/g or smaller, more
preferably 15 m.sup.2/g or smaller, even more preferably 10
m.sup.2/g or smaller. When the specific surface area thereof is
smaller than the lower limit of that range, there are cases where
use of this graphitic carbonaceous substance as a
negative-electrode material is apt to result in poor lithium
acceptance during charge and in lithium deposition on the electrode
surface. On the other hand, when the specific surface area thereof
exceeds the upper limit of that range, there are cases where use of
this graphitic carbonaceous substance as a negative-electrode
material is apt to result in enhanced reactivity with the
electrolyte and enhanced gas evolution and a preferred battery is
difficult to obtain.
[0387] BET specific surface area is defined as a value measured
with a surface area meter (e.g., a fully automatic surface area
measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by
preliminarily drying a sample at 350.degree. C. for 15 minutes in a
nitrogen stream and then measuring the specific surface area
thereof by the gas-flowing nitrogen adsorption BET one-point method
using a nitrogen/helium mixture gas precisely regulated so as to
have a nitrogen pressure of 0.3 relative to atmosphere
pressure.
(6) Pore Distribution
[0388] In the graphitic carbonaceous substance, the amount of
interstices corresponding to a pore diameter of from 0.01 .mu.m to
1 .mu.m which include pores in the particles and particle surface
irregularities formed by steps, as determined by mercury
porosimetry (mercury intrusion method), is generally 0.01 mL/g or
larger, preferably 0.05 mL/g or larger, more preferably 0.1 mL/g or
larger. The upper limit thereof is generally 0.6 mL/g or smaller,
preferably 0.4 mL/g or smaller, more preferably 0.3 mL/g or
smaller. When the amount of interstices therein is larger than the
upper limit of that range, there are cases where a large amount of
a binder is necessary in electrode plate formation. On the other
hand, when the amount thereof is smaller than the lower limit of
that range, there are cases where high-current-density
charge/discharge characteristics decrease and the effect of
diminishing electrode expansion/contraction during charge/discharge
is not obtained.
[0389] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger. The upper limit thereof is
generally 10 mL/g or smaller, preferably 5 mL/g or smaller, more
preferably 2 mL/g or smaller. When the total pore volume exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary in electrode plate formation. When the
total pore volume is smaller than the lower limit, there are cases
where the dispersing effect of a thickener or binder in electrode
plate formation is not obtained.
[0390] The average pore diameter of the graphitic carbonaceous
substance is preferably 0.05 .mu.m or larger, more preferably 0.1
.mu.m or larger, even more preferably 0.5 .mu.m or larger. The
upper limit thereof is generally 50 .mu.m or smaller, preferably 20
.mu.m or smaller, more preferably 10 .mu.m or smaller. When the
average pore diameter thereof exceeds the upper limit of that
range, there are cases where a large amount of a binder is
necessary. On the other hand, when the average pore diameter
thereof is smaller than the lower limit of that range, there are
cases where high-current-density charge/discharge characteristics
decrease.
[0391] As an apparatus for the mercury porosimetry is used a
mercury porosimeter (Autopore 9520, manufactured by Micromeritics
Corp.). About 0.2 g of a sample (negative-electrode material) is
weighed out and placed in a powder cell and this cell is closed.
The sample in the cell is pretreated by degassing it at room
temperature under vacuum (50 .mu.mHg, or lower) for 10 minutes.
Subsequently, the pressure in the cell is reduced to 4 psia (about
28 kPa) and mercury is introduced thereinto. The internal pressure
is stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia
(about 280 MPa) and then lowered to 25 psia (about 170 kPa). The
number of steps in the pressure elevation is 80 or larger. In each
step, the amount of mercury intruded is measured after an
equilibrium time of 10 seconds. A pore distribution is calculated
from the mercury intrusion curve thus obtained, using the Washburn
equation. For the calculation, the surface tension (.gamma.) and
contact angle (.psi.) of mercury are taken as 485 dyne/cm and
140.degree., respectively. The average pore diameter used is the
pore diameter corresponding to a cumulative pore volume of 50%.
(7) Roundness
[0392] With respect to the degree of sphericity of the graphitic
carbonaceous substance, the roundness of the particles having a
particle diameter in the range of 3-40 .mu.m is preferably 0.1 or
higher, especially preferably 0.5 or higher, more preferably 0.8 or
higher, even more preferably 0.85 or higher, most preferably 0.9 or
higher. High roundness values are preferred because they are
effective in improving high-current-density charge/discharge
characteristics. Roundness is defined by the following equation.
When a particle has a roundness of 1, this particle theoretically
is a true sphere.
Roundness=(length of periphery of equivalent circle having the same
area as projected particle shape)/(actual length of periphery of
projected particle shape)
[0393] The value of roundness is determined, for example, with a
flow type particle image analyzer (e.g., FPIA, manufactured by
Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a
0.2% by mass aqueous solution (about 50 mL) of poly(oxyethylene
(20)) sorbitan monolaurate as a surfactant, propagating an
ultrasonic wave of 28 kHz to the dispersion for 1 minute at an
output of 60 W, and then examining particles having a particle
diameter in the range of 3-40 .mu.m with the analyzer having a
detection range set at 0.6-400 .mu.m.
[0394] Methods for improving roundness are not particularly
limited. However, a graphitic carbonaceous substance in which the
particles have been rounded by a rounding treatment is preferred
because it gives an electrode in which the interstices among
particles are uniform in shape. Examples of the rounding treatment
include: a method in which a shear force or compressive force is
applied to thereby mechanically make the shape of the particles
close to sphere; and a method of mechanical/physical treatment in
which fine particles are aggregated into particles by means of the
bonding force of a binder or of the fine particles themselves.
(8) True Density
[0395] The true density of the graphitic carbonaceous substance is
generally 2 g/cm.sup.3 or higher, preferably 2.1 g/cm.sup.3 or
higher, more preferably 2.2 g/cm.sup.3 or higher, even more
preferably 2.22 g/cm.sup.3 or higher. The upper limit thereof may
be 2.26 g/cm.sup.3 or lower. This upper limit is a theoretical
value for graphites. When the true density of the graphitic
carbonaceous substance is lower than the lower limit of that range,
there are cases where this carbon has too low crystallinity,
resulting in an increase in initial irreversible capacity. In the
invention, true density is defined as one determined by the
liquid-phase displacement method (pycnometer method) using
butanol.
(9) Tap Density
[0396] The tap density of the graphitic carbonaceous substance
desirably is generally 0.1 g/cm.sup.3 or higher, preferably 0.5
g/cm.sup.3 or higher, more preferably 0.7 g/cm.sup.3 or higher,
especially preferably 0.9 g/cm.sup.3 or higher. The upper limit
thereof is preferably 2 g/cm.sup.3 or lower, more preferably 1.8
g/cm.sup.3 or lower, especially preferably 1.6 g/cm.sup.3 or lower.
When the tap density thereof is lower than the lower limit of that
range, there are cases where this graphitic carbonaceous substance,
when used in a negative electrode, is less apt to have a high
loading density and cannot give a battery having a high capacity.
On the other hand, when the tap density thereof exceeds the upper
limit of that range, the amount of interparticle interstices in the
electrode is too small and it is difficult to secure electrical
conductivity among the particles. There are hence cases where
preferred battery performances are difficult to obtain.
[0397] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the cell with the
sample up to the brim, subsequently conducting tapping operations
1,000 times over a stroke length of 10 mm using a powder densimeter
(e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.),
and determining the resultant bulk density. This bulk density is
defined as the tap density.
(10) Orientation Ratio (Powder)
[0398] The orientation ratio of the graphitic carbonaceous
substance is generally 0.005 or higher, preferably 0.01 or higher,
more preferably 0.015 or higher. The upper limit thereof is
theoretically 0.67 or lower. When the orientation ratio thereof is
lower than the lower limit of that range, there are cases where
high-density charge/discharge characteristics decrease.
[0399] Orientation ratio is determined by X-ray diffractometry. The
peaks attributable to the (110) diffraction and (004) diffraction
of the carbon obtained by X-ray diffractometry are subjected to
peak separation by fitting with asymmetric Pearson VII as a profile
function. Thus, the integrated intensities of the (110) diffraction
and (004) diffraction peaks are calculated. From the integrated
intensities obtained, the ratio represented by (integrated
intensity of (110) diffraction)/(integrated intensity of (004)
diffraction) is calculated. This ratio is defined as the
active-material orientation ratio.
[0400] Conditions for this X-ray diffractometry are as follows.
Incidentally, "2.theta." represents diffraction angle. [0401]
Target: Cu(K.alpha. line) graphite monochromator [0402] Slit:
[0403] Divergence slit=1 degree [0404] Receiving slit=0.1 mm [0405]
Scattering slit=1 degree [0406] Examination range and step
angle/measuring time: [0407] (110) plane:
76.5.degree..ltoreq.2.theta.78.5.degree. 0.01.degree./3 sec [0408]
(004) plane: 53.5.degree..ltoreq.2.theta.56.0.degree.
0.01.degree./3 sec
(11) Aspect Ratio (Powder)
[0409] The aspect ratio of the graphitic carbonaceous substance is
theoretically 1 or higher. The upper limit thereof is generally 10
or lower, preferably 8 or lower, more preferably 5 or lower. When
the aspect ratio thereof exceeds the upper limit, there are cases
where this graphitic carbonaceous substance causes streak lines in
electrode plate formation and an even coating surface cannot be
obtained, resulting in a decrease in high-current-density
charge/discharge characteristics.
[0410] The aspect ratio of a carbon material particle is
represented by A/B, wherein A is the length of the longest axis of
the particle in a three-dimensional examination and B is the length
of the shortest axis perpendicular to that axis. The examination of
carbon particles is made with a scanning electron microscope
capable of enlargement examination. Fifty are arbitrarily selected
from graphite particles fixed to an edge face of a metal having a
thickness of 50 .mu.m or smaller, and each particle is examined for
A and B while rotating or inclining the stage to which the sample
is fixed. The average of the A/B values is determined.
[[[[Lowly Crystalline Carbonaceous Substance]]]]
[0411] The term lowly crystalline carbonaceous substance means a
carbonaceous substance which has such low crystallinity that the
interplanar spacing (d002) for the (002) planes as determined by
wide-angle X-ray diffractometry is 0.340 nm or larger.
(Composition of Lowly Crystalline Carbonaceous Substance)
[0412] The "carbonaceous substance differing in crystallinity from
the particulate carbonaceous substance" preferably is a
carbonaceous substance having lower crystallinity than the
particulate carbonaceous substance. It preferably is a product of
the carbonization of the following (d) or (e):
(d) an organic substance capable of carbonization which is selected
from the group consisting of coal-derived heavy oil, straight-run
heavy oil, petroleum-derived cracked heavy oil, aromatic
hydrocarbons, nitrogen-ring compounds, sulfur-ring compounds,
polyphenylenes, organic synthetic polymers, natural polymers,
thermoplastic resins, and thermosetting resins; and (e) a solution
obtained by dissolving any of these organic substances capable of
carbonization in a low-molecular organic solvent.
[0413] The coal-derived heavy oil preferably is any of coal tar
pitches ranging from soft pitch to hard pitch, dry
distillation/liquefaction oil, and the like. The straight-run heavy
oil preferably is any of topping residues, vacuum distillation
residues, and the like. The petroleum-derived cracked heavy oil
preferably is, for example, ethylene tar as a by-product of the
thermal cracking of crude oil, naphtha, etc. The aromatic
hydrocarbons preferably are acenaphthylene, decacyclene,
anthracene, phenanthrene, and the like. The nitrogen-ring compounds
preferably are phenazine, acridine, and the like. The sulfur-ring
compounds preferably are thiophene, bithiophene, and the like. The
polyphenylenes preferably are biphenyl, terphenyl, and the like.
The organic synthetic polymers preferably are poly(vinyl chloride),
poly(vinyl alcohol), poly(vinyl butyral), substances obtained by
insolubilizing these compounds, polyacrylonitrile, polypyrrole,
polythiophene, polystyrene, and the like. The natural polymers
preferably are, for example, polysaccharides such as cellulose,
lignin, mannan, poly(galacturonic acid), chitosan, saccharose, and
the like. The thermoplastic resins preferably are poly(phenylene
sulfide), poly(phenylene oxide), and the like. The thermosetting
resins preferably are furfuryl alcohol resins, phenol-formaldehyde
resins, imide resins, and the like.
[0414] Although the "carbonaceous substance differing in
crystallinity from the particulate carbonaceous substance"
preferably is a product of the carbonization of the "organic
substance capable of carbonization" described above, it also
preferably is a product of the carbonization of a solution obtained
by dissolving the "organic substance capable of carbonization" in a
low-molecular organic solvent such as benzene, toluene, xylene,
quinoline, or n-hexane. It is furthermore preferred that the
"carbonaceous substance differing in crystallinity from the
particulate carbonaceous substance" should be a product of the
carbonization of coal coke or petroleum coke.
[0415] It is especially preferred that the (d) or (e) should be
liquid. Namely, to cause carbonization to proceed in a liquid phase
is preferred from the standpoint of forming an interface between
the lowly crystalline carbonaceous substance and a graphitic
carbonaceous substance part.
(Properties of Lowly Crystalline Carbonaceous Substance)
[0416] It is desirable that the lowly crystalline carbonaceous
substance should satisfy any one of the following properties (1) to
(5) or simultaneously satisfy two or more thereof. Furthermore, one
lowly crystalline carbonaceous substance having such properties may
be used alone, or any two or more such carbonaceous substances may
be used in combination in any desired proportion.
(1) X-Ray Parameter
[0417] With respect to the lowly crystalline carbonaceous substance
part, the value of d (interplanar spacing) for the lattice planes
(002), as determined by X-ray diffractometry in accordance with the
method of the Japan Society for Promotion of Scientific Research,
is 0.340 nm or larger according to the definition. However, the
value of d thereof is preferably 0.340 nm or larger, especially
preferably 0.341 nm or larger. The upper limit thereof is
preferably 0.380 nm or smaller, especially preferably 0.355 nm or
smaller, more preferably 0.350 nm or smaller. When the value of d
is too large, there are cases where the surface of this
carbonaceous substance has considerably low crystallinity to
increase irreversible capacity. When the value of d is too small,
there are cases where the effect of improving suitability for
charge which is obtained by disposing a lowly crystalline
carbonaceous substance on the surface is lessened to reduce the
effects of the invention. Furthermore, the crystallite size (Lc) of
this carbon material, as determined by X-ray diffractometry in
accordance with the method of the Japan Society for Promotion of
Scientific Research, is generally 1 nm or larger, preferably 1.5 nm
or larger. In case where the Lc thereof is smaller than the lower
limit of that range, there are cases where crystallinity decreases
to increase initial irreversible capacity.
(2) Ash Content
[0418] The ash content of the lowly crystalline carbonaceous
substance part may be 1% by mass or lower and is especially 0.5% by
mass or lower, in particular 0.1% by mass or lower, based on the
whole composite carbonaceous material. The lower limit of the ash
content thereof is preferably at least 1 ppm by mass of the whole
carbonaceous material. When the ash content thereof exceeds the
upper limit of that range, there are cases where battery
performance deterioration caused by reaction with the electrolyte
during charge/discharge becomes not negligible. When the ash
content thereof is lower than the lower limit of that range, there
are cases where the production of this carbonaceous substance
necessitates much time and energy and an apparatus for pollution
prevention, resulting in an increase in cost.
(4) Raman R Value, Raman Half-Value Width
[0419] The R value of the lowly crystalline carbonaceous substance
part as determined by the argon ion laser Raman spectroscopy is
generally 0.5 or higher, preferably 0.7 or higher, more preferably
0.9 or higher. The upper limit thereof is generally 1.5 or lower,
preferably 1.2 or lower. When the R value thereof is lower than the
lower limit of that range, the surface of such particles has too
high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. In addition, when a coating fluid containing such a
composite carbonaceous material is applied to a current collector
and the resultant coating is pressed to heighten the density of the
negative electrode, then the crystals are apt to orient in
directions parallel to the electrode plate and this may lead to a
decrease in load characteristics. On the other hand, when the R
value thereof exceeds the upper limit of that range, the surface of
such particles has reduced crystallinity and enhanced reactivity
with the electrolyte and this may lead to a decrease in efficiency
and enhanced gas evolution.
[0420] The Raman half-value width around 1,580 cm.sup.-1 of the
lowly crystalline carbonaceous substance part is not particularly
limited. However, the half-value width thereof is generally 40
cm.sup.-1 or larger, preferably 50 cm.sup.-1 or larger. The upper
limit thereof is generally 100 cm.sup.-1 or smaller, preferably 90
cm.sup.-1 or smaller, more preferably 80 cm.sup.-1 or smaller. When
the Raman half-value width thereof is smaller than the lower limit
of that range, the surface of such particles has too high
crystallinity and there are cases where the number of intercalation
sites into which lithium comes with charge/discharge decreases.
Namely, there are cases where suitability for charge decreases. In
addition, when a coating fluid containing such a composite
carbonaceous material is applied to a current collector and the
resultant coating is pressed to heighten the density of the
negative electrode, then the crystals are apt to orient in
directions parallel to the electrode plate and this may lead to a
decrease in load characteristics. On the other hand, when the Raman
half-value width thereof exceeds the upper limit of that range, the
surface of such particles has reduced crystallinity and enhanced
reactivity with the electrolyte and this may lead to a decrease in
efficiency and enhanced gas evolution.
(4) True Density
[0421] The true density of the lowly crystalline carbonaceous
substance part is generally 1.4 g/cm.sup.3 or higher, preferably
1.5 g/cm.sup.3 or higher, more preferably 1.6 g/cm.sup.3 or higher,
even more preferably 1.7 g/cm.sup.3 or higher. The upper limit
thereof is generally 2.1 g/cm.sup.3 or lower, preferably 2
g/cm.sup.3 or lower. In case where the true density thereof exceeds
the upper limit of that range, there is a possibility that
suitability for charge might be impaired. When the true density
thereof is lower than the lower limit of that range, there are
cases where this carbon has too low crystallinity, resulting in an
increase in initial irreversible capacity.
(5) Orientation Ratio
[0422] The orientation ratio of the lowly crystalline carbonaceous
substance part is generally 0.005 or higher, preferably 0.01 or
higher, more preferably 0.015 or higher. The upper limit thereof is
theoretically 0.67 or lower. Orientation ratios thereof lower than
the lower limit of that range are undesirable because high-density
charge/discharge characteristics decrease. Orientation ratio is
determined by X-ray diffractometry. The peaks attributable to the
(110) diffraction and (004) diffraction of the carbon obtained by
X-ray diffractometry are subjected to peak separation by fitting
with asymmetric Pearson VII as a profile function. Thus, the
integrated intensities of the (110) diffraction and (004)
diffraction peaks are calculated. From the integrated intensities
obtained, the ratio represented by (integrated intensity of (110)
diffraction)/(integrated intensity of (004) diffraction) is
calculated. This ratio is defined as the electrode active-material
orientation ratio.
[[[Composite Carbonaceous Material]]]
[0423] The composite carbonaceous material for use in negative
electrode [1] in a lithium secondary battery of the invention
preferably comprises a "particulate carbonaceous substance" and a
"carbonaceous substance differing in crystallinity from the
particulate carbonaceous substance" as stated above. In this case,
either of these substances may be a graphitic carbonaceous
substance and the other may be a lowly crystalline carbonaceous
substance. Preferably, the "particulate carbonaceous substance" is
a graphitic carbonaceous substance and the "carbonaceous substance
differing in crystallinity from the particulate carbonaceous
substance" is a lowly crystalline carbonaceous substance.
[0424] In the composite carbonaceous material, the proportion of
the graphitic carbonaceous substance to the lowly crystalline
carbonaceous substance by mass is preferably 50/50 or higher, more
preferably 80/20 or higher, especially preferably 90/10 or higher,
and is preferably 99.9/0.1 or lower, more preferably 99/1 or lower,
especially preferably 98/2 or lower. When the proportion thereof
exceeds the upper limit of that range, there are cases where the
possession of the two kinds of crystalline carbonaceous substances
does not produce its effect. In case where the proportion thereof
is lower than the lower limit of that range, initial irreversible
capacity tends to increase and this may pose a problem concerning
battery design. It is preferred that the graphitic carbonaceous
substance should account for 50% by mass or more of the whole
composite carbonaceous material.
(Properties of Composite Carbonaceous Material)
[0425] It is desirable that the composite carbonaceous material
should satisfy any one of the following properties (1) to (11) or
simultaneously satisfy two or more thereof. Furthermore, one
composite carbonaceous material having such properties may be used
alone, or any two or more such composite carbonaceous materials may
be used in combination in any desired proportion.
(1) X-Ray Parameter
[0426] The composite carbonaceous material preferably has a value
of d (interplanar spacing) for the lattice planes (002), as
determined by X-ray diffractometry in accordance with the method of
the Japan Society for Promotion of Scientific Research, of 0.335 nm
or larger. The value of d desirably is generally 0.350 nm or
smaller, preferably 0.345 nm or smaller, more preferably 0.340 nm
or smaller. Furthermore, the crystallite size (Lc) of this carbon
material, as determined by X-ray diffractometry in accordance with
the method of the Japan Society for Promotion of Scientific
Research, is generally 1.5 nm or larger, preferably 3.0 nm or
larger. In case where the Lc thereof is smaller than the lower
limit of that range, there is a possibility that crystallinity
might decrease to increase initial irreversible capacity.
(2) Ash Content
[0427] The ash content of the composite carbonaceous material is
generally 1% by mass or lower, especially 0.5% by mass or lower, in
particular 0.1% by mass or lower, based on the whole composite
carbonaceous material. The lower limit of the ash content thereof
is preferably at least 1 ppm by mass of the whole composite
carbonaceous material. When the ash content thereof exceeds the
upper limit of that range, there are cases where battery
performance deterioration caused by reaction with the electrolyte
during charge/discharge becomes not negligible. When the ash
content thereof is lower than the lower limit of that range, there
are cases where the production of this composite carbonaceous
material necessitates much time and energy and an apparatus for
pollution prevention, resulting in an increase in cost.
(3) Volume-Average Particle Diameter
[0428] With respect to the volume-average particle diameter of the
composite carbonaceous material, the volume-average particle
diameter (median diameter) thereof as determined by the laser
diffraction/scattering method is generally 1 .mu.m or larger,
preferably 3 .mu.m or larger, more preferably 5 .mu.m or larger,
even more preferably 7 .mu.m or larger. The upper limit thereof is
generally 100 .mu.m or smaller, preferably 50 .mu.m or smaller,
more preferably 40 .mu.m or smaller, even more preferably 30 .mu.m
or smaller, especially preferably 25 .mu.m or smaller. When the
volume-average particle diameter thereof is smaller than the lower
limit of that range, there are cases where irreversible capacity
increases, leading to a loss in initial battery capacity. When the
volume-average particle diameter thereof exceeds the upper limit of
that range, there are cases where such a composite carbonaceous
material is undesirable from the standpoint of battery production
because an uneven coating surface is apt to result when an
electrode plate is produced.
(4) Raman R Value, Raman Half-Value Width
[0429] The R value of the composite carbonaceous material as
determined by the argon ion laser Raman spectroscopy is generally
0.03 or higher, preferably 0.10 or higher, more preferably 0.15 or
higher. The upper limit thereof may be 0.60 or lower and is
preferably 0.50 or lower. When the R value thereof is lower than
the lower limit of that range, the surface of such particles has
too high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. In addition, when a coating fluid containing such a
composite carbonaceous material is applied to a current collector
and the resultant coating is pressed to heighten the density of the
negative electrode, then the crystals are apt to orient in
directions parallel to the electrode plate and this may lead to a
decrease in load characteristics. On the other hand, when the R
value thereof exceeds the upper limit of that range, the surface of
such particles has reduced crystallinity and enhanced reactivity
with the electrolyte and this may lead to a decrease in efficiency
and enhanced gas evolution.
[0430] The Raman half-value width around 1,580 cm.sup.-1 of the
composite carbonaceous material is not particularly limited.
However, the half-value width thereof is generally 15 cm.sup.-1 or
larger, preferably 20 cm.sup.-1 or larger. The upper limit thereof
is generally 70 cm.sup.-1 or smaller, preferably 60 cm.sup.-1 or
smaller, more preferably 50 cm.sup.-1 or smaller. When the Raman
half-value width thereof is smaller than the lower limit of that
range, the surface of such particles has too high crystallinity and
there are cases where the number of intercalation sites into which
lithium comes with charge/discharge decreases. Namely, there are
cases where suitability for charge decreases. In addition, when a
coating fluid containing such a composite carbonaceous material is
applied to a current collector and the resultant coating is pressed
to heighten the density of the negative electrode, then the
crystals are apt to orient in directions parallel to the electrode
plate and this may lead to a decrease in load characteristics. On
the other hand, when the Raman half-value width thereof exceeds the
upper limit of that range, the surface of such particles has
reduced crystallinity and enhanced reactivity with the electrolyte
and this may lead to a decrease in efficiency and enhanced gas
evolution.
(5) BET Specific Surface Area
[0431] The specific surface area of the composite carbonaceous
material, as determined by the BET method, is generally 0.1
m.sup.2/g or larger, preferably 0.7 m.sup.2/g or larger, more
preferably 1 m.sup.2/g or larger, even more preferably 1.5
m.sup.2/g or larger. The upper limit thereof is generally 100
m.sup.2/g or smaller, preferably 25 m.sup.2/g or smaller, more
preferably 15 m.sup.2/g or smaller, even more preferably 10
m.sup.2/g or smaller. When the specific surface area thereof is
smaller than the lower limit of that range, there are cases where
use of this composite carbonaceous material as a negative-electrode
material is apt to result in poor lithium acceptance during charge
and in lithium deposition on the electrode surface. On the other
hand, when the specific surface area thereof exceeds the upper
limit of that range, there are cases where use of this composite
carbonaceous material as a negative-electrode material is apt to
result in enhanced reactivity with the electrolyte and enhanced gas
evolution and a preferred battery is difficult to obtain.
(6) Pore Distribution
[0432] In the composite carbonaceous material, the amount of
interstices corresponding to a pore diameter of from 0.01 .mu.m to
1 .mu.m which include pores in the particles and particle surface
irregularities formed by steps, as determined by mercury
porosimetry (mercury intrusion method), is generally 0.01 mL/g or
larger, preferably 0.05 mL/g or larger, more preferably 0.1 mL/g or
larger. The upper limit thereof is generally 0.6 mL/g or smaller,
preferably 0.4 mL/g or smaller, more preferably 0.3 mL/g or
smaller. When the amount of interstices therein is larger than the
upper limit of that range, there are cases where a large amount of
a binder is necessary in electrode plate formation. When the amount
thereof is smaller than the lower limit, there are cases where
high-current-density charge/discharge characteristics decrease and
the effect of diminishing electrode expansion/contraction during
charge/discharge is not obtained.
[0433] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger. The upper limit thereof is
generally 10 mL/g or smaller, preferably 5 mL/g or smaller, more
preferably 2 mL/g or smaller. When the total pore volume exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary in electrode plate formation. When the
total pore volume is smaller than the lower limit, there are cases
where the dispersing effect of a thickener or binder in electrode
plate formation is not obtained.
[0434] The average pore diameter of the composite carbonaceous
material is preferably 0.05 .mu.m or larger, more preferably 0.1
.mu.m or larger, even more preferably 0.5 .mu.m or larger. The
upper limit thereof is generally 80 .mu.m or smaller, preferably 50
.mu.m or smaller, more preferably 20 .mu.m or smaller. When the
average pore diameter thereof exceeds the upper limit of that
range, there are cases where a large amount of a binder is
necessary. When the average pore diameter thereof is smaller than
the lower limit, there are cases where high-current-density
charge/discharge characteristics decrease.
(7) Roundness
[0435] With respect to the degree of sphericity of the composite
carbonaceous material, the roundness of the particles having a
particle diameter in the range of 3-40 .mu.m is preferably 0.85 or
higher, more preferably 0.87 or higher, even more preferably 0.9 or
higher. High roundness values are preferred because they are
effective in improving high-current-density charge/discharge
characteristics.
(8) True Density
[0436] The true density of the composite carbonaceous material is
generally 1.9 g/cm.sup.3 or higher, preferably 2 g/cm.sup.3 or
higher, more preferably 2.1 g/cm.sup.3 or higher, even more
preferably 2.2 g/cm.sup.3 or higher. The upper limit thereof may be
2.26 g/cm.sup.3 or lower. This upper limit is a theoretical value
for graphites. When the true density of the composite carbonaceous
material is lower than the lower limit of that range, there are
cases where this carbon has too low crystallinity, resulting in an
increase in initial irreversible capacity.
(9) Tap Density
[0437] The tap density of the composite carbonaceous material
desirably is generally 0.1 g/cm.sup.3 or higher, preferably 0.5
g/cm.sup.3 or higher, more preferably 0.7 g/cm.sup.3 or higher,
especially preferably 1 g/cm.sup.3 or higher. The tap density
thereof is preferably 2 g/cm.sup.3 or lower, more preferably 1.8
g/cm.sup.3 or lower, especially preferably 1.6 g/cm.sup.3 or lower.
When the tap density thereof is lower than the lower limit of that
range, there are cases where this composite carbonaceous material,
when used in a negative electrode, is less apt to have a high
loading density and cannot give a battery having a high capacity.
On the other hand, when the tap density thereof exceeds the upper
limit of that range, the amount of interparticle interstices in the
electrode is too small and it is difficult to secure electrical
conductivity among the particles. There are hence cases where
preferred battery performances are difficult to obtain. Tap density
is determined by the same method as described above and is defined
as described therein.
(10) Orientation Ratio (Powder)
[0438] The orientation ratio of the composite carbonaceous material
is generally 0.005 or higher, preferably 0.01 or higher, more
preferably 0.015 or higher. The upper limit thereof is
theoretically 0.67 or lower. When the orientation ratio thereof is
lower than the lower limit of that range, there are cases where
high-density charge/discharge characteristics decrease.
(11) Aspect Ratio (Powder)
[0439] The aspect ratio of the composite carbonaceous material is
theoretically 1 or higher. The upper limit thereof may be 10 or
lower and is preferably 8 or lower, more preferably 5 or lower.
When the aspect ratio thereof exceeds the upper limit, there are
cases where this composite carbonaceous material causes streak
lines in electrode plate formation and an even coating surface
cannot be obtained, resulting in a decrease in high-current-density
charge/discharge characteristics.
(Processes for Producing Composite Carbonaceous Material)
[0440] Processes for producing the composite carbonaceous material
are not particularly limited. Examples thereof include the
following methods.
[0441] Methods usable for combining a graphitic carbonaceous
substance with a lowly crystalline carbonaceous substance include:
a method in which a carbon precursor for obtaining a lowly
crystalline carbonaceous substance is used as it is and a mixture
of the carbon precursor and a powder of a graphitic carbonaceous
substance is heat-treated to obtain the composite powder; a method
which comprises partly carbonizing the carbon precursor to produce
a powder of a lowly crystalline carbonaceous substance beforehand,
mixing this powder with a powder of a graphitic carbonaceous
substance, and heat-treating the mixture to combine the two
substances; and a method which comprises producing the powder of a
lowly crystalline carbonaceous substance beforehand, mixing a
powder of a graphitic carbonaceous substance, the powder of a lowly
crystalline carbonaceous substance, and a carbon precursor, and
heat-treating the mixture to combine these substances. In the
latter two methods, in which a powder of a lowly crystalline
carbonaceous substance is prepared beforehand, it is preferred that
the particles of the lowly crystalline carbonaceous substance to be
used should have an average particle diameter not larger than
one-tenth the average particle diameter of the particles of the
graphitic carbonaceous substance. Also usable is a method in which
the lowly crystalline carbonaceous substance produced beforehand is
mixed with a graphitic carbonaceous substance while applying
mechanical energy thereto by, e.g., pulverization to thereby impart
thereto a structure in which one is surrounded by the other or a
structure in which one is electrostatically adherent to the
other.
[0442] In a preferred process, a mixture of particles of a
graphitic carbonaceous substance and a carbon precursor is heated
to obtain an intermediate, or a mixture of particles of a graphitic
carbonaceous substance and particles of a lowly crystalline
carbonaceous substance is mixed with a carbon precursor and this
mixture is heated to obtain an intermediate. Thereafter, the
intermediate is carbonized/heat-treated and pulverized to finally
obtain a composite carbonaceous material comprising the particles
of a graphitic carbonaceous substance and a lowly crystalline
carbonaceous substance combined therewith.
[0443] This process for obtaining a composite carbonaceous material
is divided into the following four steps.
[0444] First step: Either particles of a graphitic carbonaceous
substance or a mixture of particles of a graphitic carbonaceous
substance and particles of a lowly crystalline carbonaceous
substance is mixed with a carbon precursor for particles of a lowly
crystalline carbonaceous substance and optionally with a solvent by
means of any of various commercial mixers, kneaders, etc. to obtain
a mixture.
[0445] Second step: The mixture is heated to obtain an intermediate
from which the solvent and any volatile ingredient generating from
the carbon precursor have been removed. According to need, this
heating may be conducted while stirring the mixture. Even when a
volatile ingredient remains, this poses no problem because it will
be removed in the subsequent step, i.e., the third step.
[0446] Third step: The mixture or intermediate is heated to a
temperature of from 400.degree. C. to 3,200.degree. C. in an inert
gas atmosphere such as nitrogen gas, carbon dioxide gas, or argon
gas to obtain a graphite/lowly crystalline carbonaceous substance
composite.
[0447] Fourth step: The composite is subjected to powder processing
such as pulverization, disaggregation, classification, etc.
according to need.
[0448] Of those steps, the second step and the fourth step can be
omitted in some cases, and the fourth step may be conducted before
the third step. However, in the case where the fourth step was
conducted before the third step, powder processing such as
pulverization, disaggregation, classification, etc. is conducted
again according to need to obtain a composite carbonaceous
material.
[0449] With respect to conditions for the heat treatment in the
third step, heat hysteresis temperature conditions are important.
The lower limit of the temperature is generally 400.degree. C. or
higher, preferably 900.degree. C. or higher, although it changes
slightly depending on the kind of the carbon precursor and the heat
hysteresis thereof. On the other hand, with respect to upper-limit
temperature, the mixture or intermediate can be heated basically to
a temperature at which the carbon precursor does not come to have a
structural order higher than the crystal structure of the graphitic
carbonaceous substance particles as cores. Consequently, the
upper-limit temperature in the heat treatment is generally
3,200.degree. C. or lower, preferably 2,000.degree. C. or lower,
more preferably 1,500.degree. C. or lower. Heating rate, cooling
rate, heat treatment period, and the like, among conditions of such
heat treatment, can be determined at will according to purposes. It
is also possible to conduct a heat treatment in a relatively
low-temperature region and then heat the work to a given
temperature. The reactor to be used in the steps may be of the
batch type or continuous type, and one reactor or two or more
reactors may be used.
[[Minor-Material Mixing]]
[0450] Besides the composite carbonaceous material, one or more
carbonaceous substances (carbonaceous materials) differing in
carbonaceous-substance property from the composite carbonaceous
material may be incorporated into the negative-electrode active
material for use in a lithium secondary battery of the invention.
Thus, battery performances can be further improved. The term
"carbonaceous-substance property" herein means one or more
properties selected from X-ray diffraction parameter, median
diameter, aspect ratio, BET specific surface area, orientation
ratio, Raman R value, tap density, true density, pore distribution,
roundness, and ash content. Preferred embodiments include: one in
which the volume-based particle size distribution is not
symmetrical about the median diameter; one in which two or more
carbon materials differing in Raman R value are contained; and one
in which carbon materials differing in X-ray parameter are
contained. One example of the effects thereof is that the
incorporation of a carbon material, such as a graphite, e.g., a
natural graphite or artificial graphite, or an amorphous carbon,
e.g., a carbon black such as acetylene black or needle coke, as a
sub material serves to reduce electrical resistance. One of these
carbon materials may be used alone, or any desired two or more
thereof may be used in combination in any desired proportion. In
the case where such a carbon material is added as a sub material,
the amount thereof may be 0.1% by mass or larger and is preferably
0.5% by mass or larger, more preferably 0.6% by mass or larger. The
upper limit thereof may be 80% by mass or smaller and is preferably
50% by mass or smaller, more preferably 40% by mass or smaller,
even more preferably 30% by mass or smaller. Amounts thereof
smaller than the lower limit of that range are undesirable because
they result in difficulties in obtaining the effect of improving
conductivity. Amounts thereof exceeding the upper limit are
undesirable because they lead to an increase in initial
irreversible capacity.
[Negative Electrode [1]: Electrode Production]
[0451] Negative electrode [1] may be produced by an ordinary
method. It can be formed in the same manner as described above. The
current collector, thickness ratio between the current collector
and the active-material layer, electrode density, binder,
electrode-plate orientation ratio, impedance, etc. also are the
same as described above.
<Negative Electrode [2]>
[0452] An explanation will be given below on negative electrode [2]
for use in a lithium secondary battery of the invention, i.e., "a
negative electrode containing as a negative-electrode active
material at least one amorphous carbonaceous substance which, when
examined by wide-angle X-ray diffractometry, has an interplanar
spacing (d002) for the (002) planes of 0.337 nm or larger and a
crystallite size (Lc) of 80 nm or smaller and which, in an
examination by argon ion laser Raman spectroscopy, has a Raman R
value defined as the ratio of the peak intensity at 1,360 cm.sup.-1
to the peak intensity at 1,580 cm.sup.-1 of 0.2 or higher".
[Negative Electrode [2]: Negative-Electrode Active Material]
[0453] The negative-electrode active material for use in negative
electrode [2] is described below.
[0454] The negative-electrode active material for use in negative
electrode [2] in a lithium secondary battery of the invention
comprises an amorphous carbonaceous substance satisfying at least
the following (a), (b), and (c).
(a) The interplanar spacing (d002) for the (002) planes as
determined by wide-angle X-ray diffractometry is 0.337 nm or
larger. (b) The crystallite size (Lc) of the (002) planes as
determined by wide-angle X-ray diffractometry is 80 nm or smaller.
(c) The Raman R value defined as the ratio of the peak intensity at
1,360 cm.sup.-1 to the peak intensity at 1,580 cm.sup.-1 in an
examination by argon ion laser Raman spectroscopy (hereinafter
often referred to simply as "Raman R value") is 0.2 or higher.
[0455] Although the negative-electrode active material for use in
this invention comprises an amorphous carbonaceous substance
satisfying at least (a), (b), and (c), the content of this
amorphous carbonaceous substance in the whole negative-electrode
active material is preferably 10% by mass or higher, especially
preferably 50% by mass or higher. More preferably, the content
thereof is 100%, i.e., this amorphous carbonaceous substance
accounts for the whole negative-electrode active material.
Negative-electrode active materials usable with this amorphous
carbonaceous substance are not particularly limited, and known
negative-electrode active materials such as artificial graphites
and natural graphites may be used.
[[Interplanar Spacing (d002) and Lc]]
[0456] The interplanar spacing (d002) for the (002) planes of the
amorphous carbonaceous substance for use as the negative-electrode
active material of negative electrode [2] in a lithium secondary
battery of the invention, as determined by wide-angle X-ray
diffractometry, is 0.337 nm or larger, preferably 0.34 nm or
larger. The upper limit thereof is generally 0.39 nm or smaller,
preferably 0.38 nm or smaller, more preferably 0.37 nm or smaller,
even more preferably 0.36 or smaller, especially preferably 0.35 or
smaller. In case where the interplanar spacing (d002) thereof
exceeds the upper limit of that range, crystallinity decreases
considerably and the resultant decrease in conductivity among the
particles becomes not negligible. There are hence cases where the
effect of improving short-time high-current-density
charge/discharge characteristics is difficult to obtain. On the
other hand, when the interplanar spacing (d002) thereof is smaller
than the lower limit of that range, there are cases where
crystallinity is too high and the effect of improving short-time
high-current-density charge/discharge characteristics is difficult
to obtain.
[0457] The term "interplanar spacing (d002) for the (002) planes as
determined by wide-angle X-ray diffractometry" in the invention
means the value of d (interplanar spacing) of the lattice planes
((002) planes) as determined by X-ray diffractometry by the method
of the Japan Society for Promotion of Scientific Research.
[0458] On the other hand, the crystallite size (Lc) for the (002)
planes of the amorphous carbonaceous substance as determined by
X-ray diffractometry by the method of the Japan Society for
Promotion of Scientific Research is 80 nm or smaller, preferably 35
nm or smaller, more preferably 20 nm or smaller, even more
preferably 10 nm or smaller. The lower limit thereof is generally
0.1 nm or larger, preferably 1 nm or larger. When the Lc thereof is
smaller than the lower limit of that range, there are cases where
crystallinity decreases considerably and the resultant decrease in
conductivity among the particles becomes not negligible and where
the effect of improving suitability for short-time
high-current-density charge/discharge is difficult to obtain. On
the other hand, when the Lc thereof exceeds the upper limit of that
range, there are cases where crystallinity is too high and the
effect of improving suitability for short-time high-current-density
charge/discharge is difficult to obtain.
[[Raman R Value]]
[0459] The Raman R value of the amorphous carbonaceous substance
for use as the negative-electrode active material of negative
electrode [2] in a lithium secondary battery of the invention
should be 0.2 or higher, and is preferably 0.5 or higher,
especially preferably 0.7 or higher, more preferably 0.8 or higher.
The upper limit thereof is preferably 1.5 or lower, more preferably
1.2 or lower. In case where the Raman R value thereof is lower than
the lower limit of that range, the surface of such particles has
too high crystallinity and suitability for charge decreases. There
are hence cases where the effect of improving suitability for
short-time high-current-density charge/discharge is difficult to
obtain. On the other hand, in case where the Raman R value thereof
exceeds the upper limit of that range, the surface of such
particles has considerably reduced crystallinity, resulting in
increased contact resistance among the particles. There are hence
cases where the effect of improving suitability for short-time
high-current-density charge/discharge is difficult to obtain.
[0460] The examination for a Raman spectrum is made with a Raman
spectrometer (e.g., a Raman spectrometer manufactured b.sub.y Japan
Spectroscopic Co., Ltd.). In the examination, a sample is charged
into a measuring cell by causing the sample to fall naturally into
the cell and the surface of the sample in the cell is irradiated
with argon ion laser light while rotating the cell in a plane
perpendicular to the laser light. The Raman spectrum obtained is
examined for the intensity I.sub.A of a peak P.sub.A at 1,580
cm.sup.-1 and the intensity I.sub.B of a peak P.sub.B at 1,360
cm.sup.-1. The ratio between these intensities R
(R=I.sub.B/I.sub.A) is calculated and this ratio is defined as the
Raman R value of the amorphous carbonaceous substance. Furthermore,
the half-value width of the peak P.sub.A at 1,580 cm.sup.-1 in the
Raman spectrum obtained is measured, and this value is defined as
the Raman half-value width of the amorphous carbonaceous
substance.
[0461] Conditions for this Raman spectroscopy are as follows.
[0462] Wavelength of argon ion laser: 514.5 nm [0463] Laser power
on sample: 15-25 mW [0464] Resolution: 10-20 cm.sup.-1 [0465]
Examination range: 1,100 cm.sup.-1 to 1,730 cm.sup.-1 [0466]
Analysis for Raman R value and Raman half-value width: background
processing [0467] Smoothing: simple average; convolution, 5
points
[0468] The Raman half-value width at 1,580 cm.sup.-1 of the
amorphous carbonaceous substance for use as the negative-electrode
active material of negative electrode [2] in a lithium secondary
battery of the invention is not particularly limited. However, the
half-value width thereof is generally 20 cm.sup.-1 or larger,
preferably 25 cm.sup.-1 or larger. The upper limit thereof is
generally 150 cm.sup.-1 or smaller, preferably 140 cm.sup.-1 or
smaller. In case where the Raman half-value width thereof is
smaller than the lower limit of that range, the surface of such
particles has too high crystallinity and suitability for charge
decreases. There is hence a possibility that the effect of
improving suitability for short-time high-current-density
charge/discharge might be difficult to obtain. On the other hand,
in case where the Raman half-value width thereof exceeds the upper
limit of that range, the surface of such particles has considerably
reduced crystallinity, resulting in increased contact resistance
among the particles. There is hence a possibility that the effect
of improving suitability for short-time high-current-density
charge/discharge might be difficult to obtain. It is, however,
noted that there are cases where the Raman half-value widths of
some peaks cannot be judged because of the shapes of the peaks.
[0469] Although the amorphous carbonaceous substance for use in
this invention satisfies the requirements concerning interplanar
spacing (d002), crystallite size (Lc), and Raman R value described
above, it is preferred from the standpoint of battery performance
balance that the carbonaceous substance should further satisfy any
one of the following requirements or simultaneously satisfy two or
more thereof. It is especially preferred that any one of or two or
more of true density, H/C value, O/C value, tap density, BET
specific surface area, pore volume in the range of 0.01 .mu.m to 1
.mu.m, ash content, and volume-average particle diameter, among
those requirements, should be simultaneously satisfied.
[[True Density]]
[0470] The true density of the amorphous carbonaceous substance is
generally 2.22 g/cm.sup.3 or lower, preferably 2.2 g/cm.sup.3 or
lower, more preferably 2.1 g/cm.sup.3 or lower, even more
preferably 2.0 g/cm.sup.3 or lower. The lower limit thereof is
generally 1.4 g/cm.sup.3 or higher, preferably 1.5 g/cm.sup.3 or
higher, more preferably 1.6 g/cm.sup.3 or higher, even more
preferably 1.7 g/cm.sup.3 or higher, especially preferably 1.8
g/cm.sup.3 or higher. When the true density thereof is lower than
the lower limit of that range, there are cases where this carbon
has too low crystallinity, resulting in an increase in initial
irreversible capacity. When the true density thereof exceeds the
upper limit of that range, there are cases where this carbon has
too high crystallinity and the effect of improving suitability for
short-time high-current-density charge/discharge is difficult to
obtain.
[0471] In the invention, true density is defined as one determined
by the liquid-phase displacement method (pycnometer method) using
butanol.
[[O/C Value]]
[0472] The upper limit of the atomic amount ratio O/C value of the
amorphous carbonaceous substance is generally 0.15 or lower,
preferably 0.1 or lower, more preferably 0.05 or lower, even more
preferably 0.03 or lower. The lower limit thereof is generally 0 or
higher, preferably 0.01 or higher.
[0473] The term O/C value means the ratio of the molar
concentration of oxygen atoms in the amorphous carbonaceous
substance to the molar concentration of carbon atoms therein. It is
an index to the amount of functional groups, such as carboxyl,
phenolic hydroxyl, and carbonyl groups, present in the substance.
An amorphous carbonaceous substance having a high O/C value
frequently has an oxygen-containing functional group bonded to,
e.g., an edge face of the carbon crystallite. When the O/C value of
the amorphous carbonaceous substance exceeds the upper limit of
that range, there are cases where irreversible capacity
increases.
[[H/C Value]]
[0474] The upper limit of the atomic amount ratio H/C value of the
amorphous carbonaceous substance is generally 0.3 or lower,
preferably 0.15 or lower, more preferably 0.1 or lower, even more
preferably 0.08 or lower. The lower limit thereof is generally 0 or
higher, preferably 0.01 or higher.
[0475] The term H/C value means the ratio of the molar
concentration of hydrogen atoms in the amorphous carbonaceous
substance to the molar concentration of carbon atoms therein. It is
an index to the amount of hydrogen present in an edge face of the
crystallite of the amorphous carbonaceous substance. An amorphous
carbonaceous material having a high H/C value frequently has a
large hydrogen amount relative to carbon amount in, e.g., edge
faces of the carbon crystallites in the particle surface. When the
H/C value of the amorphous carbonaceous substance exceeds the upper
limit of that range, there are cases where irreversible capacity
increases.
[0476] The terms "O/C value" and "H/C value" in the invention are
values determined by the following CHN elemental analysis.
[[CHN Elemental Analysis]]
[0477] An amorphous carbonaceous substance to be examined was
vacuum-dried at 120.degree. C. for about 15 hours and then dried at
100.degree. C. for 1 hour on a hot plate in a dry box.
Subsequently, a sample was taken out of the dried carbonaceous
substance and placed in an aluminum cup in an argon atmosphere, and
this sample was heat-treated. The carbon content, hydrogen content,
and nitrogen content were determined respectively from the weights
of the CO2 gas, H2O, and NO2 generated by the combustion. The
inorganic content was determined from the weight of the residue
which remained after the combustion. The oxygen amount was
determined by subtracting the carbon content, hydrogen content,
nitrogen content, and inorganic content from the whole weight. From
these values, the number of moles of each element was calculated.
The O/C value and H/C value were determined from the numbers of
moles of the elements contained, using the following equations.
O/C value=(number of moles of oxygen contained)/(number of moles of
carbon contained)
H/C value=(number of moles of hydrogen contained)/(number of moles
of carbon contained)
[[Tap Density]]
[0478] The tap density of the amorphous carbonaceous substance for
use as the negative-electrode active material of negative electrode
[2] in a lithium secondary battery of the invention is preferably
0.1 g/cm.sup.3 or higher, more preferably 0.2 g/cm.sup.3 or higher,
even more preferably 0.5 g/cm.sup.3 or higher, especially
preferably 0.7 g/cm.sup.3 or higher. The upper limit thereof is
preferably 1.4 g/cm.sup.3 or lower, more preferably 1.2 g/cm.sup.3
or lower, especially preferably 1.1 g/cm.sup.3 or lower. In case
where the tap density thereof is lower than the lower limit of that
range, this amorphous carbonaceous substance, when used in a
negative electrode, is less apt to have a high loading density and
has a reduced interparticle contact area. There are hence cases
where interparticle resistance increases and short-time
high-current-density chare/discharge characteristics decrease. On
the other hand, in case where the tap density thereof exceeds the
upper limit of that range, the electrode has too small an amount of
interparticle interstices and a reduced amount of channels for the
nonaqueous electrolyte. There are hence cases where short-time
high-current-density chare/discharge characteristics decrease.
[0479] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the cell with the
sample up to the brim, subsequently conducting tapping operations
1,000 times over a stroke length of 10 mm using a powder densimeter
(e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.),
and determining a density from the resultant volume of the sample
and the weight thereof. This density is defined as the tap
density.
[[BET Specific Surface Area]]
[0480] The specific surface area of the amorphous carbonaceous
substance for use as the negative-electrode active material of
negative electrode [2] in a lithium secondary battery of the
invention, as determined by the BET method, is preferably 0.1
m.sup.2/g or larger, especially preferably 0.5 m.sup.2/g or larger,
more preferably 0.7 m.sup.2/g or larger, even more preferably 1.5
m.sup.2/g or larger. The upper limit thereof is preferably 100
m.sup.2/g or smaller, especially preferably 50 m.sup.2/g or
smaller, more preferably 25 m.sup.2/g or smaller, even more
preferably 15 m.sup.2/g or smaller. When the BET specific surface
area thereof is smaller than the lower limit of that range, there
are cases where use of this amorphous carbonaceous substance as a
negative-electrode material is apt to result in poor lithium
acceptance during charge and in lithium deposition on the electrode
surface. On the other hand, when the specific surface area thereof
exceeds the upper limit of that range, there are cases where use of
this amorphous carbonaceous substance as a negative-electrode
material is apt to result in enhanced reactivity with the
nonaqueous electrolyte and enhanced gas evolution and a preferred
battery is difficult to obtain.
[0481] BET specific surface area is defined as a value measured
with a surface area meter (e.g., a fully automatic surface area
measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by
preliminarily drying a sample at 350.degree. C. for 15 minutes in a
nitrogen stream and then measuring the specific surface area
thereof by the gas-flowing nitrogen adsorption BET one-point method
using a nitrogen/helium mixture gas precisely regulated so as to
have a nitrogen pressure of 0.3 relative to atmosphere
pressure.
[[Volume-Average Particle Diameter]]
[0482] The volume-average particle diameter of the amorphous
carbonaceous substance for use as the negative-electrode active
material of negative electrode [2] in a lithium secondary battery
of the invention, which is defined as the volume-average particle
diameter (median diameter) determined by the laser
diffraction/scattering method, is preferably 1 .mu.m or larger,
especially preferably 3 .mu.m or larger, more preferably 5 .mu.m or
larger, even more preferably 7 .mu.m or larger. The upper limit
thereof is generally 50 .mu.m or smaller, preferably 40 .mu.m or
smaller, more preferably 30 .mu.m or smaller, even more preferably
25 .mu.m or smaller. When the volume-average particle diameter
thereof is smaller than the lower limit of that range, there are
cases where irreversible capacity increases, leading to a loss in
initial battery capacity. When the volume-average particle diameter
thereof exceeds the upper limit of that range, there are cases
where such an amorphous carbonaceous substance is undesirable from
the standpoint of battery production because an uneven coating
surface is apt to result when an electrode plate is produced.
[[Pore Volume]]
[0483] With respect to the pore volume of the amorphous
carbonaceous substance for use as the negative-electrode active
material of negative electrode [2] in a lithium secondary battery
of the invention, the amount of interstices corresponding to a pore
diameter of from 0.01 .mu.m to 1 .mu.m which include pores in the
particles and particle surface irregularities formed by steps, as
determined by mercury porosimetry (mercury intrusion method), may
be 0.01 mL/g or larger and is preferably 0.05 mL/g or larger, more
preferably 0.1 mL/g or larger. The upper limit thereof may be 0.6
mL/g or smaller and is preferably 0.4 mL/g or smaller, more
preferably 0.3 mL/g or smaller. When the amount of interstices
therein is larger than the upper limit of that range, there are
cases where a large amount of a binder is necessary in electrode
plate formation. When the amount thereof is smaller than the lower
limit, there are cases where short-time high-current-density
charge/discharge characteristics decrease and the effect of
diminishing electrode expansion/contraction during charge/discharge
is not obtained.
[0484] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger. The upper limit thereof is
generally 10 mL/g or smaller, preferably 5 mL/g or smaller, more
preferably 2 mL/g or smaller. When the total pore volume exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary in electrode plate formation. When the
total pore volume is smaller than the lower limit, there are cases
where the dispersing effect of a thickener or binder in electrode
plate formation is not obtained.
[0485] The average pore diameter of the amorphous carbonaceous
substance is preferably 0.05 .mu.m or larger, more preferably 0.1
.mu.m or larger, even more preferably 0.5 .mu.m or larger. The
upper limit thereof is generally 50 .mu.m or smaller, preferably 20
.mu.m or smaller, more preferably 10 .mu.m or smaller. When the
average pore diameter thereof exceeds the upper limit of that
range, there are cases where a large amount of a binder is
necessary. When the average pore diameter thereof is smaller than
the lower limit, there are cases where short-time
high-current-density charge/discharge characteristics decrease.
[0486] As an apparatus for the mercury porosimetry is used a
mercury porosimeter (Autopore 9520, manufactured by Micromeritics
Corp.). About 0.2 g of a sample (negative-electrode material) is
weighed out and placed in a powder cell and this cell is closed.
The sample in the cell is pretreated by degassing it at room
temperature under vacuum (50 .mu.mHg or lower) for 10 minutes.
Subsequently, the pressure in the cell is reduced to 4 psia (about
28 kPa) and mercury is introduced thereinto. The internal pressure
is stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia
(about 280 MPa) and then lowered to 25 psia (about 170 kPa). The
number of steps in the pressure elevation is 80 or larger. In each
step, the amount of mercury intruded is measured after an
equilibrium time of 10 seconds. A pore distribution is calculated
from the mercury intrusion curve thus obtained, using the Washburn
equation. For the calculation, the surface tension (.gamma.) and
contact angle (.psi.) of mercury are taken as 485 dyne/cm and
140.degree., respectively. The average pore diameter used is the
pore diameter corresponding to a cumulative pore volume of 50%.
[[Ash Content]]
[0487] The ash content of the amorphous carbonaceous substance for
use as the negative-electrode active material of negative electrode
[2] in a lithium secondary battery of the invention is preferably
1% by mass or lower, especially preferably 0.5% by mass or lower,
more preferably 0.1% by mass or lower, based on the whole amorphous
carbonaceous substance. The lower limit thereof is preferably 1 ppm
by mass or higher. When the ash content thereof exceeds the upper
limit of that range, there are cases where battery performance
deterioration caused by reaction with the nonaqueous electrolyte
during charge/discharge becomes not negligible. On the other hand,
when the ash content thereof is lower than the lower limit of that
range, there are cases where the production of this carbonaceous
substance necessitates much time and energy and an apparatus for
pollution prevention, resulting in an increase in cost.
[[Roundness]]
[0488] The roundness of the amorphous carbonaceous substance for
use as the negative-electrode active material of negative electrode
[2] in a lithium secondary battery of the invention is generally
0.1 or higher, preferably 0.8 or higher, more preferably 0.85 or
higher, especially preferably 0.9 or higher. With respect to the
upper limit thereof, a particle having a roundness of 1
theoretically is a true sphere. In case where the roundness thereof
is lower than the lower limit of that range, use of this
negative-electrode active material results in a reduced loading and
interparticle resistance increases. There are hence cases where
short-time high-current-density charge/discharge characteristics
decrease.
[0489] In the invention, roundness is defined by the following
equation.
Roundness=(length of periphery of equivalent circle having the same
area as projected particle shape)/(actual length of periphery of
projected particle shape)
[0490] The value of roundness is determined, for example, with a
flow type particle image analyzer (e.g., FPIA, manufactured by
Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a
0.2% by mass aqueous solution (about 50 mL) of
poly(oxyethylene(20)) sorbitan monolaurate as a surfactant,
propagating an ultrasonic wave of 28 kHz to the dispersion for 1
minute at an output of 60 W, subsequently examining particles
having a particle diameter in the range of 3-40 .mu.m with the
analyzer having a detection range set at 0.6-400 .mu.m, and
averaging the measured values.
[[Orientation Ratio]]
[0491] The orientation ratio of the amorphous carbonaceous
substance for use as the negative-electrode active material of
negative electrode [2] in a lithium secondary battery of the
invention is generally 0.005 or higher, preferably 0.01 or higher,
more preferably 0.015 or higher. The upper limit thereof is
theoretically 0.67 or lower. When the orientation ratio thereof is
lower than the lower limit of that range, there are cases where
short-time high-density charge/discharge characteristics
decrease.
[0492] Orientation ratio is determined by X-ray diffractometry. The
peaks attributable to the (110) diffraction and (004) diffraction
of the carbon obtained by X-ray diffractometry are subjected to
peak separation by fitting with asymmetric Pearson VII as a profile
function. Thus, the integrated intensities of the (110) diffraction
and (004) diffraction peaks are calculated. From the integrated
intensities obtained, the ratio represented by (integrated
intensity of (110) diffraction)/(integrated intensity of (004)
diffraction) is calculated. This ratio is defined as the
active-material orientation ratio.
[0493] Conditions for this X-ray diffractometry are as follows.
Incidentally, "2.theta." represents diffraction angle. [0494]
Target: Cu(K.alpha. line) graphite monochromator [0495] Slit:
[0496] Divergence slit=1 degree [0497] Receiving slit=0.1 mm [0498]
Scattering slit=1 degree [0499] Examination range and step
angle/measuring time: [0500] (110) plane:
76.5.degree..ltoreq.2.theta.78.5.degree. 0.01.degree./3 sec [0501]
(004) plane: 53.5.degree..ltoreq.2.theta.56.0.degree.
0.01.degree./3 sec
[[Aspect Ratio]]
[0502] The aspect ratio of the amorphous carbonaceous substance for
use as the negative-electrode active material of negative electrode
[2] in a lithium secondary battery of the invention is
theoretically 1 or higher. The upper limit thereof is generally 10
or lower, preferably 8 or lower, more preferably 5 or lower. When
the aspect ratio thereof exceeds the upper limit, there are cases
where this amorphous carbonaceous material causes streak lines in
electrode plate formation and an even coating surface cannot be
obtained, resulting in a decrease in short-time
high-current-density charge/discharge characteristics.
[0503] The aspect ratio of a particle is represented by A/B,
wherein A is the length of the longest axis of the particle in a
three-dimensional examination and B is the length of the shortest
axis perpendicular to that axis. The examination of particles is
made with a scanning electron microscope capable of enlargement
examination. Fifty are arbitrarily selected from particles fixed to
an edge face of a metal having a thickness of 50 .mu.m or smaller,
and each particle is examined for A and B while rotating or
inclining the stage to which the sample is fixed. The average of
the A/B values is determined.
[0504] The amorphous carbonaceous substance contained as the
negative-electrode active material of negative electrode [2] in a
lithium secondary battery of the invention preferably is an
amorphous carbonaceous substance selected from the following (1) to
(4):
(1) one obtained by further heat-treating a carbonization product
selected from the group consisting of coal coke, petroleum coke,
furnace black, acetylene black, and pitch-derived carbon fibers;
(2) an organic substance selected from the group consisting of
pitch materials, aromatic hydrocarbons, nitrogen-ring compounds,
sulfur-ring compounds, polyphenylenes, organic synthetic polymers,
natural polymers, thermoplastic resins, and thermosetting resins
and/or a product of the pyrolysis of the organic substance and/or a
substance obtained by further heat-treating the pyrolysis product;
(3) a product of the pyrolysis of a solution obtained by dissolving
the organic substance (2) in a low-molecular organic solvent and/or
a substance obtained by further heat-treating the pyrolysis
product; and (4) a product of the carbonization of a gas containing
an organic substance.
[0505] The substance in (2) is not particularly limited as long as
it is capable of carbonization. Examples thereof include the
following organic substances: pitch materials; aromatic
hydrocarbons such as acenaphthylene, decacyclene, anthracene, and
phenanthrene; nitrogen-ring compounds such as phenazine and
acridine; sulfur-ring compounds such as thiophene and bithiophene;
polyphenylenes such as biphenyl and terphenyl; poly(vinyl
chloride), poly(vinyl alcohol), poly(vinyl butyral), substances
obtained by crosslinking or insolubilizing these compounds;
nitrogen-containing organic polymers such as polyacrylonitrile and
polypyrrole; sulfur-containing organic polymers such as
polythiophene; organic polymers such as polystyrene; natural
polymers such as polysaccharides represented by cellulose, lignin,
mannan, poly(galacturonic acid), chitosan, and saccharose;
thermoplastic resins such as poly(phenylene sulfide) and
poly(phenylene oxide); thermosetting resins such as furfuryl
alcohol resins, phenol-formaldehyde resins, and imide resins; and
solutions obtained by dissolving any of such organic substances in
a low-molecular organic solvent, e.g., benzene, toluene, xylene,
quinoline, or n-hexane. Examples thereof further include gases
capable of carbonization and containing any of those organic
substances.
[0506] Preferred of these are pitch materials. This is because
pitch materials have a high actual carbon ratio and, hence, can
give a material in high yield. The term "pitch material" in this
description means a substance which is a pitch or a similar
material and can be carbonized or graphitized by an appropriate
treatment. Examples of the pitch materials which can be used
include tars, heavy oils, and pitches. Examples of the tars include
coal tar and petroleum tar. Examples of the heavy oils include
petroleum-derived heavy oils such as catalytically cracked oil,
thermally cracked oil, topping residues, and vacuum distillation
residues. Examples of the pitches include coal tar pitch, petroleum
pitch, and synthetic pitch. Of these, coal tar pitch is preferred
because it has high aromaticity. Any one of these pitch materials
may be used alone, or any desired two or more thereot may be used
in combination in any desired proportion.
[0507] With respect to (3), preferred examples thereof include a
carbonization product obtained from a precursor which is a product
of the pyrolysis of a solution of the organic substance (2) in a
low-molecular organic solvent such as benzene, toluene, xylene,
quinoline, or n-hexane.
[0508] With respect to (4), examples include hydrocarbon compounds
such as methane, ethane, propane, benzene, acetylene, and ethylene;
and carbon monoxide.
[0509] To conduct a crosslinking treatment is also preferred. A
crosslinking treatment is conducted for the purpose of giving a
crosslinked pitch material or the like which, when heat-treated,
gives a carbonaceous material which is less apt to be graphitized.
By conducting this treatment, charge capacity per unit mass can be
increased.
[0510] Examples of the crosslinking treatment include: a
crosslinking treatment in which a vinyl monomer such as
divinylbenzene, trivinylbenzene, diallyl phthalate, ethylene glycol
dimethacrylate, or N,N-methylenebisacrylamide is used in
combination with a radical polymerization initiator such as
.alpha.,.alpha.'-azobisisobutyronitrile (AIBN), benzoyl peroxide
(BPO), lauryl peroxide, cumene hydroperoxide, t-butyl
hydroperoxide, or hydrogen peroxide; and an oxidizing agent
treatment in which an oxidizing gas such as oxygen, ozone, or
nitrogen dioxide or an oxidizing liquid such as sulfuric acid,
nitric acid, or an aqueous hydrogen peroxide solution is used. One
example of methods for such crosslinking treatments comprises
mixing a pitch material kept at 50.degree. C.-400.degree. C. with a
crosslinking agent, oxidizing agent, etc. to treat the
material.
[[Li-NMR Shift]]
[0511] When an amorphous carbonaceous substance for use as the
negative-electrode active material of negative electrode [2] in a
lithium secondary battery of the invention is charged to a fully
charged state and subjected, in this state, to 7Li-NMR analysis,
then there are cases where a main resonance peak is observed in a
position shifted by 80-200 ppm to the low-magnetic-field side from
the resonance line for LiCl as a reference. This amorphous
carbonaceous substance has an increased capacity per unit mass. It
is therefore preferred to use an amorphous carbonaceous substance
which has undergone a crosslinking treatment.
[[Processes for Producing Amorphous Carbonaceous Substance]]
[0512] Processes for producing the amorphous carbonaceous substance
described above are not particularly limited unless they depart
from the spirit of the invention. Examples thereof include several
processes. For producing the amorphous carbonaceous substance, it
is necessary to conduct a heat treatment one or more times.
However, a heat treatment may be conducted in two or more portions.
It is also preferred to conduct any of various treatments before or
after a heat treatment and/or during the heat treatment. Examples
of the various treatments include pulverization, classification,
and the crosslinking treatment described above. The pulverization
and classification treatments can be conducted in any stage, i.e.,
before, after, or during the heat treatment, as long as the
carbonaceous substance is solid. In the case of the crosslinking
treatment, it preferably is conducted before or during the heat
treatment. By conducting these treatments, the specific surface
area of the negative-electrode active material can be regulated and
the capacity thereof per unit mass can be increased.
[0513] Apparatus usable for the pulverization before the heat
treatment are not particularly limited. Examples of coarse
pulverizers include shearing mills, jaw crushers, impact crushers,
and cone crushers. Examples of intermediate pulverizers include
roll crushers and hammer mills. Examples of fine pulverizers
include ball mills, oscillating mills, pin mills, stirring mills,
and jet mills.
[0514] Apparatus usable for the heat treatment of a raw material
are not particularly limited. For example, use can be made of a
reaction vessel such as a shuttle furnace, tunnel kiln, electric
furnace, Riedhammer furnace, rotary kiln, or autoclave, a coker
(heat-treatment vessel for coke production), a direct voltage
application type electric furnace, or the like. During the
treatment of a raw material, stirring may be conducted according to
need.
[0515] Temperature conditions in the case of conducting the heat
treatment are not particularly limited. However, the temperature is
generally 600.degree. C. or higher, preferably 900.degree. C. or
higher. The upper limit thereof is generally 2,500.degree. C. or
lower, preferably 1,300.degree. C. or lower. In case where the
temperature conditions are lower than the lower limit of that
range, there is a possibility that crystallinity might become too
low, resulting in an increase in irreversible capacity. On the
other hand, in case where the temperature exceeds the upper limit
of that range, there is a possibility that crystallinity might
become too high, resulting in reduced short-time
high-current-density charge/discharge characteristics.
[0516] The amorphous carbonaceous substance which has undergone the
heat treatment may be subjected to pulverization and classification
according to the size of the aggregates or particles. Apparatus
usable for the pulverization are not particularly limited. Examples
of crushers include shearing mills, jaw crushers, impact crushers,
and cone crushers. Examples of intermediate pulverizers include
roll crushers and hammer mills. Examples of pulverizers include
ball mills, oscillating mills, pin mills, stirring mills, and jet
mills. Apparatus usable for the classification are not particularly
limited. However, in the case of dry sieving, use can be made of a
rotary sieve, swing sieve, revolving sieve, vibrating sieve, or the
like. In the case of dry air classification, use can be made of a
gravitational classifier, inertial classifier, or centrifugal
classifier (e.g., classifier or cyclone). Furthermore, for wet
sieving, use can be made of a mechanical wet classifier, hydraulic
classifier, sedimentation classifier, centrifugal wet classifier,
or the like.
[Negative Electrode [2]: Electrode Production]
[0517] Negative electrode [2] may be produced by an ordinary
method. It can be formed in the same manner as described above. The
current collector, thickness ratio between the current collector
and the active-material layer, electrode density, binder,
electrode-plate orientation ratio, impedance, etc. also are the
same as described above.
<Negative Electrode [3]>
[0518] An explanation will be given below on negative electrode [3]
for use in a lithium secondary battery of the invention, i.e., "a
negative electrode containing as a negative-electrode active
material a titanium-containing metal oxide capable of occluding and
releasing lithium".
[Negative Electrode [3]: Negative-Electrode Active Material]
[0519] The negative-electrode active material for use in negative
electrode [3] is described below.
[[Constitution of Negative-Electrode Active Material]]
[0520] The negative-electrode active material for use in negative
electrode [3] in a lithium secondary battery of the invention
contains a titanium-containing metal oxide capable of occluding and
releasing lithium. Preferred of such metal oxides is a composite
oxide of lithium and titanium (hereinafter abbreviated to
"lithium-titanium composite oxide"). Furthermore, the metal oxide
preferably is a titanium-containing metal oxide having a spinel
structure. A metal oxide simultaneously satisfying these, i.e., a
lithium-titanium composite oxide having a spinel structure, is
especially preferred because use of this composite oxide in the
negative-electrode active material for a lithium secondary battery
is effective in considerably reducing output resistance.
[0521] Also preferred are lithium-titanium composite oxides in
which the lithium or titanium has been replaced by one or more
other metallic elements, e.g., at least one element selected from
the group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn,
and Nb.
[0522] The metal oxide preferably is a lithium-titanium composite
oxide represented by general formula (1) wherein
0.7.ltoreq.x.ltoreq.1.5, 1.5.ltoreq.y.ltoreq.2.3, and
0.ltoreq.z.ltoreq.1.6, because the structure thereof is stable
during lithium ion doping/undoping.
Li.sub.xTi.sub.yM.sub.zO.sub.4 (1)
[In general formula (1), M represents at least one element selected
from the group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu,
Zn, and Nb.]
[0523] Of the compositions represented by general formula (1), the
following structures are especially preferred because they bring
about a satisfactory balance among battery performances.
[0524] Structures represented by general formula (1), i.e.,
Li.sub.xTi.sub.yM.sub.2O.sub.4, wherein
(a) 1.2.ltoreq.x.ltoreq.1.4, 1.5.ltoreq.y.ltoreq.1.7, and z=0; (b)
0.9.ltoreq.x.ltoreq.1.1, 1.9.ltoreq.y.ltoreq.2.1, and z=0; or (c)
0.7.ltoreq.x.ltoreq.0.9, 2.1.ltoreq.y.ltoreq.2.3, and z=0.
[0525] Especially preferred typical compositions of those compounds
are: Li.sub.4/3Ti.sub.5/3O.sub.4 for (a), Li.sub.1Ti.sub.2O.sub.4
for (b), and Li.sub.4/5Ti.sub.11/5O.sub.4 for (c).
[0526] Preferred examples of the structure wherein z.noteq.0
include Li.sub.4/3Ti.sub.4/3Al.sub.1/3O.sub.4.
[[Properties, Shape, Etc. Of Negative-Electrode Active
Material]]
[0527] It is preferred that the negative-electrode active material
for use in negative electrode [3] in a lithium secondary battery of
the invention should satisfy at least one of the following
properties besides the requirements described above. Especially
preferably, the negative-electrode active material simultaneously
satisfies two or more of the following properties besides the
requirements described above.
[[[BET Specific Surface Area]]]
[0528] The specific surface area of the titanium-containing metal
oxide for use as the negative-electrode active material in negative
electrode [3] in a lithium secondary battery of the invention, as
determined by the BET method, is preferably 0.5 m.sup.2/g or
larger, more preferably 0.7 m.sup.2/g or larger, especially
preferably 1.0 m.sup.2/g or larger, even more preferably 1.5
m.sup.2/g or larger. The upper limit thereof is preferably 200
m.sup.2/g or smaller, more preferably 100 m.sup.2/g or smaller,
especially preferably 50 m.sup.2/g or smaller, even more preferably
25 m.sup.2/g or smaller. When the BET specific surface area thereof
is smaller than the lower limit of that range, there are cases
where use of this metal oxide as a negative-electrode material
results in a reduced reaction area available for contact with the
nonaqueous electrolyte and in an increase in output resistance. On
the other hand, in case where the BET specific surface area thereof
exceeds the upper limit of that range, the proportion of surfaces
and edge faces of crystals of the titanium-containing metal oxide
increases and this causes crystal deformation. There are hence
cases where irreversible capacity becomes not negligible and a
preferred battery is difficult to obtain.
[0529] BET specific surface area is defined as a value measured
with a surface area meter (e.g., a fully automatic surface area
measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by
preliminarily drying a sample at 350.degree. C. for 15 minutes in a
nitrogen stream and then measuring the specific surface area
thereof by the gas-flowing nitrogen adsorption BET one-point method
using a nitrogen/helium mixture gas precisely regulated so as to
have a nitrogen pressure of 0.3 relative to atmosphere
pressure.
[[[Volume-Average Particle Diameter]]]
[0530] The volume-average particle diameter (secondary-particle
diameter in the case where the primary particles have aggregated to
form secondary particles) of the titanium-containing metal oxide
for use as the negative-electrode active material in negative
electrode [3] in a lithium secondary battery of the invention,
which is defined as the volume-average particle diameter (median
diameter) determined by the laser diffraction/scattering method, is
preferably 0.1 .mu.m or larger, more preferably 0.5 .mu.m or
larger, even more preferably 0.7 .mu.m or larger. The upper limit
thereof is generally 50 .mu.m or smaller, preferably 40 .mu.m or
smaller, more preferably 30 .mu.m or smaller, even more preferably
25 .mu.m or smaller. When the volume-average particle diameter
thereof is smaller than the lower limit of that range, there are
cases where a large amount of a binder is necessary in electrode
production and this results in a decrease in battery capacity. When
the volume-average particle diameter thereof exceeds the upper
limit of that range, there are cases where such a metal oxide is
undesirable from the standpoint of battery production because an
uneven coating surface is apt to result when an electrode plate is
produced.
[[[Average Primary-Particle Diameter]]]
[0531] In the case where the primary particles have aggregated to
form secondary particles, the average primary-particle diameter of
the titanium-containing metal oxide for use as the
negative-electrode active material in a lithium secondary battery
of the invention is preferably 0.01 .mu.m or larger, more
preferably 0.05 .mu.m or larger, even more preferably 0.1 .mu.m or
larger, most preferably 0.2 .mu.m or larger. The upper limit
thereof is preferably 2 .mu.m or smaller, more preferably 1.6 .mu.m
or smaller, even more preferably 1.3 .mu.m or smaller, most
preferably 1 .mu.m or smaller. In case where the average
primary-particle diameter thereof exceeds the upper limit,
spherical secondary particles are difficult to form and this
adversely influences powder loading or results in a considerably
reduced specific surface area. There may hence be a high
possibility that battery performances such as output
characteristics might decrease. Conversely, when the average
primary-particle diameter thereof is smaller than the lower limit,
crystal growth is usually insufficient and, hence, there are cases
where this positive-electrode active material poses a problem, for
example, that charge/discharge reversibility is poor.
[0532] Primary-particle diameter is determined through an
examination with a scanning electron microscope (SEM).
Specifically, arbitrarily selected 50 primary-particle images in a
photograph having a magnification capable of particle observation,
e.g., 10,000-100,000 diameters, each are examined for the length of
the longest segment of a horizontal line which extends across the
primary-particle image from one side to the other side of the
boundary. These measured lengths are averaged, whereby the average
value can be determined.
[[[Shape]]]
[0533] The particle shape of the titanium-containing metal oxide
for use in negative electrode [3] in a lithium secondary battery of
the invention may be any of massive, polyhedral, spherical,
ellipsoidal, platy, acicular, columnar, and other shapes such as
those in common use. Preferred of these is one in which the primary
particles have aggregated to form secondary particles and these
secondary particles have a spherical or ellipsoidal shape. In
electrochemical elements, the active material in each electrode
usually expands/contracts with the charge/discharge of the element
and, hence, deterioration is apt to occur, such as active-material
breakage and conduction path breakage, due to the stress caused by
the expansion/contraction. Because of this, a positive-electrode
active material in which the primary particles have aggregated to
form secondary particles is preferable to an active material
composed of primary particles only since the particles in the
former active material relieve the stress caused by
expansion/contraction to prevent deterioration. Furthermore,
particles of a spherical or ellipsoidal shape are preferable to
particles showing axial orientation, e.g., platy ones, because the
former particles are less apt to orient during electrode molding
and hence this electrode is reduced in expansion/contraction during
charge/discharge, and because these particles are apt to be evenly
mixed with a conductive material in electrode production.
[[[Tap Density]]]
[0534] The tap density of the titanium-containing metal oxide for
use as the negative-electrode active material in negative electrode
[3] in a lithium secondary battery of the invention is preferably
0.05 g/cm.sup.3 or higher, more preferably 0.1 g/cm.sup.3 or
higher, even more preferably 0.2 g/cm.sup.3 or higher, especially
preferably 0.4 g/cm.sup.3 or higher. The upper limit thereof is
preferably 2.8 g/cm.sup.3 or lower, more preferably 2.4 g/cm.sup.3
or lower, especially preferably 2 g/cm.sup.3 or lower. In case
where the tap density thereof is lower than the lower limit of that
range, this metal oxide, when used in a negative electrode, is less
apt to have a high loading density and has a reduced interparticle
contact area. There are hence cases where interparticle resistance
increases and output resistance increases. On the other hand, in
case where the tap density thereof exceeds the upper limit of that
range, the electrode has too small an amount of interparticle
interstices and a reduced amount of channels for the nonaqueous
electrolyte. There are hence cases where output resistance
increases.
[0535] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the cell with the
sample up to the brim, subsequently conducting tapping operations
1,000 times over a stroke length of 10 mm using a powder densimeter
(e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.),
and determining a density from the resultant volume of the sample
and the weight thereof. This density is defined as the tap
density.
[[[Roundness]]]
[0536] The roundness of the titanium-containing metal oxide for use
as the negative-electrode active material of negative electrode [3]
in a lithium secondary battery of the invention is generally 0.10
or higher, preferably 0.80 or higher, more preferably 0.85 or
higher, especially preferably 0.90 or higher. With respect to the
upper limit thereof, a particle having a roundness of 1
theoretically is a true sphere. In case where the roundness thereof
is lower than the lower limit of that range, use of this
negative-electrode active material results in a reduced loading and
interparticle resistance increases. There are hence cases where
short-time high-current-density charge/discharge characteristics
decrease.
[0537] In the invention, roundness is defined by the following
equation.
Roundness=(length of periphery of equivalent circle having the same
area as projected particle shape)/(actual length of periphery of
projected particle shape)
[0538] The value of roundness is determined with a flow type
particle image analyzer (e.g., FPIA, manufactured by Sysmex
Industrial Corp.) by dispersing about 0.2 g of a sample in a 0.2%
by mass aqueous solution (about 50 mL) of poly(oxyethylene(20))
sorbitan monolaurate as a surfactant, propagating an ultrasonic
wave of 28 kHz to the dispersion for 1 minute at an output of 60 W,
and then examining particles having a particle diameter in the
range of 3-40 .mu.m with the analyzer having a detection range set
at 0.6-400 .mu.m.
[[[Aspect Ratio]]]
[0539] The aspect ratio of the titanium-containing metal oxide for
use as the negative-electrode active material of negative electrode
[3] in a lithium secondary battery of the invention is
theoretically 1 or higher. The upper limit thereof may be 5 or
lower and is preferably 4 or lower, more preferably 3 or lower,
even more preferably 2 or lower. When the aspect ratio thereof
exceeds the upper limit, there are cases where this metal oxide
causes streak lines in electrode plate formation and an even
coating surface cannot be obtained, resulting in a decrease in
short-time high-current-density charge/discharge
characteristics.
[0540] The aspect ratio of a particle is represented by A/B,
wherein A is the length of the longest axis of the particle in a
three-dimensional examination and B is the length of the shortest
axis perpendicular to that axis. The examination of particles is
made with a scanning electron microscope capable of enlargement
examination. Fifty are arbitrarily selected from particles fixed to
an edge face of a metal having a thickness of 50 .mu.m or smaller,
and each particle is examined for A and B while rotating or
inclining the stage to which the sample is fixed. The average of
the A/B values is determined.
[[Processes for Producing Negative-Electrode Active Material]]
[0541] Processes for producing the negative-electrode active
material of negative electrode [3] in a lithium secondary battery
of the invention are not particularly limited unless they depart
from the spirit of the invention. Examples thereof include several
processes, and processes in general use for producing inorganic
compounds may be employed. For example, use may be made of a method
in which a titanium source, e.g., titanium oxide, is evenly mixed
with an Li source, e.g., LiOH, Li.sub.2CO.sub.3, or LiNO.sub.3, and
optionally with a source of other element(s) and this mixture is
heat-treated at a high temperature to obtain the active material.
Especially for producing spherical or ellipsoidal active materials,
various techniques are usable. Examples thereof include: a method
which comprises dissolving or pulverizing/dispersing a titanium
source, e.g., titanium oxide, optionally together with a source of
other element(s) in a solvent, e.g., water, regulating the pH of
the solution or dispersion with stirring to produce a spherical
precursor, recovering and optionally drying the precursor,
subsequently adding thereto an Li source, e.g., LiOH,
Li.sub.2CO.sub.3, or LiNO.sub.3, and heat-treating the mixture at a
high temperature to obtain the active material; a method which
comprises dissolving or pulverizing/dispersing a titanium source,
e.g., titanium oxide, optionally together with a source of other
element(s) in a solvent, e.g., water, drying and forming the
solution or dispersion with a spray dryer or the like to obtain a
spherical or ellipsoidal precursor, adding thereto an Li source,
e.g., LiOH, Li.sub.2CO.sub.3, or LiNO.sub.3, and heat-treating the
mixture at a high temperature to obtain the active material; and a
method which comprises dissolving or pulverizing/dispersing a
titanium source, e.g., titanium oxide, together with an Li source,
e.g., LiOH, Li.sub.2CO.sub.3, or LiNO.sub.3, and optionally with a
source of other element(s) in a solvent, e.g., water, drying and
forming the solution or dispersion with a spray dryer or the like
to obtain a spherical or ellipsoidal precursor, and heat-treating
the precursor at a high temperature to obtain the active
material.
[0542] In those steps, one or more of elements other than Ti, such
as, e.g., Al, Mn, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga,
Zr, C, Si, Sn, and Ag, can be caused to be present in the
titanium-containing metal oxide structure and/or present so as be
in contact with the titanium-containing oxide. The incorporation of
such elements can be used for regulating the operating voltage and
capacity of the battery.
[Negative Electrode [3]: Electrode Production]
[0543] Negative electrode [3] may be produced by an ordinary
method. It can be formed in the same manner as described above. The
current collector, thickness ratio between the current collector
and the active-material layer, electrode density, binder,
electrode-plate orientation ratio, impedance, etc. also are the
same as described above.
<Negative Electrode [4]>
[0544] An explanation will be given below on negative electrode [4]
for use in a lithium secondary battery of the invention, i.e., "a
negative electrode containing as a negative-electrode active
material a carbonaceous substance having a roundness of 0.85 or
higher and a surface functional-group amount O/C value of from 0 to
0.01".
[Negative Electrode [4]: Negative-Electrode Active Material]
[0545] The negative-electrode active material for use in negative
electrode [4] is described below.
[0546] The negative-electrode active material for use in negative
electrode [4] in a lithium secondary battery of the invention
comprises a carbonaceous substance satisfying at least the
following requirements (a) and (b).
(a) The roundness is 0.85 or higher. (b) The surface
functional-group amount O/C value is from 0 to 0.01.
[0547] Details of the carbonaceous substance for use in this
invention are explained below.
[[Roundness]]
[0548] The roundness of the carbonaceous substance is generally
0.85 or higher, preferably 0.87 or higher, more preferably 0.89 or
higher, especially preferably 0.92 or higher. With respect to the
upper limit thereof, a particle having a roundness of 1
theoretically is a true sphere. In case where the roundness thereof
is lower than the lower limit of that range, use of this
negative-electrode active material results in a reduced loading and
compaction in negative-electrode formation is difficult and leads
to particle breakage. There is hence a possibility that those inner
parts of the particles which have poor high-temperature storability
at a low state of charge are apt to be exposed.
[0549] In the invention, roundness is defined by the following
equation.
Roundness=(length of periphery of equivalent circle having the same
area as projected particle shape)/(actual length of periphery of
projected particle shape)
[0550] The value of roundness is determined, for example, with a
flow type particle image analyzer (e.g., FPIA, manufactured by
Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a
0.2% by mass aqueous solution (about 50 mL) of poly(oxyethylene
(20)) sorbitan monolaurate as a surfactant, propagating an
ultrasonic wave of 28 kHz to the dispersion for 1 minute at an
output of 60 W, and then examining particles having a particle
diameter in the range of 3-40 .mu.m with the analyzer having a
detection range set at 0.6-400 .mu.m.
[[O/C Value]]
[0551] The surface functional-group amount O/C value of the
carbonaceous substance should be from 0 to 0.01. The upper limit of
the O/C value thereof is preferably 0.005 or lower. The closer the
O/C value to 0, the more the carbonaceous substance is
preferred.
[0552] The term O/C value in the invention indicates the proportion
of the amount of surface functional groups determined by X-ray
photoelectron spectroscopy (XPS). In case where the surface
functional-group amount O/C value exceeds the upper limit of that
range, the particle surface has an increased amount of a functional
group and the SEI film which is formed on the surface during charge
in the presence of a specific compound does not have sufficient
stability. There are hence cases where high-temperature storability
at a low state of charge decreases.
[0553] Surface functional-group amount O/C value means the ratio of
the concentration of oxygen atoms present on the surface of, e.g.,
a graphite material, to the concentration of carbon atoms present
thereon. It is an index to the amount of functional groups, such as
carboxyl, phenolic hydroxyl, and carbonyl groups, present on the
surface. When a carbon material has a high surface functional-group
amount O/C value, this frequently means that an oxygen-containing
surface functional group is bonded to, e.g., edge faces of the
carbon crystallites in the particle surface. Incidentally, the
surface functional-group amount O/C value of a graphite material is
determined by determining the areas of C1s and O1s spectral peaks
in X-ray photoelectron spectroscopy and calculating the
carbon-oxygen atomic concentration ratio O/C (O atom
concentration/C atom concentration) from the peak areas. Although
specific procedures of the determination are not particularly
limited, an example is as follows.
[0554] An X-ray photoelectron spectrometer (e.g., ESCA,
manufactured by ULVAC-PHI, INC.) is used for the X-ray
photoelectron spectroscopy. A sample to be examined (here, a
graphite material) is placed on a sample table so as to form a flat
surface. An aluminum K.alpha. line is used as an X-ray source to
obtain a spectrum having C1s (280-300 eV) and O1s (525-545 eV)
peaks in the multiplex mode. Charge correction is made, with the
top of the C1s peak being at 284.3 eV, and the areas of the C1s and
O1s spectral peaks are determined. Furthermore, these areas are
multiplied by the apparatus sensitivity coefficient to calculate
the concentrations of surface C atoms and surface O atoms. From the
O and C atom concentrations obtained, the atomic concentration
ratio O/C (O atom concentration/C atom concentration) is
calculated. This ratio is defined as the surface functional-group
amount O/C value of the graphite material.
[0555] Although the carbonaceous substance for use in this
invention satisfies the requirements concerning "roundness" and
"surface functional-group amount O/C value" described above, it is
preferred from the standpoint of battery performance balance that
the carbonaceous substance should further satisfy anyone of the
following requirements or simultaneously satisfy two or more
thereof. It is preferred that any one of or two or more of tap
density, Raman R value, and volume-average particle diameter, among
those requirements, should be simultaneously satisfied.
[[Tap Density]]
[0556] The tap density of the carbonaceous substance is generally
0.55 g/cm.sup.3 or higher, preferably 0.7 g/cm.sup.3 or higher,
more preferably 0.8 g/cm.sup.3 or higher, especially preferably 0.9
g/cm.sup.3 or higher. The upper limit thereof is preferably 2
g/cm.sup.3 or lower, more preferably 1.8 g/cm.sup.3 or lower,
especially preferably 1.6 g/cm.sup.3 or lower. In case where the
tap density thereof is lower than the lower limit of that range,
this carbonaceous substance, when used in a negative electrode, is
less apt to have a high loading density. There are hence cases
where a high-capacity battery cannot be obtained. On the other
hand, in case where the tap density thereof exceeds the upper limit
of that range, the electrode has too small an amount of
interparticle interstices and electrical conductivity among the
particles is difficult to secure. There are hence cases where
preferred battery characteristics are difficult to obtain.
[0557] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the cell with the
sample up to the brim, subsequently conducting tapping operations
1,000 times over a stroke length of 10 mm using a powder densimeter
(e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.),
and calculating a density from the resultant volume of the sample
and the weight thereof. This value is defined as the tap
density.
[[Raman R Value, Half-Value Width]]
[0558] The R value of the carbonaceous substance, as determined by
argon ion laser Raman spectroscopy, is generally 0.001 or higher,
preferably 0.01 or higher. The upper limit thereof is generally 0.2
or lower, preferably 0.18 or lower, more preferably 0.15 or lower.
When the R value thereof is lower than the lower limit of that
range, the surface of such particles has too high crystallinity and
there are cases where the number of intercalation sites into which
lithium comes with charge/discharge decreases. Namely, there are
cases where suitability for charge decreases. On the other hand, in
case where the R value thereof exceeds the upper limit of that
range, the surface of such particles has reduced crystallinity and
the SEI film which is formed on the surface during charge in the
presence of a specific compound is not sufficiently stable. There
are hence cases where high-temperature storability at a low state
of charge decreases.
[0559] The Raman half-value width around 1,580 cm.sup.-1 of the
carbonaceous substance is not particularly limited. However, the
half-value width thereof is generally 10 cm.sup.-1 or larger,
preferably 15 cm.sup.-1 or larger. The upper limit thereof is
generally 35 cm.sup.-1 or smaller, preferably 30 cm.sup.-1 or
smaller. When the Raman half-value width thereof is smaller than
the lower limit of that range, the surface of such particles has
too high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. On the other hand, when the Raman half-value width
thereof exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity and the SEI film which is
formed on the surface during charge in the presence of a specific
compound is not sufficiently stable. There are hence cases where
high-temperature storability at a low state of charge
decreases.
[0560] The examination for a Raman spectrum is made with a Raman
spectrometer (e.g., a Raman spectrometer manufactured by Japan
Spectroscopic Co., Ltd.). In the examination, a sample is charged
into a measuring cell by causing the sample to fall naturally into
the cell and the surface of the sample in the cell is irradiated
with argon ion laser light while rotating the cell in a plane
perpendicular to the laser light. The Raman spectrum obtained is
examined for the intensity I.sub.A of a peak P.sub.A around 1,580
cm.sup.-1 and the intensity I.sub.B of a peak P.sub.B around 1,360
cm.sup.-1. The ratio between these intensities R
(R=I.sub.B/I.sub.A) is calculated and this ratio is defined as the
Raman R value of the carbon material. Furthermore, the half-value
width of the peak P.sub.A around 1,580 cm.sup.-1 in the Raman
spectrum obtained is measured, and this value is defined as the
Raman half-value width of the carbonaceous substance.
[0561] Conditions for this Raman spectroscopy are as follows.
[0562] Wavelength of argon ion laser: 514.5 nm [0563] Laser power
on sample: 15-25 mW [0564] Resolution: 10-20 cm.sup.-1 [0565]
Examination range: 1,100-1,730 cm.sup.-1 [0566] Analysis for R
value and half-value width: background processing [0567] Smoothing:
simple average; convolution, 5 points
[[Volume-Average Particle Diameter]]
[0568] With respect to the volume-average particle diameter of the
carbonaceous substance, the volume-average particle diameter
(median diameter) thereof as determined by the laser
diffraction/scattering method is generally 1 .mu.m or larger,
preferably 3 .mu.m or larger, more preferably 5 .mu.m or larger,
even more preferably 7 .mu.m or larger. The upper limit thereof is
generally 50 .mu.m or smaller, preferably 40 .mu.m or smaller, more
preferably 30 .mu.m or smaller, even more preferably 25 .mu.m or
smaller. When the volume-average particle diameter thereof is
smaller than the lower limit of that range, there are cases where
irreversible capacity increases, leading to a loss in initial
battery capacity. When the volume-average particle diameter thereof
exceeds the upper limit of that range, there are cases where such a
carbonaceous substance is undesirable from the standpoint of
battery production because an uneven coating surface is apt to
result when an electrode plate is produced.
[0569] Furthermore, in the volume-based particle diameter
distribution thereof, the ratio of the 90% particle diameter to the
10% particle diameter (d.sub.90/d.sub.10) may be 1.2 or higher and
is preferably 1.5 or higher, more preferably 1.7 or higher. The
upper limit thereof may be 8 or lower and is preferably 5 or lower,
more preferably 4 or lower, even more preferably 3 or lower.
[0570] Volume-average particle diameter in the invention is defined
as the median diameter determined by dispersing the carbon powder
in a 0.2% by mass aqueous solution (about 1 mL) of polyoxyethylene
(20) sorbitan monolaurate as a surfactant and examining the
dispersion with a laser diffraction type particle size distribution
analyzer (e.g., LA-700, manufactured by HORIBA, Ltd.). With respect
to the 90% particle diameter/10% particle diameter ratio
(d.sub.90/d.sub.10) the volume-based 90% particle diameter and 10%
particle diameter are determined in the same manner and the ratio
(d.sub.90/d.sub.10) can be calculated therefrom.
[[X-Ray Parameter]]
[0571] The carbonaceous substance preferably has a value of d
(interplanar spacing) for the lattice planes (002), as determined
by X-ray diffractometry in accordance with the method of the Japan
Society for Promotion of Scientific Research, of 0.335 nm or
larger. The upper limit of the value of d is 0.340 nm or smaller
and is preferably 0.337 nm or smaller. When the value of d is too
large, there are cases where such particles have reduced
crystallinity to increase initial irreversible capacity. On the
other hand, 0.335 is a theoretical value for graphites.
Furthermore, the crystallite size (Lc) of this carbon material, as
determined by X-ray diffractometry in accordance with the method of
the Japan Society for Promotion of Scientific Research, is
generally 30 nm or larger, preferably 50 nm or larger, more
preferably 80 nm or larger. In case where the Lc thereof is smaller
than the lower limit of that range, there is a possibility that
such particles might have reduced crystallinity to increase initial
irreversible capacity.
[[Ash Content]]
[0572] The ash content of the carbonaceous substance is preferably
1% by mass or lower, especially 0.5% by mass or lower, in
particular 0.1% by mass or lower, based on the whole carbonaceous
substance. The lower limit of the ash content thereof is preferably
at least 1 ppm by mass of the whole carbonaceous substance. When
the ash content thereof exceeds the upper limit of that range,
there are cases where battery performance deterioration caused by
reaction with the electrolyte during charge/discharge becomes not
negligible. When the ash content thereof is lower than the lower
limit of that range, there are cases where the production of this
carbonaceous substance necessitates much time and energy and an
apparatus for pollution prevention, resulting in an increase in
cost.
[[BET Specific Surface Area]]
[0573] The specific surface area of the carbonaceous substance, as
determined by the BET method, is generally 0.1 m.sup.2/g or larger,
preferably 0.7 m.sup.2/g or larger, more preferably 1 m.sup.2/g or
larger, even more preferably 1.5 m.sup.2/g or larger. The upper
limit thereof is generally 100 m.sup.2/g or smaller, preferably 25
m.sup.2/g or smaller, more preferably 15 m.sup.2/g or smaller, even
more preferably 10 m.sup.2/g or smaller. When the specific surface
area thereof is smaller than the lower limit of that range, there
are cases where use of this carbonaceous substance as a
negative-electrode material is apt to result in poor lithium
acceptance during charge and in lithium deposition on the electrode
surface. On the other hand, when the specific surface area thereof
exceeds the upper limit of that range, there are cases where use of
this carbonaceous substance as a negative-electrode material is apt
to result in enhanced reactivity with the electrolyte and enhanced
gas evolution and a preferred battery is difficult to obtain.
[0574] BET specific surface area is defined as a value measured
with a surface area meter (e.g., a fully automatic surface area
measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by
preliminarily drying a sample at 350.degree. C. for 15 minutes in a
nitrogen stream and then measuring the specific surface area
thereof by the gas-flowing nitrogen adsorption BET one-point method
using a nitrogen/helium mixture gas precisely regulated so as to
have a nitrogen pressure of 0.3 relative to atmosphere
pressure.
[[Pore Distribution]]
[0575] In the carbonaceous substance, the amount of interstices
corresponding to a pore diameter of from 0.01 .mu.m to 1 .mu.m
which include pores in the particles and particle surface
irregularities formed by steps, as determined by mercury
porosimetry (mercury intrusion method), is generally 0.01 mL/g or
larger, preferably 0.05 mL/g or larger, more preferably 0.1 mL/g or
larger. The upper limit thereof is generally 0.6 mL/g or smaller,
preferably 0.4 mL/g or smaller, more preferably 0.3 mL/g or
smaller. When the amount of interstices therein is larger than the
upper limit of that range, there are cases where a large amount of
a binder is necessary in electrode plate formation. On the other
hand, when the amount thereof is smaller than the lower limit of
that range, there are cases where high-current-density
charge/discharge characteristics decrease and the effect of
diminishing electrode expansion/contraction during charge/discharge
is not obtained.
[0576] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger. The upper limit thereof is
generally 10 mL/g or smaller, preferably 5 mL/g or smaller, more
preferably 2 mL/g or smaller. When the total pore volume exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary in electrode plate formation. When the
total pore volume is smaller than the lower limit, there are cases
where the dispersing effect of a thickener or binder in electrode
plate formation is not obtained.
[0577] The average pore diameter of the carbonaceous substance is
preferably 0.05 .mu.m or larger, more preferably 0.1 .mu.m or
larger, even more preferably 0.5 .mu.m or larger. The upper limit
thereof is generally 50 .mu.m or smaller, preferably 20 .mu.m or
smaller, more preferably 10 .mu.m or smaller. When the average pore
diameter thereof exceeds the upper limit of that range, there are
cases where a large amount of a binder is necessary. On the other
hand, when the average pore diameter thereof is smaller than the
lower limit of that range, there are cases where
high-current-density charge/discharge characteristics decrease.
[0578] As an apparatus for the mercury porosimetry is used a
mercury porosimeter (Autopore 9520, manufactured by Micromeritics
Corp.). About 0.2 g of a sample (negative-electrode material) is
weighed out and placed in a powder cell and this cell is closed.
The sample in the cell is pretreated by degassing it at room
temperature under vacuum (50 .mu.mHg or lower) for 10 minutes.
Subsequently, the pressure in the cell is reduced to 4 psia (about
28 kPa) and mercury is introduced thereinto. The internal pressure
is stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia
(about 280 MPa) and then lowered to 25 psia (about 170 kPa). The
number of steps in the pressure elevation is 80 or larger. In each
step, the amount of mercury intruded is measured after an
equilibrium time of 10 seconds. A pore distribution is calculated
from the mercury intrusion curve thus obtained, using the Washburn
equation. For the calculation, the surface tension (.gamma.) and
contact angle (.psi.) of mercury are taken as 485 dyne/cm and
140.degree., respectively. The average pore diameter used is the
pore diameter corresponding to a cumulative pore volume of 50%.
[[True Density]]
[0579] The true density of the carbonaceous substance is generally
2 g/cm.sup.3 or higher, preferably 2.1 g/cm.sup.3 or higher, more
preferably 2.2 g/cm.sup.3 or higher, even more preferably 2.22
g/cm.sup.3 or higher. The upper limit thereof may be 2.26
g/cm.sup.3 or lower. This upper limit is a theoretical value for
graphites. When the true density of the carbonaceous substance is
lower than the lower limit of that range, there are cases where
this carbon has too low crystallinity, resulting in an increase in
initial irreversible capacity. In the invention, true density is
defined as one determined by the liquid-phase displacement method
(pycnometer method) using butanol.
[[Orientation Ratio]]
[0580] The orientation ratio of the carbonaceous substance is
generally 0.005 or higher, preferably 0.01 or higher, more
preferably 0.015 or higher. The upper limit thereof is
theoretically 0.67 or lower. When the orientation ratio thereof is
lower than the lower limit of that range, there are cases where
high-density charge/discharge characteristics decrease.
[0581] Orientation ratio is determined by X-ray diffractometry. The
peaks attributable to the (110) diffraction and (004) diffraction
of the carbon obtained by X-ray diffractometry are subjected to
peak separation by fitting with asymmetric Pearson VII as a profile
function. Thus, the integrated intensities of the (110) diffraction
and (004) diffraction peaks are calculated. From the integrated
intensities obtained, the ratio represented by (integrated
intensity of (110) diffraction)/(integrated intensity of (004)
diffraction) is calculated. This ratio is defined as the
active-material orientation ratio.
[0582] Conditions for this X-ray diffractometry are as follows.
Incidentally, "2.theta." represents diffraction angle. [0583]
Target: Cu(K.alpha. line) graphite monochromator [0584] Slit:
[0585] Divergence slit=1 degree [0586] Receiving slit=0.1 mm [0587]
Scattering slit=1 degree [0588] Examination range and step
angle/measuring time: [0589] (110) plane:
76.5.degree..ltoreq.2.theta..ltoreq.78.5.degree. 0.01.degree./3 sec
[0590] (004) plane:
53.5.degree..ltoreq.2.theta..ltoreq.56.0.degree. 0.01.degree./3
sec
[[Aspect Ratio]]
[0591] The aspect ratio of the carbonaceous substance is
theoretically 1 or higher. The upper limit thereof may be 10 or
lower and is preferably 8 or lower, more preferably 5 or lower.
When the aspect ratio thereof exceeds the upper limit, there are
cases where this carbonaceous substance causes streak lines in
electrode plate formation and an even coating surface cannot be
obtained, resulting in a decrease in high-current-density
charge/discharge characteristics.
[0592] The aspect ratio of a particle is represented by A/B,
wherein A is the length of the longest axis of the particle in a
three-dimensional examination and B is the length of the shortest
axis perpendicular to that axis. The examination of particles is
made with a scanning electron microscope capable of enlargement
examination. Fifty are arbitrarily selected from graphite particles
fixed to an edge face of a metal having a thickness of 50 .mu.m or
smaller, and each particle is examined for A and B while rotating
or inclining the stage to which the sample is fixed. The average of
the A/B values is determined.
[[Processes for Producing Carbonaceous Substance and Raw Materials
Therefor]]
[0593] The carbonaceous substance for use in this invention may be
one which occurs naturally or one produced artificially. However,
one derived from a natural graphite is preferred. The carbonaceous
substance may be one obtained by subjecting a naturally occurring
one or artificially produced one to a specific treatment. Processes
for producing the carbonaceous substance (including classification
methods) also are not particularly limited. For example, a
carbonaceous substance obtained by a classification technique such
as, e.g., sieving or air classification so as to have the
properties described above may be employed.
[0594] Of these, a carbonaceous substance obtained by heat-treating
a naturally occurring carbon material (raw natural graphite
material) is preferred from the standpoints of availability and
processability in a preceding step. Furthermore, a carbonaceous
substance obtained by subjecting a naturally occurring carbon
material (raw natural graphite material) or an artificially
produced carbon material to a mechanical energy treatment to
modify/round the material and heat-treating the rounded
carbonaceous substance is preferred from the standpoints of loading
improvement, etc. Moreover, a carbonaceous substance prepared by
subjecting a raw natural graphite material to a mechanical energy
treatment and heat-treating the resultant rounded natural graphite
is especially preferred from the standpoints of balance among
lithium secondary battery performances, etc. Hereinafter, the
carbon material (raw material) which has not undergone a heat
treatment, e.g., a raw natural graphite material, is often referred
to simply as "raw material before heat treatment".
[[[Raw Natural Graphite Material]]]
[0595] Natural graphites are especially preferred raw materials for
the carbonaceous substance as stated above.
[0596] Natural graphites are classified into flake graphite,
crystalline (vein) graphite, and amorphous graphite according to
properties (see "Graphite" in Funrutai Purosesu Gijutsu Shusei,
published by Industrial Technology Center in 1974; and HANDBOOK OF
CARBON, GRAPHITE, DIAMOND AND FULLERENES, published by Noyes
Publications). With respect to the degree of graphitization,
crystalline graphite has a highest graphitization degree of 100%
and flake graphite is second, which has a graphitization degree of
99.9%. The graphitization degree of amorphous graphite is as low as
28%. The quality of the natural graphites is governed mainly by
locality and vein. Flake graphite occurs in Madagascar, China,
Brazil, Ukraine, Canada, etc. Crystalline graphite occurs mainly in
Sri Lanka. Amorphous graphite occurs mainly in the Korean
Peninsula, China, Mexico, etc. Of these natural graphites, flake
graphite and crystalline graphite are preferred raw materials for
the carbonaceous substance because of their advantages including a
high degree of graphitization and a low impurity content.
[[[Mechanical Energy Treatment/Rounding Treatment]]]
[0597] The mechanical energy treatment is conducted so that the
ratio between the volume-average particle diameters before and
after the treatment becomes 1 or less. The term "ratio between the
volume-average particle diameters before and after the treatment"
means a value obtained by dividing the volume-average particle
diameter as measured after the treatment by the volume-average
particle diameter as measured before the treatment. In the
mechanical energy treatment in the invention for producing a raw
material before heat treatment, it is preferred to conduct the
mechanical energy treatment so that the ratio between the average
particle diameters before and after this treatment becomes 1 or
less.
[0598] The mechanical energy treatment is performed for the
purposes of reducing the size of the powder particles to 1 or less
in terms of the ratio between the average particle diameters before
and after the treatment and simultaneously regulating the particle
shape. This mechanical energy treatment belongs to the
pulverization treatment among engineering unit operations
utilizable for particle design, such as pulverization,
classification, mixing, granulation, surface modification, and
reaction.
[0599] Pulverization means an operation in which a force is applied
to a substance to thereby reduce the size thereof and regulate the
particle diameter, particle size distribution, and loading of the
substance. Pulverization treatments are classified according to the
kind of force to be applied to the substance and the method of
treatment. Forces to be applied to the substance are roughly
divided into the following four: (1) breaking force (impact force),
(2) crushing force (compressive force), (3) milling force (grinding
force), and (4) scraping force (shear force). On the other hand,
treatment methods are roughly divided into the following two:
volume pulverization in which cracks are generated and propagated
in inner parts of the particles; and surface pulverization in which
surface parts of the particles are scraped off. Volume
pulverization proceeds by means of impact force, compressive force,
and shear force, while surface pulverization proceeds by means of
grinding force and shear force. Pulverization is a treatment based
on any of various combinations of such kinds of forces to be
applied to a substance and such treatment methods. A suitable
combination can be determined according to the purpose of the
treatment.
[0600] There are cases where pulverization is conducted by a
chemical reaction, e.g., explosion, or by volume expansion.
However, pulverization is generally conducted with a mechanical
apparatus such as a pulverizer. It is preferred that the
pulverization treatment to be used for producing a rounded
carbonaceous substance as a raw material in the invention should be
a treatment which finally involves a high proportion of surface
treatment, regardless of whether volume pulverization occurs or
not. This is because surface pulverization is important for
eliminating angles of the surface-pulverized particles and thereby
imparting roundness to the particle shape. Specifically, use may be
made of a method in which surface treatment is conducted after
volume pulverization has proceeded to some degree or a method in
which surface treatment only is conducted almost without conducting
volume pulverization. Furthermore, volume pulverization and surface
treatment may be simultaneously conducted. It is preferred to
conduct a pulverization treatment in which surface pulverization
proceeds finally and angles are eliminated from the surface of the
particles.
[0601] The apparatus to be used for conducting the mechanical
energy treatment is selected from ones usable for conducting the
preferred treatment described above. The mechanical energy
treatment may be accomplished with one or more of the four kinds of
forces applicable to a substance. However, it is preferred and
effective to repeatedly apply impact force as a main force and
other mechanical actions such as compressive, frictional, shear,
and other forces, including those caused by particle interaction,
to the particles. Consequently, a preferred apparatus specifically
is one which comprises a casing and disposed therein a rotor having
many blades and in which the rotor is rotated at a high speed to
thereby apply mechanical actions such as impact, compressive,
frictional, and shear forces to a carbon material introduced
therein and conduct surface treatment while allowing volume
pulverization to proceed. More preferred is one which has a
mechanism in which a carbonaceous substance is circulated to
thereby repeatedly apply mechanical actions thereto.
[0602] Examples of the preferred apparatus include Hybridization
System (manufactured by Nara Machinery Co., Ltd.), Kryptron (Earth
Technica Co., Ltd.), CF Mill (manufactured by Ube Industries,
Ltd.), and Mechanofusion System (manufactured by Hosokawa Micron
Corp.). Preferred of these is Hybridization System, manufactured by
Nara Machinery Co., Ltd. In the case where the treatment is
conducted with this apparatus, the rotor is rotated at a peripheral
speed of preferably 30-100 m/sec, more preferably 40-100 m/sec,
even more preferably 50-100 m/sec. Although a carbonaceous
substance can be treated by merely passing it through the
apparatus, it is preferred to treat the carbonaceous substance
while causing it to circulate or reside in the apparatus for 30
seconds or longer. More preferably, the carbonaceous substance is
treated while circulating or residing in the apparatus for 1 minute
or longer.
[0603] Through such mechanical energy treatment, the carbon
particles become particles in which only parts near the surface are
rough and which have exposed strains and edge faces, while
retaining high crystallinity as a whole. Thus, the carbon particles
come to have an increased amount of surfaces where lithium ions can
come in and out, and hence have a high capacity even at a high
current density.
[0604] In general, flaky, crystalline, and platy carbon materials
tend to become poorer in suitability for loading as the particle
diameter decreases. It is thought that as a result of
pulverization, the particles come to have more irregular shapes,
have a larger amount of surface projections such as "fine splits",
"half-peeled parts", and "bent parts", and have finer particles of
irregular shapes adherent to the particle surface at some degree of
strength. The impaired suitability for loading is thought to be
attributable to increased resistance between adjacent particles due
to such causes.
[0605] As long as such shape irregularity is reduced and the
particle shape becomes close to sphere, a decrease in particle
diameter should result in a limited decrease in loading and
large-diameter carbon particles and small-diameter carbon particles
should have almost the same tap density.
[[[Properties of Raw Material before Heat Treatment]]]
[0606] It is desirable that the raw material before heat treatment
should have properties which satisfy any one of the following items
(1) to (11) or simultaneously satisfy two or more thereof. The
methods of determining the following properties and the definitions
thereof are the same as in the carbonaceous substances described
above.
(1) X-Ray Parameter
[0607] The raw material before heat treatment preferably has a
value of d (interplanar spacing) for the lattice planes (002), as
determined by X-ray diffractometry in accordance with the method of
the Japan Society for Promotion of Scientific Research, of 0.335 nm
or larger. The lower limit of the value of d is smaller than 0.340
nm, and is preferably 0.337 nm or smaller. When the value of d is
too large, there are cases where crystallinity decreases to
increase initial irreversible capacity. On the other hand, 0.335 is
a theoretical value for graphites. Furthermore, the crystallite
size (Lc) of this carbon material, as determined by X-ray
diffractometry in accordance with the method of the Japan Society
for Promotion of Scientific Research, is generally 30 nm or larger,
preferably 50 nm or larger, more preferably 80 nm or larger. In
case where the Lc thereof is smaller than the lower limit of that
range, there is a possibility that crystallinity might decrease to
increase initial irreversible capacity.
(2) Ash Content
[0608] The ash content of the raw material before heat treatment is
generally 1% by mass or lower, especially 0.5% by mass or lower, in
particular 0.1% by mass or lower, based on the whole raw material
before heat treatment. The lower limit of the ash content thereof
is generally preferably at least 1 ppm by mass of the whole raw
material. When the ash content thereof exceeds the upper limit of
that range, there are cases where battery performance deterioration
caused by reaction with the electrolyte during charge/discharge
becomes not negligible. When the ash content thereof is lower than
the lower limit of that range, there are cases where the production
of this raw material necessitates much time and energy and an
apparatus for pollution prevention, resulting in an increase in
cost.
(3) Volume-Average Particle Diameter
[0609] With respect to the volume-average particle diameter of the
raw material before heat treatment, the volume-average particle
diameter (median diameter) thereof as determined by the laser
diffraction/scattering method is generally 1 .mu.m or larger,
preferably 3 .mu.m or larger, more preferably 5 .mu.m or larger,
even more preferably 7 .mu.m or larger. The upper limit thereof is
generally 50 .mu.m or smaller, preferably 40 .mu.m or smaller, more
preferably 30 .mu.m or smaller, even more preferably 25 .mu.m or
smaller. When the volume-average particle diameter thereof is
smaller than the lower limit of that range, there are cases where
irreversible capacity increases, leading to a loss in initial
battery capacity. When the volume-average particle diameter thereof
exceeds the upper limit of that range, there are cases where such a
raw material is undesirable from the standpoint of battery
production because an uneven coating surface is apt to result when
an electrode plate is produced.
(4) Raman R Value, Raman Half-Value Width
[0610] The R value of the raw material before heat treatment as
determined by the argon ion laser Raman spectroscopy is generally
0.10 or higher, preferably 0.15 or higher, more preferably 0.17 or
higher, even more preferably 0.2 or higher. The upper limit thereof
is generally 0.8 or lower, preferably 0.6 or lower, more preferably
0.4 or lower. When the R value thereof is lower than the lower
limit of that range, there is a possibility that the particles
might have not been sufficiently rounded and the effect of
improving suitability for loading cannot be obtained. On the other
hand, when the R value thereof exceeds the upper limit of that
range, the surface of such particles has reduced crystallinity and
enhanced reactivity with the electrolyte and this may lead to a
decrease in efficiency and enhanced gas evolution.
[0611] The Raman half-value width around 1,580 cm.sup.-1 of the raw
material before heat treatment is not particularly limited.
However, the half-value width thereof is generally 10 cm.sup.-1 or
larger, preferably 15 cm.sup.-1 or larger. The upper limit thereof
is generally 80 cm.sup.-1 or smaller, preferably 60 cm.sup.-1 or
smaller, more preferably 45 cm.sup.-1 or smaller, even more
preferably 40 cm.sup.-1 or smaller. When the Raman half-value width
thereof is smaller than the lower limit of that range, there is a
possibility that the particles might have not been sufficiently
rounded and the effect of improving suitability for loading cannot
be obtained. On the other hand, when the Raman half-value width
thereof exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity and enhanced reactivity with
the electrolyte and this may lead to a decrease in efficiency and
enhanced gas evolution.
(5) BET Specific Surface Area
[0612] The specific surface area of the raw material before heat
treatment, as determined by the BET method, is generally 0.1
m.sup.2/g or larger, preferably 0.7 m.sup.2/g or larger, more
preferably 1 m.sup.2/g or larger, even more preferably 1.5
m.sup.2/g or larger. The upper limit thereof is generally 100
m.sup.2/g or smaller, preferably 50 m.sup.2/g or smaller, more
preferably 15 m.sup.2/g or smaller, even more preferably 10
m.sup.2/g or smaller. When the specific surface area thereof is
smaller than the lower limit of that range, there are cases where
use of the carbonaceous substance obtained from this raw material
as a negative-electrode material is apt to result in poor lithium
acceptance during charge and in lithium deposition on the electrode
surface. On the other hand, when the specific surface area thereof
exceeds the upper limit of that range, there are cases where the
carbonaceous substance obtained therefrom through a heat treatment
is apt to excessively react with the electrolyte and evolve a
larger amount of a gas, although it has a reduced specific surface
area, making it difficult to obtain a preferred battery.
(6) Pore Distribution
[0613] In the raw material before heat treatment, the amount of
interstices corresponding to a pore diameter of from 0.01 .mu.m to
1 .mu.m which include pores in the particles and particle surface
irregularities formed by steps, as determined by mercury
porosimetry (mercury intrusion method), is generally 0.01 mL/g or
larger, preferably 0.05 mL/g or larger, more preferably 0.1 mL/g or
larger. The upper limit thereof is generally 0.6 mL/g or smaller,
preferably 0.4 mL/g or smaller, more preferably 0.3 mL/g or
smaller. When the amount of interstices therein is larger than the
upper limit of that range, there are cases where a large amount of
a hinder is necessary in electrode plate formation. On the other
hand, when the amount thereof is smaller than the lower limit of
that range, there are cases where high-current-density
charge/discharge characteristics decrease and the effect of
diminishing electrode expansion/contraction during charge/discharge
is not obtained.
[0614] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger. The upper limit thereof is
generally 10 mL/g or smaller, preferably 5 mL/g or smaller, more
preferably 2 mL/g or smaller. When the total pore volume exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary in electrode plate formation. When the
total pore volume is smaller than the lower limit, there are cases
where the dispersing effect of a thickener or binder in electrode
plate formation is not obtained.
[0615] The average pore diameter of the raw material before heat
treatment is preferably 0.05 .mu.m or larger, more preferably 0.1
.mu.m or larger, even more preferably 0.5 .mu.m or larger. The
upper limit thereof is generally 50 .mu.m or smaller, preferably 20
.mu.m or smaller, more preferably 10 .mu.m or smaller. When the
average pore diameter thereof exceeds the upper limit of that
range, there are cases where a large amount of a binder is
necessary. On the other hand, when the average pore diameter
thereof is smaller than the lower limit of that range, there are
cases where high-current-density charge/discharge characteristics
decrease.
(7) Roundness
[0616] With respect to the degree of sphericity of the raw material
before heat treatment, the roundness of the particles having a
particle diameter in the range of 3-40 .mu.m is generally
preferably 0.85 or higher, more preferably 0.87 or higher,
especially preferably 0.90 or higher, even more preferably 0.92 or
higher. High roundness values are preferred because they are
effective in improving high-current-density charge/discharge
characteristics and suitability for loading.
[0617] Methods for improving roundness are not particularly
limited. However, a raw material before heat treatment in which the
particles have been rounded by a rounding treatment with the
mechanical energy described above is preferred because it gives an
electrode in which the interstices among particles are uniform in
shape.
(8) True Density
[0618] The true density of the raw material before heat treatment
is generally 2 g/cm.sup.3 or higher, preferably 2.1 g/cm.sup.3 or
higher, more preferably 2.2 g/cm.sup.3 or higher, even more
preferably 2.22 g/cm.sup.3 or higher. The upper limit thereof may
be 2.26 g/cm.sup.3 or lower. This upper limit is a theoretical
value for graphites. When the true density of the raw material is
lower than the lower limit of that range, there are cases where the
carbon has too low crystallinity, resulting in an increase in
initial irreversible capacity.
(9) Tap Density
[0619] The tap density of the raw material before heat treatment is
generally 0.55 g/cm.sup.3 or higher, preferably 0.7 g/cm.sup.3 or
higher, more preferably 0.8 g/cm.sup.3 or higher, especially
preferably 0.9 g/cm.sup.3 or higher. The upper limit thereof is
preferably 2 g/cm.sup.3 or lower, more preferably 1.8 g/cm.sup.3 or
lower, especially preferably 1.6 g/cm.sup.3 or lower. When the tap
density thereof is lower than the lower limit of that range, there
are cases where the carbonaceous substance obtained from this raw
material, when used in a negative electrode, is less apt to have a
high loading density and cannot give a battery having a high
capacity. On the other hand, when the tap density thereof exceeds
the upper limit of that range, the amount of interparticle
interstices in the electrode is too small and it is difficult to
secure electrical conductivity among the particles. There are hence
cases where preferred battery performances are difficult to
obtain.
(10) Orientation Ratio
[0620] The orientation ratio of the raw material before heat
treatment is generally 0.005 or higher, preferably 0.01 or higher,
more preferably 0.015 or higher. The upper limit thereof is
theoretically 0.67 or lower. When the orientation ratio thereof is
lower than the lower limit of that range, there are cases where
high-density charge/discharge characteristics decrease.
(11) Aspect Ratio
[0621] The aspect ratio of the raw material before heat treatment
is theoretically 1 or higher. The upper limit thereof is generally
10 or lower, preferably 8 or lower, more preferably 5 or lower.
When the aspect ratio thereof exceeds the upper limit, there are
cases where the carbonaceous substance obtained from this raw
material causes streak lines in electrode plate formation and an
even coating surface cannot be obtained, resulting in a decrease in
high-current-density charge/discharge characteristics.
[[[Heat Treatment Temperature]]]
[0622] The temperature in the heat treatment of the raw material
before heat treatment is generally 600.degree. C. or higher,
preferably 1,200.degree. C. or higher, more preferably
2,000.degree. C. or higher, even more preferably 2,500.degree. C.
or higher, especially preferably 2,800.degree. C. or higher. The
upper limit thereof is generally 3,200.degree. C. or lower,
preferably 3,100.degree. C. or lower. When the temperature
conditions are lower than the lower limit of that range, there are
cases where the surface of the graphite particles which has been
disordered by the rounding treatment, etc. does not undergo a
sufficient crystal repair and the graphite particles come to have
neither a reduced Raman R value nor a reduced BET specific surface
area. On the other hand, when the temperature exceeds the upper
limit of that range, there are cases where graphite sublimation is
apt to occur in an increased amount.
[[[Method of Heat Treatment]]]
[0623] The heat treatment is accomplished by heating the raw
material once to a temperature in that range. Although the holding
time period in which the temperature conditions are kept in that
range is not particularly limited, it generally is longer than 10
seconds and is not longer than 168 hours.
[0624] In general, the heat treatment is conducted in an inert gas
atmosphere, e.g., nitrogen gas, or in a non-oxidizing atmosphere
formed by the gas generated from the raw graphite. It is, however,
noted that in a furnace of the type in which the raw material is
buried in a breeze (fine carbon powder obtained by pitch
heat-treating), there are cases where the atmosphere in an initial
stage is a mixture with the air. In such cases, it is not always
necessary to use a completely inert gas atmosphere.
[0625] Apparatus usable for the heat treatment are not particularly
limited. For example, use can be made of a shuttle furnace, tunnel
kiln, electric furnace, Riedhammer furnace, rotary kiln, direct
voltage application type electric furnace, Acheson furnace,
resistance heating furnace, induction heating furnace, or the
like.
[0626] Besides the treatments described above, various treatments
including classification can be conducted. Classification is a
treatment for removing coarse particles and fine particles in order
to obtain a target particle diameter. Apparatus usable for the
classification are not particularly limited. However, in the case
of dry sieving, use can be made of a rotary sieve, swing sieve,
revolving sieve, vibrating sieve, or the like. In the case of dry
air classification, use can be made of a gravitational classifier,
inertial classifier, or centrifugal classifier (e.g., classifier or
cyclone). In the case of wet sieving, use can be made of a
mechanical wet classifier, hydraulic classifier, sedimentation
classifier, centrifugal wet classifier, or the like. Classification
may be conducted before the heat treatment or may be conducted in
another timing, e.g., after the heat treatment. It is also possible
to omit the classification treatment itself. However, from the
standpoint of productivity of the graphite powder as a
negative-electrode material, it is preferred to conduct a
classification treatment immediately after the rounding treatment
and prior to the heat treatment.
[[Minor-Material Mixing]]
[0627] Besides the carbonaceous substance described above, one or
more carbonaceous substances (carbonaceous materials) differing in
carbonaceous-substance property from that carbonaceous substance
may be incorporated into the negative-electrode active material for
use in this invention. Thus, battery performances can be further
improved. The term "carbonaceous-substance property" herein means
one or more properties selected from X-ray diffraction parameter,
median diameter, aspect ratio, BET specific surface area,
orientation ratio, Raman R value, tap density, true density, pore
distribution, roundness, and ash content. Preferred embodiments
include: one in which the volume-based particle size distribution
is not symmetrical about the median diameter; one in which two or
more carbon materials dittering in Raman R value are contained; and
one in which carbon materials differing in X-ray parameter are
contained. One example of the effects thereof is that the
incorporation of a carbon material, such as a graphite, e.g., a
natural graphite or artificial graphite, or an amorphous carbon,
e.g., a carbon black such as acetylene black or needle coke, as a
sub material serves to reduce electrical resistance. One of these
carbon materials may be used alone, or any desired two or more
thereof may be used in combination in any desired proportion. In
the case where such a carbon material is added as a sub material,
the amount thereof is generally 0.1% by mass or larger, preferably
0.5% by mass or larger, more preferably 0.6% by mass or larger. The
upper limit thereof is generally 80% by mass or smaller, preferably
50% by mass or smaller, more preferably 40% by mass or smaller,
especially preferably 30% by mass or smaller. Amounts thereof
smaller than the lower limit of that range are undesirable because
they result in difficulties in obtaining the effect of improving
conductivity. When the amount thereof exceeds the upper limit,
there are cases where this leads to an increase in initial
irreversible capacity.
[Negative Electrode [4]: Electrode Production]
[0628] The negative electrode may be produced by an ordinary
method. It can be formed in the same manner as described above. The
current collector, thickness ratio between the current collector
and the active-material layer, electrode density, binder,
electrode-plate orientation ratio, impedance, etc. also are the
same as described above.
<Negative Electrode [5]>
[0629] An explanation will be given below on negative electrode [5]
for use in a lithium secondary battery of the invention, i.e., "a
negative electrode containing as a negative-electrode active
material an orientation-differing-carbon composite comprising two
or more carbonaceous substances differing in orientation".
[Negative Electrode [5]: Negative-Electrode Active Material]
[0630] The negative-electrode active material for use in negative
electrode [5] is described below.
[[Constitution of Orientation-Differing-Carbon Composite]]
[0631] The negative-electrode active material to be used in
negative electrode [5] in a lithium secondary battery of the
invention comprises an orientation-differing-carbon composite
comprising two or more carbonaceous substances differing in
orientation.
[0632] The term "differing in orientation" herein means that when
the powder is examined with a polarizing microscope and the
optically anisotropic structures are visually compared in the
pattern of anisotropic units, i.e., the size, direction, number,
etc. of the anisotropic units, then the structures differ in at
least any of the size, direction, number, etc. of the anisotropic
units. Examples thereof include: the case where one of carbonaceous
substance 1 and carbonaceous substance 2 has crystal orientation in
one direction and the other has random crystal orientation; and the
case where both of carbonaceous substance 1 and carbonaceous
substance 2 have crystal orientation in respective certain
directions and these directions are different. In the case where
one or each of carbonaceous substance 1 and carbonaceous substance
2 is not composed of single-crystal particles but composed of
aggregates of crystals, each unit aggregate is taken as one region
and the optically anisotropic structures are compared in the
aggregation pattern of the anisotropic units.
[0633] With respect to the state in which carbonaceous substance 1
and carbonaceous substance 2 coexist in the
orientation-differing-carbon composite, it is preferred that the
two carbonaceous substances should be contained in one secondary
particle. The term "contained in one secondary particle" herein
means, for example, the state in which the carbonaceous substances
differing in orientation are physically restrained or adhered, the
state in which the carbonaceous substances retain a shape by means
of electrostatic restraint or adhesion, or the state in which the
carbonaceous substances are restrained by bonding. The term
"physically restrained or adhered" herein means a state such as
that in which one of the carbonaceous substances is surrounded by
the other or the state in which one is caught by the other. The
term "electrostatic restraint or adhesion" means the state in which
one of the carbonaceous substances is adherent to the other by
means of electrostatic energy. As long as the carbonaceous
substances in this restrained or adhered state are different from
each other in orientation as described above, the original
carbonaceous substances may be equal. Furthermore, the term "the
state in which the carbonaceous substances are restrained by
bonding" means chemical bonding by hydrogen bonds, covalent bonds,
ionic bonds, etc.
[0634] Preferred of those is the state in which the surface of one
of the carbonaceous substances at least partly has an interface
where another carbonaceous substance, which differs in orientation
from that carbonaceous substance, is adhered and/or bonded to that
carbonaceous substance. Such composite having an interface is
preferred because this composite, as compared with
negative-electrode active materials having no interface and having
the same particle shape as the composite, is effective in
dispersing in many directions the expansion caused by lithium
intercalation during charge to thereby prevent battery
deterioration.
[0635] The part differing in orientation is not limited as to
whether it is one formed by the bonding of an externally supplied
material and/or of a product of alteration thereof or one formed by
the alteration of a surface-part material of a carbonaceous
substance. The term "coating" herein means that the interface
between one carbonaceous substance and the surface of the other
carbonaceous substance at least partly has chemical bonds and the
former carbonaceous substance is (1) in the state of covering the
whole surface, (2) in the state of locally covering the
carbonaceous particles, (3) in the state of selectively covering
part of the surface, or (4) in the state of being present in
ultrafine regions including chemical bonds.
[0636] Around the interface, the orientation of the carbonaceous
substances may change continuously or discontinuously. Namely, it
is preferred that the orientation-differing-carbon composite should
be one which has an interface where carbonaceous substances
differing in orientation are adhered and/or bonded to each other,
and in which the orientation of the carbonaceous substances at the
interface changes discontinuously and/or continuously.
[0637] The components of the orientation-differing-carbon composite
(A) are not particularly limited as long as they have
crystallinity. However, it is preferred that one or more of the
carbonaceous substances differing in orientation should be a
graphitic carbonaceous substance (B) derived from a natural
graphite (D) (hereinafter, this substance (B) is abbreviated to
"natural-graphite-derived carbonaceous substance (B)), because use
of this carbonaceous substance (B) increases charge capacity per
unit mass.
[0638] The proportion of the natural-graphite-derived carbonaceous
substance (B) contained in the orientation-differing-carbon
composite is generally 5% by mass or higher, preferably 20% by mass
or higher, more preferably 30% by mass or higher, even more
preferably 40% by mass or higher, especially preferably 50% by mass
or higher, based on the orientation-differing-carbon composite. The
upper limit thereof is generally 99.9% by mass or lower, preferably
99% by mass or lower, more preferably 95% by mass or lower, even
more preferably 90% by mass or lower. When the proportion of the
carbonaceous substance (B) is lower than the lower limit of that
range, there are cases where electrode rolling results in a
considerably increased load and this causes electrode peeling, etc.
On the other hand, when the proportion thereof exceeds the upper
limit of that range, there are cases where the composite comprising
particles differing in orientation has weak interfacial
bonding.
[0639] With respect to other components of the
orientation-differing-carbon composite, it is also preferred, from
the standpoints of interface formation during the preparation of
the orientation-differing-carbon composite and interfacial-bonding
improvement, that one or more of the carbonaceous substances
differing in orientation should be a carbonaceous substance (C)
selected from the following (1) to (5):
(1) a carbonization product selected from the group consisting of
coal coke, petroleum coke, furnace black, acetylene black, and
pitch-derived carbon fibers; (2) a carbonization product obtained
from a precursor which is an organic substance selected from the
group consisting of pitch materials, aromatic hydrocarbons,
nitrogen-ring compounds, sulfur-ring compounds, polyphenylenes,
organic synthetic polymers, natural polymers, thermoplastic resins,
and thermosetting resins and/or a product of the pyrolysis of the
organic substance; (3) a carbonization product obtained from a
precursor which is a product of the pyrolysis of a solution
obtained by dissolving the organic substance (2) in a low-molecular
organic solvent; (4) a product of the carbonization of a gas
containing an organic substance; and (5) a product of the
graphitization of any of (1) to (4) above.
[0640] The substance in (2) is not particularly limited as long as
it is capable of carbonization. Examples thereof include the
following organic substances: pitch materials; aromatic
hydrocarbons such as acenaphthylene, decacyclene, anthracene, and
phenanthrene; nitrogen-ring compounds such as phenazine and
acridine; sulfur-ring compounds such as thiophene and bithiophene;
polyphenylenes such as biphenyl and terphenyl; poly(vinyl
chloride), poly(vinyl alcohol), poly(vinyl butyral), and substances
obtained by insolubilizing these compounds; nitrogen-containing
organic polymers such as polyacrylonitrile and polypyrrole;
sulfur-containing organic polymers such as polythiophene; organic
polymers such as polystyrene; natural polymers such as
polysaccharides represented by cellulose, lignin, mannan,
poly(galacturonic acid), chitosan, and saccharose; thermoplastic
resins such as poly(phenylene sulfide) and poly(phenylene oxide);
thermosetting resins such as furfuryl alcohol resins,
phenol-formaldehyde resins, and imide resins; and solutions
obtained by dissolving any of such organic substances in a
low-molecular organic solvent, e.g., benzene, toluene, xylene,
quinoline, or n-hexane. Examples thereof further include gases
capable of carbonization.
[0641] Preferred of these are pitch materials. This is because
pitch materials have a high actual carbon ratio and, hence, can
give a material in high yield. The term "pitch material" in this
description means a substance which is a pitch or a similar
material and can be carbonized or graphitized by an appropriate
treatment. Examples of the pitch materials which can be used
include tars, heavy oils, and pitches. Examples of the tars include
coal tar and petroleum tar. Examples of the heavy oils include
petroleum-derived heavy oils such as catalytically cracked oil,
thermally cracked oil, topping residues, and vacuum distillation
residues. Examples of the pitches include coal tar pitch, petroleum
pitch, and synthetic pitch. Of these, coal tar pitch is preferred
because it has high aromaticity. Any one of these pitch materials
may be used alone, or any desired two or more thereof may be used
in combination in any desired proportion.
[0642] With respect to (3), preferred examples thereof include a
carbonization product obtained from a precursor which is a product
of the pyrolysis of a solution of the organic substance (2) in a
low-molecular organic solvent such as benzene, toluene, xylene,
quinoline, or n-hexane.
[0643] With respect to (4), examples include hydrocarbon compounds
such as methane, ethane, propane, benzene, acetylene, and ethylene
and carbon monoxide.
[0644] The proportion of the carbonaceous substance (C) contained
in the orientation-differing-carbon composite is generally 0.1% by
mass or higher, preferably 1% by mass or higher, more preferably 5%
by mass or higher, even more preferably 10% by mass or higher.
There is no particular upper limit on the proportion thereof, as
long as the composite has an interface where orientation changes.
When the proportion of the carbonaceous substance (C) is lower than
the lower limit of that range, there are cases where interfacial
bonding in the orientation-differing-carbon composite becomes weak.
Furthermore, proportions thereof lower than the lower limit of that
range may lessen the effect of inhibiting particle deformation
during pressing which is produced by the carbonaceous substance (C)
contained, resulting in reduced cycle characteristics.
[0645] From the standpoints of pressing load during electrode
rolling and of balance of interfacial bonding in the composite, a
more preferred constitution of this negative-electrode active
material for use in a lithium secondary battery of the invention is
one in which the orientation-differing-carbon composite comprises
both of one or more natural-graphite-derived carbonaceous
substances (B) and one or more carbonaceous substances (C).
[0646] For the same reasons as described above, the proportion by
mass of the natural-graphite-derived carbonaceous substance (B) to
the carbonaceous substance (C) (natural-graphite-derived
carbonaceous substance (B)/carbonaceous substance (C)) in the
orientation-differing-carbon composite is generally 20/80 or
higher, preferably 40/60 or higher, more preferably 60/40 or
higher, even more preferably 70/30 or higher. The upper limit
thereof may be 99.9/0.1 or lower and is preferably 99/1 or lower,
more preferably 95/5 or lower. When the proportion thereof is
higher than the higher limit of that range (when the proportion of
the natural-graphite-derived carbonaceous substance (B) is too
high), there is a possibility that the interfacial bonding brought
about by the carbonaceous substance (C) might decrease. On the
other hand, when the proportion thereof is lower than the lower
limit of that range (when the proportion of the
natural-graphite-derived carbonaceous substance (B) is too low),
there are cases where electrode rolling results in a considerably
increased pressing load to cause peeling during the rolling.
[0647] The orientation-differing-carbon composite comprising the
natural-graphite-derived carbonaceous substance (B) and the
carbonaceous substance (C) may be in any desired form unless this
is counter to the spirit of the invention. Examples thereof are
shown below.
(i) The form in which the surface of the natural-graphite-derived
carbonaceous substance (B) is wholly or partly adhered to and/or
covered with and/or bonded to the carbonaceous substance (C). (ii)
The form in which the carbonaceous substance (C) is bonded to the
whole or part of the surface of the natural-graphite-derived
carbonaceous substance (B) to combine two or more particles of the
natural-graphite-derived carbonaceous substance (B) and/or
carbonaceous substance (C). (iii) The form which comprises a
mixture of (i) and (ii) above in any proportion.
[0648] The natural-graphite-derived carbonaceous substance (B) and
the carbonaceous substance (C) may be replaced by each other.
Specific examples of the state of combination in the
orientation-differing-carbon composite include: the state in which
the surface of the particle serving as a core is adhered to and/or
covered with and/or bonded to a carbonaceous substance differing in
orientation; the state in which the surface of the core particle is
adhered to and/or covered with and/or bonded to two or more such
carbonaceous substances; and the state in which particles serving
as cores have been aggregated in non-parallel arrangement. The term
"the state in which particles have been aggregated in non-parallel
arrangement" herein means that particles having a certain degree of
crystallinity have been fixed, in the state of being randomly
oriented, with another carbonaceous substance and have bonds which
cause the two substances to differ in orientation.
[[Preparation of Orientation-Differing-Carbon Composite]]
[0649] The preparation of the orientation-differing-carbon
composite will be described later under "Production Process 1 and
Production Process 2 for Producing Orientation-Differing-Carbon
Composite". In preparing the orientation-differing-carbon
composite, crystallinity can be improved by conducting a heat
treatment and capacity per unit weight can hence be increased. The
temperature for the heat treatment is generally 400.degree. C. or
higher, preferably 1,000.degree. C. or higher, more preferably
2,000.degree. C. or higher, even more preferably 2,400.degree. C.
or higher, especially preferably 2,800.degree. C. or higher. The
upper limit thereof is generally 3,400.degree. C. or lower,
preferably 3,200.degree. C. or lower. In case where the temperature
is lower than the lower limit of that range, there is a possibility
that crystallinity improvement might be insufficient and the effect
of increasing capacity per unit weight cannot be obtained. On the
other hand, in case where the temperature exceeds the upper limit
of that range, there is a possibility that the loss caused by
carbon sublimation becomes not negligible, resulting in a decrease
in yield.
[[Properties of Orientation-Differing-Carbon Composite]]
[0650] It is preferred that the orientation-differing-carbon
composite should have properties which satisfy any one of the
following items (1) to (5) or simultaneously satisfy two or more
thereof.
(1) Roundness
[0651] With respect to the degree of sphericity of the
orientation-differing-carbon composite, the roundness of the
particles having a particle diameter in the range of 3-40 .mu.m is
generally 0.1 or higher, preferably 0.5 or higher, more preferably
0.8 or higher, even more preferably 0.85 or higher, most preferably
0.9 or higher. High roundness values are preferred because they are
effective in improving high-current-density charge/discharge
characteristics. Roundness is defined by the following equation.
When a particle has a roundness of 1, this particle theoretically
is a true sphere.
Roundness=(length of periphery of equivalent circle having the same
area as projected particle shape)/(actual length of periphery of
projected particle shape)
[0652] The value of roundness is determined, for example, with a
flow type particle image analyzer (e.g., FPIA, manufactured by
Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a
0.2% by mass aqueous solution (about 50 mL) of
poly(oxyethylene(20)) sorbitan monolaurate as a surfactant,
propagating an ultrasonic wave of 28 kHz to the dispersion for 1
minute at an output of 60 W, and then examining particles having a
particle diameter in the range of 3-40 .mu.m with the analyzer
having a detection range set at 0.6-400 .mu.m.
[0653] Methods for improving roundness are not particularly
limited. However, an orientation-differing-carbon composite in
which the particles have been rounded by a rounding treatment is
preferred because it gives an electrode in which the interstices
among particles are uniform in shape. Examples of the rounding
treatment include: a method in which a shear force or compressive
force is applied to thereby mechanically make the shape of the
particles close to sphere; and a method of mechanical/physical
treatment in which fine particles are aggregated into particles by
means of the bonding force of a binder or of the fine particles
themselves.
(2) Raman R Value, Raman Half-Value Width
[0654] The Raman R value of the orientation-differing-carbon
composite as determined by the argon ion laser Raman spectroscopy
is generally 0.01 or higher, preferably 0.02 or higher, more
preferably 0.04 or higher. The upper limit thereof is generally
0.35 or lower, preferably 0.30 or lower, more preferably 0.25 or
lower. When the Raman R value thereof is lower than the lower limit
of that range, the surface of such particles has too high
crystallinity and there are cases where the number of intercalation
sites into which lithium comes with charge/discharge decreases.
Namely, there are cases where suitability for charge decreases. On
the other hand, when the Raman R value thereof exceeds the upper
limit of that range, the surface of such particles has reduced
crystallinity and enhanced reactivity with the nonaqueous
electrolyte and this may lead to a decrease in efficiency and
enhanced gas evolution.
[0655] The Raman half-value width around 1,580 cm.sup.-1 of the
carbon material in this invention is not particularly limited.
However, the half-value width thereof is generally 5 cm.sup.-1 or
larger, preferably 10 cm.sup.-1 or larger. The upper limit thereof
is generally 40 cm.sup.-1 or smaller, preferably 35 cm.sup.-1 or
smaller, more preferably 30 cm.sup.-1 or smaller. When the Raman
half-value width thereof is smaller than the lower limit of that
range, the surface of such particles has too high crystallinity and
there are cases where the number of intercalation sites into which
lithium comes with charge/discharge decreases. Namely, there are
cases where suitability for charge decreases. On the other hand,
when the Raman half-value width thereof exceeds the upper limit of
that range, the surface of such particles has reduced crystallinity
and enhanced reactivity with the nonaqueous electrolyte and this
may lead to a decrease in efficiency and enhanced gas
evolution.
[0656] The examination for a Raman spectrum is made with a Raman
spectrometer (e.g., a Raman spectrometer manufactured by Japan
Spectroscopic Co., Ltd.). In the examination, a sample is charged
into a measuring cell by causing the sample to fall naturally into
the cell and the surface of the sample in the cell is irradiated
with argon ion laser light while rotating the cell in a plane
perpendicular to the laser light. The Raman spectrum obtained is
examined for the intensity I.sub.A of a peak P.sub.A around 1,580
cm.sup.-1 and the intensity I.sub.B of a peak P.sub.B around 1,360
cm.sup.-1. The ratio between these intensities R
(R=I.sub.B/I.sub.A) is calculated and this ratio is defined as the
Raman R value of the carbon material. Furthermore, the half-value
width of the peak P.sub.A around 1,580 cm.sup.-1 in the Raman
spectrum obtained is measured, and this value is defined as the
Raman half-value width of the carbon material.
[0657] Conditions for this Raman spectroscopy are as follows.
[0658] Wavelength of argon ion laser: 514.5 nm [0659] Laser power
on sample: 15-25 mW [0660] Resolution: 10-20 cm.sup.-1 [0661]
Examination range: 1,100 cm.sup.-1 to 1,730 cm.sup.-1 [0662]
Analysis for Raman R value and Raman half-value width: background
processing [0663] Smoothing: simple average; convolution, 5
points
(3) Tap Density
[0664] The tap density of the orientation-differing-carbon
composite desirably is generally 0.55 g/cm.sup.3 or higher,
preferably 0.70/cm.sup.3 or higher, more preferably 0.9 g/cm.sup.3
or higher, especially preferably 1 g/cm.sup.3 or higher. The tap
density thereof is preferably 2.0 g/cm.sup.3 or lower, more
preferably 1.8 g/cm.sup.3 or lower, even more preferably 1.7
g/cm.sup.3 or lower, especially preferably 1.5 g/cm.sup.3 or lower.
When the tap density thereof is lower than the lower limit of that
range, there are cases where this orientation-differing-carbon
composite, when used in a negative electrode, is less apt to have a
high loading density and cannot give a battery having a high
capacity. On the other hand, when the tap density thereof exceeds
the upper limit of that range, the amount of interparticle
interstices in the electrode is too small and it is difficult to
secure electrical conductivity among the particles. There are hence
cases where preferred battery performances are difficult to
obtain.
[0665] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the cell with the
sample up to the brim, subsequently conducting tapping operations
1,000 times over a stroke length of 10 mm using a powder densimeter
(e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.),
and determining a density from the resultant volume of the sample
and weight thereof. This density is defined as the tap density.
(4) BET Specific Surface Area
[0666] The specific surface area of the
orientation-differing-carbon composite as a carbon material for use
in this invention, as determined by the BET method, is generally
0.1 m.sup.2/g or larger, preferably 0.7 m.sup.2/g or larger, more
preferably 1.0 m.sup.2/g or larger, even more preferably 1.2
m.sup.2/g or larger. The upper limit thereof is generally 100
m.sup.2/g or smaller, preferably 25 m.sup.2/g or smaller, more
preferably 15 m.sup.2/g or smaller, even more preferably 10
m.sup.2/g or smaller. When the specific surface area thereof is
smaller than the lower limit of that range, there are cases where
use of this orientation-differing-carbon composite as a
negative-electrode material is apt to result in poor lithium
acceptance during charge and in lithium deposition on the electrode
surface. On the other hand, when the specific surface area thereof
exceeds the upper limit of that range, there are cases where use of
this orientation-differing-carbon composite as a negative-electrode
material is apt to result in enhanced reactivity with the
nonaqueous electrolyte and enhanced gas evolution and a preferred
battery is difficult to obtain.
[0667] BET specific surface area is defined as a value measured
with a surface area meter (e.g., a fully automatic surface area
measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by
preliminarily drying a sample at 350.degree. C. for 15 minutes in a
nitrogen stream and then measuring the specific surface area
thereof by the gas-flowing nitrogen adsorption BET one-point method
using a nitrogen/helium mixture gas precisely regulated so as to
have a nitrogen pressure of 0.3 relative to atmospheric
pressure.
(5) Volume-Average Particle Diameter
[0668] With respect to the volume-average particle diameter of the
orientation-differing-carbon composite, the volume-average particle
diameter (median diameter) thereof as determined by the laser
diffraction/scattering method is generally 1 .mu.m or larger,
preferably 3 .mu.m or larger, more preferably 5 .mu.m or larger,
even more preferably 7 .mu.m or larger. The upper limit thereof is
generally 100 .mu.m or smaller, preferably 50 .mu.m or smaller,
more preferably 40 .mu.m or smaller, even more preferably 30 .mu.m
or smaller, especially preferably 25 .mu.m or smaller. When the
volume-average particle diameter thereof is smaller than the lower
limit of that range, there are cases where irreversible capacity
increases, leading to a loss in initial battery capacity. When the
volume-average particle diameter thereof exceeds the upper limit of
that range, there are cases where such a carbon composite is
undesirable from the standpoint of battery production because an
uneven coating surface is apt to result when an electrode plate is
produced.
[0669] Volume-average particle diameter in the invention is defined
as the median diameter determined by dispersing the carbon powder
in a 0.2% by mass aqueous solution (about 1 mL) of
poly(oxyethylene(20)) sorbitan monolaurate as a surfactant and
examining the dispersion with a laser diffraction type particle
size distribution analyzer (e.g., LA-700, manufactured by HORIBA,
Ltd.).
[0670] From the standpoint of balance among battery
characteristics, the orientation-differing-carbon composite
preferably satisfies one or more of the following items (6) to (11)
besides items (1) to (5) given above.
(6) X-Ray Parameter
[0671] The orientation-differing-carbon composite preferably has a
value of d (interplanar spacing) for the lattice planes (002), as
determined by X-ray diffractometry in accordance with the method of
the Japan Society for Promotion of Scientific Research, of 0.335 nm
or larger. The lower limit of the value of d desirably is generally
0.340 nm or smaller, preferably 0.337 nm or smaller. When the value
of d exceeds the upper limit of that range, there is a possibility
that crystallinity might decrease to increase initial irreversible
capacity. The value of 0.335 nm as the lower limit is a theoretical
value for graphites. Furthermore, the crystallite size (Lc) of this
carbon material, as determined by X-ray diffractometry in
accordance with the method of the Japan Society for Promotion of
Scientific Research, is generally 30 nm or larger, preferably 50 nm
or larger, more preferably 80 nm or larger. In case where the Lc
thereof is smaller than the lower limit of that range, there is a
possibility that crystallinity might decrease to increase initial
irreversible capacity.
(7) Ash Content
[0672] The ash content of the orientation-differing-carbon
composite is generally 1% by mass or lower, especially 0.5% by mass
or lower, in particular 0.1% by mass or lower, based on the whole
orientation-differing-carbon composite. The lower limit of the ash
content thereof is preferably at least 1 ppm by mass of the whole
carbon composite. When the ash content thereof exceeds the upper
limit of that range, there are cases where battery performance
deterioration caused by reaction with the nonaqueous electrolyte
during charge/discharge becomes not negligible. When the ash
content thereof is lower than the lower limit of that range, there
are cases where the production of this carbon composite
necessitates much time and energy and an apparatus for pollution
prevention, resulting in an increase in cost.
(8) Pore Distribution
[0673] In the orientation-differing-carbon composite, the amount of
interstices corresponding to a pore diameter of from 0.01 .mu.m to
1 .mu.m which include pores in the particles and particle surface
irregularities formed by steps, as determined by mercury
porosimetry (mercury intrusion method), may be 0.001 mL/g or larger
and is preferably 0.002 mL/g or larger. The upper limit thereof is
generally 0.6 mL/g or smaller, preferably 0.4 mL/g or smaller, more
preferably 0.3 mL/g or smaller. When the amount of interstices
therein is larger than the upper limit of that range, there are
cases where a large amount of a binder is necessary in electrode
plate formation. On the other hand, when the amount thereof is
smaller than the lower limit of that range, there are cases where
high-current-density charge/discharge characteristics decrease and
the effect of diminishing electrode expansion/contraction during
charge/discharge is not obtained.
[0674] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger. The upper limit thereof is
generally 10 mL/g or smaller, preferably 5 mL/g or smaller, more
preferably 2 mL/g or smaller. When the total pore volume exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary in electrode plate formation. When the
total pore volume is smaller than the lower limit, there are cases
where the dispersing effect of a thickener or binder in electrode
plate formation is not obtained.
[0675] The average pore diameter of the
orientation-differing-carbon composite is preferably 0.05 .mu.m or
larger, more preferably 0.1 .mu.m or larger, even more preferably
0.5 .mu.m or larger. The upper limit thereof is generally 50 .mu.m
or smaller, preferably 20 .mu.m or smaller, more preferably 10
.mu.m or smaller. When the average pore diameter thereof exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary. On the other hand, when the average pore
diameter thereof is smaller than the lower limit of that range,
there are cases where high-current-density charge/discharge
characteristics decrease.
[0676] As an apparatus for the mercury porosimetry is used a
mercury porosimeter (Autopore 9520, manufactured by Micromeritics
Corp.). About 0.2 g of a sample (negative-electrode material) is
weighed out and placed in a powder cell and this cell is closed.
The sample in the cell is pretreated by degassing it at room
temperature under vacuum (50 .mu.mHg or lower) for 10 minutes.
Subsequently, the pressure in the cell is reduced to 4 psia (about
28 kPa) and mercury is introduced thereinto. The internal pressure
is stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia
(about 280 MPa) and then lowered to 25 psia (about 170 kPa). The
number of steps in the pressure elevation is 80 or larger. In each
step, the amount of mercury intruded is measured after an
equilibrium time of 10 seconds. A pore distribution is calculated
from the mercury intrusion curve thus obtained, using the Washburn
equation. For the calculation, the surface tension (.gamma.) and
contact angle (.psi.) of mercury are taken as 485 dyne/cm and
140.degree., respectively. The average pore diameter used is the
pore diameter corresponding to a cumulative pore volume of 50%.
(9) True Density
[0677] The true density of the orientation-differing-carbon
composite is generally 2.0 g/cm.sup.3 or higher, preferably 2.1
g/cm.sup.3 or higher, more preferably 2.2 g/cm.sup.3 or higher,
even more preferably 2.22 g/cm.sup.3 or higher. The upper limit
thereof may be 2.26 g/cm.sup.3 or lower. This upper limit is a
theoretical value for graphites. When the true density of the
orientation-differing-carbon composite is lower than the lower
limit of that range, there are cases where this carbon has too low
crystallinity, resulting in an increase in initial irreversible
capacity. In the invention, true density is defined as one
determined by the liquid-phase displacement method (pycnometer
method) using butanol.
(10) Orientation Ratio (Powder)
[0678] The orientation ratio of the orientation-differing-carbon
composite is generally 0.005 or higher, preferably 0.01 or higher,
more preferably 0.015 or higher. The upper limit thereof is
theoretically 0.67 or lower. When the orientation ratio thereof is
lower than the lower limit of that range, there are cases where
high-density charge/discharge characteristics decrease.
[0679] Orientation ratio is determined by X-ray diffractometry. The
peaks attributable to the (110) diffraction and (004) diffraction
of the carbon obtained by X-ray diffractometry are subjected to
peak separation by fitting with asymmetric Pearson VII as a profile
function. Thus, the integrated intensities of the (110) diffraction
and (004) diffraction peaks are calculated. From the integrated
intensities obtained, the ratio represented by (integrated
intensity of (110) diffraction)/(integrated intensity of (004)
diffraction) is calculated. This ratio is defined as the
active-material orientation ratio.
[0680] Conditions for this X-ray diffractometry are as follows.
Incidentally, "2.theta." represents diffraction angle. [0681]
Target: Cu(K.alpha. line) graphite monochromator [0682] Slit:
[0683] Divergence slit=1 degree [0684] Receiving slit=0.1 mm [0685]
Scattering slit=1 degree [0686] Examination range and step
angle/measuring time: [0687] (110) plane:
76.5.degree..ltoreq.2.theta..ltoreq.78.5.degree. 0.01.degree./3 sec
[0688] (004) plane:
53.5.degree..ltoreq.2.theta..ltoreq.56.0.degree. 0.01 0/3 sec
(11) Aspect Ratio (Powder)
[0689] The aspect ratio of the orientation-differing-carbon
composite is theoretically 1 or higher. The upper limit thereof may
be 10 or lower and is preferably 8 or lower, more preferably 5 or
lower. When the aspect ratio thereof exceeds the upper limit, there
are cases where this orientation-differing-carbon composite causes
streak lines in electrode plate formation and an even coating
surface cannot be obtained, resulting in a decrease in
high-current-density charge/discharge characteristics.
[0690] The aspect ratio of a carbon material particle is
represented by A/B, wherein A is the length of the longest axis of
the particle in a three-dimensional examination and B is the length
of the shortest axis perpendicular to that axis. The examination of
carbon particles is made with a scanning electron microscope
capable of enlargement examination. Fifty are arbitrarily selected
from graphite particles fixed to an edge face of a metal having a
thickness of 50 .mu.m or smaller, and each particle is examined for
A and B while rotating or inclining the stage to which the sample
is fixed. The average of the A/B values is determined.
[[Raw Materials for Natural-Graphite-Derived Carbonaceous Substance
(B)]]
[0691] Examples of raw materials for the natural-graphite-derived
carbonaceous substance (B) to be contained in the
orientation-differing-carbon composite include ones obtained from a
natural graphite having such high crystallinity that the
interplanar spacing (d002) for the (002) planes as determined by
wide-angle X-ray diffractometry is generally 0.340 nm or smaller.
Specifically, it is preferred to use a powder selected from natural
graphites, materials obtained by mechanically pulverizing natural
graphites to improve their roundness and/or by heat-treating
natural graphites at 1,000.degree. C. or higher, materials obtained
by heat-treating expanded graphites, and high-purity products
obtained from those graphites.
[0692] The natural graphites (D) usable as precursors for the
natural-graphite-derived carbonaceous substance (B) are classified
into flake graphite, crystalline (vein) graphite, and amorphous
graphite according to properties (see "Graphite" in Funrutai
Purosesu Gijutsu Shusei (published by Industrial Technology Center
in 1974); and HANDBOOK OF CARBON, GRAPHITE, DIAMOND AND FULLERENES
(published by Noyes Publications)). With respect to the degree of
graphitization, crystalline graphite has a highest graphitization
degree of 100% and flake graphite is second, which has a
graphitization degree as high as 99.9%. However, the graphitization
degree of amorphous graphite is as low as 28%. Flake graphite,
among such natural graphites, occurs in Madagascar, China, Brazil,
Ukraine, Canada, etc. Crystalline graphite occurs mainly in Sri
Lanka. Amorphous graphite occurs mainly in the Korean Peninsula,
China, Mexico, etc. Of these natural graphites, amorphous graphite
generally has a small particle diameter and a low purity. In
contrast, flake graphite and crystalline graphite have advantages
such as the degree of graphitization and a low impurity content,
and can hence be advantageously used in the invention.
[[Preparation of Natural-Graphite-Derived Carbonaceous Substance
(B)]]
[0693] The preparation of the natural-graphite-derived carbonaceous
substance (B) will be described later under "Production Process 1
and Production Process 2 for Producing Orientation-Differing-Carbon
Composite".
[[Properties of Natural-Graphite-Derived Carbonaceous Substance
(B)]]
[0694] It is desirable that the natural-graphite-derived
carbonaceous substance (B) should satisfy any one of the following
items (1) to (11) or simultaneously satisfy two or more thereof.
The definition of each of the following properties, method of
determining the same, etc. are the same as those described above
with regard to the orientation-differing-carbon composite.
(1) X-Ray Parameter
[0695] The natural-graphite-derived carbonaceous substance
preferably has a value of d (interplanar spacing) for the lattice
planes (002), as determined by X-ray diffractometry in accordance
with the method of the Japan Society for Promotion of Scientific
Research, of 0.335 nm or larger. The value of d thereof desirably
is generally 0.340 nm or smaller, preferably 0.337 nm or smaller.
When the value of d exceeds the upper limit of that range, there is
a possibility that crystallinity might decrease to enhance the
increase in initial irreversible capacity. The value of 0.335 nm is
a theoretical value for graphites. Furthermore, the crystallite
size (Lc) of this carbon material, as determined by X-ray
diffractometry in accordance with the method of the Japan Society
for Promotion of Scientific Research, is generally 30 nm or larger,
preferably 50 nm or larger, more preferably 90 nm or larger. In
case where the Lc thereof is smaller than the lower limit of that
range, there is a possibility that crystallinity might decrease to
enhance the increase in initial irreversible capacity.
(2) Ash Content
[0696] The ash content of the natural-graphite-derived carbonaceous
substance is generally 1% by mass or lower, especially 0.5% by mass
or lower, in particular 0.1% by mass or lower, based on the whole
natural-graphite-derived carbonaceous substance. The lower limit of
the ash content thereof is preferably at least 1 ppm by mass of the
whole carbonaceous substance. When the ash content thereof exceeds
the upper limit of that range, there are cases where battery
performance deterioration caused by reaction with the nonaqueous
electrolyte during charge/discharge becomes not negligible. When
the ash content thereof is lower than the lower limit of that
range, there are cases where the production of this carbonaceous
substance necessitates much time and energy and an apparatus for
pollution prevention, resulting in an increase in cost.
(3) Volume-Average Particle Diameter
[0697] With respect to the volume-average particle diameter of the
natural-graphite-derived carbonaceous substance, the volume-average
particle diameter (median diameter) thereof as determined by the
laser diffraction/scattering method is generally 1 .mu.m or larger,
preferably 3 .mu.m or larger, more preferably 5 .mu.m or larger,
even more preferably 7 .mu.m or larger. The upper limit thereof is
generally 50 .mu.m or smaller, preferably 40 .mu.m or smaller, more
preferably 30 .mu.m or smaller, even more preferably 25 .mu.m or
smaller. When the volume-average particle diameter thereof is
smaller than the lower limit of that range, there are cases where
irreversible capacity increases, leading to a loss in initial
battery capacity. When the volume-average particle diameter thereof
exceeds the upper limit of that range, there are cases where such a
carbonaceous substance is undesirable from the standpoint of
battery production because an uneven coating surface is apt to
result when an electrode plate is produced.
(4) Raman R Value, Raman Half-Value Width
[0698] The Raman R value of the natural-graphite-derived
carbonaceous substance as determined by the argon ion laser Raman
spectroscopy is generally 0.01 or higher, preferably 0.02 or
higher, more preferably 0.04 or higher. The upper limit thereof may
be 0.35 or lower and is preferably 0.30 or lower, more preferably
0.25 or lower. When the Raman R value thereof is lower than the
lower limit of that range, the surface of such particles has too
high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. On the other hand, when the Raman R value thereof
exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity and enhanced reactivity with
the nonaqueous electrolyte and this may lead to a decrease in
efficiency and enhanced gas evolution.
[0699] The Raman half-value width around 1,580 cm.sup.-1 of the
carbon material in the invention is not particularly limited.
However, the half-value width thereof is generally 5 cm.sup.-1 or
larger, preferably 10 cm.sup.-1 or larger. The upper limit thereof
is generally 40 cm.sup.-1 or smaller, preferably 35 cm.sup.-1 or
smaller, more preferably 30 cm.sup.-1 or smaller. When the Raman
half-value width thereof is smaller than the lower limit of that
range, the surface of such particles has too high crystallinity and
there are cases where the number of intercalation sites into which
lithium comes with charge/discharge decreases. Namely, there are
cases where suitability for charge decreases. On the other hand,
when the Raman half-value width thereof exceeds the upper limit of
that range, the surface of such particles has reduced crystallinity
and enhanced reactivity with the nonaqueous electrolyte and this
may lead to a decrease in efficiency and enhanced gas
evolution.
(5) BET Specific Surface Area
[0700] The specific surface area of the natural-graphite-derived
carbonaceous substance, as determined by the BET method, is
generally 0.1 m.sup.2/g or larger, preferably 0.7 m.sup.2/g or
larger, more preferably 1.0 m.sup.2/g or larger, even more
preferably 1.5 m.sup.2/g or larger. The upper limit thereof is
generally 100 m.sup.2/g or smaller, preferably 25 m.sup.2/g or
smaller, more preferably 15 m.sup.2/g or smaller, even more
preferably 10 m.sup.2/g or smaller. When the specific surface area
thereof is smaller than the lower limit of that range, there are
cases where use of the carbon composite containing this
carbonaceous substance as a negative-electrode material is apt to
result in poor lithium acceptance during charge and in lithium
deposition on the electrode surface. On the other hand, when the
specific surface area thereof exceeds the upper limit of that
range, there are cases where use of the carbon composite containing
this carbonaceous substance as a negative-electrode material is apt
to result in enhanced reactivity with the nonaqueous electrolyte
and enhanced gas evolution and a preferred battery is difficult to
obtain.
(6) Pore Distribution
[0701] In the natural-graphite-derived carbonaceous substance, the
amount of interstices corresponding to a pore diameter of from 0.01
.mu.m to 1 .mu.m which include pores in the particles and particle
surface irregularities formed by steps, as determined by mercury
porosimetry (mercury intrusion method), is generally 0.01 mL/g or
larger, preferably 0.05 mL/g or larger, more preferably 0.1 mL/g or
larger. The upper limit thereof is generally 0.6 mL/g or smaller,
preferably 0.4 mL/g or smaller, more preferably 0.3 mL/g or
smaller. When the amount of interstices therein is larger than the
upper limit of that range, there are cases where a large amount of
a binder is necessary in electrode plate formation. When the amount
thereof is smaller than the lower limit, there are cases where
high-current-density charge/discharge characteristics decrease and
the effect of diminishing electrode expansion/contraction during
charge/discharge is difficult to obtain.
[0702] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger. The upper limit thereof is
generally 10 mL/g or smaller, preferably 5 mL/g or smaller, more
preferably 2 mL/g or smaller. When the total pore volume exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary in electrode plate formation. When the
total pore volume is smaller than the lower limit, there are cases
where the dispersing effect of a thickener or binder in electrode
plate formation is not obtained.
[0703] The average pore diameter of the natural-graphite-derived
carbonaceous substance is preferably 0.05 .mu.m or larger, more
preferably 0.1 .mu.m or larger, especially preferably 0.5 .mu.m or
larger. The upper limit thereof is generally 50 .mu.m or smaller,
preferably 20 .mu.m or smaller, more preferably 10 .mu.m or
smaller. When the average pore diameter thereof exceeds the upper
limit of that range, there are cases where a large amount of a
binder is necessary. When the average pore diameter thereof is
smaller than the lower limit, there are cases where
high-current-density charge/discharge characteristics decrease.
(7) Roundness
[0704] With respect to the degree of sphericity of the
natural-graphite-derived carbonaceous substance, the roundness of
the particles having a particle diameter in the range of 3-40 .mu.m
is preferably 0.1 or higher, especially preferably 0.5 or higher,
more preferably 0.8 or higher, even more preferably 0.85 or higher,
most preferably 0.9 or higher. High roundness values are preferred
because they are effective in improving high-current-density
charge/discharge characteristics.
(8) True Density
[0705] The true density of the natural-graphite-derived
carbonaceous substance is generally 2.0 g/cm.sup.3 or higher,
preferably 2.1 g/cm.sup.3 or higher, more preferably 2.2 g/cm.sup.3
or higher, even more preferably 2.22 g/cm.sup.3 or higher. The
upper limit thereof may be 2.26 g/cm.sup.3 or lower. This upper
limit is a theoretical value for graphites. When the true density
of the carbonaceous substance is lower than the lower limit of that
range, there are cases where the carbon has too low crystallinity,
resulting in an increase in initial irreversible capacity.
(9) Tap Density
[0706] The tap density of the natural-graphite-derived carbonaceous
substance desirably is generally 0.1 g/cm.sup.3 or higher,
preferably 0.5 g/cm.sup.3 or higher, more preferably 0.7 g/cm.sup.3
or higher, especially preferably 0.9 g/cm.sup.3 or higher. The
upper limit thereof is preferably 2.0 g/cm.sup.3 or lower, more
preferably 1.8 g/cm.sup.3 or lower, especially preferably 1.6
g/cm.sup.3 or lower. When the tap density thereof is lower than the
lower limit of that range, there are cases where the carbon
composite containing this carbonaceous substance, when used in a
negative electrode, is less apt to have a high loading density and
cannot give a battery having a high capacity. On the other hand,
when the tap density thereof exceeds the upper limit of that range,
the amount of interparticle interstices in the electrode is too
small and it is difficult to secure electrical conductivity among
the particles. There are hence cases where preferred battery
performances are difficult to obtain.
(10) Orientation Ratio (Powder)
[0707] The orientation ratio of the natural-graphite-derived
carbonaceous substance is generally 0.005 or higher, preferably
0.01 or higher, more preferably 0.015 or higher. The upper limit
thereof is theoretically 0.67 or lower. When the orientation ratio
thereof is lower than the lower limit of that range, there are
cases where high-density charge/discharge characteristics
decrease.
(11) Aspect Ratio (Powder)
[0708] The aspect ratio of the natural-graphite-derived
carbonaceous substance is theoretically 1 or higher. The upper
limit thereof is generally 10 or lower, preferably 8 or lower, more
preferably 5 or lower. When the aspect ratio thereof exceeds the
upper limit, there are cases where the carbon composite containing
this carbonaceous substance causes streak lines in electrode plate
formation and an even coating surface cannot be obtained, resulting
in a decrease in high-current-density charge/discharge
characteristics.
[[Raw Materials for Carbonaceous Substance (C)]]
[0709] Raw materials for the carbonaceous substance (C) to be
contained in the orientation-differing-carbon composite in this
invention are not particularly limited as long as they are capable
of carbonization. Examples thereof include the following organic
substances: pitch materials; aromatic hydrocarbons such as
acenaphthylene, decacyclene, anthracene, and phenanthrene;
nitrogen-ring compounds such as phenazine and acridine; sulfur-ring
compounds such as thiophene and bithiophene; polyphenylenes such as
biphenyl and terphenyl; poly(vinyl chloride), poly(vinyl alcohol),
poly(vinyl butyral), and substances obtained by insolubilizing
these compounds; nitrogen-containing organic polymers such as
polyacrylonitrile and polypyrrole; sulfur-containing organic
polymers such as polythiophene; organic polymers such as
polystyrene; natural polymers such as polysaccharides represented
by cellulose, lignin, mannan, poly(galacturonic acid), chitosan,
and saccharose; thermoplastic resins such as poly(phenylene
sulfide) and poly(phenylene oxide); thermosetting resins such as
furfuryl alcohol resins, phenol-formaldehyde resins, and imide
resins; and solutions obtained by dissolving any of such organic
substances in a low-molecular organic solvent, e.g., benzene,
toluene, xylene, quinoline, or n-hexane. Examples thereof further
include gases capable of carbonization.
[0710] Preferred of these are pitch materials. This is because
pitch materials have a high actual carbon ratio and, hence, can
give a material in high yield. The term "pitch material" in this
description means a substance which is a pitch or a similar
material and can be carbonized or graphitized by an appropriate
treatment. Examples of the pitch materials which can be used
include tars, heavy oils, and pitches. Examples of the tars include
coal tar and petroleum tar. Examples of the heavy oils include
petroleum-derived heavy oils such as catalytically cracked oil,
thermally cracked oil, topping residues, and vacuum distillation
residues. Examples of the pitches include coal tar pitch, petroleum
pitch, and synthetic pitch. Of these, coal tar pitch is preferred
because it has high aromaticity. Any one of these pitch materials
may be used alone, or any desired two or more thereof may be used
in combination in any desired proportion.
[0711] In the pitch materials described above, the quinoline
insoluble content is not particularly limited. However, pitch
materials having a quinoline insoluble content of 30 or lower are
generally used. Quinoline insolubles are, for example, submicron
carbon particles, a fine sludge, etc. which are contained in a
slight amount in coal tar. In case where such ingredients are
contained in too large an amount, they considerably inhibit
crystallinity from improving in a graphitization step and this
leads to a considerable decrease in discharge capacity after
graphitization. For determining the content of quinoline
insolubles, use can be made of, e.g., the method as provided for in
JIS K2425.
[0712] The pitch materials described above may be used as a raw
material in combination with any of various thermosetting resins,
thermoplastic resins, and the like unless this lessens the effects
of the invention.
[[Preparation of Carbonaceous Substance (C)]]
[0713] The preparation of the carbonaceous substance (C) will be
described later under "Production Process 1 and Production Process
2 for Producing Orientation-Differing-Carbon Composite".
[[Properties of Carbonaceous Substance (C)]]
[0714] It is preferred that the carbonaceous substance (C) should
satisfy any one of the following items (1) to (4) or simultaneously
satisfy two or more thereof. The definition of each of the
following properties, method of determining the same, etc. are the
same as those described above with regard to the
orientation-differing-carbon composite.
(1) X-Ray Parameter
[0715] The carbonaceous substance (C) preferably has a value of d
(interplanar spacing) for the lattice planes (002), as determined
by X-ray diffractometry in accordance with the method of the Japan
Society for Promotion of Scientific Research, of 0.335 nm or
larger. The value of d thereof desirably is generally 0.345 nm or
smaller, preferably 0.340 nm or smaller, more preferably 0.337 nm
or smaller. Furthermore, the crystallite size (Lc) of this carbon
material, as determined by X-ray diffractometry in accordance with
the method of the Japan Society for Promotion of Scientific
Research, is generally 5 nm or larger, preferably 10 nm or larger,
more preferably 50 nm or larger, even more preferably 80 nm or
larger. In case where the Lc thereof is smaller than the lower
limit of that range, there is a possibility that crystallinity
might decrease to enhance the increase in initial irreversible
capacity.
(2) Ash Content
[0716] The ash content of the carbonaceous substance (C) is
generally 1% by mass or lower, especially 0.5% by mass or lower, in
particular 0.1% by mass or lower, based on the whole
orientation-differing-carbon composite. The lower limit of the ash
content thereof is preferably at least 1 ppm by mass of the whole
carbon composite. When the ash content thereof exceeds the upper
limit of that range, there are cases where battery performance
deterioration caused by reaction with the nonaqueous electrolyte
during charge/discharge becomes not negligible. When the ash
content thereof is lower than the lower limit of that range, there
are cases where the production of this carbonaceous substance
necessitates much time and energy and an apparatus for pollution
prevention, resulting in an increase in cost.
(3) Raman R Value, Raman Half-Value Width
[0717] The Raman R value of the carbonaceous substance (C) as
determined by the argon ion laser Raman spectroscopy is generally
0.01 or higher, preferably 0.03 or higher, more preferably 0.05 or
higher. The upper limit thereof is generally 0.60 or lower,
preferably 0.30 or lower. When the Raman R value thereof is lower
than the lower limit of that range, the surface of such particles
has too high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. On the other hand, when the Raman R value thereof
exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity and enhanced reactivity with
the nonaqueous electrolyte and this may lead to a decrease in
efficiency and enhanced gas evolution.
[0718] The Raman half-value width around 1,580 cm.sup.-1 of the
carbonaceous substance (C) in the invention is not particularly
limited. However, the half-value width thereof is generally 5
cm.sup.-1 or larger, preferably 10 cm.sup.-1 or larger. The upper
limit thereof is generally 60 cm.sup.-1 or smaller, preferably 45
cm.sup.-1 or smaller, more preferably 30 cm.sup.-1 or smaller. When
the Raman half-value width thereof is smaller than the lower limit
of that range, the surface of such particles has too high
crystallinity and there are cases where the number of intercalation
sites into which lithium comes with charge/discharge decreases. On
the other hand, when the Raman half-value width thereof exceeds the
upper limit of that range, the surface of such particles has
reduced crystallinity and enhanced reactivity with the nonaqueous
electrolyte and this may lead to a decrease in efficiency and
enhanced gas evolution.
(4) True Density
[0719] The true density of the carbonaceous substance (C) is
generally 2.0 g/cm.sup.3 or higher, preferably 2.2 g/cm.sup.3 or
higher, more preferably 2.22 g/cm.sup.3 or higher. The upper limit
thereof may be up to 2.26 g/cm.sup.3, which is a theoretical value
for graphites. When the true density of the carbonaceous substance
(C) is lower than the lower limit of that range, there are cases
where the carbon has too low crystallinity, resulting in an
increase in initial irreversible capacity.
[Negative Electrode [5]: Processes for Producing
Orientation-Differing-Carbon Composite]
[0720] Production processes are not particularly limited, and any
technique may be used unless it departs from the spirit of the
invention. However, a preferred example thereof roughly comprises
the following (1) and (2).
(1) A starting material for the carbonaceous substance (C) is
wholly or partly brought into a liquid state in any step, and the
material in this liquid state is mixed and/or kneaded with a
natural graphite(D). (2) The composite obtained in that step is
subjected to volatile removal/heat-treating, graphitized, and then
pulverized and classified to regulate the particle size. Between
these steps, a pulverization/classification step may be conducted
one or more times.
[0721] Specific examples of the process roughly explained above are
shown below.
[[Production Process 1 for Producing Orientation-Differing-Carbon
Composite]]
[0722] The orientation-differing-carbon composite in the invention
preferably comprises a natural-graphite-derived carbonaceous
substance (B) and, combined therewith, a carbonaceous substance (C)
differing in orientation from the natural-graphite-derived
carbonaceous substance (B). Examples of processes for producing it
include a process which comprises mixing the natural graphite (D)
with a precursor for the carbonaceous substance (C), i.e., a pitch
material or a material obtained by heat-treating a pitch material
or another material and pulverizing the heat-treated material
(hereinafter abbreviated to "heat-treated graphite crystal
precursor"), and with a natural polymer or the like in a given
proportion, subjecting the mixture to heat treatment A,
subsequently pulverizing the heat-treated mixture, and then
subjecting the resultant powder to heat treatment B (heat-treating
and graphitization). The pulverization may be omitted when the
heat-treated mixture has a sufficiently small median diameter and
does not necessitate pulverization.
[[[Production of Heat-Treated Graphite Crystal Precursor]]]
[0723] A pitch material is subjected beforehand to a heat treatment
to obtain a heat-treated graphite crystal precursor. This heat
treatment conducted beforehand is called pitch heat treatment. This
heat-treated graphite crystal precursor is pulverized and
subsequently mixed with a natural graphite (D), and this mixture is
subjected to heat treatment A, during which the precursor melts
partly or wholly. By regulating the content of volatiles through
the heat treatment conducted beforehand (pitch heat treatment), the
molten state can be properly controlled. Examples of the volatiles
contained in the heat-treated graphite crystal precursor generally
include hydrogen, benzene, naphthalene, anthracene, and pyrene.
[0724] Temperature conditions for the pitch heat treatment are not
particularly limited. However, the temperature is generally in the
range of from 300.degree. C. to 550.degree. C. When the heat
treatment temperature is lower than the lower limit of that range,
there are cases where volatiles remain in an increased amount and,
hence, safe pulverization in the air is difficult to conduct. On
the other hand, when the temperature exceeds the upper limit, there
are cases where the heat-treated graphite crystal precursor does
not melt partly or wholly during the heat treatment A and particles
comprising a natural-graphite-derived carbonaceous substance (B)
and the heat-treated graphite crystal precursor combined therewith
(orientation-differing-carbon composite) are difficult to obtain.
The pitch heat treatment is conducted in an inert gas atmosphere
such as, e.g., nitrogen gas or in the atmosphere of volatiles
generated from the pitch material.
[0725] Apparatus usable for the pitch heat treatment are not
particularly limited. For example, use can be made of a reaction
vessel such as a shuttle furnace, tunnel kiln, electric furnace, or
autoclave, a coker (heat-treatment vessel for coke production), or
the like. During the pitch heat treatment, stirring may be
conducted according to need.
[0726] It is preferred that the heat-treated graphite crystal
precursor to be used should be one having a volatile content of
generally 5% by mass or higher. When the graphite crystal precursor
having a volatile content in that range is used, the carbonaceous
substance (C) is combined with a natural-graphite-derived
carbonaceous substance (B) by the heat treatment A, whereby an
orientation-differing-carbon composite having the properties
specified above can be obtained.
[0727] First, a process for producing the bulk mesophase which is
the precursor for graphite crystals obtained beforehand by
heat-treating a pitch material (bulk mesophase is a graphite
crystal precursor which has been heat-treated; hereinafter it is
abbreviated to "heat-treated graphite crystal precursor) will be
explained.
(Volatile Content of Heat-Treated Graphite Crystal Precursor)
[0728] The volatile content of the graphite crystal precursor
obtained by the pitch heat treatment is not particularly limited.
However, the volatile content thereof is generally 5% by mass or
higher, preferably 6% by mass or higher, and is generally 20% by
mass or lower, preferably 15% by mass or lower. When the volatile
content thereof is lower than the lower limit of that range, there
are cases where safe pulverization in the air is difficult to
conduct because of the volatiles contained in a large amount. On
the other hand, when the volatile content thereof is higher than
the upper limit, there are cases where the graphite crystal
precursor does not melt partly or wholly during the heat treatment
A and particles comprising a natural-graphite-derived carbonaceous
substance (B) and the heat-treated graphite crystal precursor
combined therewith (orientation-differing-carbon composite) are
difficult to obtain. For determining the content of volatiles, use
is made of, e.g., the method as provided for in JIS M8812.
(Softening Point of Heat-Treated Graphite Crystal Precursor)
[0729] The softening point of the graphite crystal precursor
obtained by the pitch heat treatment is not particularly limited.
However, the softening point thereof is generally 250.degree. C. or
higher, preferably 300.degree. C. or higher, more preferably
370.degree. C. or higher, and is generally 470.degree. C. or lower,
preferably 450.degree. C. or lower, more preferably 430.degree. C.
or lower. When the softening point thereof is lower than the lower
limit, there are cases where a heat treatment of this graphite
crystal precursor results in a low carbonization yield and a
homogeneous mixture with a natural-graphite-derived carbonaceous
substance (B) is difficult to obtain. When the softening point
thereof exceeds the upper limit, there are cases where the graphite
crystal precursor does not melt partly or wholly during the heat
treatment A and particles comprising a natural-graphite-derived
carbonaceous substance (B) and the heat-treated graphite crystal
precursor combined therewith (orientation-differing-carbon
composite) are difficult to obtain.
[0730] The values of softening point used are ones measured by
examining a sample molded in a thickness of 1 mm with a tableting
press, by the penetration method using an apparatus for
thermomechanical analysis (e.g., TMA4000, manufactured by Bruker
AXS K.K.) in a nitrogen stream under the conditions of a heating
rate of 10.degree. C./min, needle tip shape of 1 mm.phi., and load
of 20 gf.
(Pulverization of Heat-Treated Graphite Crystal Precursor)
[0731] Subsequently, the heat-treated graphite crystal precursor
obtained by the pitch heat treatment is pulverized. This
pulverization is conducted for the purpose of pulverizing the
heat-treated graphite crystal precursor crystals which have come
into a half-oriented state in each of large units due to the heat
treatment and/or of evenly mixing and combining the heat-treated
graphite crystal precursor with a natural graphite (D).
[0732] The pulverization of the heat-treated graphite crystal
precursor obtained by the pitch heat treatment is not particularly
limited. However, it is conducted so that the particle size of the
heat-treated graphite crystal precursor is reduced by the
pulverization to a value which is generally 1 .mu.m or larger,
preferably 5 .mu.m or larger, and is generally 10 mm or smaller,
preferably 5 mm or smaller, more preferably 500 .mu.m or smaller,
especially preferably 200 .mu.m or smaller, even more preferably 50
.mu.m or smaller. When the particle size thereof is smaller than 1
.mu.m, there are cases where the heat-treated graphite crystal
precursor is oxidized during or after the pulverization when the
surface of the precursor comes into contact with air. This
oxidation may inhibit crystallinity from improving in the step of
graphitization, leading to a decrease in discharge capacity after
the graphitization. On the other hand, when the particle size
thereof exceeds 10 mm, the size-reducing effect of the
pulverization is lessened and this tends to result in crystal
orientation and the orientation of the carbonaceous substance (C).
There are hence cases where the electrode employing this
orientation-differing carbonaceous composite (A) has a reduced
active-material orientation ratio and the electrode expansion which
occurs upon battery charge is difficult to inhibit. Furthermore
and/or alternatively, there are cases where the increased
difference in particle diameter between the natural graphite (D)
and the heat-treated graphite crystal precursor makes it difficult
to conduct even mixing and this tends to result in a composite
which is not homogeneous.
[0733] Apparatus usable for the pulverization are not particularly
limited. Examples of crushers include shearing mills, jaw crushers,
impact crushers, and cone crushers. Examples of intermediate
pulverizers include roll crushers and hammer mills. Examples of
pulverizers include ball mills, oscillating mills, pin mills,
stirring mills, jet mills, and turbo mills.
[[[Heat Treatment of Natural Graphite (D) and Heat-Treated Graphite
Crystal Precursor]]]
[0734] A natural graphite (D) is mixed with the heat-treated
graphite crystal precursor (raw material for a carbonaceous
substance (C)) in a given proportion. This mixture is subjected to
heat treatment A, pulverization, and heat treatment B
(heat-treating and graphitization) to thereby produce an
orientation-differing-carbon composite.
(Mixing of Natural Graphite (D) and Heat-Treated Graphite Crystal
Precursor)
[0735] The mixing of the natural graphite (D) with the heat-treated
graphite crystal precursor, which is conducted before heat
treatment A, is not particularly limited in mixing ratio. However,
the two ingredients are mixed together so that the proportion of
the natural graphite (D) to the mixture is generally 20% by mass or
higher, preferably 30% by mass or higher, more preferably 40% by
mass or higher, and is generally 95% by mass or lower, preferably
90% by mass or lower. When the proportion thereof is lower than the
lower limit, the proportion of the carbonaceous substance (C) in
the orientation-differing carbonaceous composite (A) increases.
There are hence cases where use of this carbon composite (A) in
electrode formation is less apt to result in a high loading density
and necessitates an excessively high pressing load. Namely, the
effect to be produced by combining the natural-graphite-derived
carbonaceous substance (B) is difficult to obtain. When the
proportion of the natural graphite (D) exceeds the upper limit, the
natural-graphite-derived carbonaceous substance (B) in the
orientation-differing carbonaceous composite (A) comes to have an
increased amount of exposed surfaces and this may result in an
increase in the specific surface area of the orientation-differing
carbonaceous composite (A). Namely, there are cases where such
carbon composite (A) is undesirable from the standpoint of powder
properties.
[0736] Apparatus usable for mixing the natural graphite (D) with
the heat-treated graphite crystal precursor regulated so as to have
a given particle size are not particularly limited. Examples
thereof include V-shaped mixers, W-shaped mixers, change-can
mixers, kneaders, drum mixers, and shearing mixers.
(Heat Treatment A)
[0737] Subsequently, the mixture of the natural graphite (D) and
the heat-treated graphite crystal precursor is subjected to heat
treatment A. This heat treatment is conducted for the purpose of
re-melting or fusing the pulverized heat-treated graphite crystal
precursor to thereby fix the natural graphite (D) and the fine
particles of the heat-treated graphite crystal precursor while
keeping the two ingredients in contact with each other in an
unoriented state. Thus, the mixture of the natural graphite (D) and
the heat-treated graphite crystal precursor can be converted to not
a mere mixture of particles but a more homogeneous composite
mixture (hereinafter suitably referred to as "graphite composite
mixture").
[0738] Temperature conditions for the heat treatment A are not
particularly limited. However, the temperature is generally
300.degree. C. or higher, preferably 400.degree. C. or higher, more
preferably 450.degree. C. or higher, and is generally 650.degree.
C. or lower, preferably 600.degree. C. or lower. When the
temperature in heat treatment A is lower than the lower limit of
that range, a large amount of volatiles remain in the material
after heat treatment A and, hence, there is a possibility that the
powder particles might be fused to each other during the step of
heat-treating or graphitization. Namely, there are cases where
re-pulverization becomes necessary. On the other hand, when the
temperature exceeds the upper limit of that range, there are cases
where the ingredient which was re-melted is broken into an acicular
shape during pulverization, resulting in a decrease in tap density.
Heat treatment A is conducted in an inert gas atmosphere, e.g.,
nitrogen gas, or in the atmosphere of volatiles generated from the
heat-treated graphite crystal precursor which has been reduced into
fine particles by pulverization.
[0739] Apparatus usable for heat treatment A are not particularly
limited. For example, a shuttle furnace, tunnel kiln, electric
furnace, or the like can be used.
(Treatment as Substitute for Pulverization of Heat-Treated Graphite
Crystal Precursor and for Heat Treatment A)
[0740] Incidentally, a treatment as a substitute for the
pulverization and heat treatment A described above can be
conducted. Namely, it is possible to mix the heat-treated graphite
crystal precursor with a natural graphite (D) and heat-treat the
mixture while conducting a treatment in which the structure of the
graphite crystal precursor can be reduced into fine particles in an
unoriented state, e.g., a treatment in which mechanical energy is
applied at a temperature in a range where the heat-treated graphite
crystal precursor melts or softens.
[0741] This heat treatment as a substitute is not particularly
limited. However, it is conducted at a temperature which is
generally 200.degree. C. or higher, preferably 250.degree. C. or
higher, and is generally 450.degree. C. or lower, preferably
400.degree. C. or lower. When the temperature conditions are lower
than the lower limit of that range, there are cases where the
melting or softening of the graphite crystal precursor during this
substitute treatment is insufficient and this precursor is
difficult to combine with the natural graphite (D). When the
temperature exceeds the upper limit, the heat treatment is apt to
proceed rapidly and there are hence cases where the particles of
the carbonaceous heat-treated graphite crystal precursor, etc. are
broken into an acicular shape during pulverization, resulting in a
decrease in tap density.
[0742] This substitute treatment is generally conducted in an inert
atmosphere, e.g., nitrogen gas, or in an oxidizing atmosphere,
e.g., air. It should, however, be noted that when the treatment is
conducted in an oxidizing atmosphere, there are cases where high
crystallinity is difficult to obtain after graphitization. It is
therefore necessary to prevent oxygen from excessively making the
precursor infusible. Specifically, the substitute treatment is
conducted so that the graphite crystal precursor after the
treatment has an oxygen content of generally 8% by mass or lower,
preferably 5% by mass or lower.
[0743] Apparatus usable for the substitute treatment are not
particularly limited. However, a mixer, kneader, or the like can be
used.
(Pulverization)
[0744] Subsequently, the graphite composite mixture which has
undergone heat treatment A is pulverized. This graphite composite
mixture is in the form of aggregates formed through heat treatment
A by combining the finely powdered structure in an unoriented state
with the natural graphite (D) through melting or fusion. This
pulverization is conducted for the purpose of pulverizing the
aggregates to a target particle diameter.
[0745] The particle size of the graphite composite mixture after
the pulverization is not particularly limited. However, it is
generally 1 .mu.m or larger, preferably 3 .mu.m or larger, more
preferably 5 .mu.m or larger, even more preferably 7 .mu.m or
larger, and is generally 50 .mu.m or smaller, preferably 35 .mu.m
or smaller, more preferably 30 .mu.m or smaller. In case where the
particle size thereof is smaller than the lower limit of that
range, the resultant orientation-differing carbonaceous composite
(A) has a reduced tap density and gives an electrode less apt to
have an increased active-material loading density. Namely, a
high-capacity battery is difficult to obtain. On the other hand,
when the particle size thereof exceeds the upper limit of that
range, there are cases where the resultant orientation-differing
carbonaceous composite (A) is apt to cause coating unevenness when
used in producing an electrode through coating fluid
application.
[0746] Apparatus usable for the pulverization are not particularly
limited. Examples of crushers include jaw crushers, impact
crushers, and cone crushers. Examples of intermediate pulverizers
include roll crushers and hammer mills. Examples of pulverizers
include ball mills, oscillating mills, pin mills, stirring mills,
and jet mills.
(Heat Treatment B: Burning)
[0747] Heat treatment B means heat-treating and graphitization. The
heat-treating is first explained. However, this heat-treating may
be omitted. The graphite composite mixture which has been reduced
into fine particles by the pulverization is heat-treated. This
heat-treating is conducted for the purpose of removing volatiles
from the graphite composite mixture in order to control the fusion
of the graphite composite mixture in graphitization.
[0748] Temperature conditions in the heat-treating are not
particularly limited. However, the temperature is generally
600.degree. C. or higher, preferably 1,000.degree. C. or higher.
The upper limit thereof is generally 2,400.degree. C. or lower,
preferably 1,300.degree. C. or lower. When the temperature
conditions are lower than the lower limit of that range, there are
cases where the powder of the graphite composite mixture is apt to
fuse during graphitization. On the other hand, in case where the
temperature exceeds the upper limit of that range, the
heat-treating apparatus becomes costly. Consequently, the
heat-treating is conducted usually under temperature conditions
within that range.
[0749] The heat-treating is conducted in an inert gas atmosphere,
e.g., nitrogen gas, or in a non-oxidizing atmosphere comprising
gases generated from the graphite composite mixture which has been
re-pulverized. For the purpose of simplifying production steps,
graphitization may be directly conducted without via a
heat-treating step.
[0750] Apparatus usable for the heat-treating are not particularly
limited. For example, use can be made of a shuttle furnace, tunnel
kiln, electric furnace, Riedhammer furnace, rotary kiln, or the
like.
(Heat Treatment B: Graphitization)
[0751] Subsequently, the graphite composite mixture which has been
heat-treated is graphitized. This graphitization is conducted for
the purpose of improving crystallinity in order to increase
discharge capacity in battery evaluation. Through the
graphitization, an orientation-differing carbonaceous composite (A)
can be obtained.
[0752] Temperature conditions in the graphitization are not
particularly limited. However, the temperature is generally
2,800.degree. C. or higher, preferably 2,900.degree. C. or higher,
more preferably 3,000.degree. C. or higher, and is generally
3,400.degree. C. or lower, preferably 3,200.degree. C. or lower.
When the temperature exceeds the upper limit of that range, there
are cases where a battery having a reduced reversible capacity
results. Namely, there are cases where a high-capacity battery is
difficult to produce. In addition, temperatures higher than the
upper limit of that range may apt to result in enhanced graphite
sublimation.
[0753] The graphitization is conducted in an inert gas atmosphere,
e.g., argon gas, or in a non-oxidizing atmosphere comprising gases
generated from the graphite composite mixture which has been
heat-treated. Apparatus usable for the graphitization are not
particularly limited. Examples thereof include direct voltage
application type electric furnaces, Acheson furnaces, and indirect
voltage application type furnaces such as resistance heating
furnaces and induction heating furnaces.
[0754] Incidentally, a graphitization catalyst such as, e.g., Si or
B may be added to inner parts or the surface of a material (the
natural graphite (D), pitch material, or graphite crystal
precursor) during the graphitization or in any of the steps before
the graphitization, i.e., the steps ranging from the heat treatment
to the heat-treating.
(Other Treatments)
[0755] Besides the treatments described above, other various
treatments including classification can be conducted unless such
treatments lessen the effects of the invention. Classification is a
treatment for removing coarse particles and fine particles in order
to regulate the particle size after the graphitization treatment to
a target particle diameter.
[0756] Apparatus usable for the classification are not particularly
limited. However, in the case of dry sieving, use can be made of a
rotary sieve, swing sieve, revolving sieve, vibrating sieve, or the
like. In the case of dry air classification, use can be made of a
gravitational classifier, inertial classifier, or centrifugal
classifier (e.g., classifier or cyclone). Furthermore, for wet
sieving, use can be made of a mechanical wet classifier, hydraulic
classifier, sedimentation classifier, centrifugal wet classifier,
or the like.
[0757] Classification may be conducted immediately after the
pulverization after heat treatment A, or may be conducted in
another timing. For example, it may be conducted after the
heat-treating after the pulverization or conducted after the
graphitization. It is also possible to omit the classification
treatment itself. However, from the standpoints of reducing the BET
specific surface area of the orientation-differing carbonaceous
composite (A) and of productivity, it is preferred to conduct a
classification treatment immediately after the pulverization after
heat treatment A.
(Treatment after Production of Orientation-Differing Carbonaceous
Composite (A))
[0758] An artificial-graphite powder separately produced or a
natural-graphite powder may be further mixed with the
orientation-differing carbonaceous composite (A) produced by the
procedure described above, for the purposes of regulating the BET
specific surface area of the negative-electrode material, improving
suitability for electrode pressing, improving discharge capacity,
cost reduction, etc.
[Negative Electrode [5]: Production Process 2 for
Orientation-Differing-Carbon Composite]
[0759] The orientation-differing-carbon composite may be produced
also by the following process. The orientation-differing-carbon
composite in the invention preferably comprises a
natural-graphite-derived carbonaceous substance (B) and, combined
therewith, a carbonaceous substance (C) differing in orientation
from the natural-graphite-derived carbonaceous substance (B).
Examples of processes for producing it include a process in which
the natural graphite (D) and a pitch material as a precursor for
the carbonaceous substance (C) are subjected to the steps of
"kneading (mixing)", "molding", "heat-treating", "graphitization",
and "pulverization". Of these steps, "molding", "heat-treating",
and "pulverization" may be omitted and/or conducted simultaneously
with another step. Specifically, the carbon composite can be
obtained, for example, by the following production process.
[[Kneading (Mixing)]]
[0760] Raw materials comprising a natural graphite (D), a pitch
material, and optional ingredients, e.g., a graphitization
catalyst, are mixed together. In this operation, it is preferred to
conduct heating for even mixing. Thus, a mixture is obtained in
which the liquid pitch material is adherent to the natural graphite
(D) and the raw material which does not melt at the kneading
temperature. In this case, use may be made of a method in which all
raw materials are charged into a kneading machine and are kneaded
and heated simultaneously. Alternatively, use may be made of a
method in which the ingredients other than the pitch material are
charged into a kneading machine and heated with stirring and the
pitch material having ordinary temperature or in a vulcanized and
molten state is charged after the contents have been heated to a
kneading temperature.
[0761] The heating temperature for the kneading is generally a
temperature not lower than the softening point of the pitch
material, preferably a temperature higher than the softening point
by 10.degree. C. or more, more preferably a temperature higher than
the softening point by 20.degree. C. or more. The upper limit
thereof is generally 300.degree. C. or lower, preferably
250.degree. C. or lower. When the temperature is lower than the
lower limit of that range, there are cases where the pitch material
has an increased viscosity to make the mixing difficult. On the
other hand, when the temperature exceeds the upper limit of that
range, there are cases where the system being mixed comes to have
too high a viscosity due to volatilization and
polycondensation.
[0762] A mixing machine of the kind having stirring blades is
preferred. As the stirring blades, general ones can be used, such
as the Z type or the matiscator type. The amount of the raw
materials to be charged into the mixing machine generally is 10% by
volume or larger, preferably 15% by volume or larger, and is 50% by
volume or smaller, preferably 30% by volume or smaller, based on
the capacity of the mixing machine. A mixing period of 5 minutes or
longer is necessary. The mixing period is up to the time when the
volatilization of volatile ingredients results in a large viscosity
change. Usually, the mixing period is 30-120 minutes. It is
preferred that the mixing machine be preheated to the kneading
temperature before the mixing.
[[Molding]]
[0763] The mixture obtained may be subjected as it is to the step
of volatile removal/heat-treating, which is conducted for the
removal of volatile ingredients and carbonization. It is, however,
preferred to mold the mixture in order to facilitate handling
before it is subjected to the step of volatile
removal/heat-treating.
[0764] Molding methods are not particularly limited as long as the
resultant shape can be maintained. Use can be made of extrusion
molding, molding with a mold, hydraulic molding, or the like. Of
these molding techniques, extrusion molding is apt to give molded
objects in which the particles are oriented, while hydraulic
molding is inferior in productivity although the particles are in a
randomly oriented state. Molding with a mold is preferable to these
two techniques because it has advantages that the operation is
relatively easy and a molded object can be obtained without
breaking the structure in which the particles are in a randomly
oriented state due to the mixing.
[0765] The molding may be conducted either at room temperature
(cold molding) or with heating (hot molding; temperature not lower
than the softening point of the pitch material). In the case of
cold molding, it is desirable that the mixture which has been
cooled after the kneading should be crushed beforehand to a maximum
size of 1 mm or smaller in order to improve moldability and obtain
molded-object evenness. The shape and size of the molded object are
not particularly limited. In the case of hot molding, however,
formation of too large a molding necessitates much time in
conducting even preheating prior to the molding. Consequently, the
size thereof is generally preferably up to about 150 cm in terms of
maximum dimension.
[0766] With respect to molding pressure, too high a pressure makes
it difficult to remove volatile ingredients through pores of the
resultant molded object and orients particles of the natural
graphite (D) which are not completely round, making it difficult to
conduct pulverization in a later step. Consequently, the upper
limit of the molding pressure is generally 3,000 kgf/cm.sup.2 (294
MPa) or lower, preferably 500 kgf/cm.sup.2 (49 MPa) or lower, more
preferably 10 kgf/cm.sup.2 (0.98 MPa) or lower. Although the lower
limit of the pressure is not particularly limited, it is preferably
regulated to a value which enables the molded object to retain its
shape in the step of volatile removal/heat-treating.
[[Volatile Removal/Burning]]
[0767] The molded object obtained is subjected to the step of
volatile removal/heat-treating for the purpose of removing volatile
ingredients from the natural graphite (D) and pitch material to
thereby prevent the volatile ingredients from fouling a packing and
the molded object from adhering to the packing during
graphitization. The volatile removal/heat-treating is conducted at
a temperature which is generally 600.degree. C. or higher,
preferably 650.degree. C. or higher, and is generally 1,300.degree.
C. or lower, preferably 1,100.degree. C. or lower, for a period of
generally 0.1-10 hours. For the purpose of preventing oxidation,
the heating is conducted generally in the stream of an inert gas,
e.g., nitrogen or argon, or in a non-oxidizing atmosphere formed by
packing a particulate carbon material such as, e.g., a breeze or
packing coke in spaces.
[0768] Apparatus usable for the volatile removal/heat-treating are
not particularly limited as long as they can be used for
heat-treating in a non-oxidizing atmosphere, like electric
furnaces, gas furnaces, Riedhammer furnaces for electrode
materials, and the like. The heating rate in the heating desirably
is low from the standpoint of removing volatile ingredients. In
general, heating from about 200.degree. C., at which low-boiling
ingredients begin to volatilize, to about 700.degree. C., at which
hydrogen becomes the only substance which generates, is conducted
at 3-100.degree. C./hr.
[[Graphitization]]
[0769] The carbonized molding obtained by the volatile
removal/heat-treating is subsequently graphitized by
high-temperature heating. Conditions for this graphitization are
the same as described above under Production Process 1.
[0770] With respect to atmospheres for the graphitization, it is
conducted in the stream of an inert gas, e.g., nitrogen or argon,
or in a non-oxidizing atmosphere formed by packing a particulate
carbon material such as, e.g., a breeze or packing coke in spaces,
in order to prevent oxidation. Apparatus usable for the
graphitization are not particularly limited as long as they meet
the purpose. For example, an electric furnace, gas furnace, Acheson
furnace, or the like can be used. Conditions including heating
rate, cooling rate, and heat treatment period can be determined at
will within the ranges allowable for the apparatus.
[[Pulverization]]
[0771] The graphitization product thus obtained is usually in the
form of aggregates. This product, without any treatment, is usually
difficult to use as a negative-electrode active material.
Consequently, pulverization and/or removal of large particles and
small particles is conducted. Methods for pulverizing the
graphitization product are not particularly limited. Examples of
pulverization devices include devices for mechanical grinding, such
as, e.g., ball mills, hammer mills, CF mills, atomizer mills, and
pulverizers, and pulverization devices utilizing wind force, such
as, e.g., jet mills. For crushing and intermediate pulverization,
use may be made of an impact type crushing device, such as a jaw
crusher, hammer mill, or roller mill. The timing of pulverization
may be before the graphitization or after the graphitization.
[Negative Electrode [5]: Minor-Material Mixing]
[0772] Besides the orientation-differing-carbon composite, one or
more carbonaceous substances (carbonaceous materials) differing in
carbonaceous-substance property from the carbon composite may be
incorporated into the negative-electrode active material for use in
a lithium secondary battery of the invention. Thus, battery
performances can be further improved. The term
"carbonaceous-substance property" herein means one or more
properties selected from X-ray diffraction parameter, median
diameter, aspect ratio, BET specific surface area, orientation
ratio, Raman R value, tap density, true density, pore distribution,
roundness, and ash content. Preferred embodiments include: one in
which the volume-based particle size distribution is not
symmetrical about the median diameter; one in which two or more
carbon materials differing in Raman R value are contained; and one
in which carbon materials differing in X-ray parameter are
contained. One example of the effects thereof is that the
incorporation of a carbon material, such as a graphite, e.g., a
natural graphite or artificial graphite, or an amorphous carbon,
e.g., a carbon black such as acetylene black or needle coke, as a
sub material serves to reduce electrical resistance. One of these
carbon materials may be used alone, or any desired two or more
thereof may be used in combination in any desired proportion. In
the case where such a carbon material is added as a sub material,
the amount thereof may be 0.1% by mass or larger and is preferably
0.5% by mass or larger, more preferably 0.6% by mass or larger. The
upper limit thereof may be 80% by mass or smaller and is preferably
50% by mass or smaller, more preferably 40% by mass or smaller,
even more preferably 30% by mass or smaller. When the amount
thereof is smaller than the lower limit of that range, there are
cases where the effect of improving conductivity is difficult to
obtain. When the amount thereof exceeds the upper limit, there are
cases where this leads to an increase in initial irreversible
capacity.
[Negative Electrode [5]: Electrode Production]
[0773] This negative electrode may be produced by an ordinary
method. It can be formed in the same manner as described above. The
current collector, thickness ratio between the current collector
and the active-material layer, electrode density, binder,
electrode-plate orientation ratio, impedance, etc. also are the
same as described above.
<Negative Electrode [6]>
[0774] An explanation will be given below on negative electrode [6]
for use in a lithium secondary battery of the invention, i.e., "a
negative electrode containing, as a negative-electrode active
material, graphitic carbon particles which have a roundness of 0.85
or higher and an interplanar spacing (d002) for the (002) planes as
determined by wide-angle X-ray diffractometry of smaller than 0.337
nm and which, in an examination by argon ion laser Raman
spectroscopy, have a Raman R value defined as the ratio of the peak
intensity at 1,360 cm.sup.-1 to the peak intensity at 1,580
cm.sup.-1 of from 0.12 to 0.8".
[Negative Electrode [6]: Negative-Electrode Active Material]
[0775] The negative-electrode active material for use in negative
electrode [6] is described below.
[0776] The negative-electrode active material for use in negative
electrode [6] in a lithium secondary battery of the invention
comprises graphitic carbon particles satisfying at least the
following (a), (b), and (c).
(a) The roundness is 0.85 or higher. (b) The interplanar spacing
(d002) for the (002) planes as determined by wide-angle X-ray
diffractometry is smaller than 0.337 nm. (c) The Raman R value
defined as the ratio of the peak intensity at 1,360 cm.sup.-1 to
the peak intensity at 1,580 cm.sup.-1 in an examination by argon
ion laser Raman spectroscopy (hereinafter often referred to simply
as "Raman R value") is from 0.12 to 0.8.
[[Roundness]]
[0777] The roundness of the graphitic carbon particles for use as a
negative-electrode active material in a lithium secondary battery
of the invention is generally 0.85 or higher, preferably 0.87 or
higher, more preferably 0.89 or higher, especially preferably 0.92
or higher. With respect to the upper limit thereof, a particle
having a roundness of 1 theoretically is a true sphere. In case
where the roundness thereof is lower than the lower limit of that
range, use of this negative-electrode active material results in a
reduced loading and thermal conductivity decreases. There are hence
cases where a quick output recovery is inhibited. In particular,
there are cases where an output recovery from a low-output state at
low temperatures is slow.
[0778] In the invention, roundness is defined by the following
equation.
Roundness=(length of periphery of equivalent circle having the same
area as projected particle shape)/(actual length of periphery of
projected particle shape)
[0779] The value of roundness is determined, for example, with a
flow type particle image analyzer (e.g., FPIA, manufactured by
Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a
0.2% by mass aqueous solution (about 50 mL) of
poly(oxyethylene(20)) sorbitan monolaurate as a surfactant,
propagating an ultrasonic wave of 28 kHz to the dispersion for 1
minute at an output of 60 W, and then examining particles having a
particle diameter in the range of 3-40 .mu.m with the analyzer
having a detection range set at 0.6-400 .mu.m.
[[Interplanar Spacing (d002)]]
[0780] The interplanar spacing (d002) for the (002) planes of the
graphitic carbon particles for use as the negative-electrode active
material of negative electrode [6] in a lithium secondary battery
of the invention, as determined by wide-angle X-ray diffractometry,
is smaller than 0.337 nm, and is preferably 0.336 nm or smaller.
The lower limit thereof is 0.335, which is a theoretical value for
graphites. In case where the interplanar spacing (d002) thereof
exceeds the upper limit of that range, crystallinity decreases and
heat conduction by electrons decreases. There are hence cases where
the property of quickly recovering output decreases. In particular,
there are cases where an output recovery from a low-output state at
low temperatures is slow.
[0781] The term "interplanar spacing (d002) for the (002) planes as
determined by wide-angle X-ray diffractometry" in the invention
means the value of d (interplanar spacing) for the lattice planes
((002) planes) as determined by X-ray diffractometry by the method
of the Japan Society for Promotion of Scientific Research.
[0782] The crystallite size (Lc) of the graphitic carbon particles
as determined by X-ray diffractometry by the method of the Japan
Society for Promotion of Scientific Research is not particularly
limited. However, the Lc thereof is generally 10 nm or larger,
preferably 30 nm or larger, more preferably 80 nm or larger. In
case where the Lc thereof is smaller than the lower limit of that
range, crystallinity decreases and heat conduction by electrons
decreases. There are hence cases where the property of quickly
recovering output decreases.
[[Raman R Value]]
[0783] The Raman R value of the graphitic carbon particles for use
as the negative-electrode active material of negative electrode [6]
in a lithium secondary battery of the invention should be 0.12 or
higher, and is preferably 0.15 or higher, more preferably 0.17 or
higher, especially preferably 0.2 or higher. The upper limit
thereof is preferably 0.8 or lower, more preferably 0.6 or lower,
especially preferably 0.45 or lower. In case where the Raman R
value thereof is lower than the lower limit of that range, the
surface of such particles has too high crystallinity and the number
of charge/discharge sites decreases. There are hence cases where
output itself decreases accordingly. On the other hand, in case
where the Raman R value thereof exceeds the upper limit of that
range, the surface of such particles has reduced crystallinity.
There are hence cases where heat conduction by electrons decreases
and the property of recovering output decreases.
[0784] The examination for a Raman spectrum is made with a Raman
spectrometer (e.g., a Raman spectrometer manufactured by Japan
Spectroscopic Co., Ltd.). In the examination, a sample is charged
into a measuring cell by causing the sample to fall naturally into
the cell and the surface of the sample in the cell is irradiated
with argon ion laser light while rotating the cell in a plane
perpendicular to the laser light. The Raman spectrum obtained is
examined for the intensity I.sub.A of a peak P.sub.A at 1,580
cm.sup.-1 and the intensity I.sub.B of a peak P.sub.B at 1,360
cm.sup.-1. The ratio between these intensities R
(R=I.sub.B/I.sub.A) is calculated and this ratio is defined as the
Raman R value of the graphitic carbon particles. Furthermore, the
half-value width of the peak P.sub.A at 1,580 cm.sup.-1 in the
Raman spectrum obtained is measured, and this value is defined as
the Raman half-value width of the graphitic carbon particles.
[0785] Conditions for this Raman spectroscopy are as follows.
[0786] Wavelength of argon ion laser: 514.5 nm [0787] Laser power
on sample: 15-25 mW [0788] Resolution: 10-20 cm.sup.-1 [0789]
Examination range: 1,100 cm.sup.-1 to 1,730 cm.sup.-1 [0790]
Analysis for Raman R value and Raman half-value width: background
processing [0791] Smoothing: simple average; convolution, 5
points
[0792] The Raman half-value width at 1,580 cm.sup.-1 of the
graphitic carbon particles for use as the negative-electrode active
material of negative electrode [6] in a lithium secondary battery
of the invention is not particularly limited. However, the
half-value width thereof is generally 10 cm.sup.-1 or larger,
preferably 15 cm.sup.-1 or larger. The upper limit thereof is
generally 60 cm.sup.-1 or smaller, preferably 50 cm.sup.-1 or
smaller, more preferably 45 cm.sup.-1 or smaller. In case where the
Raman half-value width thereof is smaller than the lower limit of
that range, the surface of such particles has too high
crystallinity and the number of charge/discharge sites decreases.
There are hence cases where output itself decreases accordingly. On
the other hand, in case where the Raman half-value width thereof
exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity. There are hence cases where
heat conduction by electrons decreases and the property of
recovering output decreases.
[[Tap Density]]
[0793] The tap density of the graphitic carbon particles for use as
the negative-electrode active material of negative electrode [6] in
a lithium secondary battery of the invention is preferably 0.55
g/cm.sup.3 or higher, more preferably 0.7 g/cm.sup.3 or higher,
even more preferably 0.8 g/cm.sup.3 or higher, especially
preferably 1 g/cm.sup.3 or higher. The upper limit thereof is
preferably 2 g/cm.sup.3 or lower, more preferably 1.8 g/cm.sup.3 or
lower, especially preferably 1.6 g/cm.sup.3 or lower. In case where
the tap density thereof is lower than the lower limit of that
range, this graphitic carbon, when used in a negative electrode, is
less apt to have a high loading density and has a reduced
interparticle contact area. There are hence cases where thermal
conductivity decreases. On the other hand, in case where the tap
density thereof exceeds the upper limit of that range, the
electrode has too small an amount of interparticle interstices and
a reduced amount of channels for the nonaqueous electrolyte. There
are hence cases where output itself decreases.
[0794] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the cell with the
sample up to the brim, subsequently conducting tapping operations
1,000 times over a stroke length of 10 mm using a powder densimeter
(e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.),
and determining a density from the resultant volume of the sample
and the weight thereof. This density is defined as the tap
density.
[[BET Specific Surface Area]]
[0795] The specific surface area of the graphitic carbon particles
for use as the negative-electrode active material of negative
electrode [6] in a lithium secondary battery of the invention, as
determined by the BET method, is preferably 0.1 m.sup.2/g or
larger, more preferably 0.7 m.sup.2/g or larger, especially
preferably 1 m.sup.2/g or larger, even more preferably 1.5
m.sup.2/g or larger. The upper limit thereof is preferably 100
m.sup.2/g or smaller, more preferably 50 m.sup.2/g or smaller,
especially preferably 25 m.sup.2/g or smaller, even more preferably
15 m.sup.2/g or smaller. When the BET specific surface area thereof
is smaller than the lower limit of that range, there are cases
where use of this graphitic carbon as a negative-electrode material
is apt to result in poor lithium acceptance during charge and in
lithium deposition on the electrode surface. On the other hand,
when the specific surface area thereof exceeds the upper limit of
that range, there are cases where use of this graphitic carbon as a
negative-electrode material is apt to result in enhanced reactivity
with the nonaqueous electrolyte and enhanced gas evolution and a
preferred battery is difficult to obtain.
[0796] BET specific surface area is defined as a value measured
with a surface area meter (e.g., a fully automatic surface area
measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by
preliminarily drying a sample at 350.degree. C. for 15 minutes in a
nitrogen stream and then measuring the specific surface area
thereof by the gas-flowing nitrogen adsorption BET one-point method
using a nitrogen/helium mixture gas precisely regulated so as to
have a nitrogen pressure of 0.3 relative to atmospheric
pressure.
[[Volume-Average Particle Diameter]]
[0797] The volume-average particle diameter of the graphitic carbon
particles for use as the negative-electrode active material of
negative electrode [6] in a lithium secondary battery of the
invention, which is defined as the volume-average particle diameter
(median diameter) determined by the laser diffraction/scattering
method, is preferably 1 .mu.m or larger, more preferably 3 .mu.m or
larger, especially preferably 5 .mu.m or larger, even more
preferably 7 .mu.m or larger. The upper limit thereof is generally
50 .mu.m or smaller, preferably 40 .mu.m or smaller, more
preferably 30 .mu.m or smaller, especially preferably 25 .mu.m or
smaller. When the volume-average particle diameter thereof is
smaller than the lower limit of that range, there are cases where
irreversible capacity increases, leading to a loss in initial
battery capacity. When the volume-average particle diameter thereof
exceeds the upper limit of that range, there are cases where such
graphitic carbon particles are undesirable from the standpoint of
battery production because an uneven coating surface is apt to
result when an electrode plate is produced.
[[Pore Volume]]
[0798] With respect to the pore volume of the graphitic carbon
particles for use as the negative-electrode active material of
negative electrode [6] in a lithium secondary battery of the
invention, the amount of interstices corresponding to a pore
diameter of from 0.01 .mu.m to 1 .mu.m which include pores in the
particles and particle surface irregularities formed by steps, as
determined by mercury porosimetry (mercury intrusion method), is
generally 0.01 mL/g or larger, preferably 0.05 mL/g or larger, more
preferably 0.1 mL/g or larger. The upper limit thereof is generally
0.6 mL/g or smaller, preferably 0.4 mL/g or smaller, more
preferably 0.3 mL/g or smaller. When the amount of interstices
therein is larger than the upper limit of that range, there are
cases where a large amount of a binder is necessary in electrode
plate formation. When the amount thereof is smaller than the lower
limit, there are cases where high-current-density charge/discharge
characteristics decrease and the effect of diminishing electrode
expansion/contraction during charge/discharge is not obtained.
[0799] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger. The upper limit thereof is
generally 10 mL/g or smaller, preferably 5 mL/g or smaller, more
preferably 2 mL/g or smaller. When the total pore volume exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary in electrode plate formation. When the
total pore volume is smaller than the lower limit, there are cases
where the dispersing effect of a thickener or binder in electrode
plate formation is not obtained.
[0800] The average pore diameter of the graphitic carbon particles
is preferably 0.05 .mu.m or larger, more preferably 0.1 .mu.m or
larger, even more preferably 0.5 .mu.m or larger. The upper limit
thereof may be 50 .mu.m or smaller and is preferably 20 .mu.m or
smaller, more preferably 10 .mu.m or smaller. When the average pore
diameter thereof exceeds the upper limit of that range, there are
cases where a large amount of a binder is necessary. When the
average pore diameter thereof is smaller than the lower limit,
there are cases where high-current-density charge/discharge
characteristics decrease.
[0801] As an apparatus for the mercury porosimetry was used a
mercury porosimeter (Autopore 9520, manufactured by Micromeritics
Corp.). About 0.2 g of a sample (negative-electrode material) was
weighed out and placed in a powder cell and this cell was closed.
The sample in the cell was pretreated by degassing it at room
temperature under vacuum (50 .mu.mHg or lower) for 10 minutes.
Subsequently, the pressure in the cell was reduced to 4 psia (about
28 kPa) and mercury was introduced thereinto. The internal pressure
was stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia
(about 280 MPa) and then lowered to 25 psia (about 170 kPa). The
number of steps in the pressure elevation was 80 or larger. In each
step, the amount of mercury intruded was measured after an
equilibrium time of 10 seconds. A pore distribution was calculated
from the mercury intrusion curve thus obtained, using the Washburn
equation. For the calculation, the surface tension (y) and contact
angle (y) of mercury were taken as 485 dyne/cm and 140.degree.,
respectively. The average pore diameter used was the pore diameter
corresponding to a cumulative pore volume of 50%.
[[Ash Content]]
[0802] The ash content of the graphitic carbon particles for use as
the negative-electrode active material of negative electrode [6] in
a lithium secondary battery of the invention is preferably 1% by
mass or lower, especially preferably 0.5% by mass or lower, more
preferably 0.1% by mass or lower, based on the whole graphitic
carbon particles. The lower limit thereof is preferably 1 ppm or
higher. When the ash content thereof exceeds the upper limit of
that range, there are cases where battery performance deterioration
caused by reaction with the nonaqueous electrolyte during
charge/discharge becomes not negligible. On the other hand, when
the ash content thereof is lower than the lower limit of that
range, there are cases where the production of this graphitic
carbon necessitates much time and energy and an apparatus for
pollution prevention, resulting in an increase in cost.
[0803] The true density of the graphitic carbon particles is
generally 2.0 g/cm.sup.3 or higher, preferably 2.1 g/cm.sup.3 or
higher, more preferably 2.2 g/cm.sup.3 or higher, even more
preferably 2.22 g/cm.sup.3 or higher. The upper limit thereof may
be 2.26 g/cm.sup.3 or lower. This upper limit is a theoretical
value for qraphites. When the true density thereof is lower than
the lower limit of that range, there are cases where this carbon
has too low crystallinity, resulting man increase in initial
irreversible capacity. In the invention, true density is defined as
one determined by the liquid-phase displacement method (pycnometer
method) using butanol.
[[Orientation Ratio]]
[0804] The orientation ratio of the graphitic carbon particles for
use as the negative-electrode active material of negative electrode
[6] in a lithium secondary battery of the invention is generally
0.005 or higher, preferably 0.01 or higher, more preferably 0.015
or higher. The upper limit thereof is theoretically 0.67 or lower.
When the orientation ratio thereof is lower than the lower limit of
that range, there are cases where high-density charge/discharge
characteristics decrease.
[0805] Orientation ratio is determined by X-ray diffractometry. The
peaks attributable to the (110) diffraction and (004) diffraction
of the carbon obtained by X-ray diffractometry are subjected to
peak separation by fitting with asymmetric Pearson VII as a profile
function. Thus, the integrated intensities of the (110) diffraction
and (004) diffraction peaks are calculated. From the integrated
intensities obtained, the ratio represented by (integrated
intensity of (110) diffraction)/(integrated intensity of (004)
diffraction) is calculated. This ratio is defined as the
active-material orientation ratio.
[0806] Conditions for this X-ray diffractometry are as follows.
Incidentally, "2.theta." represents diffraction angle. [0807]
Target: Cu(K.alpha. line) graphite monochromator [0808] Slit [0809]
Divergence slit=1 degree [0810] Receiving slit=0.1 mm [0811]
Scattering slit=1 degree [0812] Examination range and step
angle/measuring time: [0813] (110) plane:
76.5.degree..ltoreq.2.theta..ltoreq.78.5.degree. 0.01.degree./3 sec
[0814] (004) plane:
53.5.degree..ltoreq.2.theta..ltoreq.56.0.degree. 0.01.degree./3
sec
[[Aspect Ratio]]
[0815] The aspect ratio of the graphitic carbon particles for use
as the negative-electrode active material of negative electrode [6]
in a lithium secondary battery of the invention is theoretically 1
or higher. The upper limit thereof may be 10 or lower and is
preferably 8 or lower, more preferably 5 or lower. When the aspect
ratio thereof exceeds the upper limit, there are cases where this
graphitic carbon causes streak lines in electrode plate formation
and an even coating surface cannot be obtained, resulting in a
decrease in high-current-density charge/discharge
characteristics.
[0816] The aspect ratio of a particle is represented by A/B,
wherein A is the length of the longest axis of the particle in a
three-dimensional examination and B is the length of the shortest
axis perpendicular to that axis. The examination of particles is
made with a scanning electron microscope capable of enlargement
examination. Fifty are arbitrarily selected from particles fixed to
an edge face of a metal having a thickness of 50 .mu.m or smaller,
and each particle is examined for A and B while rotating or
inclining the stage to which the sample is fixed. The average of
the A/B values is determined.
[0817] The graphitic carbon particles for use as the
negative-electrode active material of negative electrode [6] in a
lithium secondary battery of the invention may be ones which occur
naturally or ones produced artificially. However, the graphitic
carbon particles preferably are ones comprising a natural graphite.
The graphitic carbon particles may be ones obtained by subjecting
naturally occurring ones or artificially produced ones to a
specific treatment. Processes for producing the graphitic carbon
particles (including classification methods) also are not
particularly limited. For example, graphitic carbon particles
obtained by a classification technique such as, e.g., sieving or
air classification so as to have the properties described above may
be employed.
[0818] Especially preferred graphitic carbon particles, among
those, are ones produced by modifying naturally occurring
carbonaceous particles or artificially produced carbonaceous
particles by subjecting them to a mechanical energy treatment. More
preferred are ones produced by this modification in which the
carbonaceous particles to be subjected as a raw material to the
mechanical energy treatment comprise a natural graphite.
[[Mechanical Energy Treatment]]
[0819] This mechanical energy treatment is explained below. The
carbonaceous particles to be subjected as a raw material to the
mechanical energy treatment are not particularly limited. Examples
thereof include carbonaceous particles derived from a natural or
artificial graphite and carbonaceous particles which are a graphite
precursor. Properties of these raw materials will be shown
below.
[[[Graphite-Derived Carbonaceous Particles as Raw Material for
Mechanical Energy Treatment]]]
[0820] It is desirable that the graphite-derived carbonaceous
particles to be used as a raw material should have properties which
satisfy any one of the following items (1) to (11) or
simultaneously satisfy two or more thereof. The methods of
determining the following properties and the definitions thereof
are the same as in the graphitic carbon particles described
above.
(1) X-Ray Parameter
[0821] The graphite-derived carbonaceous particles as a raw
material preferably have a value of d (interplanar spacing) for the
lattice planes (002), as determined by X-ray diffractometry in
accordance with the method of the Japan Society for Promotion of
Scientific Research, of 0.335 nm or larger. The upper limit of the
value of d desirably is generally 0.340 nm or smaller, preferably
0.337 nm or smaller. Furthermore, the crystallite size (Lc) of the
graphite-derived carbonaceous particles, as determined by X-ray
diffractometry in accordance with the method of the Japan Society
for Promotion of Scientific Research, is generally 30 nm or larger,
preferably 50 nm or larger, more preferably 100 nm or larger. In
case where the Lc thereof is smaller than the lower limit of that
range, there is a possibility that crystallinity might decrease to
enhance the increase in initial irreversible capacity.
(2) Ash Content
[0822] The ash content of the graphite-derived carbonaceous
particles as a raw material is preferably 1% by mass or lower,
especially 0.5% by mass or lower, in particular 0.1% by mass or
lower, based on the whole graphite-derived carbonaceous substance.
The lower limit of the ash content thereof is preferably at least 1
ppm by mass of the whole carbonaceous substance. When the ash
content thereof exceeds the upper limit of that range, there are
cases where battery performance deterioration caused by reaction
with the nonaqueous electrolyte during charge/discharge becomes not
negligible. When the ash content thereof is lower than the lower
limit of that range, there are cases where the production of this
carbonaceous substance necessitates much time and energy and an
apparatus for pollution prevention, resulting in an increase in
cost.
(3) Volume-Average Particle Diameter
[0823] With respect to the volume-average particle diameter of the
graphite-derived carbonaceous particles as a raw material, the
volume-average particle diameter (median diameter) thereof as
determined by the laser diffraction/scattering method is generally
1 .mu.m or larger, preferably 3 .mu.m or larger, more preferably 5
.mu.m or larger, even more preferably 7 .mu.m or larger. Although
the upper limit thereof is not particularly limited, it is
generally 10 mm or smaller, preferably 1 mm or smaller, more
preferably 500 .mu.m or smaller, even more preferably 100 .mu.m or
smaller, especially preferably 50 .mu.m or smaller. When the
volume-average particle diameter thereof is smaller than the lower
limit of that range, there are cases where application of
mechanical energy results in too small a particle diameter and this
leads to an increase in irreversible capacity. When the
volume-average particle diameter thereof exceeds the upper limit of
that range, there are cases where the apparatus to be used for the
application of mechanical energy is difficult to operate
efficiently and this leads to a time loss.
(4) Raman R Value, Raman Half-Value Width
[0824] The Raman R value of the graphite-derived carbonaceous
particles as a raw material, as determined by the argon ion laser
Raman spectroscopy, is generally 0.01 or higher, preferably 0.03 or
higher, more preferably 0.1 or higher. The upper limit thereof may
be 0.6 or lower and is preferably 0.4 or lower. When the Raman R
value thereof is lower than the lower limit of that range, the
surface of such particles has too high crystallinity. Even after
the Raman R value thereof is increased by applying mechanical
energy, crystallinity remains low. There are hence cases where the
number of intercalation sites into which lithium comes with
charge/discharge decreases. Namely, there are cases where
suitability for charge decreases. On the other hand, when the Raman
R value thereof exceeds the upper limit of that range, the surface
of such particles comes to have further reduced crystallinity upon
reception of mechanical energy. There are hence cases where the
particles have enhanced reactivity with the nonaqueous electrolyte,
leading to a decrease in efficiency and enhanced gas evolution.
[0825] The Raman half-value width around 1,580 cm.sup.-1 of the
graphite-derived carbonaceous particles is not particularly
limited. However, the half-value width thereof is generally 10
cm.sup.-1 or larger, preferably 15 cm.sup.-1 or larger. The upper
limit thereof is generally 50 cm.sup.-1 or smaller, preferably 45
cm.sup.-1 or smaller, more preferably 40 cm.sup.-1 or smaller. When
the Raman half-value width thereof is smaller than the lower limit
of that range, the surface of such particles has too high
crystallinity. Even after the Raman half-value width thereof is
increased by applying mechanical energy, crystallinity remains low.
There are hence cases where the number of intercalation sites into
which lithium comes with charge/discharge decreases. Namely, there
are cases where suitability for charge decreases. On the other
hand, when the Raman half-value width thereof exceeds the upper
limit of that range, the surface of such particles comes to have
further reduced crystallinity upon reception of mechanical energy.
There are hence cases where the particles have enhanced reactivity
with the nonaqueous electrolyte, leading to a decrease in
efficiency and enhanced gas evolution.
(5) BET Specific Surface Area
[0826] The specific surface area of the graphite-derived
carbonaceous particles as a raw material, as determined by the BET
method, is generally 0.05 m.sup.2/g or larger, preferably 0.2
m.sup.2/g or larger, more preferably 0.5 m.sup.2/g or larger,
especially preferably 1 m.sup.2/g or larger. The upper limit
thereof is generally 50 m.sup.2/g or smaller, preferably 25
m.sup.2/g or smaller, more preferably 15 m.sup.2/g or smaller,
especially preferably 10 m.sup.2/g or smaller. When the BET
specific surface area thereof is smaller than the lower limit of
that range, there are cases where use of this carbonaceous
substance is apt to result in poor lithium acceptance during charge
and in lithium deposition on the electrode surface even after the
BET specific surface area thereof is increased by applying
mechanical energy. On the other hand, when the specific surface
area thereof exceeds the upper limit of that range, this
carbonaceous substance comes to have a further increased BET
specific area upon reception of mechanical energy. There are hence
cases where use of this carbonaceous substance as a
negative-electrode active material is apt to result in enhanced
reactivity with the nonaqueous electrolyte and enhanced gas
evolution and a preferred battery is difficult to obtain.
(7) Roundness
[0827] With respect to the degree of sphericity of the
graphite-derived carbonaceous particles as a raw material, the
roundness of the particles having a particle diameter in the range
of 3-40 .mu.m is preferably 0.1 or higher, more preferably 0.2 or
higher, especially preferably 0.4 or higher, even more preferably
0.5 or higher, most preferably 0.6 or higher. When the roundness
thereof is lower than the lower limit of that range, there are
cases where rounding does not proceed sufficiently even with
application of mechanical energy, resulting in a decrease in
high-current-density charge/discharge characteristics.
(8) True Density
[0828] The true density of the graphite-derived carbonaceous
particles as a raw material is generally 2 g/cm.sup.3 or higher,
preferably 2.1 g/cm.sup.3 or higher, more preferably 2.2 g/cm.sup.3
or higher, especially preferably 2.22 g/cm.sup.3 or higher. The
upper limit thereof may be 2.26 g/cm.sup.3 or lower. This upper
limit is a theoretical value for graphites. When the true density
of the carbonaceous substance is lower than the lower limit of that
range, there are cases where this carbon has too low crystallinity,
resulting man increase in initial irreversible capacity.
(9) Tap Density
[0829] The tap density of the graphite-derived carbonaceous
particles as a raw material desirably is generally 0.05 g/cm.sup.3
or higher, preferably 0.1 g/cm.sup.3 or higher, more preferably 0.2
g/cm.sup.3 or higher, especially preferably 0.5 g/cm.sup.3 or
higher. The tap density thereof is preferably 2 g/cm.sup.3 or
lower, more preferably 1.8 g/cm.sup.3 or lower, especially
preferably 1.6 g/cm.sup.3 or lower. When the tap density thereof is
lower than the lower limit of that range, there are cases where
application of mechanical energy does not result in a sufficiently
improved tap density. Namely, there are cases where the treated
carbonaceous substance, when used as a negative-electrode active
material, is less apt to have a high loading density and cannot
give a battery having a high capacity. On the other hand, when the
tap density thereof exceeds the upper limit of that range,
application of mechanical energy results in a further increased tap
density. There is hence a possibility that use of the treated
carbonaceous substance in electrode formation might give an
electrode which has too small an amount of interparticle
interstices and has a deficiency in passages for the nonaqueous
electrolyte, resulting in a decrease in high-current-density
charge/discharge characteristics. The tap density of the
graphite-derived carbonaceous particles also is measured and
defined by the same method as described above.
(10) Orientation Ratio (Powder)
[0830] The orientation ratio of the graphite-derived carbonaceous
particles as a raw material is generally 0.001 or higher,
preferably 0.005 or higher. The upper limit thereof is
theoretically 0.67 or lower. When the orientation ratio thereof is
lower than the lower limit of that range, there are cases where
application of mechanical energy does not result in a sufficiently
improved orientation ratio, resulting in a decrease in high-density
charge/discharge characteristics.
(11) Aspect Ratio (Powder)
[0831] The aspect ratio of the graphite-derived carbonaceous
particles as a raw material is theoretically 1 or higher. The upper
limit thereof is generally 10 or lower, preferably 8 or lower, more
preferably 5 or lower. When the aspect ratio thereof exceeds the
upper limit, there are cases where application of mechanical energy
does not result in a sufficiently reduced aspect ratio and where
the treated carbonaceous substance causes streak lines in electrode
plate formation and an even coating surface cannot be obtained,
resulting in a decrease in high-current-density charge/discharge
characteristics.
[0832] The graphite-derived carbonaceous particles as a raw
material described above may be ones made of a highly crystalline
carbon material in which carbon-hexagonal planar structures have
grown. Examples of this carbon material include a highly oriented
graphite in which the hexagonal planes have been grown so as to
enhance planar orientation and an isotropic high-density graphite
in which highly oriented graphite particles have been aggregated in
isotropic directions. Preferred examples of the highly oriented
graphite include natural graphites occurring in Sri Lanka and
Madagascar, the so-called Kish graphite obtained by precipitating
carbon from supersaturated molten iron, and some artificial
graphites having a high degree of graphitization.
[0833] Natural graphites are classified into flake graphite,
crystalline (vein) graphite, and amorphous graphite according to
properties (see "Graphite" in Funrutai Purosesu Gijutsu Shusei
(published by Industrial Technology Center in 1974); and HANDBOOK
OF CARBON, GRAPHITE, DIAMOND AND FULLERENES (published by Noyes
Publications)). With respect to the degree of graphitization,
crystalline graphite has a highest graphitization degree of 100%
and flake graphite is second, which has a graphitization degree as
high as 99.9%. However, the graphitization degree of amorphous
graphite is as low as 28%. Flake graphite, among such natural
graphites, occurs in Madagascar, China, Brazil, Ukraine, Canada,
etc. Crystalline graphite occurs mainly in Sri Lanka. Amorphous
graphite occurs mainly in the Korean Peninsula, China, Mexico, etc.
Of these natural graphites, amorphous graphite generally has a
small particle diameter and a low purity. In contrast, flake
graphite and crystalline graphite have advantages such as a high
degree of graphitization and a low impurity content, and can hence
be advantageously used in the invention.
[0834] Artificial graphites can be produced by heating a
petroleum-derived heavy oil, coal-derived heavy oil, petroleum
coke, or coal coke at a temperature of 1,500-3,000.degree. C. or
higher in a non-oxidizing atmosphere.
[0835] In the invention, any artificial graphite can be used as a
raw material as long as it comes to have high orientation and high
capacity through the mechanical energy treatment and a heat
treatment. Furthermore, even an incompletely graphitized material
such as, e.g., a graphite precursor, among those artificial
graphites, can be used as a raw material for the mechanical energy
treatment in the invention as long as it can become graphitic
carbon particles satisfying the properties described above through
the mechanical energy treatment.
[[[Details of Mechanical Energy Treatment]]]
[0836] The mechanical energy treatment of such graphite-derived
carbonaceous particles as a raw material is a treatment for
reducing the particle diameter thereof so that the ratio between
the volume-average particle diameters before and after the
treatment becomes 1 or lower and for heightening the tap density
thereof and increasing the Raman R value thereof 1.1 time or
more.
[0837] Through such mechanical energy treatment, the carbonaceous
particles such as graphite-derived carbonaceous particles as a raw
material become particles in which only parts near the surface are
rough and which have exposed strains and edge faces, while
maintaining high crystallinity as a whole. Thus, the particles come
to have an increased amount of surfaces where lithium ions can come
in and out, and hence have a high capacity even at a high current
density.
[0838] The "mechanical energy treatment" in the invention belongs
to a "pulverization treatment" among engineering unit operations
utilizable for particle design, such as pulverization,
classification, mixing, granulation, surface modification, and
reaction. However, it simultaneously includes a surface treatment
which causes fine structural defects in the surface structure by
impacts, friction, compression, etc.
[0839] In general, a pulverization treatment means an operation in
which a force is applied to a substance to thereby reduce the size
thereof and regulate the particle diameter, particle size
distribution, and loading of the substance. Pulverization
treatments are classified according to the kind of force to be
applied to the substance and the method of treatment. Forces to be
applied to the substance are roughly divided into the following
four: breaking force (impact force), crushing force (compressive
force), milling force (grinding force), and scraping force (shear
force). On the other hand, treatment methods are roughly divided
into the following two: volume pulverization in which cracks are
generated and propagated in inner parts of the particles; and
surface pulverization in which surface parts of the particles are
scraped off. Volume pulverization proceeds by means of impact
force, compressive force, and shear force, while surface
pulverization proceeds by means of grinding force and shear force.
Pulverization is a treatment based on any of various combinations
of such kinds of forces to be applied to a substance and such
treatment methods. A suitable combination can be determined
according to the purpose of the treatment. There are cases where a
pulverization treatment is conducted by a chemical reaction, e.g.,
explosion, or by volume expansion. However, pulverization is
generally conducted with a mechanical apparatus such as a
pulverizer.
[0840] The mechanical energy treatment use of which is preferred
for producing the graphitic carbon particles in the invention
preferably is a treatment which finally involves surface treatment
and involves a high proportion of the pulverization of particle
surface parts (surface pulverization) regardless of whether volume
pulverization occurs or not. This is because particle surface
pulverization is important for eliminating angles of the
carbonaceous particles, e.g., graphitic carbon particles, and
thereby imparting roundness to the particle shape. Specifically, a
mechanical energy treatment may be conducted in such a manner that
surface pulverization is conducted after volume pulverization has
proceeded to some degree, or may be conducted in such a manner that
surface pulverization only is conducted almost without conducting
volume pulverization. Furthermore, a mechanical energy treatment
may be conducted in such a manner that volume pulverization and
surface pulverization are simultaneously conducted. Preferred is a
mechanical energy treatment in which surface pulverization proceeds
finally and angles are eliminated from the surface of the
particles.
[0841] The mechanical energy treatment in the invention is a
treatment for reducing the particle diameter thereof so that the
ratio between the volume-average particle diameters before and
after the treatment becomes 1 or lower and for heightening the tap
density thereof and increasing the Raman R value thereof 1.1 time
or more.
[0842] The term "ratio between the volume-average particle
diameters before and after the treatment" means a value obtained by
dividing the volume-average particle diameter as measured after the
treatment by the volume-average particle diameter as measured
before the treatment. Although the value of (volume-average
particle diameter after the treatment)/(volume-average particle
diameter before the treatment) is 1 or smaller, it is preferably
0.95 or smaller. When the value thereof is substantially 1, there
are cases where the loading-improving effect of the mechanical
energy treatment through roundness improvement is low.
Incidentally, the treatment in which the particle size is reduced
so that the ratio between the average particle diameters before and
after the treatment becomes 1 or lower is effective also in
regulating the particle shape.
[0843] The mechanical energy treatment in the invention is one
which increases tap density. To increase tap density means an
improvement in the degree of sphericity represented by roundness as
will be described later. The mechanical energy treatment hence
should be one which produces such an effect. Although the value of
(tap density after the treatment)/(tap density before the
treatment) is 1 or larger, it is preferably 1.1 or larger. When the
value thereof is smaller than 1, there are cases where the effect
of improving loading through a roundness improvement is low.
[0844] The mechanical energy treatment in the invention is one
which increases Raman R value 1.1 time or more. To increase Raman R
value means that crystallinity around the particle surface
decreases as will be described later. The mechanical energy
treatment hence should be one which produces such an effect.
Although the value of (Raman R value after the treatment)/(Raman R
value before the treatment) is 1.1 or larger, it is preferably 1.4
or larger. When the value thereof is smaller than 1.1, there are
cases where the effect of improving suitability for charge through
a change in Raman R value is low.
[0845] The mechanical energy treatment in the invention imparts
roundness to particles and increases the tap density of the
particles. It is known that for heightening the tap density of
powder particles, it is effective to fill interstices among the
particles with smaller particles capable of coming into the
interstices. Although it may be thought that the tap density of
carbonaceous particles, e.g., graphite-derived carbonaceous
particles, can be increased by subjecting the particles to a
treatment such as pulverization to reduce the particle diameter
thereof, the particle diameter reduction by such a technique
generally reduces rather than increases the tap density. The reason
for this is thought to be that the pulverization makes the particle
shape more irregular.
[0846] On the other hand, the proportion of interstices in a layer
packed with powder particles decreases as the number of particles
in contact with one particle (any particle selected) increases
(i.e., as the coordination number n increases). Namely, important
factors influencing tap density include the proportions of particle
sizes, i.e., particle diameter distribution. It should, however, be
noted that this investigation was made on a model powder composed
of spherical particles. The carbonaceous particles to be handled in
the invention, e.g., graphite-derived carbonaceous particles before
treatment, are flaky, crystalline, or platy. Because of this, even
when it is attempted to heighten a tap density by regulating the
particle diameter distribution by merely conducting general
pulverization, classification, etc. only, such a highly packed
state cannot be attained.
[0847] Flaky, crystalline, and platy carbonaceous particles such
as, e.g., graphite-derived carbonaceous particles, generally tend
to decrease in tap density as the particle diameter decreases. The
reasons for this are thought to be as follows. As a result of
pulverization, the particles come to have more irregular shapes,
have a larger amount of surface projections such as "fine splits",
"half-peeled parts", and "bent parts", and have finer particles of
irregular shapes adherent to the particle surface at some degree of
strength. These are causative of increased resistance between
adjacent particles and impaired suitability for loading, resulting
in a decrease in tap density.
[0848] As long as such shape irregularity is reduced and the
particle shape becomes close to sphere, a decrease in particle
diameter should result in a limited decrease in loading and
large-diameter carbon particles and small-diameter carbon particles
should have almost the same tap density.
[0849] As a result of investigations made by the inventors on
carbonaceous or graphitic particulate materials which are almost
equal in true density and in average particle diameter, it has been
ascertained that the materials having a more spherical shape have a
higher tap density. Namely, it is important to impart roundness to
the particle shape and make the shape close to sphere. As the
particle shape becomes close to sphere, the suitability for loading
of the powder simultaneously improves greatly. For the reasons
shown above, powder tap density is employed as an index to the
application of mechanical energy. In the case where the suitability
for loading of a powdery or particulate material was improved
through a treatment, it can be thought that this improvement is a
result of the rounding of the particles due to the treatment used.
Furthermore, when particles are treated by the method according to
the invention while considerably reducing the particle diameter and
the carbon material thus obtained has a tap density which is higher
than the tap densities of carbon materials obtained by general
pulverization and having almost the same particle diameter, then
this can be thought to be a result of rounding.
[0850] As indexes to the crystallinity of particles and the
roughness of the particle surface, i.e., to the amount of crystal
edge faces present, use can be made of interplanar spacing (d002)
for the (002) planes and crystallite size (Lc) both determined by
wide-angle X-ray diffractometry and Raman R value. In general,
carbon materials having a smaller interplanar spacing (d002) for
the (002) planes and a larger crystallite size (Lc) have lower
Raman R value. Namely, carbonaceous particles such as
graphite-derived carbonaceous particles as a whole have almost the
same crystalline state. In contrast, the graphitic carbon particles
according to this invention have a small interplanar spacing (d002)
for the (002) planes and a large crystallite size (Lc) and, despite
this, have a high Raman R value. This means that in these graphitic
carbon particles, a surface layer (to a depth of about 100 .ANG.
from the particle surface) has disordered crystallinity and a large
amount of exposed edge faces although the particles have high bulk
crystallinity.
[0851] From the standpoint of improving suitability for loading,
the mechanical energy treatment in the invention preferably is one
which further increases roundness 1.02 times, especially preferably
1.04 times.
[[[Apparatus for Mechanical Energy Treatment]]]
[0852] The apparatus to be used for conducting the mechanical
energy treatment is selected from ones usable for conducting the
preferred treatment described above. An investigation made by the
inventors revealed the following. The mechanical energy treatment
may be accomplished with one or more of the four kinds of forces.
However, it is effective to employ an apparatus which repeatedly
applies impact force as amain force and other mechanical actions
such as compressive force, frictional force, shear force, etc.,
including those caused by particle interaction, to the particles.
Specifically, a preferred apparatus is one which comprises a casing
and disposed therein a rotor having blades and in which the rotor
is rotated at a high speed to thereby apply mechanical actions such
as impact, compressive, frictional, and shear forces to
carbonaceous particles introduced therein and conduct surface
pulverization while allowing volume pulverization to proceed. More
preferred is one which has a mechanism in which carbonaceous
particles are circulated to thereby repeatedly apply mechanical
actions thereto. The number of blades in the casing is preferably 3
or more, especially preferably 5 or more.
[0853] One example of preferred apparatus which satisfy such
requirements is Hybridization System, manufactured by Nara
Machinery Co., Ltd. In the case where the treatment is conducted
with this apparatus, the rotor is rotated at a peripheral speed of
preferably 30-100 m/sec, more preferably 40-100 m/sec, even more
preferably 50-100 m/sec. Although carbonaceous particles can be
treated by merely passing these through the apparatus, it is
preferred to treat the carbonaceous particles while causing these
to circulate or reside in the apparatus for 30 seconds or longer.
More preferably, the carbonaceous particles are treated while
circulating or residing in the apparatus for 1 minute or
longer.
[0854] In the case where the graphite-derived carbonaceous
particles as a raw material have a true density lower than 2.25 and
the crystallinity thereof is not so high, it is preferred that a
heat treatment for enhancing crystallinity should be further
conducted after the mechanical energy treatment. This heat
treatment is conducted at a temperature of preferably 2,000.degree.
C. or higher, more preferably 2,500.degree. C. or higher, even more
preferably 2,800.degree. C. or higher.
[Negative Electrode [6]: Electrode Production]
[0855] The negative electrode may be produced by an ordinary
method. It can be formed in the same manner as described above. The
current collector, thickness ratio between the current collector
and the active-material layer, electrode density, binder,
electrode-plate orientation ratio, impedance, etc. also are the
same as described above.
<Negative Electrode [7]>
[0856] An explanation will be given below on negative electrode [7]
for use in a lithium secondary battery of the invention, i.e., "a
negative electrode containing as a negative-electrode active
material a multielement-containing negative-electrode active
material (C) which contains at least one of a lithium-occluding
metal (A) selected from the group consisting of Al, Pb, Zn, Sn, Bi,
In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, and Sb and/or a
lithium-occluding alloy (B) and further contains C and/or N as
element Z."
[Negative Electrode [7]: Negative-Electrode Active Material]
[0857] The negative-electrode active material for use in negative
electrode [7] is described below.
[[Composition]]
[0858] The negative-electrode active material to be used in
negative electrode [7] in a lithium secondary battery of the
invention is characterized by containing at least a metal capable
of occluding lithium (lithium-occluding metal (A)) and/or an alloy
capable of occluding lithium (lithium-occluding alloy (B)) and
containing C and/or N as element Z.
[0859] The lithium-occluding metal (A) means at least one member
selected from the group consisting of Al, Pb, Zn, Sn, Bi, In, Mg,
Ga, Cd, Ag, Si, B, Au, Pt, Pd, and Sb. Preferred of these is Al,
Si, Sn, or Pb. More preferred is Si or Sn. Even more preferred is
Si. The reasons for the preference of Si include that it brings
about a high capacity per unit mass.
[0860] The lithium-occluding alloy (B) means one comprising two or
more of those lithium-occluding metals (A) or one comprising a
lithium-occluding metal (A) and at least one element other than the
lithium-occluding metal (A), C, and N. This element other than the
lithium-occluding metal (A), C, and N preferably is one or more
elements selected from Groups 4, 5, 6, 8, 9, 10, 11, 13, and 16 of
the periodic table. More preferred are one or more of the elements
Ti, Zr, V, Cr, W, B, O, and Co. Even more preferred are one or more
of the elements Ti, Zr, W, O, and Co. These elements are apt to
form high-melting compounds and, hence, can be advantageously used
from the standpoint of reactivity control. The content of the
alloy-forming element, i.e., the element other than the
lithium-occluding metal (A), C, and N, is preferably 50% by mole or
lower based on the whole lithium-occluding alloy (B) from the
standpoint of capacity.
[0861] Examples of the state in which the C and/or N as element Z
is present in the multielement-containing negative-electrode active
material (C) include:
1. the state in which element Z is contained in the
lithium-occluding metal (A) and/or lithium-occluding alloy (B)
(element-Z-internally-possessing lithium-occluding substance (D));
2. the state in which element Z is present around the
lithium-occluding metal (A) and/or lithium-occluding alloy (B)
(element-Z-externally-possessing lithium-occluding substance (F));
and 3. the state in which element Z is present in both the states
1. and 2. above.
[0862] In a preferred embodiment of the
element-Z-internally-possessing lithium-occluding substance (D),
element Z (C and/or N) is present in a non-equilibrium state in the
lithium-occluding metal (A) and/or lithium-occluding alloy (B).
Especially preferably, the lithium-occluding metal (A) is Si.
[0863] A preferred embodiment of the
element-Z-externally-possessing lithium-occluding substance (F) is
one in which element Z is C. This is because C can impart
electrical conductivity.
[0864] Although the multielement-containing negative-electrode
active material (C) for use as a negative-electrode active material
in this invention contains a lithium-occluding metal (A) and/or a
lithium-occluding alloy (B) and further contains C and/or N as an
essential component, it preferably further contains one or more
elements selected from Groups 4, 5, 6, 8, 9, 10, 11, 13, and 16 of
the periodic table. More preferred are one or more of the elements
Ti, Zr, V, Cr, W, B, O, and Co. Even more preferred are one or more
of the elements Ti, Zr, W, O, and Co.
1. Element-Z-Internally-Possessing Lithium-Occluding Substance
(D)
[0865] Of the lithium-occluding metals (A) usable in the invention,
Si is apt to produce the effect. Because of this, the
element-Z-internally-possessing lithium-occluding substance (D)
preferably is one which comprises as a main component a compound
represented by the general formula SiZ.sub.xM.sub.y (wherein Z, M,
x, and y are as defined under the following (1) to (4)) and
constituting a phase in which element Z is present in a
non-equilibrium state in Si:
(1) element Z is C and/or N; (2) element M is one or more elements
selected from elements other than Si and element Z; (3) x is such a
value that, with respect to the Z concentration (p/(a+p)) in the
compound Si.sub.aZ.sub.p (wherein a and p are integers) present in
an equilibrium state so as to have a composition closest to Si, the
value of Z concentration ratio Q(Z) calculated with the equation
Q(Z)=[x/(1+x)]/[p/(a+p)] is 0.10-0.95; and (4) y is a number in the
range of 0.ltoreq.y.ltoreq.0.50. (With Respect to
SiZ.sub.xM.sub.y)
((Element Z))
[0866] Element Z in SiZ.sub.xM.sub.y is at least one element which
is C and/or N. The reasons why C and/or N is preferred as the
element Z to be internally possessed include the following:
(1) C and N each are capable of forming a compound having a higher
melting point than Si; (2) C and N each have a smaller covalent
radius than Si; (3) C and N each have a low coefficient of
diffusion in Si; and (4) C and N each change little in volume even
when reacted with lithium.
[0867] Specifically, the elements C and N can form compounds which
have a higher melting point than Si and are present in an
equilibrium state, such as, e.g., SiC and Si.sub.3N.sub.4.
High-melting compounds generally are stable compounds having a
large negative value of free energy regarding formation and, hence,
can effectively reduce the activity of Si. From the standpoint of
controlling reactivity with the nonaqueous electrolyte, element Z
preferably is C and/or N.
[0868] Furthermore, since the elements C and N each have a smaller
covalent radius than Si, they are less apt to form a compound
present in an equilibrium state in the compound SiZ.sub.xM.sub.y
although details of this tendency are unclear. This is thought to
be effective in more evenly distributing element Z in a high
concentration. The elements C and N can hence more effectively
reduce the activity of Si and are preferred also from the
standpoint of controlling reactivity with the nonaqueous
electrolyte.
[0869] Moreover, since the elements C and N each have a low
coefficient of diffusion in Si, the elements C and N, when present
in the state of being dispersed in the Si, inhibit the Si from
aggregating or crystallizing with charge/discharge. The elements C
and N are preferred also from the standpoint of inhibiting the Si
from becoming fine particles or reacting with the nonaqueous
electrolyte. In addition, since the elements C and N change little
in volume even when reacted with lithium, they are less apt to
influence Si conduction path breakage and are hence preferred.
[0870] Incidentally, when a compound capable of being present in an
equilibrium state has a lower melting point than Si, such as, e.g.,
Cu.sub.3Si or Ni.sub.2Si, as in the case of elements such as Cu and
Ni, there are cases where the activity of Si is not effectively
reduced and reactivity with nonaqueous electrolyte is difficult to
control. Moreover, since the elements Cu and Ni have a high
coefficient of diffusion in Si, the aggregation and crystallization
of Si proceed with charge/discharge. There are hence cases where
the Si is apt to become fine particles and cycle characteristics
are not improved. When such a compound present in an equilibrium
state in the compound SiZ.sub.xM.sub.y is a main component, there
are cases where the activity of Si does not decrease and reactivity
with the nonaqueous electrolyte cannot be controlled, resulting in
impaired cycle characteristics, etc.
((Element M))
[0871] Element M in SiZ.sub.xM.sub.y is one or more elements
selected from elements other than Si and element Z. Preferably,
element M is one or more elements selected from Groups 4, 5, 6, 8,
9, 10, 11, 13, and 16 of the periodic table. From the standpoint of
reaction inhibition, the elements Ti, Zr, V, Cr, W, B, and O are
preferred because these elements are apt to form a high-melting
compound. More preferred are the elements Ti, Zr, W, and O.
[0872] With respect to the composition of SiZ.sub.xM.sub.y, x in
the SiZ.sub.xM.sub.y is such a value that, with respect to the Z
concentration (p/(a+p)) in the compound Si.sub.aZ.sub.p (wherein a
and p are integers) present in an equilibrium state so as to have a
composition closest to Si, the value of Z concentration ratio Q(Z)
calculated with the equation Q (Z)=[x/(1+x)]/[p/(a+p)] is generally
0.10 or larger, preferably 0.15 or larger, more preferably 0.30 or
larger, especially preferably 0.40 or larger. The upper limit of
the value of Q(Z) is generally 0.95 or smaller, preferably 0.85 or
smaller, more preferably 0.75 or smaller, especially preferably
0.65 or smaller. The "compound present in an equilibrium state so
as to have a composition closest to Si" means the compound
Si.sub.aZ.sub.p in which the value of p/(a+p) is minimum and which
is present in an equilibrium state.
[0873] Incidentally, the compound Si.sub.aZ.sub.p present in an
equilibrium state so as to have a composition closest to Si, in
this invention, is described in phase diagrams for Si and element Z
(e.g., Desk Handbooks Phase Diagrams for Binary Alloys, ASM
International). In this invention, the Z concentration ratio Q(Z)
described above is used for the Z concentration (p/(a+p)) of this
Si.sub.aZ.sub.p, and the range of x is specified in terms of the
numerical-value range of the Z concentration ratio Q(Z).
[0874] The term "compound present in an equilibrium state" herein
means a constant-proportion compound, such as the compound
Si.sub.aZ.sub.p (wherein a and p are integers), indicated by a
vertex in a diagram, e.g., the phase diagram given above. For
example, when Z is C, SiC is known as a stable compound and this
compound is regarded as the compound present in an equilibrium
state in the invention. Consequently, when Z is C, then SiC
corresponds to the Si.sub.aZ.sub.p according to the invention.
Furthermore, when Z is, for example, N, then Si.sub.3N.sub.4 is
known as the most stable compound. However, Si.sub.2N.sub.3 and SiN
also are known to be present as constant-proportion compounds. In
this invention, all these compounds are regarded as compounds
present in an equilibrium state. Consequently, when Z is N, then
SiN corresponds to the Si.sub.aZ.sub.p according to the
invention.
[0875] On the other hand, the term "compound constituting a phase
present in a non-equilibrium state" means any compound other than
the compounds present in an equilibrium state. In the case of the
compound present in a non-equilibrium state, no specific
constant-proportion compound is formed and Si atoms and Z atoms are
evenly dispersed when viewed macroscopically.
[0876] When the Z concentration ratio Q(Z) is lower than the lower
limit of that range, the effect of reducing the activity of Si is
low. There are hence cases where reactivity with the nonaqueous
electrolyte cannot be controlled and electrode expansion is
enhanced, making it impossible to obtain preferred cycle
characteristics. On the other hand, when the ratio Q(Z) exceeds the
upper limit of that range, a stable compound Si.sub.aZ.sub.p or the
like present in an equilibrium state is formed and the activity of
Si is not reduced even when the amount of element Z is increased.
There are hence cases where reactivity with the nonaqueous
electrolyte cannot be controlled. Furthermore, Si.sub.aZ.sub.p or
the like has low electrical conductivity. There are hence cases
where formation of such a compound results in impaired electrical
conductivity of the active material and difficulties in lithium
doping/undoping, making it impossible to conduct
charge/discharge.
[0877] When the Z concentration ratio Q(Z) is 1, this means that
the Si is present as a stable compound Si.sub.aZ.sub.p; this state
is undesirable. When the ratio Q(Z) is far higher than the upper
limit of that range, the effect of increasing capacity which is
produced by the Si contained is difficult to obtain and there are
cases where preferred battery characteristics are not obtained.
[0878] Incidentally, when the elements C and N are simultaneously
used as element Z, the values of the Z concentration ratio Q(Z) are
determined respectively for the two elements from the element Z
concentrations based on Si.sub.aZ.sub.p, and the sum of these is
taken as the Z concentration ratio Q(Z).
[0879] Symbol y in SiZ.sub.xM.sub.y is a real number satisfying
0.ltoreq.y.ltoreq.0.5. In the case where the compound
SiZ.sub.xM.sub.y contains element M and y.noteq.0, the proportion y
of element M in this compound SiZ.sub.xM.sub.y is generally 0.08 or
higher, preferably 0.10. The upper limit thereof is generally 0.50
or lower, preferably 0.40 or lower, more preferably 0.30 or lower.
When y exceeds the upper limit of that range, the content of Si is
low and there are cases where high capacity is difficult to obtain.
In the case where the compound contains substantially no element M,
the proportion y of element M is such that y=0 or y.apprxeq.0. In
this invention, y.apprxeq.0 indicates the case where the element M
unavoidably came into the compound during, e.g., steps for
producing the negative-electrode active material according to the
invention (i.e., the case where substantially no element M is
contained) and the y is, e.g., less than 0.08.
[0880] The composition of the multielement-containing
negative-electrode active material (C) can be determined in an
ordinary manner, for example, with an X-ray photoelectron
spectroscope (e.g., "ESCA" manufactured by ULVAC-PHI, INC.) by
placing the negative-electrode active material on a sample table so
that the side containing the negative-electrode compound faces
upward and forms a flat surface, making a depth profile examination
while conducting Ar sputtering using an aluminum K.alpha. line as
an X-ray source, and calculating the concentrations of the atoms of
Si, element Z, element M, etc.
(Composition of SiC.sub.xO.sub.y)
[0881] In the case where element Z is C and element M is O, then x
in the general formula SiC.sub.xO.sub.y is generally 0.053 or
larger, preferably 0.08 or larger, more preferably 0.15 or larger,
especially preferably 0.25 or larger. The upper limit of x is
generally 0.90 or smaller, preferably 0.75 or smaller, more
preferably 0.60 or smaller, especially preferably 0.45 or smaller.
Furthermore, y is generally 0 or larger, preferably a value larger
than 0, especially preferably 0.08 or larger, more preferably 0.10
or larger. The upper limit of y is generally 0.50 or smaller,
preferably 0.40 or smaller, especially preferably 0.30 or
smaller.
(State of Element Z Present in Si in
Element-Z-Internally-Possessing Lithium-Occluding Substance
(D))
[0882] With respect to the state in which element Z is present in
Si in the negative-electrode compound SiZ.sub.xM.sub.y in the
invention, the value of XIsz in X-ray diffractometry is not
particularly limited. However, in the case where element Z is C,
the value of XIsz is preferably 1.2 or smaller, more preferably 0.7
or smaller. In the case where element Z is N, the value of XIsz is
preferably 1.1 or smaller, more preferably 1.0 or smaller. When the
value of XIsz is not larger than the upper limit of that range,
this means that a phase in which element Z is present in a
non-equilibrium state in the Si constitutes a main component and
any compound present in an equilibrium state, e.g.,
Si.sub.aZ.sub.p, is not a main component. Such values of XIsz are
preferred because of freedom from the following problems associated
with too large values of XIsz. When the value of XIsz exceeds the
upper limit of that range, i.e., when the phase of a compound
present in an equilibrium state, such as, e.g., Si.sub.aZ.sub.p
(silicon carbide or silicon nitride when element Z is C or N,
respectively), is a main component, then the activity of Si is not
reduced. There are hence cases where reactivity with the nonaqueous
electrolyte cannot be controlled, resulting in impaired cycle
characteristics. Furthermore, since the Si.sub.aZ.sub.p or the like
has poor electrical conductivity, there are cases where the
active-material thin film has impaired electrical conductivity and
lithium doping/undoping becomes difficult, making it impossible to
conduct charge/discharge. There also are cases where this active
material has a reduced capacity per unit mass. Such values of XIsz
are hence undesirable. The lower limit of the value of XIsz is
generally 0.00 or larger.
((Method of X-Ray Diffractometry))
[0883] In X-ray diffractometry, the value of XIsz can be
determined, for example, by setting the negative-electrode active
material according to the invention on a surface to be irradiated
and examining the active material with an X-ray diffractometer
(e.g., "X-Ray Diffractometer" manufactured by Rigaku Corp.).
Examination conditions are the same as those shown in Examples
which will be given later.
[0884] The definitions of the value of XIsz are as follows.
(((Value of XIsz when Element Z is C)))
[0885] The peak intensity Isz at a 2.theta. of 35.7.degree. and the
peak intensity Is at a 2.theta. of 28.4.degree. are measured, and
the intensity ratio XIsz (XIsz=Isz/Is) is calculated and defined as
the XIsz of the active-material thin film. The peak at a 2.theta.
of 35.7.degree. and the peak at a 2.theta. of 28.4.degree. are
thought to be a peak attributable to SiC and a peak attributable to
silicon, respectively. When the value of XIsz is 1.2 or smaller,
this means that almost no SIC is detected.
(((Value of XIsz when Element Z is N)))
[0886] The peak intensity Isz at a 20 of 70.2.degree. and the peak
intensity Is at a 2.theta. of 28.4.degree. are measured, and the
intensity ratio XIsz (XIsz=Isz/Is) is calculated and defined as the
XIsz of the active-material thin film. The peak at a 2.theta. of
27.1.degree. and the peak at a 2.theta. of 28.4.degree. are thought
to be a peak attributable to 51.sub.3N.sub.4 and a peak
attributable to silicon, respectively. When the value of XIsz is
1.1 or smaller, this means that almost no Si.sub.3N.sub.4 is
detected.
(State of Element Z Distributed in Element-Z-Internally-Possessing
Lithium-Occluding Substance (D))
[0887] In the SiZ.sub.xM.sub.y in the invention, element Z is
present, for example, as atoms, molecules, clusters, etc. of the
size of 1 .mu.m or smaller. With respect to the state in which
element Z is distributed, it is preferably distributed evenly in
the SiZ.sub.xM.sub.y, more preferably distributed so as to have a
concentration gradient in which the concentration thereof increases
from a central part of the SiZ.sub.xM.sub.y toward the surface
thereof (in the case of the negative-electrode material in a thin
film form described later, the concentration has a gradient so that
it increases from the part in contact with the current collector
toward the surface of the thin film; in the case of the
negative-electrode material in a powder form, the concentration has
a gradient so that it increases from a central part of each
particle toward the particle surface). When element Z is unevenly
distributed and locally present in the negative-electrode active
material, the expansion/contraction of Si with charge/discharge
localizes in Si parts where element Z is absent. There are hence
cases where electrical conductivity becomes poor with the progress
of cycling. The state in which element Z is dispersed can be
ascertained, for example, by EPMA as will be shown later.
(State of Element M Distribution)
[0888] The state in which element M is distributed in the
SiZ.sub.xM.sub.y in the invention is not particularly limited.
Element M may be distributed either evenly or unevenly.
(Raman RC Value, Raman RSC Value, and Raman RS Value)
[0889] In the invention, the element-Z-internally-possessing
lithium-occluding substance (D) in analysis by Raman spectroscopy
has a Raman RC value of generally 0.0 or higher. The upper limit of
the Raman RC value thereof is preferably 2.0 or lower. In case
where the Raman RC value thereof exceeds the upper limit of that
range, the effect of increasing capacity which is produced by the
Si contained is difficult to obtain, making it difficult to obtain
preferred battery characteristics. Especially when element Z
comprises C, the Raman RC value of the negative-electrode active
material SiZ.sub.xM.sub.y according to the invention is preferably
2.0 or lower, more preferably 1.0 or lower, especially preferably
0.5 or lower. The lower limit of the Raman RC value thereof is
generally 0.0 or higher because of limitations in measurement.
[0890] In the invention, the element-Z-internally-possessing
lithium-occluding substance (D) in analysis by Raman spectroscopy
has a Raman RSC value of generally 0.0 or higher. The upper limit
of the Raman RSC value thereof is preferably 0.25 or lower. When
the Raman RSC value thereof exceeds the upper limit of that range,
there are cases where electrical conductivity becomes poor and
lithium doping/undoping becomes difficult, making it impossible to
conduct charge/discharge. Furthermore, especially when element Z
comprises C, the RSC value of the substance (D) is preferably 0.25
or lower, more preferably 0.20 or lower. The lower limit of the
Raman RSC value thereof is generally 0.0 or higher because of
limitations in measurement.
[0891] In the invention, the element-Z-internally-possessing
lithium-occluding substance (D) in analysis by Raman spectroscopy
has a Raman RS value of preferably 0.40 or higher, more preferably
0.50 or higher. The upper limit of the Raman RS value thereof is
preferably 1.00 or lower, more preferably 0.90 or lower. In case
where the Raman RS value thereof is lower than the lower limit of
that range, there is a possibility that cycle characteristics might
be impaired. On the other hand, Raman RS values thereof exceeding
the upper limit of that range are undesirable because there is a
possibility that charge/discharge might be impossible. Especially
when element Z comprises C, the Raman RS value of the substance (D)
is preferably 0.40 or higher, more preferably 0.50 or higher. The
upper limit thereof is preferably 0.75 or lower, more preferably
0.65 or lower. Furthermore, especially when element Z comprises N,
the Raman RS value of the substance (D) is preferably 0.40 or
higher, more preferably 0.50 or higher. The upper limit thereof is
preferably 1.00 or lower, more preferably 0.90 or lower.
[0892] Raman RC value, Raman RSC value, and Raman RS value in
analysis by Raman spectroscopy in the invention are determined by
Raman spectroscopy conducted by the following method of Raman
spectroscopy, and the definitions thereof are as follows.
((Method of Raman Spectroscopy))
[0893] A Raman spectrometer (e.g., "Raman Spectrometer"
manufactured by Japan Spectroscopic Co., Ltd.) is used. A negative
electrode of the invention for nonaqueous-electrolyte secondary
batteries is set in a measuring cell, and is examined while
irradiating the surface of the sample in the cell with argon ion
laser light. The Raman spectrum obtained is subjected to background
correction to thereby determine the Raman RC value, RSC value, and
RS value. The background correction is conducted by connecting the
initiation and termination points for a peak with a straight line
to determine a background and subtracting this value from the peak
intensity.
[0894] Conditions for this Raman spectroscopy are as follows.
Smoothing is conducted by determining a simple average for 15
convolution points. [0895] Wavelength of argon ion laser: 514.5 nm
[0896] Laser power on sample: 15-40 mW [0897] Resolution: 10-20
cm.sup.-1 [0898] Examination range: 200 cm.sup.-1 to 1,900
cm.sup.-1
(((Raman RC Value)))
[0899] The peak intensity Ic of a peak "c" appearing at around
1,300 cm.sup.-1 to 1,600 cm.sup.-1 and the peak intensity Ias of a
peak "as" appearing at around 300 cm.sup.-1 to 500 cm.sup.-1 are
measured, and the intensity ratio RC between these (RC=Ic/Ias) is
calculated. This ratio is defined as the Raman RC value of the
negative electrode. The peak "c" and the peak "as" are thought to
be peaks attributable to carbon and silicon, respectively.
Consequently, Raman RC value reflects the amount of carbon. When
the Raman RC value is 2.0 or lower, this means that almost no
carbon is detected.
(((Raman RSC Value)))
[0900] The peak intensity Isc of a peak "sc" appearing at around
650 cm.sup.-1 to 850 cm.sup.-1 and the peak intensity Ias of a peak
"as" appearing at around 300 cm.sup.-1 to 500 cm.sup.-1 are
measured, and the intensity ratio RSC between these (RSC=Isc/Ias)
is calculated. This ratio is defined as the Raman RSC value of the
negative electrode.
[0901] The peak "sc" and the peak "as" are thought to be peaks
attributable to SiC and silicon, respectively. Consequently, Raman
RSC value reflects the amount of SiC. When the Raman RSC value is
0.25 or lower, this means that almost no SiC is detected.
(((Raman RS Value)))
[0902] The intensity Is at 520 cm.sup.-1 and the peak intensity Ias
of a peak "as" appearing at around 300 cm.sup.-1 to 500 cm.sup.-1
are measured, and the intensity ratio RS between these (RS=Is/Ias)
is calculated. This ratio is defined as the Raman RS value of the
negative electrode. Raman RS value reflects the state of Si.
(IRsc Value)
[0903] When a negative electrode having the negative-electrode
active material for use in the invention, which has undergone
charge/discharge, is examined by infrared reflection spectroscopy,
it gives an IRsc value of preferably 0.9 or higher, more preferably
1.1 or higher, especially preferably 1.2 or higher. When the IRsc
value thereof is lower than the lower limit of that range, there
are cases where the negative electrode containing Si reacts with
the nonaqueous electrolyte during cycling and the amount of the
active material substantially capable of charge/discharge decreases
gradually, making it difficult to obtain preferred cycle
characteristics. The upper limit of the IRsc value thereof is about
3.0. The IRsc value of a negative electrode in infrared reflection
spectroscopy in the invention is determined by the following
infrared reflection spectroscopy with an infrared spectrophotometer
and is defined as shown below.
((Method of Infrared Reflection Spectroscopy with Infrared
Spectrophotometer))
[0904] An infrared spectrophotometer (e.g., "Magna 560"
manufactured by Thermo Electron Corp.) is used. The active-material
side of the negative electrode of a lithium secondary battery which
has undergone charge/discharge is set in a measuring cell and
examined by the reflection method. The examination is made in an
inert atmosphere using a sample holder for reflection examination
having a window made of diamond. The infrared absorption spectrum
obtained is subjected to background correction to thereby determine
the IRsc value. The background correction is conducted by
prolonging a straight line connecting minimum values in the range
of 2,000-4,000 cm.sup.-1 to determine a background and subtracting
this value from each intensity. The reflection intensity Isc at
1,600 cm.sup.-1 and the reflection intensity Iaco at 1,650
cm.sup.-1 are measured, and the intensity ratio IRsc between these
(IRsc=Isc/Iaco) is calculated. This ratio is defined as the IRsc
value after charge/discharge.
[0905] Isc is thought to indicate a film derived from Si, while
Iaco is thought to indicate a film derived from a lithium alkyl
carbonate, although details thereof are unclear. Consequently, the
IRsc reflects the state and amount of the negative-electrode film
(solid electrolyte interface: SEI). When the IRsc value is 0.9 or
higher, this means that the negative-electrode film is constituted
of a film derived from a lithium alkyl carbonate and a film derived
from Si.
<Effect/Principle>
[0906] Activity is explained first. In general, activity is a kind
of thermodynamic concentration. In a multicomponent system
comprising substance amounts n.sub.1, n.sub.2 . . . n.sub.i . . . ,
when the chemical potential of component i and the chemical
potential of the pure substance are expressed by .mu..sub.i and
.mu..sub.i.sup.0, respectively, then a.sub.i defined by
.mu..sub.i-.mu..sub.i.sup.0=RT log a.sub.i
is called activity.
[0907] Furthermore, the ratio .gamma..sub.i of the activity a.sub.i
to the concentration c.sub.i
a.sub.i/c.sub.i=.gamma..sub.i
is called activity coefficient.
[0908] For example, when a system comprising a solvent and a solute
is considered to be a thermodynamic solution, the activity
coefficient is an amount corresponding to a difference between the
chemical potential of a certain component of the system regarded as
an ideal solution and the true chemical potential of the certain
component of the system regarded as a real solution. In the case of
(1) the real solution in which a certain component i is the solute,
as the solute concentration decreases, the system approaches an
ideal solution in which the component i is the solute and the
activity coefficient approaches 1. Conversely, in the case of (2)
the real solution in which a certain component i is the solvent, as
the solvent concentration increases, the system approaches an ideal
solution in which the component i is the solvent and the activity
coefficient approaches 1. Furthermore, when the chemical potential
of the component i is more stable in the real solution than in the
ideal solution, then .gamma..sub.i<1.
[0909] An example of Si which showed satisfactory properties in the
invention is an element-Z-containing Si compound (solid solution;
regarded as a real solution) in which component i is Si and element
Z regarded as a solute is contained in the Si regarded as a
solvent. Because of the solute contained, the solvent Si has a
reduced activity a.sub.i and .gamma..sub.i<1. This Si compound
is more stable than Si (regarded as an ideal solution). It is
thought that this Si compound was hence inhibited from reacting
with the nonaqueous electrolyte.
[0910] It should, however, be noted that when a compound in which
Si and element Z are present in an equilibrium state, e.g.,
Si.sub.aZ.sub.p, is formed, it is impossible to efficiently reduce
the activity of Si. It is therefore important that element Z should
be present in Si in a non-equilibrium state.
[Negative Electrode [7]: Form]
[0911] In this invention, the negative-electrode active material to
be used in negative electrode [7] generally is in a thin film form
or a powder form. In the invention, a negative electrode employing
the active material in a thin film form may be obtained by forming
an active-material layer on a current collector by vapor phase
growth as described in the production processes which will be given
later. A negative electrode employing the active material in a
powder form is obtained, for example, by applying the powdery
active material, a binder, etc. on a current collector to form an
active-material layer.
[[Active Material in Thin Film Form]]
[[[Structure]]]
[0912] Examples of the structure of the active material in a thin
film form deposited on a current collector include a columnar
structure and a lamellar structure.
[[[Film Thickness]]]
[0913] The thickness of the active material in a thin film form
corresponds to the thickness of the active-material layer of the
negative electrode employing the active material. It is generally 1
.mu.m or larger, preferably 3 .mu.m or larger. The upper limit
thereof is generally 30 .mu.m or smaller, preferably 20 .mu.m or
smaller, even more preferably 15 .mu.m or smaller. In case where
the thickness of the active material in a thin film form is smaller
than the lower limit of that range, negative electrodes of the
invention employing this active material (hereinafter a negative
electrode employing a thin-film-form active material is often
referred to as "thin-film negative electrode") have a low capacity
per sheet and it is necessary to use many such negative electrodes
for obtaining a high-capacity battery. Consequently, the total
volume of the necessary positive electrodes, separators, and
current collectors for the thin-film negative electrodes themselves
also is increased. The amount of the negative-electrode active
material which can be packed per unit battery volume is
substantially reduced accordingly, making it difficult to increase
battery capacity. On the other hand, when the thickness thereof
exceeds the upper limit of that range, there are cases where the
thin-film-form active material peels off the current collector base
due to expansion/contraction with charge/discharge. There is hence
a possibility that cycle characteristics might be impaired.
[[Active Material in Powder Form]]
[[[Shape]]]
[0914] Examples of the shape of the powder-form active material
include spherical, polyhedral, and irregular shapes.
[[[Volume-Average Particle Diameter]]]
[0915] The volume-average particle diameter of the active material
in a powder form is not particularly limited. However, it is
generally 0.1 .mu.m or larger, preferably 1 .mu.m or larger, more
preferably 3 .mu.m or larger. The upper limit thereof is generally
50 .mu.m or smaller, preferably 30 .mu.m or smaller, more
preferably 25 .mu.m or smaller. When the volume-average particle
diameter of the powder-form active material is smaller than the
lower limit of that range, it is difficult to secure conduction
paths among particles of the powder-form active material and
conduction paths between the powder-form active material and the
conductive material which will be described later, because of the
too small particle diameter. There are hence cases where cycle
characteristics are impaired. On the other hand, when the
volume-average particle diameter thereof exceeds the upper limit of
that range, there are cases where formation of a negative-electrode
active-material layer on a current collector through coating fluid
application results in unevenness as will be described later.
[0916] As the volume-average particle diameter of the active
material in a powder form, use can be made of the value obtained by
mixing a sample with a 2% by volume aqueous solution (about 1 mL)
of poly(oxyethylene(20)) sorbitan monolaurate as a surfactant and
examining the sample with a laser diffraction type particle size
distribution analyzer (e.g., "LA-920" manufactured by HORIBA, Ltd.)
using ion-exchanged water as a dispersion medium to determine the
volume-average particle diameter (median diameter). In the Examples
given later, the volume-average particle diameter was determined by
this method.
[[[BET Specific Surface Area]]]
[0917] The BET specific surface area of the active material in a
powder form is not particularly limited. However, the BET specific
surface area thereof is generally 0.1 m.sup.2/g or larger,
preferably 0.5 m.sup.2/g or larger, more preferably 1.0 m.sup.2/g
or larger, and is generally 100 m.sup.2/g or smaller, preferably 30
m.sup.2/g or smaller, more preferably 15 m.sup.2/g or smaller. When
the BET specific surface area thereof is smaller than the lower
limit of that range, use of this active material in a negative
electrode is apt to result in poor lithium acceptance during
battery charge and in lithium deposition on the electrode surface.
Such small BET specific surface areas are hence undesirable from
the standpoint of safety. On the other hand, when the BET specific
surface area thereof exceeds the upper limit of that range, there
are cases where use of this active material in a negative electrode
is apt to result in enhanced reactivity with the nonaqueous
electrolyte and enhanced gas evolution and a preferred battery is
difficult to obtain.
[0918] The BET specific surface area of the active material in a
powder form is determined with a surface area meter (e.g., a fully
automatic surface area measuring apparatus manufactured by Ohukura
Riken Co., Ltd.) by preliminarily drying the powder-form active
material at 350.degree. C. for 15 minutes in a nitrogen stream and
then measuring the specific surface area thereof by the gas-flowing
nitrogen adsorption BET one-point method using a nitrogen/helium
mixture gas precisely regulated so as to have a nitrogen pressure
of 0.3 relative to atmospheric pressure.
[[[Tap Density]]]
[0919] The tap density of the active material in a powder form is
not particularly limited. However, the tap density thereof is
generally 0.2 g/cm.sup.3 or higher, preferably 0.3 g/cm.sup.3 or
higher, more preferably 0.5 g/cm.sup.3 or higher, and is generally
3.5 g/cm.sup.3 or lower, preferably 2.5 g/cm.sup.3 or lower. When
the tap density thereof is lower than the lower limit of that
range, there are cases where a negative-electrode active-material
layer having an increased loading density is difficult to form and
a high-capacity battery is difficult to obtain. On the other hand,
when the tap density thereof exceeds the upper limit of that range,
there are cases where the negative-electrode active-material layer
has a reduced amount of interstices and preferred battery
characteristics are difficult to obtain.
[0920] In the invention, the tap density of the active material in
a powder form is determined by dropping the active material through
a sieve having an opening size of 300 .mu.m into a 20-cm.sup.3
tapping cell to fill the cell therewith up to the brim,
subsequently conducting tapping operations 1,000 times over a
stroke length of 10 mm using a powder densimeter (e.g., Tap Denser,
manufactured by Seishin Enterprise Co., Ltd.), and determining a
density from the resultant volume of the sample and the weight
thereof. This density is defined as the tap density.
2. Element-Z-Externally-Possessing Lithium-Occluding Substance
(F)
[0921] The element-Z-externally-possessing lithium-occluding
substance (F) is a material comprising a lithium-occluding metal
(A) and/or lithium-occluding alloy (B) and C (carbon) (carbonaceous
substance (E)) combined therewith as element Z in the negative
electrode. The term "combined" herein means, for example, the state
in which the lithium-occluding metal (A) and/or lithium-occluding
alloy (B) and the carbonaceous substance (E) are restrained by
bonding, the state in which they are physically restrained, or the
state in which they retain a shape by means of electrostatic
restraint. The term "physical restraint" herein means a state such
as that in which the lithium-occluding metal (A) and/or
lithium-occluding alloy (B) is surrounded by the carbonaceous
substance (E) or the state in which the former is caught by the
latter. The term "electrostatic restraint" means the state in which
the lithium-occluding metal (A) and/or lithium-occluding alloy (B)
is adherent to the carbonaceous substance (E) by means of
electrostatic energy. Furthermore, the term "the state of being
restrained by bonding" means chemical bonding by hydrogen bonds,
covalent bonds, ionic bonds, etc.
[0922] Of these, the state in which the surface of the
lithium-occluding metal (A) and/or lithium-occluding alloy (B) at
least partly has an interface where a layer of the carbonaceous
substance (E) is bonded thereto is advantageous from the standpoint
of resistance reduction. The term coating herein means that the
interface between the metal (A) and/or alloy (B) and the surface of
the carbonaceous substance (E) at least partly has chemical bonds
and the metal (A) and/or alloy (B) is (1) in the state of covering
the whole surface, (2) in the state of locally covering the
carbonaceous particles, (3) in the state of selectively covering
part of the surface, or (4) in the state of being present in
ultrafine regions including chemical bonds.
[0923] At the interface, crystallinity may change continuously or
discontinuously. Namely, it is preferred that the
element-Z-externally-possessing lithium-occluding substance (F)
should be one which has an interface where the lithium-occluding
metal (A) and/or lithium-occluding alloy (B) is covered with and/or
bonded to the carbonaceous substance (E) and the interface changes
in crystallinity discontinuously and/or continuously.
[Negative Electrode [7]: Properties of Carbonaceous Substance
(E)]
[[Composition of Carbonaceous Substance (E)]]
[0924] The carbonaceous substance (E) especially preferably is a
product of the carbonization of the following (a) or (b). This
carbonization product may contain a graphitic substance (G) such as
a natural graphite or artificial graphite. Since the graphitic
substance (G) has exceedingly developed crystallinity, it generally
has higher electrical conductivity than the carbonaceous substance
(E) and is highly effective in enhancing conductivity as compared
with the carbonaceous substance (E). Because of this, coexistence
with the carbonaceous substance (E) is preferred from the
standpoint of improving electrical conductivity.
(a) An organic substance capable of carbonization which is selected
from the group consisting of coal-derived heavy oil, straight-run
heavy oil, petroleum-derived cracked heavy oil, aromatic
hydrocarbons, nitrogen-ring compounds, sulfur-ring compounds,
polyphenylenes, organic synthetic polymers, natural polymers,
thermoplastic resins, and thermosetting resins. (b) A solution
obtained by dissolving any of these organic substances capable of
carbonization in a low-molecular organic solvent.
[0925] The coal-derived heavy oil preferably is any of coal tar
pitches ranging from soft pitch to hard pitch, dry
distillation/liquefaction oil, and the like. The straight-run heavy
oil preferably is any of topping residues, vacuum distillation
residues, and the like. The petroleum-derived cracked heavy oil
preferably is, for example, ethylene tar as a by-product of the
thermal cracking of crude oil, naphtha, etc. The aromatic
hydrocarbons preferably are acenaphthylene, decacyclene,
anthracene, phenanthrene, and the like. The nitrogen-ring compounds
preferably are phenazine, acridine, and the like. The sulfur-ring
compounds preferably are thiophene, bithiophene, and the like. The
polyphenylenes preferably are biphenyl, terphenyl, and the like.
The organic synthetic polymers preferably are poly(vinyl chloride),
poly(vinyl alcohol), poly(vinyl butyral), substances obtained by
insolubilizing these compounds, polyacrylonitrile, polypyrrole,
polythiophene, polystyrene, and the like. The natural polymers
preferably are, for example, polysaccharides such as cellulose,
lignin, mannan, poly(galacturonic acid), chitosan, saccharose, and
the like. The thermoplastic resins preferably are poly(phenylene
sulfide), poly(phenylene oxide), and the like. The thermosetting
resins preferably are furfuryl alcohol resins, phenol-formaldehyde
resins, imide resins, and the like.
[0926] Although the carbonaceous substance (E) preferably is a
product of the carbonization of the "organic substance capable of
carbonization" described above, it also preferably is a product of
the carbonization of a solution obtained by dissolving the "organic
substance capable of carbonization" in a low-molecular organic
solvent such as benzene, toluene, xylene, quinoline, or
n-hexane.
[0927] It is especially preferred that the (a) or (b) should be
liquid. Namely, to cause carbonization to proceed in a liquid phase
is preferred from the standpoint of forming an interface between
the carbonaceous substance (E) and the lithium-occluding metal (A)
and/or lithium-occluding alloy (B).
[[Properties of Carbonaceous Substance (E)]]
[0928] It is desirable that the carbonaceous substance (E) should
have properties satisfying anyone of the following items (1) to (3)
or simultaneously satisfy two or more thereof. Furthermore, one
carbonaceous substance (E) having such properties may be used
alone, or any two or more such carbonaceous substances (E) may be
used in combination in any desired proportion.
(1) X-Ray Parameter
[0929] With respect to the crystallinity of the carbonaceous
substance (E), the value of d (interplanar spacing) for the lattice
planes (002) (hereinafter abbreviated to "d002"), as determined by
X-ray diffractometry in accordance with the method of the Japan
Society for Promotion of Scientific Research, is preferably 0.38 nm
or smaller, especially preferably 0.36 nm or smaller, more
preferably 0.35 nm or smaller. When the value of d is too large,
the surface of this carbonaceous substance (E) has considerably low
crystallinity to increase resistance. There are hence cases where
the effect of improving suitability for charge is lessened to
reduce the effects of the invention. The lower limit thereof is at
least 0.335 nm, which is a theoretical value for graphites.
[0930] Furthermore, the crystallite size (Lc) of this carbon
material, as determined by X-ray diffractometry in accordance with
the method of the Japan Society for Promotion of Scientific
Research, is generally 1 nm or larger, preferably 1.5 nm or larger.
When the Lc thereof is smaller than the lower limit of that range,
there are cases where resistance increases and, hence, the effect
of improving suitability for charge is lessened.
(2) Raman R Value, Raman Half-Value Width
[0931] The Raman R value of the carbonaceous substance (E) part as
determined by the argon ion laser Raman spectroscopy is generally
0.2 or higher, preferably 0.3 or higher, more preferably 0.4 or
higher. The upper limit thereof is generally 1.5 or lower,
preferably 1.2 or lower. When the Raman R value thereof is lower
than the lower limit of that range, the surface of such particles
has too high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. On the other hand, when the Raman R value thereof
exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity and enhanced reactivity with
the nonaqueous electrolyte and this may lead to a decrease in
efficiency and enhanced gas evolution.
[0932] The Raman half-value width around 1,580 cm.sup.-1 of the
carbonaceous substance (E) part is not particularly limited.
However, the half-value width thereof is generally 20 cm.sup.-1 or
larger, preferably 30 cm.sup.-1 or larger. The upper limit thereof
is generally 140 cm.sup.-1 or smaller, preferably 100 cm.sup.-1 or
smaller. When the Raman half-value width thereof is smaller than
the lower limit of that range, the surface of such particles has
too high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. On the other hand, when the Raman half-value width
thereof exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity and enhanced reactivity with
the nonaqueous electrolyte and this may lead to a decrease in
efficiency and enhanced gas evolution.
(3) True Density
[0933] The true density of the carbonaceous substance (E) part is
generally 1.4 g/cm.sup.3 or higher, preferably 1.5 g/cm.sup.3 or
higher, more preferably 1.6 g/cm.sup.3 or higher, even more
preferably 1.7 g/cm.sup.3 or higher. The upper limit thereof is up
to 2.26 g/cm.sup.3, which is a theoretical value for graphites.
When the true density thereof is lower than the lower limit of that
range, there are cases where this carbon has too low crystallinity,
resulting in an increase in initial irreversible capacity.
[0934] Although the multielement-containing negative-electrode
active material (C) to be used as the negative-electrode active
material of negative electrode [7] in a lithium secondary battery
of the invention preferably comprises the
element-Z-externally-possessing lithium-occluding substance (F)
constituted of a lithium-occluding metal (A) and/or
lithium-occluding alloy (B) which has been combined with a
carbonaceous substance (E), it is also preferred that this
negative-electrode active material (C) further contains a graphitic
substance (G) as carbon (C) as element Z besides the carbonaceous
substance (E)
[[Composition and Properties of Graphitic Substance (G)]]
[0935] Examples of the composition of the graphitic substance (G)
include natural graphites, artificial graphites, and materials
obtained by subjecting these graphites to treatments, e.g.,
pulverization. It is desirable that the graphitic substance (G)
should have properties which satisfy any one of the following items
(1) to (3) or simultaneously satisfy two or more thereof.
Furthermore, one graphitic substance (G) having such properties may
be used alone, or any two or more such graphitic substances (G) may
be used in combination in any desired proportion.
(1) X-Ray Parameter
[0936] With respect to the graphitic substance (G) part, the value
of d (interplanar spacing) for the lattice planes (002), as
determined by X-ray diffractometry in accordance with the method of
the Japan Society for Promotion of Scientific Research, is at least
0.335 nm, which is a theoretical value for graphites. The upper
limit thereof is preferably 0.340 nm or smaller, more preferably
0.338 nm or smaller, especially preferably 0.337 nm or smaller.
When the value of d is too large, the surface of this graphitic
substance (G) has considerably low crystallinity to increase
resistance. There are hence cases where the effect of improving
suitability for charge is lessened to reduce the effects of the
invention.
[0937] Furthermore, the crystallite size (Lc) of the graphitic
substance (G), as determined by X-ray diffractometry in accordance
with the method of the Japan Society for Promotion of Scientific
Research, is generally 10 nm or larger, preferably 50 nm or larger,
more preferably 80 nm or larger. When the Lc thereof is smaller
than the lower limit of that range, there are cases where
resistance increases and, hence, the effect of improving
suitability for charge is lessened.
(2) Raman R Value, Raman Half-Value Width
[0938] The Raman R value of the graphitic substance (G) part as
determined by the argon ion laser Raman spectroscopy is generally
0.01 or higher, preferably 0.10 or higher. The upper limit thereof
is generally 0.40 or lower, preferably 0.35 or lower, more
preferably 0.25 or lower. When the Raman R value thereof is lower
than the lower limit of that range, the surface of such particles
has too high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. On the other hand, when the Raman R value thereof
exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity and enhanced reactivity with
the nonaqueous electrolyte and this may lead to a decrease in
efficiency and enhanced gas evolution.
[0939] The Raman half-value width around 1,580 cm.sup.-1 of the
graphitic substance (G) part is not particularly limited. However,
the half-value width thereof is generally 10 cm.sup.-1 or larger,
preferably 15 cm.sup.-1 or larger. The upper limit thereof is
generally 50 cm.sup.-1 or smaller, preferably 40 cm.sup.-1 or
smaller. When the Raman half-value width thereof is smaller than
the lower limit of that range, the surface of such particles has
too high crystallinity and there are cases where the number of
intercalation sites into which lithium comes with charge/discharge
decreases. Namely, there are cases where suitability for charge
decreases. On the other hand, when the Raman half-value width
thereof exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity and enhanced reactivity with
the nonaqueous electrolyte and this may lead to a decrease in
efficiency and enhanced gas evolution.
(3) True Density
[0940] The true density of the graphitic substance (G) part is
generally 2.0 g/cm.sup.3 or higher, preferably 2.1 g/cm.sup.3 or
higher, more preferably 2.2 g/cm.sup.3 or higher, even more
preferably 2.22 g/cm.sup.3 or higher. The upper limit thereof is up
to 2.26 g/cm.sup.3, which is a theoretical value for graphites.
When the true density thereof is lower than the lower limit of that
range, there are cases where this carbon has too low crystallinity,
resulting in an increase in initial irreversible capacity.
[0941] In the element-Z-externally-possessing lithium-occluding
substance (F) for use in negative electrode [7] in a lithium
secondary battery of the invention, the proportion of the
lithium-occluding metal (A) and/or lithium-occluding alloy (B) to
the carbonaceous substance (E) by mass desirably is generally 20/80
or higher, preferably 50/50 or higher, more preferably 80/20 or
higher, especially preferably 90/10 or higher, and is preferably
99.9/0.1 or lower, more preferably 99/1 or lower, especially
preferably 98/2 or lower. When the proportion thereof exceeds the
upper limit of that range, there are cases where the presence of
the carbonaceous substance (E) produces no effect. When the
proportion thereof is lower than the lower limit of that range,
there are cases where the effect of increasing capacity per unit
mass is lessened. It is preferred that the proportion of the
lithium-occluding metal (A) and lithium-occluding alloy (B) should
be 20% by mass or higher based on the whole
element-Z-externally-possessing lithium-occluding substance
(F).
[0942] In the case where the graphitic substance (G) is contained,
the content of the graphitic substance (G) is preferably 5% by mass
or higher, more preferably 20% by mass or higher, even more
preferably 50% by mass or higher, based on the sum of the
carbonaceous substance (E) and the graphitic substance (G). The
upper limit thereof is preferably 99% by mass or lower, more
preferably 95% by mass or lower. When the content of the graphitic
substance (G) is too high, there are cases where interfacial
bonding is weak and the effect of improving electrical conductivity
is difficult to obtain. When the content thereof is too low, there
are cases where the effect of improving electrical conductivity
which is produced by the graphitic substance (G) contained is
difficult to obtain.
[0943] In the invention, the element-Z-externally-possessing
lithium-occluding substance (F) is generally in a thin film form or
a powder form. In the invention, a negative electrode employing the
active material in a thin film form may be obtained by forming an
active-material layer on a current collector by vapor phase growth
as described in the production processes which will be given later.
A negative electrode employing the active material in a powder form
is obtained, for example, by applying the powdery active material,
a binder, etc. on a current collector to form an active-material
layer.
[0944] In the case where the element-Z-externally-possessing
lithium-occluding substance (F) is in a powder form, the preferred
ranges of powder properties thereof are the same as the preferred
ranges of powder properties of the element-Z-internally-possessing
lithium-occluding composite (D).
[Negative Electrode [7]: Current Collector]
(Material)
[0945] Examples of the material of the current collector include
copper, nickel, and stainless steel. Of these, copper is preferred
because it can be easily processed into a thin film and is
inexpensive. Examples of copper foils include a rolled copper foil,
which is produced by the rolling process, and an electrolytic
copper foil, which is produced by the electrolytic process. Either
of these can be used as a current collector. In the case of a
copper foil having a thickness smaller than 25 .mu.m, use can be
made of a copper alloy (e.g., phosphor bronze, titanium-copper,
Corson alloy, or Cu--Cr--Zr alloy) having a higher strength than
pure copper.
[0946] The current collector comprising a copper foil produced by
the rolling process is less apt to crack even when the negative
electrode is rolled tightly or rolled at an acute angle, because
the copper crystals are oriented in the rolling direction. This
current collector can be advantageously used in small cylindrical
batteries. The electrolytic copper foil is obtained by immersing a
metallic drum in a nonaqueous electrolyte containing copper ions
dissolved therein, causing current to flow through the system while
rotating the drum to thereby deposit copper on the drum surface,
and peeling the copper deposit from the drum. Copper may have been
deposited on a surface of the rolled copper foil by the
electrolytic process. One or each side of such a copper foil may
have undergone a surface-roughening treatment or a surface
treatment (e.g., a chromate treatment in a thickness of from
several nanometers to about 1 .mu.m or a priming treatment with
titanium).
(Thickness)
[0947] The current collector base comprising a copper foil or the
like preferably is thinner from the standpoint that a thin-film
negative electrode having a small thickness can be produced
therefrom and the thin-film negative electrode having a larger
surface area can be packed into a battery can having the same
volume for packing. However, a copper foil having an excessively
small thickness is deficient in strength and may break upon
winding, etc. in battery production. Consequently, the thickness of
the foil is preferably about 10-70 .mu.m. In the case where an
active-material thin film is to be formed on each side of a copper
foil, this copper foil preferably is even thinner. However, from
the standpoint of preventing the copper foil from being cracked by
the expansion/contraction of the active-material thin films with
charge/discharge, the thickness of the copper foil in this case is
more preferably 8-35 .mu.m. In the case where metal foils other
than copper foils are used as the current collector, these metal
foils may have respective thicknesses suitable for these. However,
the thicknesses thereof are roughly in the range of about 10-70
.mu.m.
(Properties)
[0948] The current collector base is desired to further have the
following properties.
(1) Average Surface Roughness (Ra)
[0949] The average surface roughness (Ra) of that side of the
current collector base on which an active-material thin film is to
be formed, as determined by the method provided for in JIS B
0601-1994, is not particularly limited. However, the average
surface roughness thereof is generally 0.01 .mu.m or higher,
preferably 0.03 .mu.m or higher, and is generally 1.5 .mu.m or
lower, preferably 1.3 .mu.m or lower, especially preferably 1.0
.mu.m or lower.
[0950] By regulating the average surface roughness (Ra) of the
current collector base so as to be in the range of from the lower
limit to upper limit, satisfactory charge/discharge cycle
characteristics can be expected. Namely, by regulating the average
surface roughness thereof to a value not lower than the lower
limit, the area of the interface between the base and an
active-material thin film is increased and adhesion to the
active-material thin film is improved. The upper limit of the
average surface roughness (Ra) thereof is not particularly limited.
However, it is preferably 1.5 .mu.m or lower because a foil having
a practical thickness for batteries and having an average surface
roughness (Ra) exceeding 1.5 .mu.m is generally difficult to
procure.
(2) Tensile Strength
[0951] The tensile strength of the current collector base is not
particularly limited. However, it is generally 50 N/mm.sup.2 or
higher, preferably 100 N/mm.sup.2 or higher, more preferably 150
N/mm.sup.2 or higher. Tensile strength is a quotient obtained by
dividing the maximum tensile force required before test piece
breakage by the sectional area of the test piece. In the invention,
the tensile strength is determined through a measurement conducted
with the same apparatus and by the same method as for elongation. A
current collector base having a high tensile strength can be
inhibited from cracking with the expansion/contraction of the
active-material thin film which occur upon charge/discharge. With
this current collector base, satisfactory cycle characteristics can
be obtained.
(3) 0.2% Proof Stress
[0952] The 0.2% proof stress of the current collector base is not
particularly limited. However, it is generally 30 N/mm.sup.2 or
higher, preferably 100 N/mm.sup.2 or higher, especially preferably
150 N/mm.sup.2 or higher. The term 0.2% proof stress means the
degree of load necessary for imparting a plastic (permanent)
deformation of 0.2%. Namely, it means that application of that
degree of load and subsequent removal thereof result in a 0.2%
deformation. In the invention, the 0.2% proof stress is determined
through a measurement conducted with the same apparatus and by the
same method as for elongation. A current collector base having a
high 0.2% proof stress can be inhibited from plastically deforming
with the expansion/contraction of the active-material thin film
which occur upon charge/discharge. With this current collector
base, satisfactory cycle characteristics can be obtained.
[Negative Electrode [7]: Processes for Producing
Multielement-Containing Negative-Electrode Active Materials
(C)]
[0953] Processes for producing the multielement-containing
negative-electrode active materials (C)
(element-Z-internally-possessing lithium-occluding substance (D)
and element-Z-externally-possessing lithium-occluding substance
(F)) according to the invention are not particularly limited. For
example, the negative-electrode active materials can be produced by
the following processes.
1. Processes for Producing Element-Z-Internally-Possessing
Lithium-Occluding Substance (D)
<Process 1>
[0954] Any one of the following is used as a vapor deposition
source, sputtering source, or thermal spraying source:
[0955] (i) a composition of Si, element Z, and element M (provided
that it is a composition of Si and element Z when y=0 or
y.apprxeq.0);
[0956] (ii) a mixture of Si, element Z, and element M (provided
that it is a mixture of Si and element Z when y=0 or
y.apprxeq.0);
[0957] (iii) Si, element Z, and element M which are separate
materials (the separate materials each may be a gas comprising the
element) (provided that (iii) is Si and element Z as separate
materials when y=0 or y.apprxeq.0);
[0958] (iv) a combination of a composition or mixture of Si and
element Z with element M as a separate material (which may be a gas
comprising M);
[0959] (v) a gas comprising Si, element Z, and element M (provided
that it is a gas comprising Si and element Z when y=0 or
y.apprxeq.0);
[0960] (vi) a combination of Si as a separate material with a
composition or mixture of element Z and element M; and
[0961] (vii) a combination of a composition or mixture of Si and
element M with element Z as a separate material (which may be a gas
comprising element Z).
Si, element Z, and element M (or Si and element Z when y=0 or
y.apprxeq.0) are simultaneously deposited on the current collector
base described above by the vapor deposition method and/or
sputtering method and the thermal spraying method to forma film
having a thickness of 1-30 .mu.m, preferably in the thickness
described above in "Film Thickness" under "Active Material in Thin
Film Form".
(Raw Materials)
[0962] Of the vapor deposition source, sputtering source, or
thermal spraying source (hereinafter often referred to suitably as
"raw material"), the Si separate-material raw material to be used
can be, for example, crystalline Si or amorphous Si. As the
raw-material Z, use can be made of the element C or N. With respect
to element Z, two or more elements may be simultaneously used as
long as they satisfy the items described above.
[0963] Of the raw materials, the raw material (i), i.e., a
composition of Si, element Z, and element M (provided that it is a
composition of Si and element Z when y=0 or y.apprxeq.0), may be a
single compound comprising a combination of Si, element Z, and
element M or of Si and element Z, or may be two or more compounds
of these elements. The Si, raw-material Z, and raw-material M may
be used in the form of, e.g., a powder, granules, pellets, mass, or
plate.
[0964] In the case where y.noteq.0 in the general formula
SiZ.sub.xM.sub.y, i.e., when the lithium-occluding substance (D)
contains element M, the element M may be one or more elements
selected from Groups 2, 4, 8, 9, 10, 11, 13, 14, 15, and 16 of the
periodic table excluding Si and element Z. Preferably, one or more
of the elements Ti, Zr, W, O, and Co may be used. More preferably,
the element O may be used.
(Methods of Film Deposition)
[0965] Examples of methods for forming the active-material thin
film include vapor phase growth methods such as, e.g., the vapor
deposition method (vacuum deposition, CVD, or ion plating),
sputtering method, and thermal spraying method (flame spraying or
plasma spraying). Furthermore, a combination of the sputtering
method and vapor deposition method or a combination of the
sputtering method and thermal spraying method may be used for film
deposition.
[0966] Such methods for forming a thin film of the
negative-electrode active material will be explained below.
A. Sputtering Method
[0967] In the sputtering method, a raw material for the active
material which has been emitted from a target comprising any of the
raw materials by means of a plasma at a reduced pressure is caused
to strike and deposit on a current collector base to thereby forma
thin film. According to the sputtering method, the interface
between the active-material thin film formed and the current
collector base is in a satisfactory state and this active-material
thin film has high adhesion to the current collector.
[0968] With respect to the method of applying a sputtering voltage
to the target, either of a direct-current voltage and an
alternating-current voltage may be used. In this operation, a
substantially negative bias voltage may be applied to the current
collector base to thereby control the collision energy of ions from
the plasma. The degree of vacuum to be reached in the chamber
before the initiation of thin-film formation is generally 0.1 Pa or
lower from the standpoint of preventing impurity inclusion.
[0969] As a sputtering gas is used an inert gas such as, e.g., Ne,
Ar, Kr, or Xe. Of these, argon gas is preferred from the
standpoints of sputtering efficiency, etc. In the case where
element Z in the compound SiZ.sub.xM.sub.y is N, it is preferred
from the standpoint of production that a slight amount of nitrogen
gas be caused to coexist with the inert gas. The sputtering gas
pressure is generally about 0.05-70 Pa. In forming an
active-material thin film by the sputtering method, the temperature
of the current collector base may be regulated by water cooling or
with a heater, etc. The temperature of the current collector base
is generally in the range of from room temperature to 900.degree.
C., and is preferably 150.degree. C. or lower. The rate of film
deposition in forming an active-material thin film by the
sputtering method is generally 0.01-0.5 .mu.m/min.
[0970] Prior to the formation of an active-material thin film, the
surface of the current collector base can be etched by a
pretreatment, e.g., reverse sputtering or another treatment such as
plasma treatment. Such a pretreatment is effective in removing
contaminants and an oxide film from the surface of the copper foil
as the current collector base and improving the adhesion of the
active-material thin film.
B. Vacuum Deposition Method
[0971] The vacuum deposition method is a technique in which the raw
material for the active material is melted/vaporized and deposited
on a current collector base. This method generally has an advantage
that a thin film can be formed at a higher deposition rate than in
the sputtering method. Compared to the sputtering method, the
vacuum deposition method can be advantageously utilized from the
standpoint of production cost because the time period necessary for
forming an active-material thin film having a given thickness can
be reduced. Specific examples thereof include the induction heating
method, resistance heating method, and electron beam heating
deposition method. In the induction heating method, a deposition
material placed in an evaporation crucible of, e.g., graphite, is
heated with an induction current. In the resistance heating method,
a deposition material in an evaporation boat or the like is heated
by applying a heating current thereto. In the electron beam heating
deposition method, a deposition material is heated with electron
beams. The deposition material is thus melted and vaporized to
deposit a film.
[0972] With respect to the atmosphere for the vacuum deposition
method, the deposition is generally conducted under vacuum. In the
case where element Z in the compound SiZ.sub.xM.sub.y is N, it is
possible to use a technique in which the pressure in the chamber is
reduced while introducing a slight amount of nitrogen gas together
with an inert gas to conduct simultaneous deposition under vacuum
and thereby form the SiZ.sub.xM.sub.y. The degree of vacuum to be
reached in the chamber before the initiation of thin-film formation
is generally 0.1 Pa or lower from the standpoint of preventing
impurity inclusion.
[0973] In forming an active-material thin film by the vacuum
deposition method, the temperature of the current collector base
may be regulated with a heater, etc. The temperature of the current
collector base is generally in the range of from room temperature
to 900.degree. C., and is preferably 150.degree. C. or lower. The
rate of film deposition in forming a negative-electrode
active-material thin film by the vacuum deposition method is
generally 0.1-50 .mu.m/min.
[0974] As in the case of the sputtering method, the surface of the
current collector base may be etched by ion irradiation with an ion
gun or the like before the deposition of an active-material thin
film on the current collector base. By such an etching treatment,
adhesion between the base and the active-material thin film can be
further enhanced. Furthermore, the current collector base may be
bombarded with ions during thin-film formation, whereby the
adhesion of the active-material thin film to the current collector
base can be further improved.
C. CVD Method
[0975] In the CVD method, the raw material for the active material
is deposited on a current collector base through a vapor-phase
chemical reaction. A feature of the CVD method generally resides in
that since compound gases in a reaction chamber are regulated by
gas introduction, a variety of materials having a high purity can
be synthesized. Specific examples of this method include the
thermal CVD method, plasma CVD method, photo-CVD method, and
cat-CVD method. The thermal CVD method is a technique in which a
raw-material gas of a halogen compound having a high vapor pressure
is introduced, together with a carrier gas or reaction gas, into a
reaction vessel heated at around 1,000.degree. C. to thermally
cause a chemical reaction and thereby form a thin film. The plasma
CVD method is a technique employing a plasma in place of heat
energy. The photo-CVD method is a technique employing light energy
in place of heat energy. The cat-CVD method, i.e., the catalytic
chemical vapor deposition method, is a technique in which a
catalytic decomposition reaction of a raw-material gas with a
heated catalyst is utilized to thereby form a thin film.
[0976] Examples of the raw-material gases for use in the CVD method
include SiH.sub.4 and SiCl.sub.4 as element Si sources and
NH.sub.3, N.sub.2, BCl.sub.3, CH.sub.4, O.sub.2H.sub.6, and
O.sub.3H.sub.8 as element Z sources.
D. Ion Plating Method
[0977] In the ion plating method, the raw material for the active
material is melted/vaporized, and the vapor particles are ionized
and excited in the presence of a plasma to thereby tenaciously
deposit a film on a current collector base. Specifically, examples
of methods for melting/vaporizing the raw material include the
induction heating method, resistance heating method, and electron
beam heating deposition method. Examples of methods for ionization
and excitation include the activation reaction deposition method,
multi-cathode thermoelectronic irradiation method, high-frequency
excitation method, HCD method, cluster ion beam method, and
multi-arc method. Techniques suitably selected respectively from
those methods for raw-material vaporization and from those methods
for ionization and excitation may be used in combination.
E. Thermal Spraying Method
[0978] In the thermal spraying method, the raw material for the
active material is melted or softened by heating and formed into
fine particles, which are accelerated toward a current collector
base and solidified/deposited thereon. Specific examples thereof
include the flame spraying method, arc spraying method,
direct-current plasma spraying method, RF plasma spraying method,
and laser spraying method.
F. Combination of Sputtering Method and Vapor Deposition Method
[0979] The high film deposition rate which is an advantage of the
vapor deposition method and the tenacious deposit adhesion to a
current collector base which is an advantage of the sputtering
method can be utilized, for example, in the following manner. A
first thin film layer is formed by the sputtering method and a
second thin film layer is thereafter formed at a high rate by the
vapor deposition method. Thus, an interference region having
satisfactory adhesion to the current collector base is formed and
an active-material thin film can be formed at a high deposition
rate. By such a hybrid combination of film deposition techniques, a
thin-film negative electrode having a high charge/discharge
capacity and excellent charge/discharge cycle characteristics can
be efficiently produced.
[0980] It is preferred that the formation of an active-material
thin film using a combination of the sputtering method and the
vapor deposition method should be conducted successively while
maintaining a reduced-pressure atmosphere. This is because impurity
inclusion can be prevented by successively forming a first thin
film layer and a second thin film layer while avoiding exposure to
the air. For example, it is preferred to use an apparatus for
thin-film formation in which sputtering and vapor deposition are
successively conducted in the same vacuum environment while moving
the current collector base.
[0981] In the case where an active-material thin film is formed on
each side of a current collector base by such a film deposition
method in the invention, it is preferred that the formation of an
active-material thin film layer (which may be the combination of a
first thin film layer and a second thin film layer) on one side of
the current collector base and the formation of an active-material
thin film layer (which may be the combination of a first thin film
layer and a second thin film layer) on the other side of the
current collector base should be successively conducted while
maintaining a reduced-pressure atmosphere.
<Process 2>
[0982] A production process for use in the case where element Z in
the general formula SiZ.sub.xM.sub.y is C is described below.
[0983] Any one of the following is used as a vapor deposition
source, sputtering source, or thermal spraying source:
[0984] (i) a composition of Si, C, and element M (provided that it
is a composition of Si and C when y=0 or y.apprxeq.0);
[0985] (ii) a mixture of Si, C, and element M (provided that it is
a mixture of Si and C when y=0 or y.apprxeq.0);
[0986] (iii) Si, C, and element M which are separate materials
(provided that (iii) is Si and C as separate materials when y=0 or
y.apprxeq.0);
[0987] (iv) a combination of a composition or mixture of Si and C
with element M as a separate material (which may be a gas
comprising M);
[0988] (v) a gas comprising Si, C, and element M (provided that it
is a gas comprising Si and C when y=0 or y.apprxeq.0);
[0989] (vi) a combination of Si as a separate material with a
composition or mixture of C and element M; and
[0990] (vii) a combination of a composition or mixture of Si and
element M with C as a separate material.
Si, C, and element M (or Si and C when y=0 or y.apprxeq.0) are
simultaneously deposited on the current collector base described
above by the vapor deposition method and/or sputtering method and
the thermal spraying method to form a film having a thickness of
1-30 .mu.m, preferably in the thickness described above in "Film
Thickness" under "Active Material in Thin Film Form".
(Raw Materials)
[0991] Of the vapor deposition source or sputtering source
(hereinafter often referred to suitably as "raw material"), the
raw-material Si to be used can be, for example, crystalline Si or
amorphous Si. As the raw-material C, use can be made of, for
example, a carbon material such as a natural graphite or artificial
graphite. As the raw-material M can generally be used one or more
of the elements in Groups 2, 4, 8, 9, 10, 11, 13, 14, 15, and 16 of
the periodic table excluding Si and element Z. Preferably, one or
more of the elements Ti, Zr, W, O, and Co may be used. Especially
preferably, the element O may be used.
[0992] Of the raw materials, the raw material (i), i.e., a
composition of Si, C, and element M, may be a single compound
comprising a combination of Si, C, and element M, or may be two or
more compounds of these elements. The raw-material Si, C, and M may
be used in the form of, e.g., a powder, granules, pellets, mass, or
plate. The raw material for element M may be a nitride or oxide of
Si or C. However, in the case of a raw material which is present as
a gas at ordinary temperature, e.g., O, it is preferred from the
standpoint of production that the raw-material gas, e.g., O gas,
should be caused to coexist during the deposition of Si and C.
(Methods of Film Deposition)
[0993] The same film deposition methods as in Process 1 may be
used.
A. Sputtering Method
[0994] As a sputtering gas is used an inert gas such as, e.g., Ne,
Ar, Kr, or Xe. Of these, argon gas is preferred from the
standpoints of sputtering efficiency, etc. In the case where
element M in the general formula SiC.sub.xM.sub.y is O, it is
preferred from the standpoint of production that a slight amount of
oxygen gas be caused to coexist with each of those inert gases. The
sputtering gas pressure is generally about 0.05-70 Pa.
B. Vacuum Deposition Method
[0995] With respect to the atmosphere for the vacuum deposition
method, the deposition is generally conducted under vacuum. In the
case where element M in the general formula SiC.sub.xM.sub.y is O,
it is possible to use a technique in which the pressure in the
chamber is reduced while introducing a slight amount of oxygen gas
together with each of those inert gases to conduct simultaneous
deposition under vacuum and thereby form Si/C/N.
C. CVD Method
[0996] Examples of the raw-material gases for use in the CVD method
include SiH.sub.4 and SiCl.sub.4 as element Si sources and
CH.sub.4, C.sub.2H.sub.6, and C.sub.3H.sub.8 as element C
sources.
<Process 3>
[0997] A production process for use in the case where element Z and
element M in general formula SiZ.sub.xM.sub.y are C and O,
respectively, is described below.
[0998] Any one of the following is used as a vapor deposition
source, sputtering source, or thermal spraying source:
(I) a composition of Si and C; (II) a mixture of Si and C; (III) Si
and C as separate materials; and (IV) a gas containing Si and C. Si
and C are simultaneously deposited by the vapor deposition method
and/or sputtering method and the thermal spraying method in such an
atmosphere that the oxygen concentration in the deposition gas (or
in the residual gas in the case of film deposition under vacuum) is
0.0001-0.125%. Thus, a film is deposited on the current collector
base described above in a thickness of 1-30 .mu.m, preferably in
the thickness described above in "Film Thickness" under "Active
Material in Thin Film Form".
(Raw Materials)
[0999] Of the vapor deposition source, sputtering source, or
thermal spraying source, the raw-material Si to be used can be, for
example, crystalline Si or amorphous Si. As the raw-material C, use
can be made of, for example, a carbon material such as a natural
graphite or artificial graphite. An O-element-containing gas, e.g.,
oxygen, is used as oxygen in the deposition gas either alone or in
combination with an inert gas. The raw-material Si and C may be
used in the form of, e.g., a powder, granules, pellets, mass, or
plate. It is preferred from the standpoint of production that
oxygen gas should be caused to coexist as a raw-material gas during
the deposition of Si and C.
(Methods of Film Deposition)
[1000] The same film deposition methods as in Process 1 may be
used.
(Oxygen Concentration during Film Deposition)
[1001] During vapor deposition and/or sputtering and thermal
spraying, the oxygen concentration in the deposition gas (or in the
residual gas in the case of film deposition under vacuum) is
generally 0.0001% or higher and is generally 0.125% or lower,
preferably 0.100% or lower, more preferably 0.020% or lower. When
the oxygen concentration in the deposition gas exceeds the upper
limit of that range, the Si/C/O thin film has an increased element
O amount. There are hence cases where this negative-electrode
active material has enhanced reactivity with the nonaqueous
electrolyte, leading to a decrease in charge/discharge efficiency.
When the oxygen concentration therein is too low, there are cases
where an Si/C/O thin film cannot be deposited.
[1002] Incidentally, the oxygen concentration in a deposition gas
is obtained, for example, by analysis for mass spectrum with a
quadrupole mass filter. In the case where argon gas with which
oxygen gas coexists is used as a deposition gas, the oxygen
concentration therein is determined by examining this argon gas
with an oxygen analyzer.
<Process 4>
[1003] A production process for use in the case where element Z in
the general formula SiZ.sub.xM.sub.y is N and y therein is y=0 or
y.apprxeq.0 is described below.
[1004] Any one of the following is used as a vapor deposition
source, sputtering source, or thermal spraying source:
(I) Si as a separate material; (II) a composition containing Si;
(III) a mixture containing Si; and (IV) a gas containing Si. Si and
N are simultaneously deposited by the vapor deposition method
and/or sputtering method and the thermal spraying method in such an
atmosphere that the nitrogen concentration in the deposition gas
(or in the residual gas in the case of film deposition under
vacuum) is 1-22%. Thus, a film is deposited on the current
collector base described above in a thickness of 1-30 .mu.m,
preferably in the thickness described above in "Film Thickness"
under "Active Material in Thin Film Form".
(Raw Materials)
[1005] Of the vapor deposition source, sputtering source, or
thermal spraying source, the Si separate-material raw material to
be used can be, for example, crystalline Si or amorphous Si. An
N-element-containing gas, e.g., nitrogen, is used as N in the
deposition gas either alone or in combination with an inert gas.
The Si, etc. may be used in the form of, e.g., a powder, granules,
pellets, mass, or plate. It is preferred from the standpoint of
production that nitrogen gas should be caused to coexist as a
raw-material gas during the deposition of Si.
(Methods of Film Deposition)
[1006] The same film deposition methods as in Process 1 may be
used.
(Nitrogen Concentration During Film Deposition)
[1007] During vapor deposition and/or sputtering and thermal
spraying, the nitrogen concentration in the deposition gas (or in
the residual gas in the case of film deposition under vacuum) is
generally 1% or higher and is generally 22% or lower, preferably
15% or lower, more preferably 10% or lower. When the nitrogen
concentration in the deposition gas exceeds the upper limit of that
range, the SiN.sub.x thin film has an increased element N amount.
There are hence cases where a silicon nitride not participating in
charge/discharge is yielded, leading to a decrease in
charge/discharge capacity. When the nitrogen concentration therein
is too low, there are cases where an SiN.sub.x thin film containing
N cannot be deposited, leading to a decrease in cycle
characteristics. Incidentally, the nitrogen concentration in a
deposition gas is obtained, for example, by analysis for mass
spectrum with a quadrupole mass filter.
2. Processes for Producing Element-Z-Externally-Possessing
Lithium-Occluding Substance (F)
<Process 5>
[1008] A lithium-occluding metal (A) and/or a lithium-occluding
alloy (B) is mixed with the organic substance capable of
carbonization described above under "Composition of Carbonaceous
Substance (E)". This mixture is heated to decompose the organic
substance and carbonize it through a solid phase and/or a liquid
phase and/or a gas phase. Thus, a carbonaceous substance (E) is
formed to obtain a composite. Furthermore, the resultant particles
are pulverized/classified to obtain an appropriate value of
volume-average particle diameter.
(Raw Materials)
[1009] The lithium-occluding metal (A) and/or lithium-occluding
alloy (B) as a raw material has a volume-average particle diameter
of generally 100 .mu.m or smaller, preferably 10 .mu.m or smaller,
more preferably 1 .mu.m or smaller. The lower limit thereof is 1 nm
or larger. When the volume-average particle diameter thereof
exceeds the upper limit, expansion during charge is difficult to
diminish and there are cases where capacity retention through
cycling decreases. When the volume-average particle diameter
thereof is smaller than the lower limit, pulverization is difficult
and this may lead to a loss in time or cost. Although the raw
material for the carbonaceous substance (E) is as described above,
it preferably is a substance which undergoes a liquid phase when
carbonized.
<Process 6>
[1010] A lithium-occluding metal (A) and/or lithium-occluding alloy
(B) is mixed with a graphitic substance (G) and with the
carbonaceous substance capable of carbonization described above
under "Composition of Carbonaceous Substance (E)". This mixture is
heated to decompose the organic substance and carbonize it in a
liquid phase. Thus, a carbonaceous substance (E) was formed to
obtained a composite. Furthermore, the resultant particles are
pulverized/classified to obtain an appropriate value of
volume-average particle diameter.
(Raw Materials)
[1011] The raw materials are the same as in <Process 5>.
<Process 7>
[1012] A lithium-occluding metal (A) and/or lithium-occluding alloy
(B) is mixed with a graphitic substance (G) and with the
carbonaceous substance capable of carbonization described above
under "Composition of Carbonaceous Substance (E)". This mixture is
heated to decompose the organic substance and carbonize it in a
solid phase. Thus, a carbonaceous substance (E) was formed to
obtained a composite. Furthermore, the resultant particles are
pulverized/classified to obtain an appropriate value of
volume-average particle diameter.
(Raw Materials)
[1013] The raw materials are the same as in <Process 5>.
<Process 8>
[1014] A lithium-occluding metal (A) and/or lithium-occluding alloy
(B) is mixed with a graphitic substance (G) and with the
carbonaceous substance capable of carbonization described above
under "Composition of Carbonaceous Substance (E)". This mixture is
heated to decompose the organic substance and carbonize it in a gas
phase. Thus, a carbonaceous substance (E) was formed to obtained a
composite. Furthermore, the resultant particles are
pulverized/classified to obtain an appropriate value of
volume-average particle diameter.
(Raw Materials)
[1015] The raw materials are the same as in <Process 5>.
[Negative Electrode 7: Electrode Formation with Active Material in
Powder Form]
[1016] The negative electrode may be produced by an ordinary
method. For example, it can be formed in the same manner as
described above by adding a binder and a solvent to the
negative-electrode active material optionally together with a
thickener, conductive material, filler, etc. to obtain a slurry,
applying it to a current collector, drying the slurry applied, and
then pressing the coated current collector. The thickness of the
negative-electrode active-material layer per one side in the stage
just before the step of injecting an electrolyte in battery
fabrication is generally 5 .mu.m or larger, preferably 10 .mu.m or
larger, more preferably 15 .mu.m or larger. The upper limit thereof
may be 150 .mu.m or smaller and is preferably 120 .mu.m or smaller,
more preferably 100 .mu.m or smaller. When the thickness thereof is
larger than the upper limit of that range, a nonaqueous electrolyte
is less apt to infiltrate into around the interface of the current
collector and, hence, there are cases where high-current-density
charge/discharge characteristics decrease. When the thickness
thereof is smaller than the lower limit of that range, the
proportion by volume of the current collector to the
negative-electrode active material increases and there are cases
where battery capacity decreases. The negative-electrode active
material may be molded by rolling to obtain a sheet electrode. It
is also possible to subject the negative-electrode active material
to compression molding to obtain a pellet electrode.
[1017] The binder, thickener, etc. to be used are the same as those
described above.
<Negative Electrode [8]>
[1018] Negative electrode [8], i.e., "a negative electrode
containing as a negative-electrode active material two or more
negative-electrode active materials differing in property", for use
in a lithium secondary battery of the invention will be explained
below.
[Negative Electrode [8]: Negative-Electrode Active Material]
[1019] The negative-electrode active material for use in negative
electrode [8] is described below.
[1020] The negative-electrode active material for use in negative
electrode [8] in a lithium secondary battery of the invention is
characterized by comprising two or more negative-electrode active
materials differing in property.
[1021] The term "differing in property" used herein not only means
that the active materials differ in any of powder particle shape
and powder properties represented by X-ray diffraction parameter,
median diameter, aspect ratio, BET specific surface area,
orientation ratio, Raman R value, tap density, true density, pore
distribution, roundness, ash content, etc., but also involves the
following cases: the case where the active materials differ in
material constitution, such as "a composite carbonaceous material
comprising two or more carbonaceous substances differing in
crystallinity" and "an orientation-differing-carbon composite
comprising two or more carbonaceous substances differing in
orientation", and the case where the active materials differ in
processing or treatment, such as the case including "a
negative-electrode active material which has undergone a heat
treatment" or "a negative-electrode active material which has
undergone a mechanical energy treatment".
[Difference in Shape, Property, etc.]
[1022] The negative-electrode active material to be used in
negative electrode [8] in a lithium secondary battery of the
invention may comprise two or more negative-electrode active
materials differing in volume-average particle diameter (median
diameter) among those properties. This negative-electrode active
material can improve cycle characteristics while maintaining
low-temperature output. The difference in volume-average particle
diameter (median diameter) desirably is generally 1 .mu.m or
larger, preferably 2 .mu.m or larger, more preferably 5 .mu.m or
larger. The upper limit thereof is generally 30 .mu.m or smaller,
preferably 25 .mu.m or smaller. When the difference therein exceeds
the upper limit of that range, the material having a larger median
diameter tends to have too large a particle diameter and this may
pose a problem that use of this negative-electrode active material
in electrode production results in streak lines in the coating
surface. On the other hand, when the difference therein is smaller
than the lower limit of that range, there are cases where the
effect to be produced by the mixing of two kinds is less apt to be
obtained.
[1023] Furthermore, the negative-electrode active material to be
used in negative electrode [8] in a lithium secondary battery of
the invention may be a material having a volume-based particle size
distribution which is not normal. This material also can impart
satisfactory properties for the same reasons as described above.
The term "having a volume-based particle size distribution which is
not normal" means that the volume-based particle size distribution
curve drawn with the logarithmic scale as abscissa is not
symmetrical about the volume-average particle diameter (median
diameter). With respect to the degree of asymmetry, the value of Z
represented by the following equation (1) is generally 0.3 .mu.m or
larger, preferably 0.5 .mu.m or larger, more preferably 1 .mu.m or
larger. When Z is smaller than that value, there are cases where
the effect of improving cycle characteristics which is produced by
a particle size distribution which is not normal is difficult to
obtain.
Z=|(mode diameter)-(median diameter)| (1)
[1024] In equation (1), the unit of mode diameter and that of
median diameter each are ".mu.m", and "| |" indicates absolute
value.
[1025] In this invention, the volume-average particle diameter
(median diameter) and the mode diameter are defined as values
determined by dispersing the negative-electrode active material in
a 0.2% by mass aqueous solution (about 1 mL) of poly(oxyethylene
(20)) sorbitan monolaurate as a surfactant and examining the
dispersion with a laser diffraction type particle size distribution
analyzer (LA-700, manufactured by HORIBA, Ltd.). The "median
diameter", which is generally referred to also as d.sub.50, means
the particle diameter at which the volume-based distribution of the
particles is divided into two parts, i.e., a larger-size part and a
smaller-size part, which are equal in amount. The "mode diameter"
means the particle diameter at which the volume-based particle size
distribution has a maximum value. On LA-700, manufactured by
HORIBA, Ltd., those diameters are univocally shown simply as
"median diameter" and "mode diameter" respectively.
[1026] It is also preferred to use a negative-electrode active
material having a median diameter of 10 .mu.m or smaller as at
least one of the negative-electrode active materials. This is
because use of this negative-electrode active material is effective
in improving cycle characteristics while maintaining
low-temperature output. The median diameter thereof is especially
preferably 8 .mu.m or smaller. It is especially preferred that the
negative-electrode active material having a median diameter of 10
.mu.m or smaller be used in an amount in the range of 0.5-10% by
mass based on all negative-electrode active materials.
[1027] The negative-electrode active material for use in negative
electrode [8] in a lithium secondary battery of the invention may
comprise two or more negative-electrode active materials differing
in Raman R value as determined by argon ion laser Raman
spectroscopy. This negative-electrode active material can improve
low-temperature output while maintaining cycle characteristics. The
difference in Raman R value is generally 0.1 or larger, preferably
0.2 or larger, more preferably 0.3 or larger. The upper limit
thereof is generally 1.4 or smaller, preferably 1.3 or smaller,
more preferably 1.2 or smaller. When the difference therein is
smaller than the lower limit of that range, there are cases where
the effect to be produced by a difference in Raman R value is
difficult to obtain. On the other hand, when the difference therein
exceeds the upper limit of that range, there are cases where
irreversible capacity increases due to the part having a higher
Raman R value.
[1028] In this invention, the examination for a Raman spectrum is
made with a Raman spectrometer (e.g., a Raman spectrometer
manufactured by Japan Spectroscopic Co., Ltd.). In the examination,
a sample is charged into a measuring cell by causing the sample to
fall naturally into the cell and the surface of the sample in the
cell is irradiated with argon ion laser light while rotating the
cell in a plane perpendicular to the laser light. The Raman
spectrum obtained is examined for the intensity I.sub.A of a peak
P.sub.A at 1,580 cm.sup.-1 and the intensity I.sub.B of a peak
P.sub.B at 1,360 cm.sup.-1. The ratio between these intensities R
(R=I.sub.B/I.sub.A) is calculated and this ratio is defined as the
Raman R value of the graphitic carbon particles. Furthermore, the
half-value width of the peak P.sub.A at 1,580 cm.sup.-1 in the
Raman spectrum obtained is measured, and this value is defined as
the Raman half-value width of the graphitic carbon particles.
[1029] Conditions for this Raman spectroscopy are as follows.
[1030] Wavelength of argon ion laser: 514.5 nm [1031] Laser power
on sample: 15-25 mW [1032] Resolution: 10-20 cm.sup.-1 [1033]
Examination range: 1,100 cm.sup.-1 to 1,730 cm.sup.-1 [1034]
Analysis for Raman R value and Raman half-value width: background
processing [1035] Smoothing: simple average; convolution, 5
points
[1036] The Raman half-value width at 1,580 cm.sup.-1 of the
negative-electrode active materials for use as the
negative-electrode active material of negative electrode [8] in a
lithium secondary battery of the invention is not particularly
limited. However, the half-value width thereof is generally 10
cm.sup.-1 or larger, preferably 15 cm.sup.-1 or larger. The upper
limit thereof is generally 150 cm.sup.-1 or smaller, preferably 140
cm.sup.-1 or smaller. In case where the Raman half-value width
thereof is smaller than the lower limit of that range, the surface
of such particles has too high crystallinity and there are cases
where low-temperature output decreases. On the other hand, in case
where the Raman half-value width thereof exceeds the upper limit of
that range, the surface of such particles has reduced crystallinity
and, hence, there area cases where irreversible capacity
increases.
[1037] The negative-electrode active material to be used in
negative electrode [8] in a lithium secondary battery of the
invention may comprise two or more negative-electrode active
materials differing in crystallinity. This negative-electrode
active material also can improve low-temperature output while
maintaining cycle characteristics. The term crystallinity herein
means a multilayer structure such as the repeating-structure
thickness of superposed carbon-hexagonal planar structures, spacing
thereof, etc. Although there are no particular limitations on
specific properties usable for indicating a difference in
crystallinity, the properties include interplanar spacing,
crystallite size, and the like. It is preferred that the two or
more negative-electrode active materials to be used in a lithium
secondary battery of the invention should differ in any of these
properties. When the difference in crystallinity is too small,
there are cases where the effect to be produced by the mixing is
difficult to obtain.
[1038] The negative-electrode active material to be used in
negative electrode [8] in a lithium secondary battery of the
invention may comprise two or more negative-electrode active
materials differing in interplanar spacing (d002) for the (002)
planes as determined by wide-angle X-ray diffractometry. This
negative-electrode active material can improve low-temperature
output while maintaining cycle characteristics. The difference in
interplanar spacing (d002) is generally 0.0005 nm or larger,
preferably 0.001 nm or larger, more preferably 0.003 nm or larger,
even more preferably 0.004 nm or larger. The upper limit thereof is
generally 0.05 or smaller, preferably 0.04 or smaller, more
preferably 0.03 nm or smaller, even more preferably 0.02 or
smaller. When the difference therein is smaller than the lower
limit of that range, there are cases where the effect to be
produced by a difference in crystallinity is difficult to obtain.
On the other hand, when the difference therein exceeds the upper
limit of that range, there are cases where irreversible capacity
increases due to the part having lower crystallinity. The term
"interplanar spacing (d002) for the (002) planes as determined by
wide-angle X-ray diffractometry" in the invention means the value
of d (interplanar spacing) for the lattice planes (002) as
determined by X-ray diffractometry in accordance with the method of
the Japan Society for Promotion of Scientific Research.
[1039] The negative-electrode active material to be used in
negative electrode [8] in a lithium secondary battery of the
invention may comprise two or more negative-electrode active
materials differing in crystallite size (Lc) as determined by X-ray
diffractometry in accordance with the method of the Japan society
for Promotion of Scientific Research. This negative-electrode
active material can improve low-temperature output while
maintaining cycle characteristics. The difference in crystallite
size (Lc) as determined by X-ray diffractometry in accordance with
the method of the Japan Society for Promotion of Scientific
Research is generally 1 nm or larger, preferably 10 nm or larger,
more preferably 50 nm or larger. When the difference therein is
smaller than the lower limit of that range, there are cases where
the effect to be produced by a difference in crystallite size is
difficult to obtain.
[1040] The negative-electrode active material to be used in
negative electrode [8] in a lithium secondary battery of the
invention may comprise two or more negative-electrode active
materials differing in true density. This negative-electrode active
material also can improve low-temperature output while maintaining
cycle characteristics. The difference in true density is generally
0.03 g/cm.sup.3 or larger, preferably 0.05 g/cm.sup.3 or larger,
more preferably 0.1 g/cm.sup.3 or larger, even more preferably 0.2
g/cm.sup.3 or larger. The upper limit thereof is generally 0.7
g/cm.sup.3 or smaller, preferably 0.5 g/cm.sup.3 or smaller, more
preferably 0.4 g/cm.sup.3 or smaller. When the difference therein
is smaller than the lower limit of that range, there are cases
where the effect to be produced by a difference in true density is
difficult to obtain. On the other hand, when the difference therein
exceeds the upper limit of that range, there are cases where
irreversible capacity increase due to the part having a lower true
density.
[1041] True density in the invention is defined as one determined
by the liquid-phase displacement method (pycnometer method) using
butanol.
[1042] The negative-electrode active material to be used in this
invention may comprise two or more negative-electrode active
materials differing in roundness. This negative-electrode active
material also can improve cycle characteristics while maintaining
low-temperature output. The difference in roundness is generally
0.01 or larger, preferably 0.02 or larger, more preferably 0.03 or
larger. The upper limit thereof is generally 0.3 or smaller,
preferably 0.2 or smaller, more preferably 0.1 or smaller. When the
difference therein is smaller than the lower limit of that range,
there are cases where the effect to be produced by a difference in
roundness is difficult to obtain. On the other hand, when the
difference therein exceeds the upper limit of that range, there are
cases where the part having a lower roundness poses a problem, for
example, that streak lines generate in electrode formation.
[1043] Roundness in the invention is defined by the following
equation.
Roundness=(length of periphery of equivalent circle having the same
area as projected particle shape)/(actual length of periphery of
projected particle shape)
[1044] The value of roundness is determined with a flow type
particle image analyzer (e.g., FPIA, manufactured by Sysmex
Industrial Corp.) by dispersing about 0.2 g of a sample in a 0.2%
by mass aqueous solution (about 50 mL) of poly(oxyethylene(20))
sorbitan monolaurate as a surfactant, propagating an ultrasonic
wave of 28 kHz to the dispersion for 1 minute at an output of 60 W,
and then examining particles having a particle diameter in the
range of 3-40 .mu.m with the analyzer having a detection range set
at 0.6-400 .mu.m.
[1045] The negative-electrode active material to be used in
negative electrode [8] in a lithium secondary battery of the
invention may comprise two or more negative-electrode active
materials differing in tap density. This negative-electrode active
material also can improve cycle characteristics while maintaining
low-temperature output. The difference in tap density is generally
0.1 g/cm.sup.3 or larger, preferably 0.2 g/cm.sup.3 or larger, more
preferably 0.3 g/cm.sup.3 or larger. When the difference therein is
smaller than the lower limit of that range, there are cases where
the effect to be produced by the mixing of materials differing in
tap density is difficult to obtain.
[1046] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the cell with the
sample up to the brim, subsequently conducting tapping operations
1,000 times over a stroke length of 10 mm using a powder densimeter
(e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.),
and determining a density from the resultant volume of the sample
and the weight thereof. This density is defined as the tap
density.
[1047] The negative-electrode active material to be used in
negative electrode [8] in a lithium secondary battery of the
invention may comprise two or more negative-electrode active
materials differing in BET specific surface area. This
negative-electrode active material also can improve low-temperature
output while maintaining cycle characteristics. The difference in
BET specific surface area is generally 0.1 m.sup.2/g or larger,
preferably 0.5 m.sup.2/g or larger, more preferably 1 m.sup.2/g or
larger. The upper limit thereof is generally 20 m.sup.2/g or
smaller, preferably 15 m.sup.2/g or smaller, more preferably 12
m.sup.2/g or smaller. When the difference therein is smaller than
the lower limit of that range, there are cases where the effect to
be produced by the mixing of materials differing in BET specific
surface area is difficult to obtain. On the other hand, when the
difference therein exceeds the upper limit of that range, there are
cases where irreversible capacity increases due to the part having
a larger BET specific surface area.
[1048] In the invention, BET specific surface area is defined as a
value measured with a surface area meter (a fully automatic surface
area measuring apparatus manufactured by Ohukura Riken Co., Ltd.)
by preliminarily drying a sample at 350.degree. C. for 15 minutes
in a nitrogen stream and then measuring the specific surface area
thereof by the gas-flowing nitrogen adsorption BET one-point method
using a nitrogen/helium mixture gas precisely regulated so as to
have a nitrogen pressure of 0.3 relative to atmosphere
pressure.
[1049] With respect to the mixing ratio between the two or more
different negative-electrode active materials for use in negative
electrode [8] in a lithium secondary battery of the invention, the
proportion of one negative-electrode active material is generally
0.1% by mass or higher, preferably 1% by mass or higher, more
preferably 10% by mass or higher, even more preferably 20% by mass
or higher, based on all negative-electrode active materials. The
upper limit thereof is generally 99.9% by mass or lower, preferably
99% by mass or lower, more preferably 90% by mass or lower, even
more preferably 80% by mass or lower. When the proportion thereof
is outside the range, there are cases where the effect to be
produced by the mixing of two or more different negative-electrode
active materials is difficult to obtain.
[1050] From the standpoint of high cost performance, it is
preferred that at least one of the two different negative-electrode
active materials in each of those embodiments should comprise a
natural graphite and/or a product of processing of a natural
graphite.
[1051] Natural graphites are classified into flake graphite,
crystalline (vein) graphite, and amorphous graphite according to
properties (see "Graphite" in Funrutai Purosesu Gijutsu Shusei
(published by Industrial Technology Center in 1974); and HANDBOOK
OF CARBON, GRAPHITE, DIAMOND AND FULLERENES (published by Noyes
Publications)). With respect to the degree of graphitization,
crystalline graphite has a highest graphitization degree of 100%
and flake graphite is second, which has a graphitization degree as
high as 99.9%. However, the graphitization degree of amorphous
graphite is as low as 28%. Flake graphite, among such natural
graphites, occurs in Madagascar, China, Brazil, Ukraine, Canada,
etc. Crystalline graphite occurs mainly in Sri Lanka. Amorphous
graphite occurs mainly in the Korean Peninsula, China, Mexico, etc.
Of these natural graphites, amorphous graphite generally has a
small particle diameter and a low purity. In contrast, flake
graphite and crystalline graphite have advantages such as a high
degree of graphitization and a low impurity content, and can hence
be advantageously used in the invention.
[Negative Electrode [8]: Difference in Treatment]
[1052] The negative-electrode active material for use in this
invention may comprise two or more negative-electrode active
materials differing in processing or treatment. This
negative-electrode active material also can improve low-temperature
output while maintaining cycle characteristics. Examples of methods
for processing a natural graphite include a method in which the
graphite is subjected to a heat treatment and a treatment in which
mechanical energy is applied to the graphite. One example of the
heat treatment is shown below.
[[Heat Treatment Temperature]]
[1053] The temperature in a heat treatment for producing the
negative-electrode active material is generally 600.degree. C. or
higher, preferably 1,200.degree. C. or higher, more preferably
2,000.degree. C. or higher, even more preferably 2,500.degree. C.
or higher, especially preferably 2,800.degree. C. or higher. The
upper limit thereof is generally 3,200.degree. C. or lower,
preferably 3,100.degree. C. or lower. When the temperature
conditions are lower than the lower limit of that range, there are
cases where the surface of the natural-graphite particles does not
undergo a sufficient crystal repair. On the other hand, when the
temperature exceeds the upper limit of that range, there are cases
where graphite sublimation is apt to occur in an increased amount.
It is also preferred that the negative-electrode active material
for use in this invention should comprise two or more
negative-electrode active materials differing in heat treatment
temperature.
[[Method of Heat Treatment]]
[1054] The heat treatment is accomplished by heating the natural
graphite once to a temperature in that range. Although the holding
time period in which the temperature conditions are kept in that
range is not particularly limited, it generally is longer than 10
seconds and is not longer than 168 hours.
[1055] In general, the heat treatment is conducted in an inert gas
atmosphere, e.g., nitrogen gas, or in a non-oxidizing atmosphere
formed by the gas generated from the raw natural graphite. It is,
however, noted that in a furnace of the type in which the raw
material is buried in a breeze (fine carbon powder obtained by
pitch heat-treating), there are cases where the atmosphere in an
initial stage is a mixture with the air. In such cases, it is not
always necessary to use a completely inert gas atmosphere.
Apparatus usable for the heat treatment are not particularly
limited. For example, use can be made of a shuttle furnace, tunnel
kiln, electric furnace, Riedhammer furnace, rotary kiln, direct
voltage application type electric furnace, Acheson furnace,
resistance heating furnace, induction heating furnace, or the like.
It is also preferred that the negative-electrode active material
for use in this invention should comprise two or more
negative-electrode active materials differing in heat treatment
method.
[1056] Besides the treatments described above, various treatments
including classification can be conducted. Classification is a
treatment for removing coarse particles and fine particles in order
to obtain a target particle diameter. Apparatus usable for the
classification are not particularly limited. However, in the case
of dry sieving, use can be made of a rotary sieve, swing sieve,
revolving sieve, vibrating sieve, or the like. In the case of dry
air classification, use can be made of a gravitational classifier,
inertial classifier, or centrifugal classifier (e.g., classifier or
cyclone). In the case of wet sieving, use can be made of a
mechanical wet classifier, hydraulic classifier, sedimentation
classifier, centrifugal wet classifier, or the like. Classification
may be conducted before the heat treatment or may be conducted in
another timing, e.g., after the heat treatment. It is also possible
to omit the classification treatment itself. It is also preferred
that the negative-electrode active material for use in this
invention should comprise two or more negative-electrode active
materials differing in classification treatment conditions.
[1057] The negative-electrode active material for use in this
invention may comprise two or more negative-electrode active
materials differing in mechanical energy treatment, which will be
described below. This negative-electrode active material also can
improve low-temperature output while maintaining cycle
characteristics. One example of the mechanical energy treatment is
shown below.
[[Mechanical Energy Treatment]]
[1058] The mechanical energy treatment is conducted so that the
ratio between the volume-average particle diameters before and
after the treatment becomes 1 or less. The term "ratio between the
volume-average particle diameters before and after the treatment"
means a value obtained by dividing the volume-average particle
diameter as measured after the treatment by the volume-average
particle diameter as measured before the treatment. In the
mechanical energy treatment in the invention for producing a raw
material before heat treatment, it is preferred to conduct the
mechanical energy treatment so that the ratio between the average
particle diameters before and after this treatment becomes 1 or
less. The mechanical energy treatment is performed for the purposes
of reducing the size of the powder particles to 1 or less in terms
of the ratio between the average particle diameters before and
after the treatment and simultaneously regulating the particle
shape. This mechanical energy treatment belongs to the
pulverization treatment among engineering unit operations
utilizable for particle design, such as pulverization,
classification, mixing, granulation, surface modification, and
reaction.
[1059] Pulverization means an operation in which a force is applied
to a substance to thereby reduce the size thereof and regulate the
particle diameter, particle size distribution, and loading of the
substance. Pulverization treatments are classified according to the
kind of force to be applied to the substance and the method of
treatment. Forces to be applied to the substance are roughly
divided into the following four: (1) breaking force (impact force),
(2) crushing force (compressive force), (3) milling force (grinding
force), and (4) scraping force (shear force). On the other hand,
treatment methods are roughly divided into the following two:
volume pulverization in which cracks are generated and propagated
in inner parts of the particles; and surface pulverization in which
surface parts of the particles are scraped off. Volume
pulverization proceeds by means of impact force, compressive force,
and shear force, while surface pulverization proceeds by means of
grinding force and shear force. Pulverization is a treatment based
on any of various combinations of such kinds of forces to be
applied to a substance and such treatment methods. A suitable
combination can be determined according to the purpose of the
treatment.
[1060] There are cases where pulverization is conducted by a
chemical reaction, e.g., explosion, or by volume expansion.
However, pulverization is generally conducted with a mechanical
apparatus such as a pulverizer. It is preferred that the
pulverization treatment to be used for producing a rounded
carbonaceous substance as a raw material in the invention should be
a treatment which finally involves a high proportion of surface
treatment, regardless of whether volume pulverization occurs or
not. This is because surface pulverization is important for
eliminating angles of the surface-pulverized particles and thereby
imparting roundness to the particle shape. Specifically, use may be
made of a method in which surface treatment is conducted after
volume pulverization has proceeded to some degree or a method in
which surface treatment only is conducted almost without conducting
volume pulverization. Furthermore, volume pulverization and surface
treatment may be simultaneously conducted. It is preferred to
conduct a pulverization treatment in which surface pulverization
proceeds finally and angles are eliminated from the surface of the
particles. The negative-electrode active material for use in this
invention may comprise two or more negative-electrode active
materials differing in the degree of such a surface treatment. This
negative-electrode active material also can improve low-temperature
output while maintaining cycle characteristics.
[1061] The apparatus to be used for conducting the mechanical
energy treatment is selected from ones usable for conducting the
preferred treatment described above. The mechanical energy
treatment may be accomplished with one or more of the four kinds of
forces applicable to a substance. However, it is preferred and
effective to repeatedly apply impact force as a main force and
other mechanical actions such as compressive, frictional, shear,
and other forces, including those caused by particle interaction,
to the particles. Consequently, a preferred apparatus specifically
is one which comprises a casing and disposed therein a rotor having
many blades and in which the rotor is rotated at a high speed to
thereby apply mechanical actions such as impact, compressive,
frictional, and shear forces to a carbon material introduced
therein and conduct surface treatment while allowing volume
pulverization to proceed. More preferred is one which has a
mechanism in which a carbonaceous substance is circulated to
thereby repeatedly apply mechanical actions thereto.
[1062] Examples of the preferred apparatus include Hybridization
System (manufactured by Nara Machinery Co., Ltd.), Kryptron
(manufactured by Earth Technica Co., Ltd.), CF Mill (manufactured
by Ube Industries, Ltd.), and Mechanofusion System (manufactured by
Hosokawa Micron Corp.). Preferred of these is Hybridization System,
manufactured by Nara Machinery Co., Ltd. In the case where the
treatment is conducted with this apparatus, the rotor is rotated at
a peripheral speed of preferably 30-100 m/sec, more preferably
40-100 m/sec, even more preferably 50-100 m/sec. Although a
carbonaceous substance can be treated by merely passing it through
the apparatus, it is preferred to treat the carbonaceous substance
while causing it to circulate or reside in the apparatus for 30
seconds or longer. More preferably, the carbonaceous substance is
treated while circulating or residing in the apparatus for 1 minute
or longer.
[1063] Through such mechanical energy treatment, the carbon
particles become particles in which only parts near the surface are
rough and which have exposed strains and edge faces, while
retaining high crystallinity as a whole. Thus, the carbon particles
come to have an increased amount of surfaces where lithium ions can
come in and out, and hence have a high capacity even at a high
current density.
[1064] In general, flaky, crystalline, and platy carbon materials
tend to become poorer in suitability for loading as the particle
diameter decreases. It is thought that as a result of
pulverization, the particles come to have more irregular shapes,
have a larger amount of surface projections such as "fine splits",
"half-peeled parts", and "bent parts", and have finer particles of
irregular shapes adherent to the particle surface at some degree of
strength. The impaired suitability for loading is thought to be
attributable to increased resistance between adjacent particles due
to such causes. As long as such shape irregularity is reduced and
the particle shape becomes close to sphere, a decrease in particle
diameter should result in a limited decrease in loading and
large-diameter carbon particles and small-diameter carbon particles
should have almost the same tap density.
[1065] The proportion of the natural graphite and/or product of
processing of a natural graphite is generally 0.1% by mass or
higher, preferably 1% by mass or higher, more preferably 10% by
mass or higher, even more preferably 20% by mass or higher. The
upper limit thereof is generally 99.9% by mass or lower, preferably
99% by mass or lower, more preferably 90% by mass or lower, even
more preferably 80% by mass or lower. When the proportion thereof
is lower than the lower limit of that range, there are cases where
the addition of the natural graphite and/or product of processing
of a natural graphite is less apt to attain an improvement in cost
performance. On the other hand, when the proportion thereof exceeds
the upper limit of that range, there are cases where the improving
effect of different negative-electrode active materials is
difficult to obtain.
[[Pore Volume, etc.]]
[1066] With respect to the pore volume of the negative-electrode
active materials for use as the negative-electrode active material
of a lithium secondary battery of the invention, the amount of
interstices corresponding to a pore diameter of from 0.01 .mu.m to
1 .mu.m which include pores in the particles and particle surface
irregularities formed by steps, as determined by mercury
porosimetry (mercury intrusion method) (hereinafter abbreviated to
"pore volume"), is generally 0.01 mL/g or larger, preferably 0.05
mL/g or larger, more preferably 0.1 mL/g or larger. The upper limit
thereof is generally 0.6 mL/g or smaller, preferably 0.4 mL/g or
smaller, more preferably 0.3 mL/g or smaller. When the amount of
interstices therein is larger than the upper limit of that range,
there are cases where a large amount of a binder is necessary in
electrode plate formation. When the amount thereof is smaller than
the lower limit, there are cases where high-current-density
charge/discharge characteristics decrease and the effect of
diminishing electrode expansion/contraction during charge/discharge
is not obtained.
[1067] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger. The upper limit thereof is
generally 10 mL/g or smaller, preferably 5 mL/g or smaller, more
preferably 2 mL/g or smaller. When the total pore volume exceeds
the upper limit of that range, there are cases where a large amount
of a binder is necessary in electrode plate formation. When the
total pore volume is smaller than the lower limit, there are cases
where the dispersing effect of a thickener or binder in electrode
plate formation is not obtained.
[1068] The average pore diameter of this negative-electrode active
material is preferably 0.05 .mu.m or larger, more preferably 0.1
.mu.m or larger, even more preferably 0.5 .mu.m or larger. The
upper limit thereof is generally 50 .mu.m or smaller, preferably 20
.mu.m or smaller, more preferably 10 .mu.m or smaller. When the
average pore diameter thereof exceeds the upper limit of that
range, there are cases where a large amount of a binder is
necessary. When the average pore diameter thereof is smaller than
the lower limit, there are cases where high-current-density
charge/discharge characteristics decrease.
[1069] As an apparatus for the mercury porosimetry is used a
mercury porosimeter (Autopore 9520, manufactured by Micromeritics
Corp.). About 0.2 g of a sample (negative-electrode material) is
weighed out and placed in a powder cell and this cell is closed.
The sample in the cell is pretreated by degassing it at room
temperature under vacuum (50 .mu.mHg or lower) for 10 minutes.
Subsequently, the pressure in the cell is reduced to 4 psia (about
28 kPa) and mercury is introduced thereinto. The internal pressure
is stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia
(about 280 MPa) and then lowered to 25 psia (about 170 kPa). The
number of steps in the pressure elevation is 80 or larger. In each
step, the amount of mercury intruded is measured after an
equilibrium time of 10 seconds. A pore distribution is calculated
from the mercury intrusion curve thus obtained, using the Washburn
equation. For the calculation, the surface tension (.gamma.) and
contact angle (.psi.) of mercury are taken as 485 dyne/cm and
140.degree., respectively. The average pore diameter used is the
pore diameter corresponding to a cumulative pore volume of 50%.
[[Ash Content]]
[1070] The ash content of this negative-electrode active material
for a lithium secondary battery of the invention is preferably 1%
by mass or lower, more preferably 0.5% by mass or lower, especially
preferably 0.1% by mass or lower, based on the whole graphitic
carbon particles. The lower limit of the ash content thereof is
preferably at least 1 ppm. When the ash content thereof exceeds the
upper limit of that range, there are cases where battery
performance deterioration caused by reaction with the nonaqueous
electrolyte during charge/discharge becomes not negligible. On the
other hand, when the ash content thereof is lower than the lower
limit of that range, there are cases where the production of this
active material necessitates much time and energy and an apparatus
for pollution prevention, resulting in an increase in cost.
[[Orientation Ratio]]
[1071] The orientation ratio of the graphitic carbon particles for
use as the negative-electrode active material in a lithium
secondary battery of the invention is generally 0.005 or higher,
preferably 0.01 or higher, more preferably 0.015 or higher. The
upper limit thereof is theoretically 0.67 or lower. When the
orientation ratio thereof is lower than the lower limit of that
range, there are cases where high-density charge/discharge
characteristics decrease.
[1072] Orientation ratio is determined by X-ray diffractometry. The
peaks attributable to the (110) diffraction and (004) diffraction
of the carbon obtained by X-ray diffractometry are subjected to
peak separation by fitting with asymmetric Pearson VII as a profile
function. Thus, the integrated intensities of the (110) diffraction
and (004) diffraction peaks are calculated. From the integrated
intensities obtained, the ratio represented by (integrated
intensity of (110) diffraction)/(integrated intensity of (004)
diffraction) is calculated. This ratio is defined as the
orientation ratio of the active material.
[1073] Conditions for this X-ray diffractometry are as follows.
Incidentally, "2.theta." represents diffraction angle. [1074]
Target: Cu(K.alpha. line) graphite monochromator [1075] Slit:
[1076] Divergence slit=1 degree [1077] Receiving slit=0.1 mm [1078]
Scattering slit=1 degree [1079] Examination range and step
angle/measuring time: [1080] (110) plane:
76.5.degree..ltoreq.2.theta..ltoreq.78.5.degree. 0.01.degree./3 sec
(004) plane: 53.5.degree..ltoreq.2.theta..ltoreq.56.0.degree.
0.01.degree./3 sec
[[Aspect Ratio]]
[1081] The aspect ratio of the graphitic carbon particles for use
as the negative-electrode active material in a lithium secondary
battery of the invention is theoretically 1 or higher. The upper
limit thereof is generally 10 or lower, preferably 8 or lower, more
preferably 5 or lower. When the aspect ratio thereof exceeds the
upper limit, there are cases where this graphitic material causes
streak lines in electrode plate formation and an even coating
surface cannot be obtained, resulting in a decrease in
high-current-density charge/discharge characteristics.
[1082] The aspect ratio of a particle is represented by A/B,
wherein A is the length of the longest axis of the particle in a
three-dimensional examination and B is the length of the shortest
axis perpendicular to that axis. The examination of particles is
made with a scanning electron microscope capable of enlargement
examination. Fifty are arbitrarily selected from graphite particles
fixed to an edge face of a metal having a thickness of 50 .mu.m or
smaller, and each particle is examined for A and B while rotating
or inclining the stage to which the sample is fixed. The average of
the A/B values is determined.
[1083] The lithium secondary battery of the invention having
negative electrode [8] is not particularly limited in the kinds of
negative-electrode active materials as long as the negative
electrode contains two or more negative-electrode active materials
differing in property. However, the following should be noted. With
respect to properties determined by X-ray diffractometry, such as
interplanar spacing (d002), crystallite size (Lc), orientation
ratio, and electrode plate orientation ratio, properties relating
to a Raman spectrum, such as Raman R value and Raman half-value
width, and ash content, those numerical values are for carbonaceous
substances. Consequently, those numerical-value differences are
applied to carbonaceous substances. On the other hand, with respect
to properties relating to a particle size distribution, such as
median diameter, mode diameter, and Z, BET specific surface area,
properties determined by mercury porosimetry, such as pore volume,
total pore volume, and average pore diameter, true density,
roundness, tap density, and aspect ratio, those numerical values
are not only for carbonaceous substances but for all substances
usable as negative-electrode active materials. Those
numerical-value differences are hence applied to all such
substances. However, it is preferred that the substances having
such two kinds of properties should be carbonaceous substances. In
this case, the numerical values each are regarded as a value
showing the property of carbonaceous substances and it is preferred
to use two or more carbonaceous substances differing in this
property as negative-electrode active materials.
[Negative Electrode [8]: Method of Mixing Two or More
Negative-Electrode Active Materials]
[1084] Apparatus usable for mixing two or more negative-electrode
active materials are not particularly limited. Examples thereof
include twin-cylinder mixers, W-shaped mixers, change-can mixers,
kneaders, drum mixers, and shearing mixers.
[Negative Electrode [8]: Electrode Production]
[1085] Negative electrode [8] may be produced by an ordinary
method. It can be formed in the same manner as described above. The
current collector, thickness ratio between the current collector
and the active-material layer, electrode density, binder,
electrode-plate orientation ratio, impedance, etc. also are the
same as described above.
<Negative Electrodes [9] and [10]>
[1086] An explanation will be given below on negative electrode [9]
(embodiment A) for use in a lithium secondary battery of the
invention, i.e., "a negative electrode containing a
negative-electrode active material which has a tap density of 0.1
g/cm.sup.3 or higher and has a pore volume, in terms of the volume
of particle pores corresponding to the diameter range of from 0.01
.mu.m to 1 .mu.m as determined by mercury porosimetry, of 0.01 mL/g
or larger", and on negative electrode [10] (embodiment B) for use
in a lithium secondary battery of the invention, i.e., "a negative
electrode which, when charged to 60% of a nominal capacity, has a
reaction resistance in a negative-electrode opposing cell of
500.OMEGA. or lower".
With Respect to Negative Electrode [9]
Embodiment A
[1087] Embodiment A of the invention is one in which the negative
electrode used in this invention is specified by properties of the
negative-electrode active material to be contained therein.
[1088] Namely, this embodiment relates to a lithium secondary
battery having a nonaqueous electrolyte containing the specific
compound shown above and a negative electrode containing a
negative-electrode active material having the specific properties
described below. Embodiment A of the invention is described
below.
[Negative Electrode [9]: Negative-Electrode Active Material]
[1089] The negative-electrode active material in embodiment A of
the invention is one which is capable of electrochemically
occluding/releasing lithium ions and satisfies at least the
following requirements (a) and (b).
(a) The tap density is 0.1 g/cm.sup.3 or higher. (b) The volume of
pores in the range of from 0.01 .mu.m to 1 .mu.m as determined by
mercury porosimetry is 0.01 mL/g or larger.
[[Tap Density]]
[1090] The tap density of the negative-electrode active material to
be contained in negative electrode [9] in a lithium secondary
battery of the invention is preferably 0.1 g/cm.sup.3 or higher,
more preferably 0.5 g/cm.sup.3 or higher, even more preferably 0.7
g/cm.sup.3 or higher, especially preferably 0.9 g/cm.sup.3 or
higher. The upper limit thereof is preferably 2 g/cm.sup.3 or
lower, more preferably 1.8 g/cm.sup.3 or lower, especially
preferably 1.6 g/cm.sup.3 or lower. In case where the tap density
thereof is lower than the lower limit of that range, the effect of
attaining, in particular, high output is not attained. On the other
hand, in case where the tap density thereof exceeds the upper limit
of that range, the electrode has too small an amount of
interparticle interstices and a reduced amount of channels for the
nonaqueous electrolyte. There are hence cases where output itself
decreases.
[1091] In the invention, the tap density of a sample is determined
by dropping the sample through a sieve having an opening size of
300 .mu.m into a 20-cm.sup.3 tapping cell to fill the cell with the
sample up to the brim, subsequently conducting tapping operations
1,000 times over a stroke length of 10 mm using a powder densimeter
(e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.),
and determining a density from the resultant volume of the sample
and the weight thereof. This density is defined as the tap
density.
[[Pore Volume]]
[1092] With respect to the pore volume of the negative-electrode
active material to be contained in negative electrode [9] in a
lithium secondary battery of the invention, the amount of
interstices corresponding to a pore diameter of from 0.01 .mu.m to
1 .mu.m which include pores in the particles and particle surface
irregularities formed by steps, as determined by mercury
porosimetry (mercury intrusion method) (hereinafter abbreviated to
"pore volume"), may be 0.01 mL/g or larger and is preferably 0.05
mL/g or larger, more preferably 0.1 mL/g or larger. The upper limit
thereof is generally 0.6 mL/g or smaller, preferably 0.4 mL/g or
smaller, more preferably 0.3 mL/g or smaller. When the amount of
interstices therein is larger than the upper limit of that range,
there are cases where a large amount of a binder is necessary in
electrode plate formation. On the other hand, when the amount
thereof is smaller than the lower limit of that range, it is
impossible to attain long life and high output.
[1093] The total pore volume is preferably 0.1 mL/g or larger, more
preferably 0.25 mL/g or larger, even more preferably 0.4 mL/g or
larger. The upper limit thereof is generally 10 mL/g or smaller,
preferably 5 mL/g or smaller, more preferably 2 mL/g or smaller.
When the total pore volume exceeds the upper limit of that range,
there are cases where a large amount of a binder is necessary in
electrode plate formation. When the total pore volume is smaller
than the lower limit, there are cases where the dispersing effect
of a thickener or binder in electrode plate formation is not
obtained. The term "total pore volume" herein means the sum of pore
volumes measured in the whole range under the following examination
conditions.
[1094] The average pore diameter of the negative-electrode active
material is preferably 0.05 .mu.m or larger, more preferably 0.1
.mu.m or larger, even more preferably 0.5 .mu.m or larger. The
upper limit thereof is generally 50 .mu.m or smaller, preferably 20
.mu.m or smaller, more preferably 10 .mu.m or smaller. When the
average pore diameter thereof exceeds the upper limit of that
range, there are cases where a large amount of a binder is
necessary. When the average pore diameter thereof is smaller than
the lower limit, there are cases where high-current-density
charge/discharge characteristics decrease.
[1095] As an apparatus for the mercury porosimetry is used a
mercury porosimeter (Autopore 9520, manufactured by Micromeritics
Corp.). About 0.2 g of a sample (negative-electrode material) is
weighed out and placed in a powder cell and this cell is closed.
The sample in the cell is pretreated by degassing it at room
temperature under vacuum (50 .mu.mHg or lower) for 10 minutes.
Subsequently, the pressure in the cell is reduced to 4 psia (about
28 kPa) and mercury is introduced thereinto. The internal pressure
is stepwise elevated from 4 psia (about 28 kPa) to 40,000 psia
(about 280 MPa) and then lowered to 25 psia (about 170 kPa). The
number of steps in the pressure elevation is 80 or larger. In each
step, the amount of mercury intruded is measured after an
equilibrium time of 10 seconds. A pore distribution is calculated
from the mercury intrusion curve thus obtained, using the Washburn
equation. For the calculation, the surface tension (.gamma.) and
contact angle (.psi.) of mercury are taken as 485 dyne/cm and
140.degree., respectively. The average pore diameter used is the
pore diameter corresponding to a cumulative pore volume of 50%.
[1096] The lithium secondary battery as embodiment A of the
invention can produce the effects of the invention described above
and sufficiently exhibit its performances, as long as the
negative-electrode active material satisfies requirements (a) and
(b) described above. It is, however, preferred that the
negative-electrode active material should further satisfy any one
of the following items concerning properties or simultaneously
satisfy two or more thereat. With respect to this negative
electrode, it is especially preferred that it should satisfy any
one of or simultaneously satisfy two or more of the items
concerning the properties or constitution of the negative electrode
in embodiment B, which will be described later.
[[BET Specific Surface Area]]
[1097] The specific surface area of the negative-electrode active
material to be contained in negative electrode [9] in a lithium
secondary battery of the invention, as determined by the BET
method, is preferably 0.1 m.sup.2/g or larger, especially
preferably 0.7 m.sup.2/g or larger, more preferably 1 m.sup.2/g or
larger, even more preferably 1.5 m.sup.2/g or larger. The upper
limit thereof is preferably 100 m.sup.2/g or smaller, especially
preferably 50 m.sup.2/g or smaller, more preferably 25 m.sup.2/g or
smaller, even more preferably 15 m.sup.2/g or smaller. When the BET
specific surface area thereof is smaller than the lower limit of
that range, there are cases where use of this negative-electrode
material is apt to result in poor lithium acceptance during charge
and in lithium deposition on the electrode surface. There also are
cases where high output is not obtained. On the other hand, when
the specific surface area thereof exceeds the upper limit of that
range, there are cases where use of this negative-electrode
material is apt to result in enhanced reactivity with the
nonaqueous electrolyte and enhanced gas evolution and a preferred
battery is difficult to obtain.
[1098] BET specific surface area is defined as a value measured
with a surface area meter (e.g., a fully automatic surface area
measuring apparatus manufactured by Ohukura Riken Co., Ltd.) by
preliminarily drying a sample at 350.degree. C. for 15 minutes in a
nitrogen stream and then measuring the specific surface area
thereof by the gas-flowing nitrogen adsorption BET one-point method
using a nitrogen/helium mixture gas precisely regulated so as to
have a nitrogen pressure of 0.3 relative to atmosphere
pressure.
[[Volume-Average Particle Diameter]]
[1099] The volume-average particle diameter of the
negative-electrode active material to be contained in negative
electrode [9] in a lithium secondary battery of the invention,
which is defined as the volume-average particle diameter (median
diameter) as determined by the laser diffraction/scattering method,
is preferably 1 .mu.m or larger, especially preferably 3 .mu.m or
larger, more preferably 5 .mu.m or larger, even more preferably 7
.mu.m or larger. The upper limit thereof is generally 50 .mu.m or
smaller, preferably 40 .mu.m or smaller, more preferably 30 .mu.m
or smaller, even more preferably 25 .mu.m or smaller. When the
volume-average particle diameter thereof is smaller than the lower
limit of that range, there are cases where irreversible capacity
increases, leading to a loss in initial battery capacity. When the
volume-average particle diameter thereof exceeds the upper limit of
that range, there are cases where such a negative-electrode active
material is undesirable from the standpoint of battery production
because an uneven coating surface is apt to result when an
electrode plate is produced.
[1100] Volume-average particle diameter in the invention is defined
as the median diameter determined by dispersing the carbon powder
in a 0.2% by mass aqueous solution (about 1 mL) of
poly(oxyethylene(20)) sorbitan monolaurate as a surfactant and
examining the dispersion with a laser diffraction type particle
size distribution analyzer (e.g., LA-700, manufactured by HORIBA,
Ltd.).
[[Roundness]]
[1101] The roundness of the negative-electrode active material to
be contained in negative electrode [9] in a lithium secondary
battery of the invention is generally 0.1 or higher, preferably 0.5
or higher, more preferably 0.8 or higher, especially preferably
0.85 or higher, even more preferably 0.9 or higher. With respect to
the upper limit thereof, a particle having a roundness of 1
theoretically is a true sphere. In case where the roundness thereof
is lower than the lower limit of that range, there are cases where
this negative-electrode active material poses a problem, for
example, that streak lines generate in electrode formation.
[1102] In the invention, roundness is defined by the following
equation.
Roundness=(length of periphery of equivalent circle having the same
area as projected particle shape)/(actual length of periphery of
projected particle shape)
[1103] The value of roundness is determined, for example, with a
flow type particle image analyzer (e.g., FPIA, manufactured by
Sysmex Industrial Corp.) by dispersing about 0.2 g of a sample in a
0.2% by mass aqueous solution (about 50 mL) of
poly(oxyethylene(20)) sorbitan monolaurate as a surfactant,
propagating an ultrasonic wave of 28 kHz to the dispersion for 1
minute at an output of 60 W, subsequently examining particles
having a particle diameter in the range of 3-40 .mu.m with the
analyzer having a detection range set at 0.6-400 .mu.m, and
averaging the measured values.
[[Interplanar Spacing (d002)]]
[1104] The interplanar spacing (d002) for the (002) planes of the
negative-electrode active material to be contained in negative
electrode [9] in a lithium secondary battery of the invention, as
determined by wide-angle X-ray diffractometry, is generally 0.38 nm
or smaller, preferably 0.36 nm or smaller, more preferably 0.35 nm
or smaller, even more preferably 0.345 nm or smaller. The lower
limit thereof is at least 0.335 nm, which is a theoretical value
for graphites. When the interplanar spacing (d002) thereof exceeds
the upper limit of that range, there are cases where this
negative-electrode active material has considerably reduced
crystallinity, resulting in an increase in irreversible
capacity.
[1105] The term "interplanar spacing (d002) for the (002) planes as
determined by wide-angle X-ray diffractometry" in the invention
means the value of d (interplanar spacing) for the lattice planes
(002) as determined by X-ray diffractometry in accordance with the
method of the Japan Society for Promotion of Scientific
Research.
[[Crystallite Size (Lc)]]
[1106] The crystallite size (Lc) of the negative-electrode active
material, as determined by X-ray diffractometry in accordance with
the method of the Japan Society for Promotion of Scientific
Research, is not particularly limited. However, it is generally 0.1
nm or larger, preferably 0.5 nm or larger, more preferably 1 nm or
larger. In case where the Lc thereof is smaller than the lower
limit of that range, there are cases where crystallinity decreases
considerably to increase irreversible capacity.
[[True Density]]
[1107] The true density of the negative-electrode active material
to be contained in negative electrode [9] in a lithium secondary
battery of the invention is generally 1.5 g/cm.sup.3 or higher,
preferably 1.7 g/cm.sup.3 or higher, more preferably 1.8 g/cm.sup.3
or higher, even more preferably 1.85 g/cm.sup.3 or higher. The
upper limit thereof may be 2.26 g/cm.sup.3 or lower. This upper
limit is a theoretical value for graphites. When the true density
of the negative-electrode active material is lower than the lower
limit of that range, there are cases where this carbon has too low
crystallinity, resulting in an increase in irreversible capacity.
In the invention, true density is defined as one determined by the
liquid-phase displacement method (pycnometer method) using
butanol.
[[Raman R Value]]
[1108] The Raman R value of the negative-electrode active material
to be contained in negative electrode [9] in a lithium secondary
battery of the invention is generally 0.01 or higher, preferably
0.03 or higher, more preferably 0.1 or higher. The upper limit
thereof is preferably 1.5 or lower, more preferably 1.2 or lower,
especially preferably 0.5 or lower. When the Raman R value thereof
is lower than the lower limit of that range, the surface of such
particles has too high crystallinity and there are cases where the
number of charge/discharge sites decreases, resulting in a decrease
in output itself. On the other hand, when the Raman R value thereof
exceeds the upper limit of that range, the surface of such
particles has reduced crystallinity and there are hence cases where
irreversible capacity increases.
[1109] The examination for a Raman spectrum is made with a Raman
spectrometer (e.g., a Raman spectrometer manufactured by Japan
Spectroscopic Co., Ltd.). In the examination, a sample is charged
into a measuring cell by causing the sample to fall naturally into
the cell and the surface of the sample in the cell is irradiated
with argon ion laser light while rotating the cell in a plane
perpendicular to the laser light. The Raman spectrum obtained is
examined for the intensity I.sub.A of a peak P.sub.A at 1,580
cm.sup.-1 and the intensity I.sub.B of a peak P.sub.B at 1,360
cm.sup.-1. The ratio between these intensities R
(R=I.sub.B/I.sub.A) is calculated and this ratio is defined as the
Raman R value of the negative-electrode active material.
Furthermore, the half-value width of the peak P.sub.A at 1,580
cm.sup.-1 in the Raman spectrum obtained is measured, and this
value is defined as the Raman half-value width of the
negative-electrode active material.
[1110] Conditions for this Raman spectroscopy are as follows.
[1111] Wavelength of argon ion laser: 514.5 nm [1112] Laser power
on sample: 15-25 mW [1113] Resolution: 10-20 cm.sup.-1 [1114]
Examination range: 1,100 cm.sup.-1 to 1,730 cm.sup.-1 [1115]
Analysis for Raman R value and Raman half-value width: background
processing [1116] Smoothing: simple average; convolution, 5
points
[1117] The Raman half-value width at 1,580 cm.sup.-1 of the
negative-electrode active material to be contained in negative
electrode [9] in a lithium secondary battery of the invention is
not particularly limited. However, the half-value width thereof is
generally 10 cm.sup.-1 or larger, preferably 15 cm.sup.-1 or
larger. The upper limit thereof is generally 150 cm.sup.-1 or
smaller, preferably 100 cm.sup.-1 or smaller, more preferably 60
cm.sup.-1 or smaller. When the Raman half-value width thereof is
smaller than the lower limit of that range, the surface of such
particles has too high crystallinity and there are cases where the
number of charge/discharge sites decreases, resulting in a decrease
in output itself. On the other hand, when the Raman half-value
width thereof exceeds the upper limit of that range, the surface of
such particles has reduced crystallinity and there are hence cases
where irreversible capacity increases.
[[Ash Content]]
[1118] The ash content of the negative-electrode active material to
be contained in negative electrode [9] in a lithium secondary
battery of the invention is preferably 1% by mass or lower, more
preferably 0.5% by mass or lower, especially preferably 0.1% by
mass or lower. The lower limit thereof is preferably 1 ppm or
higher. When the ash content thereof exceeds the upper limit of
that range, there are cases where battery performance deterioration
caused by reaction with the nonaqueous electrolyte during
charge/discharge becomes not negligible, resulting in a decrease in
capacity retention through cycling. On the other hand, when the ash
content thereof is lower than the lower limit of that range, there
are cases where the production of this negative-electrode active
material necessitates much time and energy and an apparatus for
pollution prevention, resulting in an increase in cost.
[[Aspect Ratio]]
[1119] The aspect ratio of the negative-electrode active material
to be contained in negative electrode [9] in a lithium secondary
battery of the invention is theoretically 1 or higher. The upper
limit thereof is generally 10 or lower, preferably 8 or lower, more
preferably 5 or lower. When the aspect ratio thereof exceeds the
upper limit, there are cases where this active material causes
streak lines in negative-electrode production and an even coating
surface cannot be obtained, resulting in a decrease in
high-current-density charge/discharge characteristics.
[1120] The aspect ratio of a particle is represented by A/B,
wherein A is the length of the longest axis of the particle in a
three-dimensional examination and B is the length of the shortest
axis perpendicular to that axis. The examination of particles is
made with a scanning electron microscope capable of enlargement
examination. Fifty are arbitrarily selected from particles fixed to
an edge face of a metal having a thickness of 50 .mu.m or smaller,
and each particle is examined for A and B while rotating or
inclining the stage to which the sample is fixed. The average of
the A/B values is determined.
[[Orientation Ratio]]
[1121] The orientation ratio of the negative-electrode active
material to be contained in negative electrode [9] in a lithium
secondary battery of the invention is generally 0.005 or higher,
preferably 0.01 or higher, more preferably 0.015 or higher. The
upper limit thereof is theoretically 0.67 or lower. When the
orientation ratio thereof is lower than the lower limit of that
range, there are cases where high-density charge/discharge
characteristics decrease.
[1122] Orientation ratio is determined by X-ray diffractometry. The
peaks attributable to the (110) diffraction and (004) diffraction
of the carbon obtained by X-ray diffractometry are subjected to
peak separation by fitting with asymmetric Pearson VII as a profile
function. Thus, the integrated intensities of the (110) diffraction
and (004) diffraction peaks are calculated. From the integrated
intensities obtained, the ratio represented by (integrated
intensity of (110) diffraction)/(integrated intensity of (004)
diffraction) is calculated. This ratio is defined as the
orientation ratio of the negative-electrode active material.
[1123] Conditions for this X-ray diffractometry are as follows.
Incidentally, "2.theta." represents diffraction angle. [1124]
Target: Cu(K.alpha. line) graphite monochromator [1125] Slit:
[1126] Divergence slit=1 degree [1127] Receiving slit=0.1 mm [1128]
Scattering slit=1 degree [1129] Examination range and step
angle/measuring time: [1130] (110) plane:
76.5.degree..ltoreq.2.theta..ltoreq.78.5.degree. 0.01.degree./3 sec
[1131] (004) plane:
53.5.degree..ltoreq.2.theta..ltoreq.56.0.degree. 0.01.degree./3
sec
[Negative Electrode [9]: Electrode Production]
[1132] The negative electrode may be produced by an ordinary
method. It can be formed in the same manner as described above. The
current collector, thickness ratio between the current collector
and the active-material layer, electrode density, binder,
electrode-plate orientation ratio, impedance, etc. also are the
same as described above.
With Respect to Negative Electrode [10]
Embodiment B
[1133] Negative electrode [10] according to the invention
(embodiment B) is one in which the negative electrode used in the
invention is specified by a property and/or constitution of the
negative electrode itself. This embodiment relates to a lithium
secondary battery having a nonaqueous electrolyte containing the
specific compound shown above and a negative electrode having the
specific property and/or constitution shown below. Negative
electrode [10] of the invention (embodiment (B)) is described
below.
[Negative Electrode [10]: Properties and Constitution of the
Negative Electrode]
[1134] The properties and constitution of the negative electrode
used in the lithium secondary battery as embodiment B are explained
below. The negative electrode comprises a current collector and a
negative-electrode active-material layer formed thereon.
[[Reaction Resistance]]
[1135] The reaction resistance of the negative electrode used in
embodiment B of the invention, as determined through an opposing
impedance measurement, should be 500.OMEGA. or lower and is
preferably 100.OMEGA. or lower, more preferably 50.OMEGA. or lower.
There is no particular lower limit thereon. In case where the
reaction resistance thereof exceeds the upper limit of that range,
this negative electrode gives a lithium secondary battery which
shows reduced output characteristics before and after a cycle test.
Furthermore, capacity retention through cycling decreases due to
the high resistance.
[1136] A method for the measurement of opposing impedance is
described below. In a general impedance measurement in a lithium
secondary battery, the alternating-current impedance between the
positive-electrode terminal and negative-electrode terminal of the
battery is measured. This method, however, has a drawback that the
value measured indicates impedance for the system including both
the positive electrode and the negative electrode and whether the
impedance is attributable to the resistance of the positive
electrode or that of the negative electrode cannot be
unconditionally judged. Because of this, the reaction resistance in
an opposing cell employing the negative electrode only is measured
in the invention. The method of measuring reaction resistance in
the invention is described below.
[1137] The lithium secondary battery to be examined is one which,
when charged at a current at which the nominal capacity can be
charged over 5 hours, subsequently kept for 20 minutes in the state
of being neither charged nor discharged, and then discharged at a
current at which the nominal capacity can be discharged over 1
hour, has a capacity of at least 80% of the nominal capacity. This
lithium secondary battery in a discharged state is charged to 60%
of the nominal capacity at a current at which the nominal capacity
can be charged over 5 hours. Immediately thereafter, the lithium
secondary battery is transferred to a gloved box filled with an
argon atmosphere. In this gloved box, the lithium secondary battery
is rapidly disassembled to take out the negative electrode while
preventing the negative electrode from suffering discharge or
short-circuiting. In the case where this negative electrode is a
double-sided electrode, the electrode active material on one side
is stripped off without marring the electrode active material on
the other side. Two disks of 12.5 mm.phi. are punched out of this
negative electrode, and are superposed through a separator so that
the active-material sides rightly face each other. Sixty
microliters of the nonaqueous electrolyte which has been used in
the battery is dropped onto the separator and between the two
negative electrodes, and the negative electrodes and separator are
brought into tight contact with each other. While keeping this
assemblage isolated from the surrounding air, the current
collectors of the respective negative electrodes are electrically
connected to each other and the assemblage is examined by the
alternating-current impedance method. In the examination, the
complex impedance is measured at a temperature of 25.degree. C. in
a frequency region of 10.sup.2-10.sup.5 Hz to determine a Cole-Cole
plot. The circular arc for the negative-electrode resistance
component in the plot is approximated to a semicircle to determine
the reaction resistance (Rct) and the double-layer capacity
(Cdl).
[1138] The lithium secondary battery as embodiment B of the
invention can produce the effects of the invention described above
and sufficiently exhibit its performances, as long as the negative
electrode satisfies the requirement concerning the reaction
resistance of a negative-electrode opposing cell. It is, however,
preferred that the negative electrode should further satisfy any
one of or simultaneously satisfy two or more of the following items
concerning the properties or constitution of the negative
electrode. With respect to the negative-electrode active material
also, it is especially preferred that it should satisfy any one of
or simultaneously satisfy two or more of the items concerning
properties of the negative-electrode active material in embodiment
A described above.
[[Double-Layer Capacity (Cdl)]]
[1139] The double-layer capacity (Cdl) of the negative electrode
for use in this invention, as determined through the opposing
impedance measurement, is preferably 1.times.10.sup.-6 F or higher,
especially preferably 1.times.10.sup.-5 F or higher, even more
preferably 3.times.10.sup.-5 F or higher. When the double-layer
capacity thereof is lower than the lower limit of that range, this
negative electrode has a reduced area available for reaction and
there are hence cases where output decreases accordingly.
[[Binder]]
[1140] The binder for binding the negative-electrode active
material to a current collector is not particularly limited as long
as it is a material stable to the nonaqueous electrolyte and the
solvent to be used for electrode production. Usable substances, the
proportion of the binder to the whole negative-electrode
active-material layer, etc. are the same as described above.
[Negative Electrode [10]: Electrode Production]
[1141] Negative electrode [10] may be produced by an ordinary
method. It can be formed in the same manner as described above. The
current collector, thickness ratio between the current collector
and the active-material layer, electrode density, binder,
electrode-plate orientation ratio, impedance, etc. also are the
same as described above.
<Nonaqueous Electrolytic Solution>
[1142] The nonaqueous electrolyte to be used in the invention is
not particularly limited as long as it comprises an electrolyte
(one or more lithium salts), a nonaqueous solvent for dissolving
the electrolyte, and the specific compound. However, this
nonaqueous electrolyte preferably is one which satisfies any
requirement selected from the group consisting of the following
electrolyte [1] to electrolyte [9]:
electrolyte [1]: the nonaqueous solvent as a component of the
electrolyte is a mixed solvent at least containing ethylene
carbonate and the proportion of the ethylene carbonate to the whole
nonaqueous solvent is from 1% by volume to 25% by volume;
electrolyte [2]: the nonaqueous solvent as a component of the
electrolyte contains at least one asymmetric chain carbonate and
the proportion of the asymmetric chain carbonate to the whole
nonaqueous solvent is from 5% by volume to 90% by volume;
electrolyte [3]: the nonaqueous solvent as a component of the
electrolyte contains at least one chain carboxylic acid ester;
electrolyte [4]: the nonaqueous solvent as a component of the
electrolyte contains at least one solvent having a flash point of
70.degree. C. or higher in an amount of 60% by volume or larger
based on the whole nonaqueous solvent; electrolyte [5]: the lithium
salts as a component of the electrolyte comprise
LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an integer of
1-4) and/or lithium bis(oxalato)borate; electrolyte [6]: the
lithium salts as a component of the electrolyte comprise a
fluorine-containing lithium salt and the nonaqueous electrolyte
contains hydrogen fluoride (HF) in an amount of from 10 ppm to 300
ppm of the whole electrolyte; electrolyte [7]: the electrolyte
contains vinylene carbonate and the content of the vinylene
carbonate is in the range of from 0.001% by mass to 3% by mass
based on the whole electrolyte; electrolyte [8]: the electrolyte
further contains at least one compound selected from the group
consisting of compounds represented by general formula (4) given
above, heterocyclic compounds containing nitrogen and/or sulfur,
cyclic carboxylic acid esters, and fluorine-containing cyclic
carbonates in an amount in the range of from 0.001% by mass to 5%
by mass based on the whole nonaqueous electrolyte; and electrolyte
[9]: the electrolyte further contains an overcharge inhibitor.
[1143] The nonaqueous electrolyte for general use in the lithium
secondary battery of this invention is first described below.
[Lithium Salt]
[1144] The electrolyte is not particularly limited as long as it
comprises one or more lithium salts known to be usable as
electrolytes for lithium secondary batteries. Examples thereof
include the following.
[1145] Inorganic lithium salts: inorganic fluoride salts such as
LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, and LiSbF.sub.6; perhalogen
acid salts such as LiClO.sub.4, LiBrO.sub.4, and LiIO.sub.4; and
inorganic chloride salts such as LiAlCl.sub.4.
[1146] Fluorine-containing organolithium salts:
perfluoroalkanesulfonic acid salts such as LiCF.sub.3SO.sub.3;
perfluoroalkanesulfonylimide salts such as
LiN(CF.sub.3SO.sub.2).sub.2, LiN(CF.sub.3CF.sub.2SO.sub.2).sub.2,
and LiN(CF.sub.3SO.sub.2) (C.sub.4F.sub.9SO.sub.2);
perfluoroalkanesulfonyl methide salts such as
LiC(CF.sub.3SO.sub.2).sub.3; and fluoroalkyl fluorophosphates such
as Li[PF.sub.5(CF.sub.2CF.sub.2CF.sub.3)],
Li[PF.sub.4(CF.sub.2CF.sub.2CF.sub.3).sub.2],
Li[PF.sub.3(CF.sub.2CF.sub.2CF.sub.3).sub.3],
Li[PF.sub.5(CF.sub.2CF.sub.2CF.sub.2CF.sub.3)],
Li[PF.sub.4(CF.sub.2CF.sub.2CF.sub.2CF.sub.3).sub.2], and Li
[PF.sub.3 (CF.sub.2CF.sub.2CF.sub.2CF.sub.3).sub.3].
[1147] Oxalatoborate salts: lithium bis(oxalato)borate and lithium
difluorooxalatoborate.
[1148] One of these lithium salts may be used alone, or any desired
two or more thereof may be used in combination in any desired
proportion. Of those, LiPF.sub.6 is preferred when solubility in
the nonaqueous solvent and secondary-battery performances such as
charge/discharge characteristics, output characteristics, and cycle
characteristics are comprehensively judged.
[1149] A preferred example of use of two or more in combination is
a combination of LiPF.sub.6 and LiBF.sub.4. In this case, the
proportion of LiBF.sub.4 to the sum of both is preferably from
0.01% by mass to 20% by mass, especially preferably from 0.1% by
mass to 5% by mass.
[1150] Another example is a combination of an inorganic fluoride
salt and a perfluoroalkanesulfonylimide salt. In this case, the
proportion of the inorganic fluoride salt to the sum of both is
preferably from 70% by mass to 99% by mass, more preferably from
80% by mass to 98% by mass. This combination is effective in
inhibiting deterioration in high-temperature storage.
[1151] The concentration of the lithium salts in the nonaqueous
electrolyte is not particularly limited. However, the concentration
thereof is generally 0.5 mol/L or higher, preferably 0.6 mol/L or
higher, more preferably 0.7 mol/L or higher. The upper limit
thereof is generally 2 mol/L or lower, preferably 1.8 mol/L or
lower, more preferably 1.7 mol/L or lower. When the concentration
thereof is too low, there are cases where the nonaqueous
electrolyte has an insufficient electrical conductivity. On the
other hand, when the concentration thereof is too high, there are
cases where the electrolyte has an increased viscosity and hence a
reduced conductivity, and this may result in a decrease in
performances of the lithium secondary battery.
[Nonaqueous Solvent]
[1152] The nonaqueous solvent to be used also can be one or more
solvents suitably selected from those which have been proposed as
solvents for nonaqueous electrolytes. Examples thereof include the
following.
1) Cyclic Carbonate:
[1153] The alkylene group as a component of the cyclic carbonate
has preferably 2-6 carbon atoms, especially preferably 2-4 carbon
atoms. Examples of the cyclic carbonate include ethylene carbonate,
propylene carbonate, and butylene carbonate. Preferred of these are
ethylene carbonate and propylene carbonate.
2) Chain Carbonate:
[1154] A preferred chain carbonate is a dialkyl carbonate, in which
the alkyl groups as components of the carbonate each have 1-5
carbon atoms, especially preferably 1-4 carbon atoms. Examples
thereof include dialkyl carbonates such as dimethyl carbonate,
diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate,
methyl n-propyl carbonate, and ethyl n-propyl carbonate. Preferred
of these are dimethyl carbonate, diethyl carbonate, and ethyl
methyl carbonate.
3) Cyclic Ester:
[1155] Examples thereof include .gamma.-butyrolactone and
.gamma.-valerolactone.
4) Chain Ester:
[1156] Examples thereof include methyl acetate, ethyl acetate,
propyl acetate, and methyl propionate.
5) Cyclic Ether:
[1157] Examples thereof include tetrahydrofuran,
2-methyltetrahydrofuran, and tetrahydropyran.
6) Chain Ether:
[1158] Examples thereof include dimethoxyethane and
dimethoxymethane.
7) Sulfur-Containing Organic Solvent:
[1159] Examples thereof include sulfolane and diethyl sulfone.
[1160] Those solvents may be used alone or in combination of two or
more thereof. However, it is preferred to use a combination of two
or more compounds. For example, it is preferred to use a
high-permittivity solvent, such as a cyclic carbonate or cyclic
ester, in combination with a low-viscosity solvent, such as a chain
carbonate or chain ester.
[1161] One preferred combination of solvents for constituting the
nonaqueous solvent is a combination consisting mainly of at least
one cyclic carbonate and at least one chain carbonate. In
particular, the content of the sum of the cyclic carbonate and the
chain carbonate in the nonaqueous solvent is 80% by volume or
higher, preferably 85% by volume or higher, more preferably 90% by
volume or higher. This combination may be one in which the
proportion by volume of the cyclic carbonate to the sum of the
cyclic carbonate and the chain carbonate is 5% or higher,
preferably 10% or higher, more preferably 15% or higher, and is
generally 50% or lower, preferably 35% or lower, more preferably
30% or lower. It is especially preferred that the preferred range
of the total content by volume of the carbonates in the whole
nonaqueous solvent should be employed in combination with the
preferred range of the proportion by volume of the cyclic carbonate
to the cyclic and chain carbonates. Use of such combination of
nonaqueous solvents is preferred because the battery produced using
this combination has a satisfactory balance between cycle
characteristics and high-temperature storability (in particular,
residual capacity and high-load discharge capacity after
high-temperature storage).
[1162] A nonaqueous electrolyte which comprises that mixed solvent
and, contained therein, one or more lithium salts and at least one
compound selected from the group consisting of cyclic siloxane
compounds represented by general formula (1) given above,
fluorosilane compounds represented by general formula (2) given
above, compounds represented by general formula (3) given above,
compounds having an S--F bond in the molecule, nitric acid salts,
nitrous acid salts, monofluorophosphoric acid salts,
difluorophosphoric acid salts, acetic acid salts, and propionic
acid salts is preferred because the battery produced using this
electrolyte has a satisfactory balance among cycle characteristics,
high-temperature storability (in particular, residual capacity and
high-load discharge capacity after high-temperature storage), and
inhibition of gas evolution.
[1163] Examples of the preferred combination of at least one cyclic
carbonate and at least one chain carbonate include: ethylene
carbonate and dimethyl carbonate; ethylene carbonate and diethyl
carbonate; ethylene carbonate and ethyl methyl carbonate; ethylene
carbonate, dimethyl carbonate, and diethyl carbonate; ethylene
carbonate, dimethyl carbonate, and ethyl methyl carbonate; ethylene
carbonate, diethyl carbonate, and ethyl methyl carbonate; and
ethylene carbonate, dimethyl carbonate, diethyl carbonate, and
ethyl methyl carbonate. Examples of preferred combinations further
include such ethylene carbonate/chain carbonate combinations to
which propylene carbonate has been further added. In the case where
propylene carbonate is contained, the proportion by volume of
ethylene carbonate to propylene carbonate is generally from 99:1 to
40:60, preferably from 95:5 to 50:50.
[1164] It is preferred that the content of propylene carbonate in
the whole nonaqueous solvent should be regulated to 0.1% by volume
or higher, preferably 1% by volume or higher, more preferably 2% by
volume or higher, and to generally 10% by volume or lower,
preferably 8% by volume or lower, more preferably 5% by volume or
lower. This is because this combination brings about excellent
low-temperature characteristics while maintaining the properties
brought about by the combination of ethylene carbonate with one or
more chain carbonates.
[1165] More preferred of those are ones which include an asymmetric
chain carbonate. Especially preferred are ones comprising ethylene
carbonate, a symmetric chain carbonate, and an asymmetric chain
carbonate, such as: ethylene carbonate, dimethyl carbonate, and
ethyl methyl carbonate; ethylene carbonate, diethyl carbonate, and
ethyl methyl carbonate; and ethylene carbonate, dimethyl carbonate,
diethyl carbonate, and ethyl methyl carbonate. This is because such
combinations bring about a satisfactory balance between cycle
characteristics and high-current discharge characteristics.
Preferred of these are ones in which the asymmetric chain carbonate
is ethyl methyl carbonate. Furthermore, the alkyl groups as
components of each dialkyl carbonate preferably have 1-2 carbon
atoms.
[1166] Other preferred examples of the nonaqueous solvent are ones
containing a chain ester. Especially preferred chain esters are
methyl acetate, ethyl acetate, and the like. The content by volume
of the chain ester in the nonaqueous solvent is generally 5% or
higher, preferably 8% or higher, more preferably 15% or higher, and
is generally 50% or lower, preferably 35% or lower, more preferably
30% or lower, even more preferably 25% or lower. In particular, a
nonaqueous solvent comprising the cyclic carbonate/chain carbonate
mixed solvent and a chain ester is preferred from the standpoint of
improving the low-temperature characteristics of the battery.
[1167] Still another preferred example of the nonaqueous solvent is
one in which either one organic solvent selected from the group
consisting of ethylene carbonate, propylene carbonate,
.gamma.-butyrolactone, and .gamma.-valerolatone or a mixed solvent
composed of two or more organic solvents selected from that group
accounts for 60% by volume or more of the whole nonaqueous solvent.
The mixed solvent preferably is one having a flash point of
50.degree. C. or higher, and especially preferably is one having a
flash point of 70.degree. C. or higher. The nonaqueous electrolyte
employing this solvent is less apt to suffer solvent vaporization
and liquid leakage even when used at high temperatures. In
particular, such nonaqueous solvent may be: one in which the
content of .gamma.-butyrolactone in the nonaqueous solvent is 60%
by volume or higher; one in which the total content of ethylene
carbonate and .gamma.-butyrolactone in the nonaqueous solvent is
80% by volume or higher, preferably 90% by volume or higher, and
the proportion by volume of ethylene carbonate to
.gamma.-butyrolactone is from 5:95 to 45:55; or one in which the
total content of ethylene carbonate and propylene carbonate in the
nonaqueous solvent is 80% by volume or higher, preferably 90% by
volume or higher, and the proportion by volume of ethylene
carbonate to propylene carbonate is from 30:70 to 60:40. Use of
this nonaqueous solvent generally results in a satisfactory balance
among cycle characteristics, high-current discharge
characteristics, etc.
[Specific Compound]
[1168] As stated above, the nonaqueous electrolyte of the invention
is characterized in that it contains at least one compound selected
from the group consisting of cyclic siloxane compounds represented
by general formula (1), fluorosilane compounds represented by
general formula (2), compounds represented by general formula (3),
compounds having an S--F bond in the molecule, nitric acid salts,
nitrous acid salts, monofluorophosphoric acid salts,
difluorophosphoric acid salts, acetic acid salts, and propionic
acid salts (hereinafter, these compounds are often abbreviated to
"specific compounds"), or that the at least one compound has been
added to the electrolyte.
[[Cyclic Siloxane Compounds Represented by General Formula
(1)]]
[1169] R.sup.1 and R.sup.2 in the cyclic siloxane compounds
represented by general formula (1) are organic groups which have
1-12 carbon atoms and may be the same or different. Examples of
R.sup.1 and R.sup.2 include chain alkyl groups such as methyl,
ethyl, n-propyl, isopropyl, butyl, isobutyl, sec-butyl, and
t-butyl; cycloalkyl groups such as cyclohexyl and norbornanyl;
alkenyl groups such as vinyl, 1-propenyl, allyl, butenyl, and
1,3-butadienyl; alkynyl groups such as ethynyl, propynyl, and
butynyl; halogenated alkyl groups such as trifluoromethyl; alkyl
groups having a saturated heterocyclic group, such as
3-pyrrolidinopropyl; aryl groups such as phenyl which may have an
alkyl substituent; aralkyl groups such as phenylmethyl and
phenylethyl; trialkylsilyl groups such as trimethylsilyl; and
trialkylsiloxy groups such as trimethylsiloxy.
[1170] Of those groups, ones having a smaller number of carbon
atoms are more effective in enabling the compound to exhibit its
properties. Organic groups having 1-6 carbon atoms are preferred.
Alkenyl groups are preferred because they act on the nonaqueous
electrolyte and the coating film on an electrode surface to improve
input/output characteristics. Aryl groups are preferred because
they function to scavenge radicals generating in the battery during
charge/discharge to improve overall battery performances.
Consequently, R.sup.1 and R.sup.2 especially preferred are methyl,
vinyl, or phenyl.
[1171] In general formula (1), n represents an integer of 3-10, and
is preferably an integer of 3-6, especially preferably 3 or 4.
[1172] Examples of the cyclic siloxane compounds represented by
general formula (1) include cyclotrisiloxanes such as
hexamethylcyclotrisiloxane, hexaethylcyclotrisiloxane,
hexaphenylcyclotrisiloxane, and
1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane, cyclotetrasiloxanes
such as octamethylcyclotetrasiloxane, and cyclopentasiloxanes such
as decamethylcyclopentasiloxane. Especially preferred of these are
cyclotrisiloxanes.
[[Fluorosilane Compounds Represented by General Formula (2)]]
[1173] R.sup.3 to R.sup.5 in general formula (2) are organic groups
which have 1-12 carbon atoms and may be the same or different.
Examples thereof include the chain alkyl groups, cycloalkyl groups,
alkenyl groups, alkynyl groups, halogenated alkyl groups, alkyl
groups having a saturated heterocyclic group, aryl groups such as
phenyl which may have an alkyl group, aralkyl groups, trialkylsilyl
groups, and trialkylsiloxy groups enumerated above as examples of
R.sup.1 and R.sup.2 in general formula (1). Examples thereof
further include carbonyl groups such as ethoxycarbonylethyl;
carboxyl groups such as acetoxy, acetoxymethyl, and
trifluoroacetoxy; oxy groups such as methoxy, ethoxy, propoxy,
butoxy, phenoxy, and allyloxy; amino groups such as allylamino; and
benzyl.
[1174] In general formula (2), x represents an integer of 1-3.
Symbols p, q, and r each represent an integer of 0-3, provided that
1.ltoreq.p+q+r.ltoreq.3. Furthermore, x+p+q+r=4 holds
inevitably.
[1175] Examples of the fluorosilane compounds represented by
general formula (2) include monofluorosilanes such as
trimethylfluorosilane, triethylfluorosilane, tripropylfluorosilane,
phenyldimethylfluorosilane, triphenylfluorosilane,
vinyldimethylfluorosilane, vinyldiethylfluorosilane,
vinyldiphenylfluorosilane, trimethoxyfluorosilane, and
triethoxyfluorosilane. Examples thereof further include
difluorosilanes such as dimethyldifluorosialne,
diethyldifluorosilane, divinyldifluorosilane, and
ethylvinyldifluorosilane; and trifluorosilanes such as
methyltrifluorosilane and ethyltrifluorosilane.
[1176] When a fluorosilane compound represented by general formula
(2) has a low boiling point, there are cases where it is difficult
to incorporate this compound into the nonaqueous electrolyte in a
given amount because this compound volatilizes. Even after
incorporation into the nonaqueous electrolyte, there is a
possibility that the compound might volatilize under such
conditions that the battery heats up upon charge/discharge or the
environment comes to have a high temperature. Consequently,
compounds having a boiling point of 50.degree. C. or higher at 1
atm are preferred. Especially preferred are compounds having a
boiling point of 60.degree. C. or higher.
[1177] As in the case of the compounds represented by general
formula (1), organic groups having a smaller number of carbon atoms
are more effective in producing the effects. Alkenyl groups having
1-6 carbon atoms act on the nonaqueous electrolyte and the coating
film on an electrode surface to improve input/output
characteristics, while aryl groups function to scavenge radicals
generating in the battery during charge/discharge to improve
overall battery performances. Consequently, from this standpoint,
the organic groups preferably are methyl, vinyl, or phenyl.
Especially preferred examples of the compounds include
trimethylfluorosilane, vinyldimethylfluorosilane,
phenyldimethylfluorosilane, and vinyldiphenylfluorosilane.
[[Compounds Represented by General Formula (3)]]
[1178] R.sup.6 to R.sup.8 in general formula (3) are organic groups
which have 1-12 carbon atoms and may be the same or different.
Examples thereof include the chain alkyl groups, cycloalkyl groups,
alkenyl groups, alkynyl groups, halogenated alkyl groups, alkyl
groups having a saturated heterocyclic group, aryl groups such as
phenyl which may have an alkyl group, aralkyl groups, trialkylsilyl
groups, trialkylsiloxy groups, carbonyl groups, carboxyl groups,
oxy groups, amino groups, and benzyl group enumerated above as
examples of R.sup.3 to R.sup.5 in general formula (2).
[1179] Symbol A in the compounds represented by general formula (3)
is not particularly limited as long as it is a group constituted of
H, C, N, O, F, S, Si, and/or P. However, the element which is
directly bonded to the oxygen atom in general formula (3)
preferably is C, S, Si, or P. With respect to the state in which
these atoms are present, the atoms each preferably are one
contained in, e.g., a chain alkyl, cycloalkyl, alkenyl, alkynyl,
halogenated alkyl, carbonyl, sulfonyl, trialkylsilyl, phosphoryl,
or phosphinyl group or the like.
[1180] The compounds represented by general formula (3) have a
molecular weight of preferably 1,000 or lower, especially
preferably 800 or lower, more preferably 500 or lower. Examples of
the compounds represented by general formula (3) include siloxane
compounds such as hexamethyldisiloxane,
1,3-diethyltetramethyldisiloxane, hexaethyldisiloxane, and
octamethyltrisiloxane; alkoxysilanes such as methoxytrimethylsilane
and ethoxytrimethylsilane; peroxides such as bis(trimethylsilyl)
peroxide; carboxylic acid esters such as trimethylsilyl acetate,
triethylsilyl acetate, trimethylsilyl propionate, trimethylsilyl
methacrylate, and trimethylsilyl trifluoroacetate; sulfonic acid
esters such as trimethylsilyl methanesulfonate, trimethylsilyl
ethanesulfonate, triethylsilyl methanesulfonate, and trimethylsilyl
fluoromethanesulfonate; sulfuric acid esters such as
bis(trimethylsilyl) sulfate; boric acid esters such as
tris(trimethylsiloxy)boron; and phosphoric or phosphorous acid
esters such as tris(trimethylsilyl) phosphate and
tris(trimethylsilyl) phosphite.
[1181] Preferred of these are siloxane compounds, sulfonic acid
esters, and sulfuric acid esters. Especially preferred are sulfonic
acid esters. Preferred of the siloxane compounds is
hexamethyldisiloxane. Preferred of the sulfonic acid esters is
trimethylsilyl methanesulfonate. Preferred of the sulfuric acid
esters is bis(trimethylsilyl) sulfate.
[[Compounds Having S--F Bond in Molecule]]
[1182] The compounds having an S--F bond in the molecule are not
particularly limited. However, sulfonyl fluorides and
fluorosulfonic acid esters are preferred.
[1183] Examples thereof include methanesulfonyl fluoride,
ethanesulfonyl fluoride, methanebis(sulfonyl fluoride),
ethane-1,2-bis(sulfonyl fluoride), propane-1,3-bis(sulfonyl
fluoride), butane-1,4-bis(sulfonyl fluoride),
difluoromethanebis(sulfonyl fluoride),
1,1,2,2-tetrafluoroethane-1,2-bis(sulfonyl fluoride),
1,1,2,2,3,3-hexafluoropropane-1,3-bis(sulfonyl fluoride), methyl
fluorosulfonate, and ethyl fluorosulfonate. Preferred of these is
methanesulfonyl fluoride, methanebis(sulfonyl fluoride), or methyl
fluorosulfonate.
[[Nitric Acid Salts, Nitrous Acid Salts, Monofluorophosphoric Acid
Salts, Difluorophosphoric Acid Salts, Acetic Acid Salts, and
Propionic Acid Salts]]
[1184] The counter cations of the nitric acid salts, nitrous acid
salts, monofluorophosphoric acid salts, difluorophosphoric acid
salts, acetic acid salts, and propionic acid salts are not
particularly limited. Examples thereof include metallic elements
such as Li, Na, K, Mg, Ca, Fe, and Cu and ammonium and quaternary
ammoniums represented by NR.sup.9R.sup.10R.sup.11R.sup.12 (wherein
R.sup.9 to R.sup.12 each independently represent a hydrogen atom or
an organic group having 1-12 carbon atoms). Examples of the organic
groups having 1-12 carbon atoms represented by R.sup.9 to R.sup.12
include alkyl groups which may be substituted by one or more
halogen atoms, cycloalkyl groups which may be substituted by one or
more halogen atoms, aryl group which may be substituted by one or
more halogen atoms, and heterocyclic groups containing a nitrogen
atom. R.sup.9 to R.sup.12 each preferably are a hydrogen atom,
alkyl group, cycloalkyl group, nitrogen-atom-containing
heterocyclic group, or the like. Lithium, sodium, potassium,
magnesium, calcium, or NR.sup.9R.sup.10R.sup.11R.sup.12 is
preferred of those counter cations from the standpoint of the
characteristics of the lithium secondary battery employing such
compounds. Lithium is especially preferred. Of those compounds,
nitric acid salts or difluorophosphoric acid salts are preferred
from the standpoints of the percentage improvement in output
characteristics and of cycle characteristics. Especially preferred
is lithium difluorophosphate. These compounds each may be one
synthesized in a nonaqueous solvent and used substantially as it
is, or may be one synthesized separately, substantially isolated,
and added to a nonaqueous solvent.
[1185] One of the specific compounds, i.e., cyclic siloxane
compounds represented by general formula (1), fluorosilane
compounds represented by general formula (2), compounds represented
by general formula (3), compounds having an S--F bond in the
molecule, nitric acid salts, nitrous acid salts,
monofluorophosphoric acid salts, difluorophosphoric acid salts,
acetic acid salts, and propionic acid salts, may be used alone.
Alternatively, any desired two or more compounds selected from
these may be used in combination in any desired proportion. With
respect to the compounds in each group in the specific compounds,
one of these may be used alone or any desired two or more thereof
may be used in combination in any desired proportion.
[1186] The proportion of those specific compounds in the nonaqueous
electrolyte should be 10 ppm or higher (0.001% by mass or higher)
in terms of total content thereof based on the whole nonaqueous
electrolyte, and is preferably 0.01% by mass or higher, more
preferably 0.05% by mass or higher, even more preferably 0.1% by
mass or higher. The upper limit thereof is preferably 5% by mass or
lower, more preferably 4% by mass or lower, even more preferably 3%
by mass or lower. When the concentration of the specific compounds
is too low, there are cases where the effect of improving battery
output or the effect of improving battery life is difficult to
obtain. On the other hand, too high concentrations thereof may lead
to a decrease in charge/discharge efficiency.
[1187] When any of those specific compounds is incorporated into a
nonaqueous electrolyte and actually used in fabricating a secondary
battery and this battery is disassembled to take out the nonaqueous
electrolyte again, then there frequently are cases where the
content of the specific compounds therein has decreased
considerably. Because of this, in the case where the specific
compound can be at least detected in the nonaqueous electrolyte
discharged from a battery, this battery is regarded as within the
scope of the invention. The nonaqueous electrolyte according to the
invention can be prepared by dissolving one or more electrolyte
lithium salts and the specific compound in a nonaqueous solvent and
further dissolving one or more other compounds therein according to
need. In preparing the nonaqueous electrolyte, it is preferred to
dehydrate each raw material beforehand to a water content of
generally 50 ppm or lower, preferably 30 ppm or lower, especially
preferably 10 ppm or lower.
[Other Compounds]
[1188] Although the nonaqueous electrolyte in the invention
comprises a nonaqueous solvent and one or more lithium salts as an
electrolyte and the specific compound which are contained as
essential ingredients in the solvent, other compounds can be
incorporated therein in any desired amount according to need unless
this incorporation lessens the effects of the invention. Examples
of such other compounds include:
(1) overcharge inhibitors such as aromatic compounds, e.g.,
biphenyl, alkylbiphenyls, terphenyl, partially hydrogenated
terphenyls, cyclohexylbenzene, t-butylbenzene, t-amylbenzene,
diphenyl ether, and dibenzofuran; products of the partial
fluorination of these aromatic compounds, e.g., 2-fluorobiphenyl,
o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; and
fluorine-containing anisole compounds, e.g., 2,4-difluoroanisole,
2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole;
(2) negative-electrode coating agents such as vinylene carbonate,
vinylethylene carbonate, fluoroethylene carbonate,
trifluoropropylene carbonate, succinic anhydride, glutaric
anhydride, maleic anhydride, citraconic anhydride, glutaconic
anhydride, itaconic anhydride, and cyclohexanedicarboxylic
anhydride; and (3) positive-electrode protective agents such as
ethylene sulfite, propylene sulfite, dimethyl sulfite,
propanesultone, butanesultone, methyl methanesulfonate, busulfan,
methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate,
sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide,
diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide,
thioanisole, diphenyl disulfide, and dipyridinium disulfide.
[1189] Preferred overcharge inhibitors are aromatic compounds such
as biphenyl, alkylbiphenyls, terphenyl, partially hydrogenated
terphenyls, cyclohexylbenzene, t-butylbenzene, t-amylbenzene,
diphenyl ether, and dibenzofuran. Two or more of these may be used
in combination. In the case where two or more are used in
combination, it is especially preferred to use cyclohexylbenzene or
terphenyl (or a product of partial hydrogenation thereof) in
combination with t-butylbenzene or t-amylbenzene.
[1190] Preferred negative-electrode coating agents are vinylene
carbonate, vinylethylene carbonate, fluoroethylene carbonate,
succinic anhydride, and maleic anhydride. Two or more of these may
be used in combination. In the case where two or more are used in
combination, it is preferred to use vinylene carbonate in
combination with vinylethylene carbonate, fluoroethylene carbonate,
succinic anhydride, or maleic anhydride. Preferred
positive-electrode protective agents are ethylene sulfite,
propylene sulfite, propanesultone, butanesultone, methyl
methanesulfonate, and busulfan. Two or more of these may be used in
combination. It is especially preferred to use a combination of a
negative-electrode coating agent and a positive-electrode
protective agent or a combination of an overcharge inhibitor, a
negative-electrode coating agent, and a positive-electrode
protective agent.
[1191] The contents of such other compounds in the nonaqueous
electrolyte are not particularly limited. However, the content of
each compound is preferably 0.01% by mass or higher, more
preferably 0.1% by mass or higher, even more preferably 0.2% by
mass or higher, based on the whole nonaqueous electrolyte. The
upper limit thereof is preferably 5% by mass or lower, more
preferably 3% by mass or lower, even more preferably 2% by mass or
lower. The addition of those compounds is effective in inhibiting
the battery from rupturing/firing in an abnormal state caused by
overcharge and in improving capacity retentivity after
high-temperature storage and cycle characteristics.
[1192] Methods for preparing the nonaqueous electrolyte for the
lithium secondary battery of the invention are not particularly
limited. The nonaqueous electrolyte can be prepared by dissolving
at least one lithium salt and the specific compound optionally
together with other compounds in a nonaqueous solvent in an
ordinary manner.
<Electrolytic Solution [1]>
[1193] The nonaqueous electrolyte preferably is one in which the
nonaqueous solvent as a component of the electrolyte is a mixed
solvent at least containing "ethylene carbonate" and the proportion
of the ethylene carbonate to the whole nonaqueous solvent is from
1% by volume to 25% by volume (electrolyte [1]).
[1194] In this invention (electrolyte [1]), the kinds and contents
of the "lithium salt", "nonaqueous solvent other than ethylene
carbonate (EC)", "specific compound", and "other compounds", use
conditions therefor, method of preparing the nonaqueous
electrolyte, etc. are the same as described above.
<Electrolytic Solution [2]>
[1195] The nonaqueous electrolyte preferably is one in which the
nonaqueous solvent as a component of the electrolyte contains "at
least one asymmetric chain carbonate" and the proportion of the
asymmetric chain carbonate to the whole nonaqueous solvent is from
5% by volume to 90% by volume (electrolyte [2]).
[1196] In this invention (electrolyte [2]), the kinds and contents
of the "lithium salt", "nonaqueous solvent other than the
asymmetric chain carbonate", "specific compound", and "other
compounds", use conditions therefor, method of preparing the
nonaqueous electrolyte, etc. are the same as described above.
[1197] In this invention (electrolyte [2]), at least one asymmetric
chain carbonate is contained in an amount of from 5% by volume to
90% by volume based on the whole nonaqueous solvent. The content of
the at least one asymmetric chain carbonate in the whole nonaqueous
solvent is preferably 8% by volume or higher, especially preferably
10% by volume or higher, more preferably 15% by volume or higher,
even more preferably 20% by volume or higher. The upper limit
thereof is preferably 85% by volume or lower, especially preferably
70% by volume or lower, more preferably 60% by volume or lower,
even more preferably 45% by volume or lower. To regulate the
content of the asymmetric chain carbonate to such a value is
desirable for reconciling the high low-temperature characteristics
with cycle characteristics, etc. according to the invention.
[1198] The asymmetric chain carbonate is not particularly limited.
However, asymmetric alkyl carbonates are preferred, and ones in
which the alkyl groups have 1-4 carbon atoms are suitable. Examples
of such asymmetric alkyl carbonates include ethyl methyl carbonate,
methyl n-propyl carbonate, ethyl n-propyl carbonate, methyl
isopropyl carbonate, ethyl isopropyl carbonate, n-butyl methyl
carbonate, and n-butyl ethyl carbonate. Preferred of these are
ethyl methyl carbonate, methyl n-propyl carbonate, ethyl n-propyl
carbonate, n-butyl methyl carbonate, and n-butyl ethyl carbonate.
More preferred are ethyl methyl carbonate, methyl n-propyl
carbonate, and n-butyl methyl carbonate. Especially preferred is
ethyl methyl carbonate. A mixture of two or more of these
asymmetric chain carbonates may be used.
[1199] As stated above, the nonaqueous solvent may be a mixture of
two or more nonaqueous solvents. In particular, it is preferred
that the nonaqueous solvent should comprise, besides the asymmetric
chain carbonate, at least one cyclic carbonate from the standpoints
of improving the cycle characteristics and storability of the
secondary battery, etc. In the case where at least one cyclic
carbonate is contained, the proportion of the cyclic carbonate to
the whole nonaqueous solvent is generally 5% by volume or higher,
preferably 10% by volume or higher, more preferably 15% by volume
or higher. The upper limit thereof is generally 50% by volume or
lower, preferably 35% by volume or lower, more preferably 30% by
volume or lower, even more preferably 25% by volume or lower. When
the proportion of the cyclic carbonate to the whole nonaqueous
solvent is too low, there are cases where the cycle characteristics
and storability of the secondary battery are not improved. On the
other hand, too high proportions thereof may result in a decrease
in low-temperature discharge characteristics.
[1200] Furthermore, it is preferred that the nonaqueous solvent
should comprise, besides the asymmetric chain carbonate, at least
one symmetric chain carbonate from the standpoints of improving a
balance among the cycle characteristics, storability, and
low-temperature discharge characteristics of the secondary battery,
etc. In the case where at least one symmetric chain carbonate is
contained, the proportion of the symmetric chain carbonate to the
whole nonaqueous solvent is generally 5% by volume or higher,
preferably 10% by volume or higher, more preferably 15% by volume
or higher. The upper limit thereof is generally 80% by volume or
lower, preferably 70% by volume or lower, more preferably 50% by
volume or lower, even more preferably 40% by volume or lower. When
the proportion of the symmetric chain carbonate to the whole
nonaqueous solvent is too low, there are cases where an improved
balance among cycle characteristics, storability, and
low-temperature discharge characteristics is not obtained. On the
other hand, when the proportion thereof is too high, there are
cases where satisfactory cycle characteristics are not
obtained.
[1201] Moreover, it is preferred that the nonaqueous solvent should
be a mixed solvent comprising the asymmetric chain carbonate and
two or more other nonaqueous solvents. Namely, the nonaqueous
solvent preferably is a mixed solvent composed of three or more
ingredients including the asymmetric chain carbonate. The mixed
solvent composed of two or more nonaqueous solvents, excluding the
asymmetric chain carbonate, preferably comprises a combination of a
high-permittivity solvent, such as a cyclic carbonate or cyclic
ester, and a low-viscosity solvent such as a symmetric chain
carbonate or chain ester, because this mixed solvent is effective
in enhancing the overall battery performance including
charge/discharge characteristics and battery life. Especially
preferred is a combination of a cyclic carbonate and a symmetric
chain carbonate. The mixing ratio in this case is not particularly
limited. However, the amount of the high-permittivity solvent such
as a cyclic carbonate or cyclic ester and that of the low-viscosity
solvent such as a symmetric chain carbonate or chain ester are
preferably 10-400 parts by volume and 10-800 parts by volume,
respectively, per 100 parts by volume of the asymmetric chain
carbonate from the standpoints of improving cycle characteristics
and storability, etc.
[1202] One preferred combination of two or more nonaqueous solvents
other than the asymmetric chain carbonate is a combination
consisting mainly of at least one cyclic carbonate and at least one
symmetric chain carbonate. Examples of the preferred combination of
at least one cyclic carbonate and at least one symmetric chain
carbonate include: ethylene carbonate and dimethyl carbonate;
ethylene carbonate and diethyl carbonate; and ethylene carbonate,
dimethyl carbonate, and diethyl carbonate. The contents of these
ingredients in the whole nonaqueous solvent are not particularly
limited. However, the contents of the asymmetric chain carbonate,
cyclic carbonate, and symmetric chain carbonate are preferably
8-80% by volume, 10-35% by volume, and 10-70% by volume,
respectively.
[1203] Examples of preferred combinations further include a
combination comprising such ethylene carbonate/symmetric chain
carbonate combination and propylene carbonate added thereto. In the
case where propylene carbonate is contained, the proportion by
volume of ethylene carbonate to propylene carbonate is preferably
from 99:1 to 40:60, especially preferably from 95:5 to 50:50.
[1204] Another preferred combination of two or more nonaqueous
solvents other than the asymmetric chain carbonate is a combination
including a chain ester. In particular, one comprising the cyclic
carbonate and at least one of the chain esters is preferred from
the stand point of improving the low-temperature characteristics of
the battery. The chain ester especially preferably is methyl
acetate, ethyl acetate, or the like. The proportion of the chain
ester to the whole nonaqueous solvent is preferably 5% by volume or
higher, especially preferably 8% by volume or higher, more
preferably 15% by volume or higher. The upper limit thereof is
preferably 50% by volume or lower, especially preferably 35% by
volume or lower, more preferably 30% by volume or lower, even more
preferably 25% by volume or lower. Consequently, in this case, the
contents of the asymmetric chain carbonate, cyclic carbonate, and
chain ester in the whole nonaqueous solvent are preferably 8-85% by
volume, 10-35% by volume, and 5-50% by volume, respectively.
[1205] Examples of preferred combinations further include a
combination comprising such cyclic carbonate/chain ester
combination and a symmetric chain carbonate added thereto. In the
case where a symmetric chain carbonate is contained, the content of
the symmetric chain carbonate in the whole nonaqueous solvent is
preferably 10-60% by volume.
[1206] Specific examples of the preferred solvent combinations for
use as the nonaqueous solvent of the nonaqueous electrolyte for a
secondary battery of the invention include the following.
Combinations of an asymmetric chain carbonate and one or more
cyclic carbonates, such as: ethyl methyl carbonate and ethylene
carbonate; methyl n-propyl carbonate and ethylene carbonate; ethyl
methyl carbonate, ethylene carbonate, and propylene carbonate; and
methyl n-propyl carbonate, ethylene carbonate, and propylene
carbonate. Combinations of an asymmetric chain carbonate, a cyclic
carbonate, and one or more symmetric chain carbonates, such as:
ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate;
ethyl methyl carbonate, ethylene carbonate, and diethyl carbonate;
ethyl methyl carbonate, ethylene carbonate, diethyl carbonate, and
dimethyl carbonate; methyl n-propyl carbonate, ethylene carbonate,
and dimethyl carbonate; methyl n-propyl carbonate, ethylene
carbonate, and diethyl carbonate; and methyl n-propyl carbonate,
ethylene carbonate, diethyl carbonate, and dimethyl carbonate.
Combinations of an asymmetric chain carbonate, a cyclic carbonate,
and a chain ester, such as: ethyl methyl carbonate, ethylene
carbonate, and methyl acetate; and ethyl methyl carbonate, ethylene
carbonate, and ethyl acetate. Combinations of an asymmetric chain
carbonate, a cyclic carbonate, a symmetric chain carbonate, and a
chain ester, such as: ethyl methyl carbonate, ethylene carbonate,
dimethyl carbonate, and methyl acetate; and ethyl methyl carbonate,
ethylene carbonate, diethyl carbonate, and methyl acetate.
[1207] A nonaqueous electrolyte which comprises this mixed solvent,
a lithium salt contained therein, and the specific compound, e.g.,
a difluorophosphoric acid salt, contained in the solvent in an
amount of 10 ppm or more by mass of the whole electrolyte and in
which the content of the asymmetric chain carbonate in the whole
nonaqueous solvent is from 5% by volume to 90% by volume is
preferred. This is because the secondary battery produced with this
electrolyte has an improved balance among cycle characteristics,
low-temperature discharge characteristics, high-temperature
storability (in particular, residual capacity and high-load
discharge capacity after high-temperature storage), and the
inhibition of gas evolution.
<Electrolytic Solution [3]>
[1208] The nonaqueous electrolyte preferably is one in which the
nonaqueous solvent as a component of the electrolyte contains "at
least one chain carboxylic acid ester" (electrolyte [3]).
[1209] In this invention (electrolyte [3]), the kinds and contents
of the "lithium salt", "nonaqueous solvent other than the chain
carboxylic acid ester", "specific compound", and "other compounds",
use conditions therefor, method of preparing the nonaqueous
electrolyte, etc. are the same as described above.
[1210] The chain carboxylic acid ester to be used in this invention
(electrolyte [3]) is not particularly limited. However, it
preferably is a C.sub.1-4-alkyl ester of a carboxylic acid in which
the number of carbon atoms including that in the carboxyl group(s)
is 1-5. Although the valence of this carboxylic acid also is not
particularly limited, a monocarboxylic acid or a dicarboxylic acid
is preferred.
[1211] Preferred of such carboxylic acid esters are fatty acid
esters such as formic esters, acetic esters, propionic esters, and
butyric esters and esters of various dicarboxylic acids. Especially
preferred are acetic esters or propionic esters.
[1212] Specifically, methyl acetate, ethyl acetate, propyl acetate,
methyl propionate, or ethyl propionate is preferred. Especially
preferred is methyl acetate, ethyl acetate, or methyl
propionate.
[1213] A mixture of two or more of those chain carboxylic acid
esters may be used. The combination of esters to be mixed is not
particularly limited. Examples of preferred combinations include:
methyl acetate and ethyl acetate; methyl acetate and methyl
propionate; ethyl acetate and methyl propionate; and methyl
acetate, ethyl acetate, and methyl propionate. By using a suitable
combination, a balance among output characteristics,
high-temperature storability, etc. can be regulated according to
purposes.
[1214] In the case where a mixed solvent composed of the chain
carboxylic acid ester and other nonaqueous solvent(s) is used, the
proportion of the chain carboxylic acid ester to all nonaqueous
solvents is preferably 3% by volume or higher, especially
preferably 5% by volume or higher, more preferably 8% by volume or
higher, even more preferably 10% by volume or higher. The upper
limit thereof is preferably 50% by volume or lower, especially
preferably 35% by volume or lower, more preferably 30% by volume or
lower, even more preferably 25% by volume or lower. To regulate the
proportion of the ester to such a value is desirable for
reconciling the high low-temperature characteristics with cycle
characteristics, etc. according to the invention.
[1215] The use of a chain carboxylic acid ester in combination with
the specific compound greatly improves low-temperature output
characteristics. Although the reasons for this are unclear, the
improvement is thought to be obtained by the following mechanism.
There are cases where the specific compound, even in the absence of
a chain carboxylic acid ester, has the effect of improving
low-temperature characteristics to some degree. It is thought that
the specific compound exerts some action on the electrodes and this
brings about that effect. It is hence thought that the chain
carboxylic acid ester enhances that action. Namely, it is thought
that the presence of the chain carboxylic acid ester, which is
highly flowable even at low temperatures, enables the specific
compound to infiltrate into inner parts of the electrode plates and
effectively produce its effect. Alternatively, it is thought that
the chain carboxylic acid ester serves as a medium for interaction
between the specific compound and the electrodes.
<Electrolytic Solution [4]>
[1216] The nonaqueous electrolyte preferably is one in which the
nonaqueous solvent as a component of the electrolyte contains at
least one solvent having a flash point of 70.degree. C. or higher
in an amount of 60% by volume or larger based on the whole
nonaqueous solvent (electrolyte [4]).
[1217] In this invention (electrolyte [4]), the kinds and contents
of the "lithium salt", "nonaqueous solvent other than the solvent
having a flash point of 70.degree. C. or higher", "specific
compound", and "other compounds", use conditions therefor, method
of preparing the nonaqueous electrolyte, etc. are the same as
described above.
[1218] In this invention, the nonaqueous solvent should contain at
least one solvent having a flash point of 70.degree. C. or higher
in a total amount of 60% by volume or larger based on the whole
nonaqueous solvent. It is preferred that the nonaqueous solvent
should contain at least one solvent having a flash point of
80.degree. C. or higher in a total amount of 60% by volume or
larger based on the whole nonaqueous solvent. Especially
preferably, the nonaqueous solvent contains at least one solvent
having a flash point of 90.degree. C. or higher in a total amount
of 60% by volume or larger based on the whole nonaqueous
solvent.
[1219] The solvent having a flash point of 70.degree. C. or higher
is not particularly limited. Preferred examples thereof include
ethylene carbonate, propylene carbonate, butylene carbonate,
.gamma.-butyrolactone, and .gamma.-valerolactone. Especially
preferred of these is ethylene carbonate, propylene carbonate, or
.gamma.-butyrolactone. These solvents may be used either alone or
as a mixture of two or more thereof. Combinations thereof are not
particularly limited.
[1220] From the standpoint of improving cycle characteristics, the
nonaqueous solvent preferably has a composition in which ethylene
carbonate or propylene carbonate is contained in an amount of 10%
by volume or larger based on the whole nonaqueous solvent. On the
other hand, since too high contents of ethylene carbonate result in
a decrease in low-temperature characteristics, the content of
ethylene carbonate is regulated to preferably 70% by volume or
lower, especially preferably 60% by volume or lower, based on the
whole nonaqueous solvent.
[1221] Preferred examples of the nonaqueous solvent containing at
least one solvent having a flash point of 70.degree. C. or higher
in a total amount of 60% by volume or larger based on the whole
nonaqueous solvent include:
(1) one in which the content of .gamma.-butyrolactone in the whole
nonaqueous solvent is 60% by volume or higher; (2) one in which the
total content of ethylene carbonate and .gamma.-butyrolactone in
the whole nonaqueous solvent is 80% by volume or higher, preferably
85% by volume or higher, and the proportion by volume of ethylene
carbonate to .gamma.-butyrolactone is from 5:95 to 45:55; and (3)
one in which the total content of ethylene carbonate and propylene
carbonate in the whole nonaqueous solvent is 80% by volume or
higher, preferably 85% by volume or higher, and the proportion by
volume of ethylene carbonate to propylene carbonate is from 30:70
to 60:40.
[1222] Use of those nonaqueous solvents results in an improved
balance especially between cycle characteristics and high-current
discharge characteristics, etc.
[1223] Furthermore, this nonaqueous electrolyte preferably has a
flash point of 40.degree. C. or higher. The flash point thereof is
especially preferably 50.degree. C. or higher, more preferably
60.degree. C. or higher, even more preferably 70.degree. C. or
higher. When the nonaqueous electrolyte has too low a flash point,
there is a fear that the electrolyte may catch fire when the
battery is exposed to a high temperature.
[1224] The nonaqueous solvent for use in this nonaqueous
electrolyte for a secondary battery of the invention may comprise
the solvent having a flash point of 70.degree. C. or higher and,
incorporated therein, a nonaqueous-solvent ingredient having a
flash point lower than 70.degree. C. (hereinafter abbreviated to
"other nonaqueous-solvent ingredient"). This other
nonaqueous-solvent ingredient may be one suitably selected from
those which have been proposed as solvents for nonaqueous
electrolytes. Examples thereof include the following.
(1) Chain Carbonates
[1225] Preferred chain carbonates are dialkyl carbonates, in which
the constituent alkyl groups each have preferably 1-5, especially
preferably 1-4 carbon atoms. Examples thereof include dialkyl
carbonates such as dimethyl carbonate, diethyl carbonate,
di-n-propyl carbonate, ethyl methyl carbonate, methyl n-propyl
carbonate, and ethyl n-propyl carbonate. Preferred of these are
dimethyl carbonate, diethyl carbonate, and ethyl methyl
carbonate.
(2) Chain Esters
[1226] Examples thereof include methyl acetate, ethyl acetate,
propyl acetate, and methyl propionate.
(3) Cyclic Ethers
[1227] Examples thereof include tetrahydrofuran,
2-methyltetrahydrofuran, and tetrahydropyran.
(4) Chain Ethers
[1228] Examples thereof include dimethoxyethane and
dimethoxymethane.
[1229] One of such other nonaqueous-solvent ingredients may be used
in combination with the "solvent having a flash point of 70.degree.
C. or higher", or two or more thereof may be used in combination
with the solvent. In this case, to use, in particular, a chain
carbonate and/or a chain ester in combination with the "solvent
having a flash point of 70.degree. C. or higher" is preferred from
the standpoints of cycle characteristics and high-current discharge
characteristics.
[1230] The content of the solvent having a flash point of
70.degree. C. or higher in the whole nonaqueous solvent should be
60% by volume or higher, and is preferably 70% by volume or higher,
more preferably 75% by volume or higher, especially preferably 80%
by volume or higher, even more preferably 85% by volume or higher.
The upper limit thereof is preferably 100% by volume or lower,
especially preferably 90% by volume or lower. When the content of
the solvent having a flash point of 70.degree. C. or higher is too
low, there are cases where the desired effects are not obtained.
For example, there are cases where the battery is apt to suffer an
internal-pressure increase when stored at high temperatures. On the
other hand, when the content thereof is too high, there are cases
where the electrolyte has an increased viscosity and hence a
reduced electrical conductivity, and this may result in a decrease
in performances of the lithium secondary battery.
[1231] The nonaqueous-electrolyte secondary battery employing the
nonaqueous electrolyte (electrolyte [4]) according to this
invention has excellent output characteristics despite the fact
that a solvent having a high flash point is used in an amount not
smaller than a certain value. Although the reasons for this are
unclear, a possible explanation may be as follows; the following
explanation should not be construed as limiting the invention.
Namely, it is thought that the specific compound acts on the
electrodes and reduces the reaction resistance relating to lithium
ion elimination/insertion to improve output characteristics.
Furthermore, solvents having a high flash point, such as ethylene
carbonate, propylene carbonate, butylene carbonate,
.gamma.-butyrolactone, and .gamma.-valerolactone, have a higher
permittivity than chain carbonates, and that effect is thought to
be produced remarkably when such a solvent having a high
permittivity is present in an amount not below a certain level.
Moreover, when the battery elements held in each battery case of
the secondary battery have an electric capacity of 3 ampere-hours
(Ah) or higher and/or when the secondary battery has a
direct-current resistance component of 10 milliohms (.OMEGA.) or
less, then the contribution of the direct-current resistance
component is small. The effects inherent in this nonaqueous
electrolyte are thought to be more apt to be produced in such
battery than in batteries in which the contribution of a
direct-current resistance component is large.
<Electrolytic Solution [5]>
[1232] The nonaqueous electrolyte preferably is one in which the
"lithium salts" as a component of the electrolyte comprise
LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an integer of
1-4) and/or lithium bis(oxalato)borate (electrolyte [5]).
[1233] In this invention (electrolyte [5]), the kinds and contents
of the "nonaqueous solvent", "specific compound", and "other
compounds", use conditions therefor, method of preparing the
nonaqueous electrolyte, etc. are the same as described above.
[1234] Although this nonaqueous electrolyte (electrolyte [5]) for a
secondary battery of the invention comprises a nonaqueous solvent
and one or more lithium salts dissolved therein, these lithium
salts comprise at least one member selected from the group
consisting of LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an
integer of 1-4) and lithium bis(oxalato)borate. One of these
lithium salts may be used alone, or any desired two or more of
these may be used in combination in any desired proportion.
[1235] When the "specific compound" (b) is used in combination with
the following (a) in an amount of 10 ppm or larger in the
nonaqueous electrolyte for secondary battery, the nonaqueous
electrolyte provided can realize a secondary battery having greatly
improved output characteristics and excellent in high-temperature
storability and cycle characteristics. (a)
LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an integer of
1-4) and/or lithium bis(oxalato)borate
[1236] Ingredient (a) is not particularly limited. However,
LiN(CF.sub.3SO.sub.2).sub.2 or lithium bis(oxalato)borate is
preferred because these salts are especially effective in producing
those effects.
[1237] Although this nonaqueous electrolyte for a secondary battery
of the invention contains LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2
(wherein n is an integer of 1-4) and/or lithium bis(oxalato)borate
as an essential lithium salt ingredient as described above, other
known lithium salts (hereinafter abbreviated to "other lithium
salts") may be used therewith.
[1238] The other lithium salts are not particularly limited.
Examples thereof include the following.
[1239] Inorganic lithium salts: inorganic fluoride salts such as
LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, and LiSbF.sub.6; perhalogen
acid salts such as LiClO.sub.4, LiBrO.sub.4, and LiIO.sub.4; and
inorganic chloride salts such as LiAlCl.sub.4.
[1240] Fluorine-containing organolithium salts:
perfluoroalkanesulfonic acid salts such as LiCF.sub.3SO.sub.3;
perfluoroalkanesulfonyl methide salts such as
LiC(CF.sub.3SO.sub.2).sub.3; and fluoroalkyl fluorophosphates such
as Li[PF.sub.5(CF.sub.2CF.sub.2CF.sub.3)], Li
[PF.sub.4(CF.sub.2CF.sub.2CF.sub.3).sub.2],
Li[PF.sub.3(CF.sub.2CF.sub.2CF.sub.3).sub.3],
Li[PF.sub.5(CF.sub.2CF.sub.2CF.sub.2CF.sub.3)], Li
[PF.sub.4(CF.sub.2CF.sub.2CF.sub.2CF.sub.3).sub.2], and Li
[PF.sub.3 (CF.sub.2CF.sub.2CF.sub.2CF.sub.3).sub.3].
[1241] Other lithium oxalatoborates: lithium
difluorooxalatoborate.
[1242] One of these lithium salts may be used alone, or any desired
two or more thereof may be used in combination in any desired
proportion. Of these "other lithium salts", LiPF.sub.6 and
LiBF.sub.4 are preferred when solubility in nonaqueous solvents and
secondary-battery performances such as charge/discharge
characteristics, output characteristics, and cycle characteristics
are comprehensively judged. Especially preferred is LiPF.sub.6.
[1243] In the case where the LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2
(wherein n is an integer of 1-4) and/or lithium bis(oxalato)borate
is used as the main salt, the concentration thereof in the
nonaqueous electrolyte is generally 0.3 mol/L or higher, preferably
0.5 mol/L or higher, more preferably 0.7 mol/L or higher. The upper
limit thereof is generally 2 mol/L or lower, preferably 1.8 mol/L
or lower, more preferably 1 mol/L or lower. In the case where other
lithium salt, e.g., LiPF.sub.6 or LiBF.sub.4, is used as the main
salt, the concentration of the LiN(C.sub.nF.sub.2n-1SO.sub.2).sub.2
(wherein n is an integer of 1-4) and/or lithium bis(oxalato)borate
may be 0.001 mol/L or higher and is preferably 0.01 mol/L or
higher, more preferably 0.03 mol/L or higher. The upper limit
thereof is generally 0.3 mol/L or lower, preferably 0.2 mol/L or
lower, more preferably 0.1 mol/L or lower. The term main salt
herein means the lithium salt contained in a highest concentration
in the nonaqueous electrolyte.
[1244] The total concentration of the lithium salts in the
nonaqueous electrolyte is not particularly limited. However, it is
generally 0.5 mol/L or higher, preferably 0.6 mol/L or higher, more
preferably 0.7 mol/L or higher. The upper limit thereof is
generally 2 mol/L or lower, preferably 1.8 mol/L or lower, more
preferably 1.7 mol/L or lower. When the concentration thereof is
too low, there are cases where this nonaqueous electrolyte has an
insufficient electrical conductivity. On the other hand, when the
concentration thereof is too high, there are cases where this
electrolyte has an increased viscosity and hence a reduced
electrical conductivity, and this may result in a decrease in
performances of the lithium secondary battery.
[1245] One preferred example of combinations of two or more lithium
salts is a combination of at least one lithium salt selected from
the group consisting of LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2
(wherein n is an integer of 1-4) and lithium bis(oxalato)borate
with LiPF.sub.6. This combination can improve, in particular,
output characteristics and storability. When this combination is
employed and the at least one lithium salt selected from the group
consisting of LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an
integer of 1-4) and lithium bis(oxalato)borate is used as the main
salt, then the concentration of the at least one lithium salt in
the nonaqueous electrolyte is preferably 0.4 mol/L or higher, more
preferably 0.5 mol/L or higher, especially preferably 0.6 mol/L or
higher. The upper limit thereof is preferably 1.8 mol/L or lower,
more preferably 1.5 mol/L or lower, especially preferably 1.2 mol/L
or lower. The concentration of LiPF.sub.6 in this case is
preferably 0.001 mol/L or higher, more preferably 0.01 mol/L or
higher, especially preferably 0.1 mol/L or higher. The upper limit
thereof is preferably 1 mol/L or lower, more preferably 0.8 mol/L
or lower, especially preferably 0.3 mol/L or lower, provided that
the LiPF.sub.6 concentration is lower than the concentration of the
at least one lithium salt selected from the group consisting of
LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an integer of
1-4) and lithium bis(oxalato)borate.
[1246] In the case where LiPF.sub.6 is used as the main salt, the
concentration of the at least one lithium salt selected from the
group consisting of LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n
is an integer of 1-4) and lithium bis(oxalato)borate is preferably
0.001 mol/L or higher, more preferably 0.01 mol/L or higher,
especially preferably 0.03 mol/L or higher. The upper limit thereof
is preferably 0.3 mol/L or lower, more preferably 0.2 mol/L or
lower, especially preferably 0.1 mol/L or lower. In this case, the
concentration of LiPF.sub.6 is preferably 0.5 mol/L or higher, more
preferably 0.6 mol/L or higher, especially preferably 0.7 mol/L or
higher. The upper limit thereof is preferably 1.8 mol/L or lower,
more preferably 1.7 mol/L or lower, especially preferably 1.5 mol/L
or lower.
[1247] Furthermore, LiBF.sub.4 may be used in combination with the
combination of at least one lithium salt selected from the group
consisting of LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an
integer of 1-4) and lithium bis(oxalato)borate with LiPF.sub.6.
This combination is preferred because it is effective in inhibiting
the deterioration caused by high-temperature storage. In this case,
the concentration of LiBF.sub.4 is generally 0.001 mol/L or higher,
preferably 0.01 mol/L or higher, more preferably 0.03 mol/L or
higher. The upper limit thereof is generally 0.4 mol/L or lower,
preferably 0.15 mol/L or lower, more preferably 0.1 mol/L or
lower.
[1248] The nonaqueous electrolyte is preferred which contains
LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an integer of
1-4) and/or lithium bis(oxalato)borate and further contains at
least one compound belonging to the group consisting of cyclic
compounds represented by general formula (1) given above, compounds
represented by general formula (2), chain compounds having in the
molecule a structure represented by general formula (3), compounds
having an S--F bond in the molecule, nitric acid salts, nitrous
acid salts, monofluorophosphoric acid salts, difluorophosphoric
acid salts, acetic acid salts, and propionic acid salts. This is
because the battery produced with this electrolyte has an improved
balance between cycle characteristics and high-temperature
storability (in particular, residual capacity and high-load
discharge capacity after high-temperature storage).
[1249] The nonaqueous-electrolyte secondary battery employing the
nonaqueous electrolyte according to this invention is excellent in
output characteristics and in high-temperature storability and
cycle characteristics. Although the reasons for this are unclear,
it is thought that the specific compound exerts some action on the
electrodes and reduces the reaction resistance relating to lithium
ion elimination/insertion to improve output characteristics. When
LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an integer of
1-4) and/or lithium bis(oxalato)borate is used as the lithium salt,
this lithium salt is thought to moderately reacts on the surfaces
of the negative electrode and positive electrode to form a stable
composite protective coating film which has excellent lithium ion
permeability and is derived from other components of the nonaqueous
electrolyte and from this lithium salt. It is presumed that this
composite coating film inhibits the highly active electrodes from
undergoing unnecessary side reactions with the nonaqueous
electrolyte and, as a result, high-temperature storability and
cycle characteristics are improved while improving output
characteristics.
<Electrolytic Solution [6]>
[1250] The nonaqueous electrolyte preferably is one in which the
lithium salts as a component of the electrolyte comprise a
fluorine-containing lithium salt and the nonaqueous electrolyte
contains hydrogen fluoride (HF) in an amount of from 10 ppm to 300
ppm of the whole electrolyte (electrolyte [6]).
[1251] In this invention (electrolyte [6]), the kinds and contents
of the "nonaqueous solvent", "specific compound", and "other
compounds", use conditions therefor, method of preparing the
nonaqueous electrolyte, etc. are the same as described above. In
this invention (electrolyte [6]), the "fluorine-containing lithium
salt" is not particularly limited as long as it is a
fluorine-containing lithium salt known to be usable as an
electrolyte for nonaqueous electrolytes for lithium secondary
batteries. For example, those shown above can be used. One of these
may be used alone, or any desired two or more thereof may be used
in combination in any desired proportion. Of these, LiPF.sub.6,
LiBF.sub.4, or the like is preferred from the standpoint of the
ease of generating hydrogen fluoride (HF) in the presence of the
alcohol which will be described later. LiPF.sub.6 is preferred when
solubility in nonaqueous solvents and secondary-battery
performances such as charge/discharge characteristics, output
characteristics, and cycle characteristics are comprehensively
judged.
[1252] Besides the fluorine-containing lithium salt, a lithium salt
containing no fluorine may be used as a mixture therewith in the
electrolyte. Examples thereof include the following.
[1253] Inorganic lithium salts; perhalogen acid salts such as
LiClO.sub.4, LiBrO.sub.4, and LiIO.sub.4; and inorganic chloride
salts such as LiAlCl.sub.4.
[1254] Oxalatoborate salts: lithium bis(oxalato)borate.
[1255] The concentration of that lithium salt in the nonaqueous
electrolyte is not particularly limited. However, it is generally
0.5 mol/L or higher, preferably 0.6 mol/L or higher, more
preferably 0.7 mol/L or higher. The upper limit thereof is
generally 2 mol/L or lower, preferably 1.8 mol/L or lower, more
preferably 1.7 mol/L or lower. When the concentration thereof is
too low, there are cases where the nonaqueous electrolyte has an
insufficient electrical conductivity. On the other hand, when the
concentration thereof is too high, there are cases where this
electrolyte has an increased viscosity and hence a reduced
electrical conductivity, and this may result in a decrease in
performances of the lithium secondary battery.
[1256] The concentration of the fluorine-containing lithium salt in
the nonaqueous electrolyte is not particularly limited. However,
the concentration thereof is generally 0.5 mol/L or higher,
preferably 0.6 mol/L or higher, more preferably 0.7 mol/L or
higher. The upper limit thereof is generally 2 mol/L or lower,
preferably 1.8 mol/L or lower, more preferably 1.7 mol/L or lower.
When the concentration thereof is too low, there are cases where
this nonaqueous electrolyte has an insufficient electrical
conductivity or the generation of hydrogen fluoride (HF) is
insufficient. On the other hand, when the concentration thereof is
too high, there are cases where this nonaqueous electrolyte has an
increased viscosity and hence a reduced electrical conductivity or
the generation of hydrogen fluoride (HF) proceeds excessively, and
this may result in a decrease in performances of the lithium
secondary battery.
[1257] The proportion of the fluorine-containing lithium salt to
all lithium salts in the nonaqueous electrolyte is preferably 50%
by mass or higher, especially preferably 70% by mass or higher,
based on all lithium salts. It is also especially preferred that
all the lithium salt(s) mixed should be one or more
fluorine-containing lithium salts. When the proportion of the
fluorine-containing lithium salt(s) is too low, there are cases
where the generation of hydrogen fluoride is insufficient.
[1258] One lithium salt may be used alone, or any desired two or
more lithium salts may be used in any desired proportion. In the
case where two or more lithium salts are used in combination, a
preferred example thereof is a combination of LiPF.sub.6 and
LiBF.sub.4. In this case, the proportion of LiBF.sub.4 to the sum
of both is especially preferably from 0.01% by mass to 20% by mass,
more preferably from 0.1% by mass to 5% by mass. Another preferred
example is a combination of an inorganic fluoride salt and a
perfluoroalkanesulfonylimide salt. In this case, the proportion of
the inorganic fluoride salt to the sum of both is especially
preferably from 70% by mass to 99% by mass, more preferably from
80% by mass to 98% by mass. This combination is effective in
inhibiting the deterioration caused by high-temperature
storage.
[1259] The nonaqueous electrolyte comprising a nonaqueous solvent
and a fluorine-containing lithium salt, e.g., LiPF.sub.6, dissolved
therein frequently contains hydrogen fluoride (HF). With respect to
the causes of the presence of hydrogen fluoride (HF) in this
electrolyte, the hydrogen fluoride can include one derived from an
impurity contained in the fluorine-containing lithium salt and one
yielded by the reaction of water or an alcohol present in a slight
amount in the nonaqueous solvent with the fluorine-containing
lithium salt. In patent document 16, which was cited above, there
is a statement to the effect that the hydrogen fluoride (HF)
contained in a nonaqueous electrolyte should be removed as much as
possible, especially preferably to 15 ppm or lower. In the Examples
given therein, the nonaqueous electrolyte having a concentration of
9 ppm gave most satisfactory results concerning cycle
characteristics.
[1260] In the case of this invention (electrolyte [6]), the content
of hydrogen fluoride (HF) is generally 10 ppm or higher, preferably
12 ppm or higher, more preferably 15 ppm or higher, especially
preferably 20 ppm or higher, and is generally 300 ppm or lower,
preferably 250 ppm or lower, more preferably 200 ppm or lower, even
more preferably 150 ppm or lower. When the content thereof is too
low, there are cases where the effect of improving output is
insufficient. When the content thereof exceeds the upper limit of
that range, there are cases where cycle characteristics as well as
output are adversely influenced.
[1261] In order that hydrogen fluoride (HF) be contained, hydrogen
fluoride may be directly added to the nonaqueous electrolyte or to
a nonaqueous solvent as a raw material, or may be generated within
the nonaqueous electrolyte by utilizing the reaction of water or an
alcohol with a fluorine-containing lithium salt. For attaining
this, use may be made of, for example, a method in which water or
an alcohol is added to the nonaqueous electrolyte or either of
these ingredients is incorporated beforehand into a raw-material
nonaqueous solvent in an appropriate concentration. In this method,
there are cases where a certain time period is required for the
reaction to be completed. Namely, when a fluorine-containing
lithium salt is dissolved in a nonaqueous solvent containing water
or an alcohol and the specific compound is added thereto to prepare
an electrolyte, then the reaction of the water or alcohol with the
fluorine-containing lithium salt requires some time before
completion. However, in the case where this method is used in
battery fabrication, it is not always necessary to complete the
reaction before initiation of the next step. In this invention,
hydrogen fluoride (HF) may be generated within the battery as long
as hydrogen fluoride (HF) is present in a concentration within the
specific range when this battery functions as a battery. In the
case where hydrogen fluoride (HF) is generated in a raw-material
nonaqueous solvent, use may be made of a method in which hydrogen
fluoride (HF) is generated in part of nonaqueous solvents to be
used as a raw material and this solvent is mixed with a nonaqueous
solvent containing neither water nor an alcohol.
[1262] In the case where water or an alcohol is to be incorporated
beforehand into a nonaqueous solvent, there are cases where the
solvent to be used, according to the purity thereof, originally
contains water or an alcohol in an amount larger than a necessary
amount. In this case, it is preferred to purify the nonaqueous
solvent by an adsorption treatment, distillation, crystallization,
or another technique to remove the water or alcohol before the
solvent is used. A nonaqueous solvent which has undergone such
water or alcohol removal and in which water or the alcohol remains
in a given amount may be used as it is. Alternatively, a purified
nonaqueous solvent may be used after water or an alcohol is added
thereto so as to result in a given content.
[1263] The adsorption treatment can be conducted as long as the
nonaqueous solvent is liquid. The nonaqueous solvent can be
purified with an adsorbent which neither dissolves in nor reacts
with the solvent, such as alumina, activated carbon, silica gel,
Molecular Sieves (trade name) 4A, and/or Molecular Sieves 5A. In
this case, raw materials which are liquid at ordinary temperature,
such as dimethyl carbonate, may be separately purified. Use may
also be made of a method in which raw materials which are solid at
ordinary temperature, such as ethylene carbonate, are mixed with
other raw materials to prepare a liquid and these raw materials are
purified together. Examples of contacting methods include a method
in which the nonaqueous solvent is continuously passed (hereinafter
referred to as continuous process) and a method in which an
adsorbent is added to the nonaqueous solvent and this mixture is
allowed to stand or stirred (hereinafter referred to as batch
process). In the continuous process, the contact time is preferably
0.1-5/hr in terms of liquid hourly space velocity (LHSV). The
contact temperature is preferably 10-60.degree. C. In the batch
process, it is preferred that an adsorbent should be added in an
amount of 0.1-30% by mass based on the nonaqueous solvent to treat
the solvent for from 0.25 hours to 24 hours.
[1264] Raw materials which are solid at ordinary temperature, such
as ethylene carbonate, may be subjected to a crystallization
treatment. Crystallization can be conducted using a solvent such as
acetonitrile, acetone, or toluene.
[1265] It is preferred that those purification conditions should be
suitably regulated according to the kind and purity of the raw
material to be used and the target content of water or an
alcohol.
[1266] In producing the nonaqueous electrolyte according to this
invention (electrolyte [6]) without adding hydrogen fluoride (HF),
a nonaqueous solvent containing water or an alcohol is used for the
nonaqueous electrolyte. Namely, water or an alcohol is added to a
nonaqueous solvent before the solvent is used, or a nonaqueous
solvent is used without removing water or an alcohol therefrom. It
is preferred that an alcohol, in particular, a monohydric or
dihydric alcohol, is contained. The alcohol is not particularly
limited, and the kind of the alkyl group and the valence of the
alcohol also are not particularly limited. Preferred examples
thereof include monohydric alcohols such as methyl alcohol, ethyl
alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, and
t-butyl alcohol; diols such as ethylene glycol and propylene
glycol; and triols such as glycerol. When "water or an alcohol" is
added, especially preferred examples thereof include methyl
alcohol, ethyl alcohol, ethylene glycol, and propylene glycol.
[1267] From the standpoints of productivity, cost, etc., it is
industrially preferred that an alcohol which comes in due to
nonaqueous-solvent production steps, etc. should be used for the
nonaqueous solvent to be used for the electrolyte. Methyl alcohol,
ethyl alcohol, ethylene glycol, or propylene glycol is especially
preferred partly because these alcohols are contained in preferred
nonaqueous solvents to be used.
[1268] It is desirable that the nonaqueous solvent to be used in
the nonaqueous electrolyte according to this invention (electrolyte
[6]) should contain water or an alcohol in an amount of 3 ppm or
larger, preferably 10 ppm or larger, more preferably 20 ppm or
larger, even more preferably 30 ppm or larger. The upper limit of
the content thereof is desirably 150 ppm or lower, preferably 130
ppm or lower, more preferably 120 ppm or lower, even more
preferably 100 ppm or lower. When the content of water or an
alcohol in the nonaqueous solvent is too low, there are cases where
high output characteristics, which are a feature of this invention,
are not sufficiently obtained. When the content thereof is too
high, there are cases where cycle characteristics and
high-temperature storability are impaired.
[1269] In the case of a monohydric alcohol, among those alcohols,
the content thereof in the nonaqueous solvent is preferably 5 ppm
or higher, more preferably 10 ppm or higher, even more preferably
15 ppm or higher, and is preferably 100 ppm or lower, more
preferably 80 ppm or lower, even more preferably 50 ppm or lower.
In the case of a diol, the content thereof in the nonaqueous
solvent is 3 ppm or higher, preferably 10 ppm or higher, more
preferably 15 ppm or higher, even more preferably 20 ppm or higher,
and is preferably 100 ppm or lower, more preferably 90 ppm or
lower, even more preferably 80 ppm or lower, especially preferably
70 ppm or lower. In the case of water, the content thereof in the
nonaqueous solvent is 3 ppm or higher, preferably 5 ppm or higher,
more preferably 10 ppm or higher, and is preferably 100 ppm or
lower, more preferably 80 ppm or lower, even more preferably 70 ppm
or lower.
[1270] When hydrogen fluoride (HF) is caused to coexist in the
certain amount with the specific compound in the nonaqueous
electrolyte, the output of the lithium secondary battery can be
improved without adversely influencing cycle characteristics.
[1271] Although the reasons why the coexistence of a certain amount
of hydrogen fluoride (HF) with the specific compound in the
nonaqueous electrolyte according to this invention improves output
without adversely influencing cycle characteristics are unclear,
they are thought to be as follows. However, the following
functions/principle should not be construed as limiting the
invention. There are cases where the specific compound has the
effect of improving output characteristics to some degree
regardless of the content of hydrogen fluoride (HF). It is thought
that the specific compound exerts some action on the electrodes of
the battery and reduces the reaction resistance relating to lithium
ion elimination/insertion to thereby produce that effect. There is
a possibility that hydrogen fluoride (HF) might enhance that action
or serve as a medium for the action. For example, both of the
specific compound and hydrogen fluoride (HF) act on the electrodes,
or hydrogen fluoride (HF) serves as a medium when the specific
compound acts on the electrodes. It is further thought that the
hydrogen fluoride (HF) which thus serves has become capable of
being stably present in the battery and is less apt to exert an
adverse influence, e.g., to reduce cycle characteristics.
<Electrolytic Solution [7]>
[1272] The nonaqueous electrolyte preferably is one in which the
electrolyte contains vinylene carbonate and the content of the
vinylene carbonate is in the range of from 0.001% by mass to 3% by
mass based on the whole electrolyte (electrolyte [7]).
[1273] In this invention (electrolyte [7]), the kinds and contents
of the "lithium salt", "nonaqueous solvent", "specific compound",
and "other compounds", use conditions therefor, method of preparing
the nonaqueous electrolyte, etc. are the same as described
above.
[1274] The nonaqueous electrolyte in this invention is
characterized by containing vinylene carbonate as stated above. In
this invention, the content of vinylene carbonate in the whole
nonaqueous electrolyte is generally 0.001% by mass or higher,
preferably 0.01% by mass or higher, more preferably 0.1% by mass or
higher. The content thereof is generally 3% by mass or lower,
preferably 2.8% by mass or lower, more preferably 2.5% by mass or
lower. When the concentration of vinylene carbonate is too low,
there are cases where the effect of improving cycle characteristics
is difficult to obtain. On the other hand, too high concentrations
thereof may lead to a decrease in low-temperature characteristics
of the battery.
[1275] The proportion of the content of vinylene carbonate to the
content of the specific compound in the nonaqueous electrolyte is
not particularly limited. However, the proportion thereof in terms
of ratio by mass is preferably 0.01 or higher, more preferably 0.1
or higher, especially preferably 0.3 or higher. The upper limit
thereof is preferably 300 or lower, more preferably 100 or lower,
especially preferably 30 or lower. When the proportion thereof is
far outside that range, there are cases where the purpose of
simultaneously attaining satisfactory cycle characteristics and
satisfactory low-temperature characteristics is not
accomplished.
[1276] When vinylene carbonate is used in combination with the
specific compound, e.g., a difluorophosphoric acid salt, cycle
characteristics are improved even when the vinylene carbonate is
contained in a small amount. Although the reasons for this are
unclear, the following are thought. The specific compound, e.g., a
difluorophosphoric acid salt, reduces the amount of vinylene
carbonate to be consumed at the positive electrode by the
charge/discharge of the battery to thereby enable the vinylene
carbonate to effectively form a coating film on the negative
electrode. Furthermore, the coexistence of the specific compound,
e.g., a difluorophosphoric acid salt, with vinylene carbonate
changes the quality of the coating film to be formed on the
negative electrode and gives a coating film of satisfactory quality
which is thin and low in resistance and, despite this, can more
highly inhibit the solvent and the electrolyte lithium salt from
decomposing. These are thought to be causes of the improvement in
cycle characteristics. In addition, these functions are thought to
further realize an improvement in low-temperature
characteristics.
<Electrolytic Solution [8]>
[1277] The nonaqueous electrolyte preferably is one in which the
electrolyte further contains at least one compound selected from
the group consisting of compounds represented by general formula
(4) given above, heterocyclic compounds containing nitrogen and/or
sulfur, cyclic carboxylic acid esters, and fluorine-containing
cyclic carbonates in an amount in the range of from 0.001% by mass
to 5% by mass based on the whole nonaqueous electrolyte
(electrolyte [8]).
[1278] In this invention (electrolyte [8]), the kinds and contents
of the "lithium salt", "nonaqueous solvent", "specific compound",
and "other compounds", use conditions therefor, method of preparing
the nonaqueous electrolyte, etc. are the same as described
above.
[1279] The nonaqueous electrolyte in this invention (electrolyte
[8]) further contains at least one compound selected from the group
consisting of compounds represented by general formula (4) given
above, heterocyclic compounds containing nitrogen and/or sulfur,
cyclic carboxylic acid esters, and fluorine-containing cyclic
carbonates (hereinafter the at least one compound is often
abbreviated to "specific compound(s) B") in an amount in the range
of from 0.001% by mass to 5% by mass based on the whole nonaqueous
electrolyte.
[[Compounds Represented by General Formula (4)]]
[1280] In general formula (4) given above, R.sup.9 to R.sup.12 may
be the same or different and each represent a group constituted of
one or more elements selected from the group consisting of H, C, N,
O, F, S, and P.
[1281] With respect to the state in which those atoms are present,
the atoms each preferably are one constituting or contained in,
e.g., a hydrogen atom, a fluorine atom, an alkyl, cycloalkyl,
alkenyl, alkynyl, halogenated alkyl, alkoxy, carbonyl, carbonyloxy,
oxycarbonyl, oxycarbonyloxy, sulfonyl, oxysulfonyl, sulfonyloxy,
phosphoryl, or phosphinyl group, or the like. The compounds
represented by general formula (4) have a molecular weight of
preferably 500 or lower, especially preferably 300 or lower, more
preferably 200 or lower.
[1282] Examples of the compounds represented by general formula (4)
include carbonates such as vinylethylene carbonate, divinylethylene
carbonate, methyl vinyl carbonate, ethyl vinyl carbonate, propyl
vinyl carbonate, divinyl carbonate, allyl methyl carbonate, allyl
ethyl carbonate, allyl propyl carbonate, and diallyl carbonate;
esters such as vinyl acetate, vinyl propionate, vinyl acrylate,
vinyl crotonate, vinyl methacrylate, allyl acetate, allyl
propionate, methyl acrylate, ethyl acrylate, propyl acrylate,
methyl methacrylate, ethyl methacrylate, and propyl methacrylate;
sulfones such as divinyl sulfone, methyl vinyl sulfone, ethyl vinyl
sulfone, propyl vinyl sulfone, diallyl sulfone, allyl methyl
sulfone, allyl ethyl sulfone, and allyl propyl sulfone; sulfites
such as divinyl sulfite, methyl vinyl sulfite, ethyl vinyl sulfite,
and diallyl sulfite; sulfonates such as vinyl methanesulfonate,
vinyl ethanesulfonate, allyl methanesulfonate, allyl
ethanesulfonate, methyl vinylsulfonate, and ethyl vinylsulfonate;
and sulfates such as divinyl sulfate, methyl vinyl sulfate, ethyl
vinyl sulfate, and diallyl sulfate. Especially preferred of these
are vinylethylene carbonate, divinylethylene carbonate, vinyl
acetate, vinyl propionate, vinyl acrylate, divinyl sulfone, and
vinyl methanesulfonate.
[[Heterocyclic Compounds Containing Nitrogen and/or Sulfur]]
[1283] The heterocyclic compounds containing nitrogen and/or sulfur
are not particularly limited. Examples thereof include
pyrrolidinone compounds such as 1-methyl-2-pyrrolidinone,
1,3-dimethyl-2-pyrrolidinone, 1,5-dimethyl-2-pyrrolidinone,
1-ethyl-2-pyrrolidinone, and 1-cyclohexyl-2-pyrrolidinone;
oxazolidinone compounds such as 3-methyl-2-oxazolidinone,
3-ethyl-2-oxazolidinone, and 3-cycicohexyl-2-oxazolidinone;
piperidone compounds such as 1-methyl-2-piperidone and
1-ethyl-2-piperidone; imidazolidinone compounds such as
1,3-dimethyl-2-imidazolidinone and 1,3-diethyl-2-imidazolidinone;
sulfolane compounds such as sulfolane, 2-methylsulfolane, and
3-methylsulfolane; sulfolene; sulfites such as ethylene sulfite and
propylene sulfite; and sultones such as 1,3-propanesultone,
1-methyl-1,3-propanesultone, 3-methyl-1,3-propanesultone,
1,4-butanesultone, 1,3-propenesultone, and 1,4-butenesultone.
Especially preferred of these are 1-methyl-2-pyrrolidinone,
1-methyl-2-piperidone, 1,3-propanesultone, 1,4-butanesultone,
1,3-propenesultone, and 1,4-butenesultone.
[[Cyclic Carboxylic Acid Esters]]
[1284] The cyclic carboxylic acid esters are not particularly
limited. Examples thereof include .gamma.-butyrolactone,
.gamma.-valerolactone, .gamma.-hexalactone, .gamma.-heptalactone,
.gamma.-octalactone, .gamma.-nonalactone, .gamma.-decalactone,
.gamma.-undecalactone, .gamma.-dodecalactone,
.alpha.-methyl-.gamma.-butyrolactone,
.alpha.-ethyl-.gamma.-butyrolactone,
.alpha.-propyl-.gamma.-butyrolactone,
.alpha.-methyl-.gamma.-valerolactone,
.alpha.-ethyl-.gamma.-valerolactone,
.alpha.,.alpha.-dimethyl-.gamma.-butyrolactone,
.alpha.,.alpha.-dimethyl-.gamma.-valerolactone,
.delta.-valerolactone, .delta.-hexalactone, .delta.-octalactone,
.delta.-nonalactone, .delta.-decalactone, .delta.-undecalactone,
and .delta.-dodecalactone. Especially preferred of these are
.gamma.-butyrolactone and .gamma.-valerolactone.
[[Fluorine-Containing Cyclic Carbonates]]
[1285] The fluorine-containing cyclic carbonates are not
particularly limited. Examples thereof include fluoroethylene
carbonate, difluoroethylene carbonate, trifluoroethylene carbonate,
tetrafluoroethylene carbonate, and trifluoropropylene carbonate.
Especially preferred of these is fluoroethylene carbonate.
[1286] One of the specific compounds B, i.e., at least one compound
selected from the group consisting of compounds represented by
general formula (4), heterocyclic compounds containing nitrogen
and/or sulfur, cyclic carboxylic acid esters, and
fluorine-containing cyclic carbonates, may be used alone.
Alternatively, any desired two or more compounds selected from
these may be used in combination in any desired proportion. With
respect to the compounds in each group in the specific compounds B,
one of these may be used alone or any desired two or more thereof
may be used in combination in any desired proportion.
[1287] The proportion of those specific compounds B in the
nonaqueous electrolyte is generally 0.001% by mass or higher, more
preferably 0.05% by mass or higher, even more preferably 0.1% by
mass or higher, in terms of total content thereof based on the
whole nonaqueous electrolyte. The upper limit of the total content
thereof is generally 5% by mass or lower, more preferably 4% by
mass or lower, even more preferably 3% by mass or lower. When the
concentration of the specific compounds B is too low, there are
cases where the effect of improving cycle characteristics and
storability is difficult to obtain. On the other hand, too high
concentrations thereof may lead to a decrease in charge/discharge
efficiency.
[1288] The nonaqueous-electrolyte secondary battery employing the
nonaqueous electrolyte according to this invention is excellent in
low-temperature discharge characteristics and in high-temperature
storability and cycle characteristics. Although the reasons for
this are unclear, they are thought to be as follows. However, the
following functions/principle should not be construed as limiting
the invention. The specific compound B undergoes reductional
decomposition on the negative electrode during initial charge to
form on the surface of the negative electrode a stable coating film
derived from the specific compound B to thereby improve storability
and cycle characteristics. This coating film, however, has
considerably increased resistance at low temperatures. There has
hence been a problem that the coating film reduces low-temperature
discharge characteristics. Coexistence with the specific compound A
is thought to inhibit the specific compound B from excessively
reacting and enable it to form a stable composite protective
coating film having excellent lithium ion permeability even at low
temperatures. High-temperature storability and cycle
characteristics are presumed to be thus improved while improving
low-temperature discharge characteristics.
[1289] Moreover, when the battery elements held in each battery
case of the secondary battery have an electric capacity of 3
ampere-hours (Ah) or higher and/or when the secondary battery has a
direct-current resistance component of 10 milliohms (.OMEGA.) or
less, then the contribution of the direct-current resistance
component is small. Those effects inherent in the nonaqueous
electrolyte according to this invention are thought to be more apt
to be produced in such battery than in batteries in which the
contribution of a direct-current resistance component is large.
<Electrolytic Solution [9]>
[1290] The nonaqueous electrolyte preferably is one in which the
electrolyte further contains an overcharge inhibitor (electrolyte
[9]).
[1291] In this invention (electrolyte [9]), the kinds and contents
of the "lithium salt", "nonaqueous solvent", "specific compound",
and "other compounds", use conditions therefor, method of preparing
the nonaqueous electrolyte, etc. are the same as described
above.
[1292] The nonaqueous electrolyte for secondary battery according
to this invention (electrolyte [9]) is characterized by containing
an overcharge inhibitor. The overcharge inhibitor is not
particularly limited. For example, the compound shown under the
following (1), (2), or (3) is preferred.
(1) Biphenyl, terphenyl, diphenyl ether, or dibenzofuran which each
may be substituted by one or more alkyl groups and/or one or more
fluorine atoms (2) Partially hydrogenated terphenyl (3) Benzene
substituted by one or more tertiary alkyl groups, one or more
cycloalkyl groups, one or more fluorine atoms, and/or one or more
methoxy groups
[1293] Compound (1) is not particularly limited. Examples thereof
include benzene ring assembly compounds such as biphenyl,
alkylbiphenyls, and terphenyl; fluorine-containing benzene ring
assembly compounds such as 2-fluorobiphenyl; aromatic ethers such
as diphenyl ether; and heterocycle/aromatic-ring assembly compounds
such as dibenzofuran.
[1294] Compound (3) is not particularly limited. Examples thereof
include (cyclo)alkylbenzenes such as cyclohexylbenzene,
t-butylbenzene, and t-amylbenzene; fluorine-atom-substituted
benzene compounds such as o-cyclohexylfluorobenzene and
p-cyclohexylfluorobenzene; and fluorine-containing anisoles such as
2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and
3,5-difluoroanisole.
[1295] Preferred examples include aromatic compounds such as
biphenyl, alkylbiphenyls, terphenyl, partially hydrogenated
terphenyls, cyclohexylbenzene, t-butylbenzene, t-amylbenzene,
diphenyl ether, and dibenzofuran. These compounds are especially
preferred because they enhance the effect of improving rate
characteristics after high-temperature storage in this
invention.
[1296] Two or more overcharge inhibitors may be used in
combination. When two or more are used in combination, it is
especially preferred to use cyclohexylbenzene or terphenyl (or
product of partial hydrogenation thereof) in combination with
t-butylbenzene or t-amylbenzene.
[1297] The term partially hydrogenated terphenyl herein means one
formed by causing the double bonds of the benzene rings of
terphenyl to partly add hydrogen. The partially hydrogenated
terphenyl may be a single compound or may be a mixture of two or
more compounds. For example, it may be a mixture of two or more
partially hydrogenated terphenyls differing in the degree of
partial hydrogenation, or may be partially hydrogenated terphenyls
equal in the degree of partial hydrogenation. Furthermore, it may
be a mixture of partially hydrogenated terphenyls differing in the
position of the hydrogenated benzene ring, or may be a mixture of
compounds differing in the positions of double bonds or a mixture
comprising constitutional isomers.
[1298] The degree of partial hydrogenation of terphenyl is a value
calculated on the assumption that the degree of partial
hydrogenation of terphenyl in which no hydrogen has added to the
double bonds of the benzene rings is 0% and the degree of partial
hydrogenation of completely hydrogenated terphenyl, i.e., the case
where hydrogen has added to all double bonds (the case where 18 mol
of hydrogen atoms have added to 1 mol of terphenyl), is 100%. In
the case of a mixture, a mole-average value is used. For example,
in the case where 2 mol of hydrogen atoms have added to 1 mol of
terphenyl, the degree of partial hydrogenation is 11.1%
(=2/18).
[1299] When the degree of partial hydrogenation defined above is
used, the degree of partial hydrogenation of the partially
hydrogenated terphenyl for use in this invention can be a value
which is higher than 0% and lower than 100%. Although the partially
hydrogenated terphenyl can include terphenyl (degree of partial
hydrogenation, 0%) or completely hydrogenated m-terphenyl (degree
of hydrogenation, 100%), such a mixture preferably is one which has
a degree of partial hydrogenation higher than 0% and lower than
100%. From the standpoints of battery storability and solubility in
the electrolyte, the degree of partial hydrogenation of the
terphenyl is preferably 30-70%, more preferably 35-60%. Although
the terphenyl or the partially hydrogenated terphenyl is not
particularly limited, m-terphenyl or a partially hydrogenated
m-terphenyl is especially preferred.
[1300] The content of each of those overcharge inhibitors in the
nonaqueous electrolyte is generally 0.01% by mass or higher,
preferably 0.1% by mass or higher, especially preferably 0.2% by
mass or higher. The upper limit thereof is generally 5% by mass or
lower, preferably 3% by mass or lower, especially preferably 2% by
mass or lower. When the content thereof is lower than the lower
limit, there are cases where safety in overcharge cannot be
sufficiently secured. When the content thereof exceeds the upper
limit, there are cases where this nonaqueous electrolyte, even when
the specific compound is incorporated therein, cannot give a
battery having satisfactory storability.
[1301] When the specific compound is present, the deterioration of
rate characteristics after storage to be caused by an overcharge
additive is less apt to occur. Although unclear, the reasons for
this are presumed to be as follows. In general, an overcharge
inhibitor upon overcharge forms a polymer film on the surface of
the positive electrode to considerably increase the internal
resistance of the battery or generates a gas during the
polymerization to thereby operate the current breaker in the
battery can. An overcharge inhibitor thus functions to improve the
safety of the secondary battery. However, it is thought that the
polymer film formation partly occurs also during the
high-temperature storage of the battery in a charged state and this
is a cause of a decrease in battery discharge capacity and
deterioration of rate characteristics. In the case where the
specific compound according to the invention is present in such
system, this compound acts on the surface of the positive electrode
to form a weak barrier and thereby inhibit the reaction between the
overcharge inhibitor and the positive-electrode active material in
a usual charged state. However, when the battery is overcharged and
the positive electrode comes into an excessively active state, then
the weak barrier breaks or the reaction between the positive
electrode and the overcharge inhibitor is accelerated despite
inhibition by the barrier. It is thought that polymerization
reaction thus proceeds and the desired safety in overcharge can be
secured.
<Battery Design (Battery Constitution)>
[1302] The constitution of the lithium secondary battery (battery
constitution) of the invention will be described below in
detail.
[1303] The lithium secondary battery of the invention, which is
capable of charge/discharge, is constituted at least of a positive
electrode and a negative electrode which are capable of
occluding/releasing lithium ions, the nonaqueous electrolyte
described above, a microporous film separator disposed between the
positive electrode and negative electrode, current collector
terminals, and a case or the like. A protective element may be
attached to an inner part and/or outer part of the battery
according to need. In this description, characteristic parts of the
constitution of the lithium secondary battery (battery
constitution) of this invention are often referred to as structure
[1] to structure [6].
[Discharge Capacity] (Structure [3])
[1304] The lithium secondary battery of this invention preferably
is one in which the battery elements held in each battery case of
the secondary battery have an electric capacity (electric capacity
as measured when the battery in a fully charged state is discharged
to a discharged state) (this capacity is often abbreviated to
"battery capacity") of 3 ampere-hours (Ah) or higher. This is
because such electric capacity enhances the effect of improving
output characteristics. Consequently, it is preferred that the
positive-electrode plate should be designed to have a discharge
capacity, as measured in discharge from a fully charged state, of
from 3 ampere-hours (Ah) to 20 Ah, more preferably from 4 Ah to 10
Ah (structure [3]). When the discharge capacity thereof is lower
than 3 Ah, there are cases where high-current discharge results in
a large decrease in voltage due to electrode reaction resistance
and hence in a poor power efficiency. On the other hand, when the
discharge capacity thereof is higher than 20 Ah, electrode reaction
resistance is low and a satisfactory power efficiency is obtained.
In this case, however, the battery has a wide temperature
distribution during pulse charge/discharge due to internal heat
generation and has poor durability in repetitions of
charge/discharge. In addition, the efficiency of heat dissipation
in the case of abrupt heat generation in an abnormality, such as
overcharge or internal short-circuiting, is also poor. There are
hence cases where the internal pressure increases and this not only
results in the phenomenon in which the gas release valve works
(valve working) but also increases the probability that the
phenomenon in which the battery contents are vigorously ejected
outside (rupture) occurs.
[Current Collector Structure] (Structure [2] and Structure [4])
[1305] The current collector structure is not particularly limited.
However, for more effectively realizing an improvement in output
characteristics of the lithium secondary battery of this invention,
it is necessary to employ a structure reduced in the resistance of
wiring parts and joint parts. In the case where the battery has
such reduced internal resistance, use of the nonaqueous electrolyte
described above produces its effects especially satisfactorily.
[1306] In the case where electrode groups are assembled into the
multilayer structure which will be described later, a structure in
which the current collector tabs for respective electrode layers
are bundled together and connected to a terminal is suitable. When
each electrode has a large area, this results in increased internal
resistance. In this case, it is preferred to dispose two or more
current collector tabs in each electrode to reduce the resistance.
In the case of an electrode group having the wound structure which
will be described later, two or more current collector tabs may be
disposed on each of the positive electrode and negative electrode
and bundled into a terminal to thereby reduce internal
resistance.
[1307] By optimizing the current collector structure, internal
resistance can be minimized. In a battery to be used at a high
current, the impedance as measured by the 10-kHz
alternating-current method (hereinafter abbreviated to
"direct-current resistance component") is regulated preferably to
20 milliohms (m.OMEGA.) or lower, more preferably 10 milliohms
(m.OMEGA.) or lower, even more preferably 5 milliohms (m.OMEGA.) or
lower (structure [2]). On the other hand, when the direct-current
resistance component is reduced to below 0.1 milliohm, high-output
characteristics improve. In this case, however, the proportion by
volume of the structural materials for current collection increases
and this may result in a decrease in battery capacity.
[1308] For an impedance measurement, use was made of battery
analyzer 1470, manufactured by Solartron, and frequency response
analyzer 1255B, manufactured by Solartron. An alternating current
of 10 kHz was applied with a bias of 5 mV, and the resultant
resistance was measured and taken as direct-current resistance
component.
[1309] The nonaqueous electrolyte in this invention is effective in
reducing the reaction resistance relating to lithium elimination
from and insertion into the electrode active materials, and this is
thought to be a factor which realizes satisfactory output
characteristics. However, it was found that in ordinary batteries
having a high direct-current resistance component, the effect of
reducing reaction resistance cannot be completely reflected in
output characteristics because of inhibition by the direct-current
resistance component. This drawback is mitigated by using a battery
having a low direct-current resistance component, whereby the
effects of the invention can be sufficiently produced.
[1310] From the standpoint of enabling the nonaqueous electrolyte
to bring about its effects to fabricate a battery having high
output characteristics, it is especially preferred that the
requirement concerning impedance should be satisfied simultaneously
with the requirement described above that the battery elements held
in each battery case of the secondary battery have an electric
capacity (electric capacity as measured when the battery in a fully
charged state is discharged to a discharged state) (battery
capacity) of 3 ampere-hours (Ah) or higher.
[1311] It is preferred that the connection between the current
collector tabs and the terminal should have been made by any of
spot welding, high-frequency welding, and ultrasonic welding
(structure [4]). Although these welding techniques are easy welding
techniques reduced in resistance, they have been regarded as
disadvantageous because the welds react with impurities contained
in the nonaqueous electrolyte and with by-products, etc. during
long-term use and thus deteriorate to increase the direct-current
resistance component. However, it is thought that when the
nonaqueous electrolyte containing the specific compound is used, a
stable coating film can be formed on the welds and the nonaqueous
electrolyte is inhibited from undergoing side reactions on the
positive electrode. It is therefore thought that even in long-term
use, deterioration of the welds is less apt to proceed and this
battery suffers no increase indirect-current resistance component
and can retain high output.
[Battery Case 1] (Structure [5])
[1312] The material of the battery case is not particularly limited
as long as it is a substance stable to the nonaqueous electrolyte
to be used. Preferred examples thereof are metals such as
nickel-plated steel sheets, stainless steel, aluminum or aluminum
alloys, and magnesium alloys or laminated films comprising a resin
and an aluminum foil. Especially preferred from the standpoint of
weight reduction is a metal which is aluminum or an aluminum alloy
or a laminated film including the metal.
[1313] When the nonaqueous electrolyte described above is used in
this invention, it is especially preferred to use a metal which is
aluminum or an aluminum alloy (structure [5]). Aluminum or aluminum
alloys are lightweight materials having high formability. However,
when such material is used as a battery case over long, it reacts
with impurities contained in the nonaqueous electrolyte and with
by-products, etc. and deteriorates. There are cases where with the
progress of the deterioration, the case decreases in strength or is
holed. When the nonaqueous electrolyte containing the specific
compound is used, a stable coating film can be formed on the
aluminum surface or aluminum alloy surface and this nonaqueous
electrolyte is inhibited from undergoing side reactions on the
positive electrode. Because of this, deterioration of the case is
less apt to proceed even in long-term use.
[1314] Examples of the case made of the metal include one of a
sealed structure formed by fusion-bonding metallic members to each
other by laser welding, resistance welding, or ultrasonic welding
and one of a caulked structure obtained by caulking members of the
metal through a resinous gasket. Examples of the case made of the
laminated film include a sealed structure formed by thermally
fusion-bonding resin layers to each other. For the purpose of
enhancing sealability, a resin different from the resin used in the
laminated film may be interposed between the resin layers.
Especially when resin layers are thermally fusion-bonded to each
other through a current collector terminal to produce a sealed
structure, metal/resin bonding is necessary and it is therefore
preferred that the resin to be interposed should be a resin having
polar groups or a modified resin having polar groups introduced
therein.
[Battery Case 2] (Structure [6])
[1315] In this invention, it is also preferred that the battery
case should be made of a casing material in which at least part of
the battery inner side comprises a sheet formed from a
thermoplastic resin and with which the electrode group can be
enclosed by placing the electrode group in the casing material and
bonding layers of the thermoplastic resin to each other by heat
sealing (structure [6]).
[1316] The material of the battery case in structure [6] should be
a material which is lightweight and stable to the nonaqueous
electrolyte to be used, for the purpose of battery weight
reduction, and in which at least part of the battery inner side
comprises a sheet formed from a thermoplastic resin, from the
standpoint of the necessity of easily enclosing an electrode group
without fail.
[1317] In structure [6], the electrode group is enclosed by bonding
layers of the thermoplastic resin to each other by heat sealing.
The term "heat sealing" herein means an operation in which the
thermoplastic-resin layers are brought into intimate contact with
each other and the layers in this state are heated to a temperature
not lower than the melting point of the thermoplastic resin to
thereby bond the thermoplastic-resin layers to each other. It is
preferred to use a sealer which has a strip-form heating part and
is capable of heating with pressing. Although at least part of the
battery inner side is made of a thermoplastic resin, the term "at
least part" herein means that the sheet has a peripheral region
sufficient for enclosing the electrode group. Only the part which
is to be heat-sealed may be made of a thermoplastic resin. From the
standpoint of the efficiency of a sheet production step, it is
preferred that the battery inner side of the sheet should be wholly
covered with a thermoplastic-resin layer.
[1318] Furthermore, in structure [6], when the microporous film
separator which will be described later has the property of closing
the pores upon heating, then the battery case preferably is made of
a casing material in which at least part of the battery inner side
comprises a sheet formed from a thermoplastic resin having a
melting point higher than the pore closing initiator temperature of
the microporous film separator. This casing material is preferred
from the standpoint of safety in an abnormality, e.g., overcharge.
The reasons for this are as follows. In case of abnormal heat
generation due to, e.g., overcharge, the battery temperature rises.
When the temperature exceeds the melting point of the thermoplastic
resin constituting the casing material, the battery case ruptures
or the electrolyte leaks out. There are cases where such troubles
lead to a fire. However, when the microporous film separator has
the property of closing the pores upon heating, then the pores of
the microporous film separator are closed before the electrolyte
leaks out through the casing material, whereby further heat
generation can be inhibited. Namely, that constitution is preferred
because there are cases where the abnormal heat generation leads to
neither a rupture nor a fire. The term melting point herein means
melting temperature such as that measured by the JIS K7121
method.
[1319] The thermoplastic resin in structure [6] is not particularly
limited. Preferred examples thereof include polyolefins such as
polyethylene, polypropylene, modified polyolefins, and polyolefin
copolymers; polyesters such as poly(ethylene terephthalate); and
polyamides such as nylons. One thermoplastic resin may be used
alone or two or more thermoplastic resins may be used.
[1320] In this structure [6], a thermoplastic resin only may be
used as the "at least part of the casing material". However, it is
also preferred to use a composite material comprising a
thermoplastic resin and another material such as, e.g., a
thermosetting resin, elastomer, metallic material, glass fibers, or
carbon fibers. The thermoplastic resin or the composite material
may contain a filler or the like. The composition material
especially preferably is a laminated sheet comprising a combination
of a thermoplastic-resin layer with an elemental metal, such as
aluminum, iron, copper, nickel, titanium, molybdenum, or gold, or
an alloy such as stainless steel or Hastelloy. More preferred is a
laminated sheet which comprises a combination with aluminum metal
and has excellent processability. Namely, it is more preferred that
the casing material should comprise a laminated sheet composed of
superposed layers comprising an aluminum layer and a
thermoplastic-resin layer. These metals or alloys each may be used
as a foil of the metal or the like or as a vapor-deposited metal
film.
[1321] In structure [6], examples of the case employing the casing
material described above include one of a sealed structure formed
by fusion-bonding resin layers to each other. For the purpose of
enhancing sealability, a resin different from the resin used in the
casing material may be interposed between the resin layers.
Especially when resin layers are thermally fusion-bonded to each
other through a current collector terminal to produce a sealed
structure, metal/resin bonding is necessary and it is therefore
preferred that the resin to be interposed should be a resin having
polar groups or a modified resin having polar groups introduced
therein.
[1322] In structure [6], the thickness of the casing material is
not particularly limited. However, it is preferably 0.03 mm or
larger, more preferably 0.04 mm or larger, even more preferably
0.05 mm or larger. The upper limit thereof is preferably 0.5 mm or
smaller, more preferably 0.3 mm or smaller, even more preferably
0.2 mm or smaller. When the casing material is thinner than the
lower limit of that range, it has reduced strength and there are
cases where this material is apt to suffer deformation, breakage,
etc. On the other hand, when the casing material is thicker than
the upper limit of that range, the case has an increased mass and
there are hence cases where the desired battery weight reduction
cannot be attained.
[1323] Batteries of structure [6] employing the casing material
described above have advantages that they are lightweight and have
a high degree of freedom of shape. These batteries, on the other
hand, have a disadvantage that gas evolution within the batteries
results in an increase in internal pressure and this may be apt to
deform the casing material. In the lithium secondary battery of
this invention, the specific compound in the nonaqueous electrolyte
is adsorbed onto the surface of the positive-electrode active
material and this inhibits the positive electrode from undergoing
side reactions and thereby inhibits the generation of a gas
ingredient. Because of this, even when the casing material
described above is used, this battery is free from that
disadvantage, and those advantages of this casing material only are
obtained. The lithium secondary battery of this invention is
preferred in this respect.
[Microporous Film Separator]
[1324] The microporous film separator to be used in the invention
is not particularly limited as long as it is a material which has
given mechanical strength enabling the separator to electronically
insulate the two electrodes from each other and which has high ion
permeability and has oxidative resistance and reductional
resistance respectively on the side to be in contact with the
positive electrode and on the negative-electrode side. The material
of the microporous film separator having such properties may be a
resin, inorganic material, glass fibers, etc.
[1325] The resin is not particularly limited. Examples thereof
include olefin polymers, fluoropolymers, cellulosic polymers,
polyimides, and nylons. Specifically, it is preferred to select
from materials which are stable to the nonaqueous electrolyte and
have excellent liquid retentivity. It is preferred to use a porous
sheet, nonwoven fabric, or the like formed from a polyolefin such
as polyethylene or polypropylene. The inorganic material is not
particularly limited. For example, use may be made of: oxides such
as alumina and silicon dioxide; nitrides such as aluminum nitride
and silicon nitride; and sulfuric acid salts such as barium sulfate
and calcium sulfate. With respect to shape, particulate or fibrous
materials may be used.
[1326] With respect to form, a separator of a thin film form is
preferred, such as a nonwoven fabric, woven fabric, or microporous
film. Suitable ones of a thin film form have a pore diameter of
0.01-1 .mu.m and a thickness of 5-50 .mu.m. Besides such a
separator in an independent thin film form, use can be made of a
separator obtained by forming a composite porous layer containing
particles of the inorganic material with a resinous binder on a
surface layer of the positive electrode and/or negative electrode.
Examples of separators in this form include a porous layer formed
by fixing alumina particles having a 90% particle diameter of 1
.mu.m or smaller with a fluororesin as a binder on both sides of a
positive electrode.
[1327] The microporous film separator to be used in the invention
preferably has the property of closing the pores upon heating. When
this microporous film separator is used in combination with a
casing material in which at least part of the battery inner side
has been formed from a thermoplastic resin having a melting point
higher than the pore closing initiation temperature of the
microporous film separator, then there is an effect that in case of
abnormal heat generation due to, e.g., overcharge, the heat
generation can be stopped before the battery case ruptures or the
electrolyte leaks out. The term "property of closing the pores upon
heating" herein means the following property of a porous layer
which is made of a thermoplastic resin and has been formed so as to
enable an electrolyte to move between the positive and negative
electrodes. When heated to around the melting point of the
thermoplastic resin, the porous layer closes its pores and thereby
prevents the electrolyte from moving between the positive and
negative electrodes.
[Battery Shape]
[1328] The battery shape is not particularly limited. Examples
thereof include a bottomed cylindrical shape, bottomed polygonal
shape, thin shape, sheet shape, and paper shape. From the
standpoints of heightening volume efficiency and thereby enhancing
suitability for mounting for incorporation into a system or
appliance, the battery may have a modified shape suitable for
mounting in peripheral systems to be disposed around the battery,
such as a horse shoe shape or comb shape. From the standpoint of
efficiently radiating the internal heat of the battery outward, a
prismatic shape having at least one side which is relatively flat
and has a large area is preferred.
[1329] In the battery of a bottomed cylindrical shape, the area of
the outer surface is small for the power-generating elements
packed. It is therefore preferred to employ a design which enables
the Joule's heat generated upon charge or discharge due to internal
resistance to efficiently escape outward. It is also preferred to
design the battery so that a material having high thermal
conductivity is packed in a higher proportion to enable the battery
to have a narrow temperature distribution inside.
[1330] In the case of a bottomed polygonal shape, the proportion of
double the area S.sub.1 of the largest face (product of the width
and height of the outer dimensions excluding the terminal parts;
unit, cm.sup.2) to the thickness T (unit, cm) of the battery outer
shape, i.e., the value of 2.times.S.sub.1/T, is preferably 100 or
larger, more preferably 200 or larger. By enlarging the largest
face, the battery can be improved in cycle characteristics,
high-temperature storability, and other properties and be increased
in the efficiency of heat dissipation in abnormal heating-up, even
when it is a high-output high-capacity battery. Thus, the battery
can be inhibited from coming into a dangerous state such as
"rupture" or "fire".
[Electrode Group]
[1331] The electrode group may be either of: one having a
multilayer structure in which the positive-electrode plate and
negative-electrode plate described later have been superposed
through the microporous film separator described later; and one
having a wound structure in which the positive-electrode plate and
negative-electrode plate described later have been spirally wound
through the microporous film separator described later. The
proportion of the volume of the electrode group to the volume of
the battery case which excludes any projection (hereinafter
abbreviated to "electrode group proportion") is regulated to
preferably from 0.3 to 0.7, more preferably from 0.4 to 0.6. In
case where the electrode group proportion is lower than 0.3, a
decrease in battery capacity results. In case where the electrode
group proportion exceeds 0.7, this battery has a reduced space
volume and, hence, a volume necessary for the current collector
structure described later cannot be sufficiently secured. There are
hence cases where battery resistance increases. In addition, there
are cases where battery heating-up causes members to expand and a
liquid component of the electrolyte to have a heightened vapor
pressure, resulting in an increased internal pressure. This battery
is reduced in various properties including charge/discharge cycling
performance and high-temperature storability, and there are even
cases where the gas release valve, which releases the gas from the
internal pressure, works.
[1332] In the case where the electrode group has a multilayer
structure, the positive electrode and the negative electrode each
are an electrode in which the ratio of the overall length L (unit,
cm) of the periphery of the electrode to double the electrode area
S.sub.2 (unit, cm.sup.2), i.e., L/(2.times.S.sub.2), is preferably
1 or smaller, more preferably 0.5 or smaller, even more preferably
0.3 or smaller. The lower limit thereof is preferably 0.02 or
larger, more preferably 0.03 or larger, even more preferably 0.05
or larger. In the multilayer structure, there are cases where those
parts of the electrode which are near to the periphery have poor
electrode film adhesion due to residual stress or cutting impact.
Because of this, when L/(2.times.S.sub.2) is larger than the upper
limit of that range, there are cases where output characteristics
decrease. When L/(2.times.S.sub.2) is smaller than the lower limit
of that range, there are cases where the battery area is too large
and this battery is impracticable.
[1333] In the case where the electrode group has a wound structure,
the proportion of the length-direction dimension to the
width-direction dimension in the positive electrode is preferably
from 15 to 200. When that proportion is lower than the lower limit
of that range, there are cases where the bottomed cylindrical shape
of the battery case has too large a height for the bottom area.
Namely, this shape has a poor balance and is impracticable.
Alternatively, there are cases where the positive-electrode
active-material layer is too thick to give high output. When that
proportion is below that range, there are cases where the bottomed
cylindrical shape of the battery case has too small a height for
the bottom area. Namely, this shape has a poor balance and is
impracticable. Alternatively, there are cases where the proportion
of the current collectors increases and the battery capacity
decreases.
[Positive-Electrode Area] (Structure [1])
[1334] When the nonaqueous electrolyte is used in this invention,
it is preferred that the area of the positive-electrode
active-material layer relative to the outer surface area of the
battery case should be large from the standpoints of high output
and enhanced high-temperature stability. Specifically, from the
standpoints of improving output and enabling the heat generated
upon charge/discharge to be efficiently dissipated through the
current collector, the overall electrode area of the positive
electrode is preferably at least 15 times, more preferably at least
20 times the surface area of the case of the secondary battery
(structure [1]). In the case of a bottomed polygonal shape, the
term outer surface area of the case means the total area determined
through calculation from the length, width, and thickness
dimensions of the case part packed with power-generating elements
and excluding the terminal projections. In the case of a bottomed
cylindrical shape, that term means a geometric surface area
determined through the approximation to a cylinder of the case part
packed with power-generating elements and excluding the terminal
projection. The term overall electrode area of the positive
electrode means the geometric surface area of the positive
electrode mix layer facing the mix layer containing a
negative-electrode active material. In a structure in which a
positive-electrode mix layer has been formed on each side of a
current collector foil, that term means the sum of the areas
calculated respectively for both sides.
[Protective Element]
[1335] Examples of the protective element include a PTC (positive
temperature coefficient), which increases in resistance upon
abnormal heating-up or when an excessive current flows, a
temperature fuse, a thermister, and a valve (current breaker valve)
which breaks current flow through the circuit in abnormal
heating-up based on an abrupt increase in the internal pressure or
internal temperature of the battery. It is preferred to select such
a protective element which does not work under ordinary
high-current use conditions. From the standpoint of high output, it
is preferred to employ a design which prevents abnormal heating-up
and thermal run-away even without a protective element.
[Battery Assembly]
[1336] When lithium secondary batteries of the invention are
subjected as a power source to practical use, they are preferably
used in the following manner. In practical use, power sources are
frequently required to have a voltage not lower than the cell
voltages. It is therefore necessary to take a measure, e.g., to
serially connect cells or to use a booster. Because of this,
lithium secondary batteries of the invention are preferably used as
a battery assembly in which the batteries are serially connected.
By fabricating a battery assembly, the resistance of connection
parts can be reduced. The battery assembly can hence be inhibited
from decreasing in output. From the standpoint of power-source
voltage, it is preferred to fabricate the battery assembly by
serially connecting five or more batteries.
[1337] Such a battery assembly obtained by connecting batteries is
more influenced by heat generation with charge/discharge. It is
therefore preferred that the battery assembly should have a cooling
mechanism for keeping the batteries at a temperature not higher
than a certain value. The battery temperature is preferably
10.degree. C.-40.degree. C. It is preferred to cool the batteries
with water or surrounding air.
[Uses]
[1338] The lithium secondary batteries of the invention and the
battery assembly comprising two or more serially connected lithium
secondary batteries of the invention have a high output, long life,
high safety, etc. It is therefore preferred that the batteries and
the battery assembly should be used in applications where they are
mounted on a vehicle and the power thereof is supplied at least to
the driving system of the vehicle.
[1339] Incidentally, it is thought that the specific compound is
adsorbed onto the surface of the positive-electrode active material
and the surface of metallic materials to thereby inhibit side
reactions with the electrolyte, etc. This inhibition of side
reactions on the surface of the positive-electrode active material
is preferred because it is thought to diminish gas evolution and
enable the battery to be less apt to deform or suffer an increase
in internal pressure even when a sheet-form casing material is
employed. The inhibition of side reactions on the surface of the
positive-electrode active material improves the life, output, and
safety in overcharge of the battery and this is thought to be
preferable for use in large batteries. Furthermore, the inhibition
of side reactions on the surface of metallic materials is effective
in inhibiting a direct-current resistance component from increasing
over long-term use even when the connection between each current
collector tab and the terminal was made by a simple welding
technique attaining low resistance. This is thought to be
preferable for use in large batteries. In addition, the specific
compound reduces the reaction resistance of the positive electrode,
and this is thought to be preferable because it is effective in
further improving output in a battery designed to have a battery
shape, positive-electrode area, etc. which are suitable for high
output.
ExampleS
[1340] The invention will be explained below in more detail by
reference to Examples and Comparative Examples. However, the
invention should not be construed as being limited to the following
Examples unless the invention departs from the spirit thereof.
Positive Electrode [1] [Positive-Electrode Active Materials]
[1341] The kinds and properties of the positive-electrode active
materials used in the following Examples and Comparative Examples
are as shown below.
TABLE-US-00001 Positive Electrode [1] Table 1 Positive- electrode
BET specific Average primary- Median active surface area particle
diameter diameter d.sub.50 Tap density material m.sup.2/g .mu.m
.mu.m g/cm.sup.3 A 1.2 0.8 4.4 1.6 B 3.0 0.2 0.9 1.0 C 1.2 0.8 4.4
1.6 D 1.0 0.5 8.0 2.3 E 0.4 9.0 9.0 2.6
[1342] With respect to the properties of the positive-electrode
active materials shown in Positive Electrode [1] Table 1, the BET
specific surface area, average primary-particle diameter (measured
with an SEM), median diameter d.sub.50, and tap density were
determined by the methods described above.
[Positive-Electrode Active Material A]
[1343] Positive-electrode active material A is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
LiMn.sub.0.33Ni.sub.0.33Co.sub.0.33O.sub.2. Mn.sub.3O.sub.4, NiO,
and Co(OH).sub.2 were weighed out as a manganese source, nickel
source, and cobalt source, respectively, in such amounts as to
result in an Mn:Ni:Co molar ratio of 1:1:1. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring to a median diameter of 0.2 .mu.m.
[1344] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 5 .mu.m consisting only of the manganese source, nickel
source, and cobalt source. An LiOH powder having a median diameter
of 3 .mu.m was added to the resultant granulated particles in such
an amount that the ratio of the number of moles of the Li to the
total number of moles of the Mn, Ni, and Co became 1.05. The
ingredients were mixed together by means of a high-speed mixer to
obtain a powder mixture of the lithium source and the granulated
particles of the nickel source, cobalt source, and manganese
source. This powder mixture was heat-treated at 950.degree. C. for
12 hours (heating/cooling rate, 5.degree. C./min) in an air stream,
subsequently disaggregated, and passed through a sieve having an
opening size of 45 .mu.m to obtain positive-electrode active
material A.
[Positive-Electrode Active Material B]
[1345] Positive-electrode active material B is a lithium-transition
metal composite oxide synthesized in the same manner as for
positive-electrode active material A, except that the spray drying
conditions were changed to obtain granulated particles having a
particle diameter of about 1 .mu.m and that the heat-treating
temperature was changed to 930.degree. C. This active material is
represented by the composition formula
LiMn.sub.0.33Ni.sub.0.33Co.sub.0.33O.sub.2.
[Positive-Electrode Active Material C]
[1346] Positive-electrode active material C is a positive-electrode
active material synthesized by the following method and comprising
positive-electrode active material A and a sulfur compound and an
antimony compound which are adherent to the surface of the active
material A. While 96.7 parts by weight of positive-electrode active
material A was being stirred in a fluidizing vessel, an aqueous
solution of 1.3 parts by weight of lithium sulfate
(Li.sub.2SO.sub.4.H.sub.2O) was sprayed over the active material.
To the mixture obtained was added 2.0 parts by weight of antimony
trioxide (Sb.sub.2O.sub.3; median diameter of the particles, 0.8
.mu.m). The ingredients were sufficiently mixed together. This
mixture was transferred to an alumina vessel and heat-treated at
680.degree. C. for 2 hours in an air atmosphere to obtain
positive-electrode active material C.
[Positive-Electrode Active Material D]
[1347] Positive-electrode active material D is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.04Mn.sub.1.84Al.sub.0.12O.sub.4. LiOH, Mn.sub.2O.sub.3,
and AlOOH were weighed out as a lithium source, manganese source,
and aluminum source, respectively, in such amounts as to result in
an Li:Mn:Al molar ratio of 1.04:1.84:0.12. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring to a median diameter of 0.5 .mu.m.
[1348] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 10 .mu.m comprising the lithium source, manganese source, and
aluminum source. These granulated particles were heat-treated at
900.degree. C. for 3 hours (heating rate, 5.degree. C./min) in a
nitrogen stream. Thereafter, the flowing gas was changed from
nitrogen to air, and the particles were further heat-treated at
900.degree. C. for 2 hours (cooling rate, 1.degree. C./min). The
particles were cooled to room temperature, subsequently taken out
and disaggregated, and then passed through a sieve having an
opening size of 45 .mu.m to obtain positive-electrode active
material D.
[Positive-Electrode Active Material E]
[1349] Positive-electrode active material E is a commercial product
(lithium cobalt oxide manufactured by Nippon Chemical Industrial
Co., Ltd.). It is a lithium cobalt oxide represented by the
composition formula Li.sub.1.03CoO.sub.2.
Positive Electrode [1]
Example 1
Production of Positive Electrode
[1350] Ninety percents by mass positive-electrode active material A
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having a positive-electrode
active-material layer size with a width of 100 mm and a length of
100 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, positive electrodes were
obtained. The positive-electrode active-material layers had a
density of 2.35 g/cm.sup.3, and the value of (thickness of the
positive-electrode active-material layer on one side)/(thickness of
the current collector) was 2.2.
<<Production of Negative Electrode>>
[1351] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having a
negative-electrode active-material layer size with a width of 104
mm and a length of 104 mm and including an uncoated part having a
width of 30 mm were cut out of the rolled foil. Thus, negative
electrodes were obtained.
[1352] <<Production of Electrolytic Solution>>
[1353] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixed solvent composed of purified ethylene
carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl
carbonate (EMC) in a volume ratio of 3:3:4. Furthermore,
hexamethylcyclotrisiloxane was added thereto so as to result in a
concentration of 0.3% by mass.
<<Production of Battery>>
[1354] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of the electrolyte was introduced into the
battery can packed with the electrode group and sufficiently
infiltrated into the electrodes. This can was sealed to fabricate a
battery. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
20.6.
<<Battery Evaluation>>
(Method of Measuring Battery Capacity)
[1355] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.1-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as initial capacity. The results of battery evaluation are shown in
Positive Electrode [1] Table 2.
(Method of Measuring Initial Output)
[1356] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W). The results of battery evaluation are shown in
Positive Electrode [1] Table 2.
(Cycle Test (Method of Measuring Battery Capacity after Durability
Test and Output after Durability Test))
[1357] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.1 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment at a current of 0.2
C. The 0.2-C discharge capacity thereof as measured in the third
cycle was taken as battery capacity after durability test.
Furthermore, the battery which had undergone the cycle test was
examined for output. This output was taken as output after
durability test. The results of battery evaluation are shown in
Positive Electrode [1] Table 2.
Positive Electrode [1]
Example 2
[1358] The same procedure as in Positive Electrode [1] Example 1
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [1] Table 2.
Positive Electrode [1]
Example 3
[1359] The same procedure as in Positive Electrode [1] Example 1
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [1] Table 2.
Positive Electrode [1]
Example 4
[1360] The same procedure as in Example 1 was conducted, except
that 0.3% by mass lithium difluorophosphate was incorporated into
the electrolyte in place of the hexamethylcyclotrisiloxane. The
results of battery evaluation are shown in Positive Electrode [1]
Table 2.
Positive Electrode [1]
Comparative Example 1
[1361] The same procedure as in Positive Electrode [1] Example 1
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive electrode [1] Table 2.
TABLE-US-00002 Positive Electrode [1] Table 2 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 1
hexamethylcyclo- 6.4 605 5.4 401 trisiloxane Example 2
trimethylsilyl 6.4 608 5.5 407 methanesulfonate Example 3
phenyldimethyl- 6.4 600 5.4 398 fluorosilane Example 4
LiPO.sub.2F.sub.2 6.4 618 5.5 415 Comparative none 6.4 525 5.1 325
Example 1
Positive Electrode [1]
Example 5
[1362] The same procedure as in Positive Electrode [1] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
A with positive-electrode active material B in a mass ratio of 2:1
was used to produce positive electrodes. This positive-electrode
active material obtained by mixing had a BET specific surface area
of 1.8 m.sup.2/g, average primary-particle diameter of 0.22 .mu.m,
median diameter d.sub.50 of 3.2 .mu.m, and tap density of 1.5
g/cm.sup.3. The results of battery evaluation are shown in Positive
Electrode [1] Table 3.
Positive Electrode [1]
Example 6
[1363] The same procedure as in Positive Electrode [1] Example 5
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [1] Table 3.
Positive Electrode [1]
Example 7
[1364] The same procedure as in Positive Electrode [1] Example 5
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [1] Table 3.
Positive Electrode [1]
Example 8
[1365] The same procedure as in Positive Electrode [1] Example 5
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [1] Table 3.
Positive Electrode [1]
Comparative Example 2
[1366] The same procedure as in Positive Electrode [1] Example 5
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive Electrode [1] Table 3.
TABLE-US-00003 Positive Electrode [1] Table 3 Battery Output
capacity Output after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 5
hexamethylcyclo- 6.4 641 5.2 406 trisiloxane Example 6
trimethylsilyl 6.4 648 5.2 409 methanesulfonate Example 7
phenyldimethyl- 6.4 637 5.1 403 fluorosilane Example 8
LiPO.sub.2F.sub.2 6.4 657 5.2 419 Comparative none 6.4 558 4.9 328
Example 2
Positive Electrode [1]
Example 9
[1367] The same procedure as in Positive Electrode [1] Example 1
was conducted, except that positive-electrode active material C was
used as a positive-electrode active material to produce positive
electrodes. The results of battery evaluation are shown in Positive
Electrode [1] Table 4.
Positive Electrode [1]
Example 10
[1368] The same procedure as in Positive Electrode [1] Example 9
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [1] Table 4.
Positive Electrode [1]
Example 11
[1369] The same procedure as in Positive Electrode [1] Example 9
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [1] Table 4.
Positive Electrode [1]
Example 12
[1370] The same procedure as in Positive Electrode [1] Example 9
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [1] Table 4.
Positive Electrode [1]
Comparative Example 3
[1371] The same procedure as in Positive Electrode [1] Example 9
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive electrode [1] Table 4.
TABLE-US-00004 Positive Electrode [1] Table 4 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 9
hexamethylcyclo- 6.4 604 5.6 412 trisiloxane Example 10
trimethylsilyl 6.4 609 5.6 419 methanesulfonate Example 11
phenyldimethyl- 6.4 599 5.6 409 fluorosilane Example 12
LiPO.sub.2F.sub.2 6.4 618 5.6 427 Comparative none 6.4 525 5.3 337
Example 3
Positive Electrode [1]
Example 13
[1372] A battery was produced in the same manner as in Positive
Electrode [1] Example 1, except that positive-electrode active
material D was used as a positive-electrode active material in
positive-electrode production, that the coated foil was rolled with
a pressing machine to a thickness of 108 .mu.m, and that 28
positive electrodes thus produced and 29 negative electrodes were
used. The value of (thickness of the positive-electrode
active-material layer on one side of the positive
electrode)/(thickness of the positive-electrode current collector)
was 3.1. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
18.1. This battery was evaluated in the same manner as in Positive
Electrode [1] Example 1, except that the voltage range in the
capacity measurement was changed to 3.0-4.2 V and the final upper
voltage in the cycle test was changed to 4.2 V. The results of
battery evaluation are shown in Positive Electrode [1] Table 5.
Positive Electrode [1]
Example 14
[1373] The same procedure as in Positive Electrode [1] Example 13
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [1] Table 5.
Positive Electrode [1]
Example 15
[1374] The same procedure as in Positive Electrode [1] Example 13
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [1] Table 5.
Positive Electrode [1]
Example 16
[1375] The same procedure as in Positive Electrode [1] Example 13
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [1] Table 5.
Positive Electrode [1]
Comparative Example 4
[1376] The same procedure as in Positive Electrode [1] Example 13
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive electrode [1] Table 5
TABLE-US-00005 Positive Electrode [1] Table 5 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 13
hexamethylcyclo- 5.6 792 3.5 392 trisiloxane Example 14
trimethylsilyl 5.6 800 3.6 398 methanesulfonate Example 15
phenyldimethyl- 5.6 788 3.5 388 fluorosilane Example 16
LiPO.sub.2F.sub.2 5.6 811 3.6 407 Comparative none 5.6 689 3.2 304
Example 4
Positive Electrode [1]
Example 17
[1377] The same procedure as in Positive Electrode [1] Example 13
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
D with positive-electrode active material E in a mass ratio of 2:1
was used in positive-electrode production, that the coated foil was
rolled with a pressing machine to a thickness of 94 .mu.m, and that
29 positive electrodes thus produced and 30 negative electrodes
were used. The value of (thickness of the positive-electrode
active-material layer on one side of the positive
electrode)/(thickness of the positive-electrode current collector)
was 2.6. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
18.7. The positive-electrode active material obtained by mixing had
a BET specific surface area of 0.8 m.sup.2/g, average
primary-particle diameter of 0.50 .mu.m, median diameter d.sub.50
of 8.3 .mu.m, and tap density of 2.5 g/cm.sup.3. The results of
battery evaluation are shown in Positive Electrode [1] Table 6.
Positive Electrode [1]
Example 18
[1378] The same procedure as in Positive Electrode [1] Example 17
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [1] Table 6.
Positive Electrode [1]
Example 19
[1379] The same procedure as in Positive Electrode [1] Example 17
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [1] Table 6.
Positive Electrode [1]
Example 20
[1380] The same procedure as in Positive Electrode [1] Example 17
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [1] Table 6.
Positive Electrode [1]
Comparative Example 5
[1381] The same procedure as in Positive Electrode [1] Example 17
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive Electrode [1] Table 6.
TABLE-US-00006 Positive Electrode [1] Table 6 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 17
hexamethylcyclo- 5.8 694 4.2 397 trisiloxane Example 18
trimethylsilyl 5.8 700 4.2 401 methanesulfonate Example 19
phenyldimethyl- 5.8 690 4.2 393 fluorosilane Example 20
LiPO.sub.2F.sub.2 5.8 711 4.3 411 Comparative none 5.8 604 3.9 313
Example 5
[1382] The results given in Positive Electrode [1] Table 2 to
Positive Electrode [1] Table 6 show the following. With respect to
each of the positive electrodes, the incorporation of the specific
compounds in the electrolyte brought about improvements in output
and capacity retentivity. Furthermore, even after the cycle test,
the battery capacity and the output were sufficiently retained.
Positive Electrode [2]
Positive-Electrode Active Materials
[1383] The kinds and properties of the positive-electrode active
materials used in the following Examples and Comparative Examples
are as shown below
TABLE-US-00007 Positive Electrode [2] Table 1 Average Positive- BET
specific primary particle Median Tap electrode surface area
diameter diameter d.sub.50 density active material m.sup.2/g .mu.m
.mu.m g/cm.sup.3 A 0.6 0.7 9.0 2.1 B 3.1 0.2 0.8 1.2 C 0.6 0.7 9.0
2.1 D 0.5 0.6 10.9 2.1 E 1.0 0.5 8.0 2.3
[1384] With respect to the properties of the positive-electrode
active materials shown in Positive Electrode [2] Table 1, the BET
specific surface area, average primary-particle diameter (measured
with an SEM), median diameter d.sub.50, and tap density were
determined by the methods described above.
[Positive-Electrode Active Material A]
[1385] Positive-electrode active material A is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.05Ni.sub.0.80Co.sub.0.15Al.sub.0.05O.sub.2. NiO,
Co(OH).sub.2, AlOOH were weighed out as a nickel source, cobalt
source, and aluminum source, respectively, in such amounts as to
result in an Ni:Co:Al molar ratio of 80:15:5. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring to a median diameter of 0.25
.mu.m.
[1386] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 10 .mu.m consisting only of the nickel source, cobalt source,
and aluminum source. An LiOH powder having a median diameter of 3
.mu.m was added to the resultant granulated particles in such an
amount that the ratio of the number of moles of the Li to the total
number of moles of the Ni, Co, and Al became 1.05. The ingredients
were mixed together by means of a high-speed mixer to obtain a
powder mixture of the lithium source and the granulated particles
of the nickel source, cobalt source, and aluminum source. This
powder mixture was heat-treated at 740.degree. C. for 6 hours
(heating/cooling rate, 5.degree. C./min) in an oxygen stream,
subsequently disaggregated, and passed through a sieve having an
opening size of 45 .mu.m to obtain positive-electrode active
material A.
[Positive-Electrode Active Material B]
[1387] Positive-electrode active material B is a lithium-transition
metal composite oxide synthesized in the same manner as for
positive-electrode active material A, except that the spray drying
conditions were changed to obtain granulated particles having a
particle diameter of about 1 .mu.m and that the heat-treating
temperature was changed to 720.degree. C. This active material is
represented by the composition formula
Li.sub.1.05Ni.sub.0.80Co.sub.0.15Al.sub.0.05O.sub.2.
[Positive-Electrode Active Material C]
[1388] Positive-electrode active material C is a positive-electrode
active material synthesized by the following method and comprising
positive-electrode active material A and a sulfur compound and an
antimony compound which are adherent to the surface of the active
material A. While 96.7 parts by weight of positive-electrode active
material A was being stirred in a fluidizing vessel, an aqueous
solution of 1.3 parts by weight of lithium sulfate
(Li.sub.2SO.sub.4H.sub.2O) was sprayed over the active material. To
the mixture obtained was added 2.0 parts by weight of antimony
trioxide (Sb.sub.2O.sub.3; median diameter of the particles, 0.8
.mu.m). The ingredients were sufficiently mixed together. This
mixture was transferred to an alumina vessel and heat-treated at
680.degree. C. for 2 hours in an air atmosphere to obtain
positive-electrode active material C.
[Positive-Electrode Active Material D]
[1389] Positive-electrode active material D is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.03Ni.sub.0.65Co.sub.0.20Mn.sub.0.15O.sub.2. Ni(OH).sub.2,
Co(OH).sub.2, and Mn.sub.2O.sub.3 were weighed out as a nickel
source, cobalt source, and manganese source, respectively, in such
amounts as to result in an Ni:Co:Mn molar ratio of 65:20:15. Pure
water was added thereto to prepare a slurry. Using a circulating
medium-stirring type wet bead mill, the solid matters in the slurry
were wet-pulverized with stirring to a median diameter of 0.2
.mu.m.
[1390] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 12 .mu.m consisting only of the nickel source, cobalt source,
and manganese source. An LiOH powder having a median diameter of 3
.mu.m was added to the resultant granulated particles in such an
amount that the ratio of the number of moles of the Li to the total
number of moles of the Ni, Co, and Mn became 1.05. The ingredients
were mixed together by means of a high-speed mixer to obtain a
powder mixture of the lithium source and the granulated particles
of the nickel source, cobalt source, and manganese source. This
powder mixture was heat-treated at 950.degree. C. for 12 hours
(heating/cooling rate, 5.degree. C./min) in an air stream,
subsequently disaggregated, and passed through a sieve having an
opening size of 45 .mu.m to obtain positive-electrode active
material D.
[Positive-Electrode Active Material E]
[1391] Positive-electrode active material E is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.04Mn.sub.1.84Al.sub.0.12O.sub.4. LiOH, Mn.sub.2O.sub.3,
and AlOOH were weighed out as a lithium source, manganese source,
and aluminum source, respectively, in such amounts as to result in
an Li:Mn:Al molar ratio of 1.04:1.84:0.12. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring to a median diameter of 0.5 .mu.m.
[1392] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 10 .mu.m comprising the lithium source, manganese source, and
aluminum source. These granulated particles were heat-treated at
900.degree. C. for 3 hours (heating rate, 5.degree. C./min) in a
nitrogen stream. Thereafter, the flowing gas was changed from
nitrogen to air, and the particles were further heat-treated at
900.degree. C. for 2 hours (cooling rate, 1.degree. C./min). The
particles were cooled to room temperature, subsequently taken out
and disaggregated, and then passed through a sieve having an
opening size of 45 .mu.m to obtain positive-electrode active
material E.
Positive Electrode [2]
Example 1
Production of Positive Electrode
[1393] Ninety percents by mass positive-electrode active material A
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 66 .mu.m. Pieces each having a positive-electrode
active-material layer size with a width of 100 mm and a length of
100 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, positive electrodes were
obtained. The positive-electrode active-material layers had a
density of 2.35 g/cm.sup.3, and the value of (thickness of the
positive-electrode active-material layer on one side)/(thickness of
the current collector) was 1.7.
<<Production of Negative Electrode>>
[1394] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having a
negative-electrode active-material layer size with a width of 104
mm and a length of 104 mm and including an uncoated part having a
width of 30 mm were cut out of the rolled foil. Thus, negative
electrodes were obtained.
<<Production of Electrolytic Solution>>
[1395] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixed solvent composed of purified ethylene
carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl
carbonate (EMC) in a volume ratio of 3:3:4. Furthermore,
hexamethylcyclotrisiloxane was added thereto so as to result in a
concentration of 0.3% by mass.
<<Production of Battery>>
[1396] Thirty-four positive electrodes and 35 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of the electrolyte was introduced into the
battery can packed with the electrode group and sufficiently
infiltrated into the electrodes. This can was sealed to fabricate a
battery. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
21.9.
<<Battery Evaluation>>
(Method of Measuring Battery Capacity)
[1397] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.1-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as initial capacity. The results of battery evaluation are shown in
Positive Electrode [2] Table 2.
(Method of Measuring Initial Output)
[1398] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W). The results of battery evaluation are shown in
Positive Electrode [2] Table 2.
(Cycle Test (Method of Measuring Battery Capacity after Durability
Test and Output after Durability Test))
[1399] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.1 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment at a current of 0.2
C. The 0.2-C discharge capacity thereof as measured in the third
cycle was taken as battery capacity after durability test.
Furthermore, the battery which had undergone the cycle test was
examined for output. This output was taken as output after
durability test. The results of battery evaluation are shown in
Positive Electrode [2] Table 2.
Positive Electrode [2]
Example 2
[1400] The same procedure as in Positive Electrode [2] Example 1
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [2] Table 2.
Positive Electrode [2]
Example 3
[1401] The same procedure as in Positive Electrode [2] Example 1
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 2.
Positive Electrode [2]
Example 4
[1402] The same procedure as in Positive Electrode [2] Example 1
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 2.
Positive Electrode [2]
Comparative Example 1
[1403] The same procedure as in Positive Electrode [2] Example 1
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive electrode [2] Table 2.
TABLE-US-00008 Positive Electrode [2] Table 2 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 1
hexamethylcyclo 6.1 551 5.4 377 trisiloxane Example 2
trimethylsilyl 6.1 556 5.4 381 methanesulfonate Example 3
phenyldimethyl 6.1 545 5.3 371 fluorosilane Example 4
LiPO.sub.2F.sub.2 6.1 563 5.4 387 Comparative none 6.1 478 5.0 303
Example 1
Positive Electrode [2]
Example 5
[1404] The same procedure as in Positive Electrode [2] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
A with positive-electrode active material B in a mass ratio of 2:1
was used to produce positive electrodes. This positive-electrode
active material obtained by mixing had a BET specific surface area
of 1.4 m.sup.2/g, average primary-particle diameter of 0.2 .mu.m,
median diameter d.sub.50 of 6.3 .mu.m, and tap density of 1.9
g/cm.sup.3. The results of battery evaluation are shown in Positive
Electrode [2] Table 3.
Positive Electrode [2]
Example 6
[1405] The same procedure as in Positive Electrode [2] Example 5
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [2] Table 3.
Positive Electrode [2]
Example 7
[1406] The same procedure as in Positive Electrode [2] Example 5
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 3.
Positive Electrode [2]
Example 8
[1407] The same procedure as in Positive Electrode [2] Example 5
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 3.
Positive Electrode [2]
Comparative Example 2
[1408] The same procedure as in Positive Electrode [2] Example 5
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive Electrode [2] Table 3.
TABLE-US-00009 Positive Electrode [2] Table 3 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 5
hexamethylcyclo 6.1 590 5.0 375 trisiloxane Example 6
trimethylsilyl 6.1 596 5.0 378 methanesulfonate Example 7
phenyldimethyl 6.1 585 4.9 369 fluorosilane Example 8
LiPO.sub.2F.sub.2 6.1 604 5.0 385 Comparative none 6.1 514 4.8 310
Example 2
Positive Electrode [2]
Example 9
[1409] The same procedure as in Positive Electrode [2] Example 1
was conducted, except that positive-electrode active material C was
used as a positive-electrode active material to produce positive
electrodes. The results of battery evaluation are shown in Positive
Electrode [2] Table 4.
Positive Electrode [2]
Example 10
[1410] The same procedure as in Positive Electrode [2] Example 9
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [2] Table 4.
Positive Electrode [2]
Example 11
[1411] The same procedure as in Positive Electrode [2] Example 9
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 4.
Positive Electrode [2]
Example 12
[1412] The same procedure as in Positive Electrode [2] Example 9
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 4.
Positive Electrode [2]
Comparative Example 3
[1413] The same procedure as in Positive Electrode [2] Example 9
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive electrode [2] Table 4.
TABLE-US-00010 Positive Electrode [2] Table 4 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 9
hexamethylcyclo- 6.1 548 5.3 374 trisiloxane Example 10
trimethylsilyl 6.1 555 5.4 379 methanesulfonate Example 11
phenyldimethyl- 6.1 546 5.3 371 fluorosilane Example 12
LiPO.sub.2F.sub.2 6.1 563 5.4 386 Comparative none 6.1 478 5.1 311
Example 3
Positive Electrode [2]
Example 13
[1414] A battery was produced in the same manner as in Positive
Electrode [2] Example 1, except that positive-electrode active
material D was used as a positive-electrode active material in
positive-electrode production, that the coated foil was rolled with
a pressing machine to a thickness of 68 .mu.m, and that 34 positive
electrodes thus produced and 35 negative electrodes were used. The
value of (thickness of the positive-electrode active-material layer
on one side of the positive electrode)/(thickness of the
positive-electrode current collector) was 1.8. The ratio of the
total electrode area of the positive electrodes to the overall
surface area of the battery case was 21.9. This battery was
evaluated in the same manner as in Positive Electrode [2] Example
1, except that the voltage range in the capacity measurement was
changed to 3.0-4.2 V and the final upper voltage in the cycle test
was changed to 4.2 V. The results of battery evaluation are shown
in Positive Electrode [2] Table 5.
Positive Electrode [2]
Example 14
[1415] The same procedure as in Positive Electrode [2] Example 13
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [2] Table 5.
Positive Electrode [2]
Example 15
[1416] The same procedure as in Positive Electrode [2] Example 13
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 5.
Positive Electrode [2]
Example 16
[1417] The same procedure as in Positive Electrode [2] Example 13
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 5.
Positive Electrode [2]
Comparative Example 4
[1418] The same procedure as in Positive Electrode [2] Example 13
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive electrode [2] Table 5
TABLE-US-00011 Positive Electrode [2] Table 5 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 13
hexamethylcyclo- 6.1 580 5.0 369 trisiloxane Example 14
trimethylsilyl 6.1 586 5.0 374 methanesulfonate Example 15
phenyldimethyl- 6.1 578 5.0 366 fluorosilane Example 16
LiPO.sub.2F.sub.2 6.1 594 5.0 380 Comparative none 6.1 505 4.7 300
Example 4
Positive Electrode [2]
Example 17
[1419] The same procedure as in Positive Electrode [2] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
A with positive-electrode active material E in a mass ratio of 2:1
was used in positive-electrode production, that the coated foil was
rolled with a pressing machine to a thickness of 76 .mu.m, and that
32 positive electrodes thus produced and 33 negative electrodes
were used. The value of (thickness of the positive-electrode
active-material layer on one side of the positive
electrode)/(thickness of the positive-electrode current collector)
was 2.0. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
20.6. The positive-electrode active material obtained by mixing had
a BET specific surface area of 0.7 m.sup.2/g, average
primary-particle diameter of 0.6 .mu.m, median diameter d.sub.50 of
8.7 .mu.m, and tap density of 2.2 g/cm.sup.3. The results of
battery evaluation are shown in Positive Electrode [2] Table 6.
Positive Electrode [2]
Example 18
[1420] The same procedure as in Positive Electrode [2] Example 17
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [2] Table 6.
Positive Electrode [2]
Example 19
[1421] The same procedure as in Positive Electrode [2] Example 17
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 6.
Positive Electrode [2]
Example 20
[1422] The same procedure as in Positive Electrode [2] Example 17
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [2] Table 6.
Positive Electrode [2]
Comparative Example 5
[1423] The same procedure as in Positive Electrode [2] Example 17
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive electrode [2] Table 6.
TABLE-US-00012 Positive Electrode [2] Table 6 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 17
hexamethylcyclo- 6.0 617 4.7 381 trisiloxane Example 18
trimethylsilyl 6.0 623 4.7 384 methanesulfonate Example 19
phenyldimethyl- 6.0 615 4.7 378 fluorosilane Example 20
LiPO.sub.2F.sub.2 6.0 632 4.8 392 Comparative none 6.0 538 4.6 316
Example 5
[1424] The results given in Positive Electrode [2] Table 2 to
Positive Electrode [2] Table 6 show the following. With respect to
each of the positive electrodes, the incorporation of the specific
compounds in the electrolyte brought about improvements in output
and capacity retentivity. Furthermore, even after the cycle test,
the battery capacity and the output were sufficiently retained.
Positive Electrode [3]
Positive-Electrode Active Materials
[1425] The kinds and properties of the positive-electrode active
materials used in the following Examples and Comparative Examples
are as shown below.
TABLE-US-00013 Positive Electrode [3] Table 1 Positive- Average
electrode BET specific primary-particle Median active surface
diameter diameter d.sub.50 Tap density material area m.sup.2/g
.mu.m .mu.m g/cm.sup.3 A 1.3 0.7 6.0 2.0 B 2.9 0.2 0.9 1.1 C 1.3
0.7 6.0 2.0 D 1.0 0.5 8.0 2.3
[1426] With respect to the properties of the positive-electrode
active materials shown in Positive Electrode [3] Table 1, the BET
specific surface area, average primary-particle diameter (measured
with an SEM), median diameter d.sub.50, and tap density were
determined by the methods described above.
[Positive-Electrode Active Material A]
[1427] Positive-electrode active material A is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.05Ni.sub.0.8Co.sub.0.2O.sub.2. NiO and Co(OH).sub.2 were
weighed out as a nickel source and a cobalt source, respectively,
in such amounts as to result in an Ni:Co molar ratio of 80:20. Pure
water was added thereto to prepare a slurry. Using a circulating
medium-stirring type wet bead mill, the solid matters in the slurry
were wet-pulverized with stirring to a median diameter of 0.25
.mu.m.
[1428] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 8 .mu.m consisting only of the nickel source and cobalt
source. An LiOH powder having a median diameter of 3 .mu.m was
added to the resultant granulated particles in such an amount that
the ratio of the number of moles of the Li to the total number of
moles of the --Ni and Co became 1.05. The ingredients were mixed
together by means of a high-speed mixer to obtain a powder mixture
of the lithium source and the granulated particles of the nickel
source and cobalt source. This powder mixture was heat-treated at
740.degree. C. for 6 hours (heating/cooling rate, 5.degree. C./min)
in an oxygen stream, subsequently disaggregated, and passed through
a sieve having an opening size of 45 .mu.m to obtain
positive-electrode active material A.
[Positive-Electrode Active Material B]
[1429] Positive-electrode active material B is a lithium-transition
metal composite oxide synthesized in the same manner as for
positive-electrode active material A, except that the spray drying
conditions were changed to obtain granulated particles having a
particle diameter of about 1 .mu.m and that the heat-treating
temperature was changed to 720.degree. C. This active material is
represented by the composition formula
Li.sub.1.05Ni.sub.0.8Co.sub.0.2O.sub.2.
[Positive-Electrode Active Material C]
[1430] Positive-electrode active material C is a positive-electrode
active material synthesized by the following method and comprising
positive-electrode active material A and a sulfur compound and an
antimony compound which are adherent to the surface of the active
material A. While 96.7 parts by weight of positive-electrode active
material A was being stirred in a fluidizing vessel, an aqueous
solution of 1.3 parts by weight of lithium sulfate
(Li.sub.2SO.sub.4.H.sub.2O) was sprayed over the active material.
To the mixture obtained was added 2.0 parts by weight of antimony
trioxide (Sb.sub.2O.sub.3; median diameter of the particles, 0.8
.mu.m). The ingredients were sufficiently mixed together. This
mixture was transferred to an alumina vessel and heat-treated at
680.degree. C. for 2 hours in an air atmosphere to obtain
positive-electrode active material C.
[Positive-Electrode Active Material D]
[1431] Positive-electrode active material D is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.04Mn.sub.1.84Al.sub.0.12O.sub.4. LiOH, Mn.sub.2O.sub.3,
and AlOOH were weighed out as a lithium source, manganese source,
and aluminum source, respectively, in such amounts as to result in
an Li:Mn:Al molar ratio of 1.04:1.84:0.12. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring to a median diameter of 0.5 .mu.m.
[1432] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 10 .mu.m comprising the lithium source, manganese source, and
aluminum source. These granulated particles were heat-treated at
900.degree. C. for 3 hours (heating rate, 5.degree. C./min) in a
nitrogen stream. Thereafter, the flowing gas was changed from
nitrogen to air, and the particles were further heat-treated at
900.degree. C. for 2 hours (cooling rate, 1.degree. C./min). The
particles were cooled to room temperature, subsequently taken out
and disaggregated, and then passed through a sieve having an
opening size of 45 .mu.m to obtain positive-electrode active
material D.
Positive Electrode [3]
Example 1
Production of Positive Electrode
[1433] Ninety percents by mass positive-electrode active material A
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 65 .mu.m. Pieces each having a positive-electrode
active-material layer size with a width of 100 mm and a length of
100 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, positive electrodes were
obtained. The positive-electrode active-material layers had a
density of 2.35 g/cm.sup.3, and the value of (thickness of the
positive-electrode active-material layer on one side)/(thickness of
the current collector) was 1.7.
<<Production of Negative Electrode>>
[1434] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having a
negative-electrode active-material layer size with a width of 104
mm and a length of 104 mm and including an uncoated part having a
width of 30 mm were cut out of the rolled foil. Thus, negative
electrodes were obtained.
<<Production of Electrolytic Solution>>
[1435] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixed solvent composed of purified ethylene
carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl
carbonate (EMC) in a volume ratio of 3:3:4. Furthermore,
hexamethylcyclotrisiloxane was added thereto so as to result in a
concentration of 0.3% by mass.
<<Production of Battery>>
[1436] Thirty-four positive electrodes and 35 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of the electrolyte was introduced into the
battery can packed with the electrode group and sufficiently
infiltrated into the electrodes. This can was sealed to fabricate a
battery. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
21.9.
<<Battery Evaluation>>
(Method of Measuring Battery Capacity)
[1437] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.1-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as initial capacity. The results of battery evaluation are shown in
Positive Electrode [3] Table 2.
(Method of Measuring Initial Output)
[1438] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W). The results of battery evaluation are shown in
Positive Electrode [3] Table 2.
(Cycle Test (Method of Measuring Battery Capacity after Durability
Test and Output after Durability Test))
[1439] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.1 V
and then discharged at a constant current of 2 C to a final
discharge'voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment at a current of 0.2
C. The 0.2-C discharge capacity thereof as measured in the third
cycle was taken as battery capacity after durability test.
Furthermore, the battery which had undergone the cycle test was
examined for output. This output was taken as output after
durability test. The results of battery evaluation are shown in
Positive Electrode [3] Table 2.
Positive Electrode [3]
Example 2
[1440] The same procedure as in Positive Electrode [3] Example 1
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [3] Table 2.
Positive Electrode [3]
Example 3
[1441] The same procedure as in Positive Electrode [3] Example 1
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [3] Table 2.
Positive Electrode [3]
Example 4
[1442] The same procedure as in Positive Electrode [3] Example 1
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [3] Table 2.
Positive Electrode [3]
Comparative Example 1
[1443] The same procedure as in Positive Electrode [3] Example 1
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive electrode [3] Table 2.
TABLE-US-00014 Positive Electrode [3] Table 2 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 1
hexamethylcyclo- 6.5 505 4.8 290 trisiloxane Example 2
trimethylsilyl 6.5 509 4.8 293 methanesulfonate Example 3
phenyldimethyl- 6.5 500 4.8 286 fluorosilane Example 4
LiPO.sub.2F.sub.2 6.5 516 4.8 297 Comparative none 6.5 438 4.7 244
Example 1
Positive Electrode [3]
Example 5
[1444] The same procedure as in Positive Electrode [3] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
A with positive-electrode active material B in a mass ratio of 2:1
was used to produce positive electrodes. This positive-electrode
active material obtained by mixing had a BET specific surface area
of 1.8 m.sup.2/g, average primary-particle diameter of 0.2 .mu.m,
median diameter d.sub.50 of 4.3 .mu.m, and tap density of 1.8
g/cm.sup.3. The results of battery evaluation are shown in Positive
Electrode [3] Table 3.
Positive Electrode [3]
Example 6
[1445] The same procedure as in Positive Electrode [3] Example 5
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [3] Table 3.
Positive Electrode [3]
Example 7
[1446] The same procedure as in Positive Electrode [3] Example 5
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [3] Table 3.
Positive Electrode [3]
Example 8
[1447] The same procedure as in Positive Electrode [3] Example 5
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [3] Table 3.
Positive Electrode [3]
Comparative Example 2
[1448] The same procedure as in Positive Electrode [3] Example 5
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive Electrode [3] Table 3.
TABLE-US-00015 Positive Electrode [3] Table 3 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 5
hexamethylcyclo- 6.5 524 4.6 288 trisiloxane Example 6
trimethylsilyl 6.5 529 4.6 290 methanesulfonate Example 7
phenyldimethyl- 6.5 519 4.6 284 fluorosilane Example 8
LiPO.sub.2F.sub.2 6.5 536 4.6 294 Comparative none 6.5 456 4.6 247
Example 2
Positive Electrode [3]
Example 9
[1449] The same procedure as in Positive Electrode [3] Example 1
was conducted, except that positive-electrode active material C was
used as a positive-electrode active material to produce positive
electrodes. The results of battery evaluation are shown in Positive
Electrode [3] Table 4.
Positive Electrode [3]
Example 10
[1450] The same procedure as in Positive Electrode [3] Example 9
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [3] Table 4.
Positive Electrode [3]
Example 11
[1451] The same procedure as in Positive Electrode [3] Example 9
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [3] Table 4.
Positive Electrode [3]
Example 12
[1452] The same procedure as in Positive Electrode [3] Example 9
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [3] Table 4.
Positive Electrode [3]
Comparative Example 3
[1453] The same procedure as in Positive Electrode [3] Example 9
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive electrode [3] Table 4.
TABLE-US-00016 Positive Electrode [3] Table 4 Battery Output
capacity after after dura- Battery Initial durability bility
Specific capacity output test test compound Ah W Ah W Example 9
hexamethylcyclo- 6.5 503 4.2 252 trisiloxane Example 10
trimethylsilyl 6.5 509 4.2 255 methanesulfonate Example 11
phenyldimethyl- 6.5 500 4.2 251 fluorosilane Example 12
LiPO.sub.2F.sub.2 6.5 516 4.2 259 Comparative none 6.5 438 4.2 217
Example 3
Positive Electrode [3]
Example 13
[1454] The same procedure as in Positive Electrode [3] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
A with positive-electrode active material D in a mass ratio of 2:1
was used in positive-electrode production, that the coated foil was
rolled with a pressing machine to a thickness of 74 .mu.m, and that
32 positive electrodes thus produced and 33 negative electrodes
were used. The value of (thickness of the positive-electrode
active-material layer on one side of the positive
electrode)/(thickness of the positive-electrode current collector)
was 2.0. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
20.6. The positive-electrode active material obtained by mixing had
a BET specific surface area of 1.2 m.sup.2/g, average
primary-particle diameter of 0.6 .mu.m, median diameter d.sub.50 of
6.7 .mu.m, and tap density of 2.2 g/cm.sup.3. The results of
battery evaluation are shown in Positive Electrode [3] Table 5.
Positive Electrode [3]
Example 14
[1455] The same procedure as in Positive Electrode [3] Example 13
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the electrolyte in place of
the hexamethylcyclotrisiloxane. The results of battery evaluation
are shown in Positive Electrode [3] Table 5.
Positive Electrode [3]
Example 15
[1456] The same procedure as in Positive Electrode [3] Example 13
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [3] Table 5.
Positive Electrode [3]
Example 16
[1457] The same procedure as in Positive Electrode [3] Example 13
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [3] Table 5.
Positive Electrode [3]
Comparative Example 4
[1458] The same procedure as in Positive Electrode [3] Example 13
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the electrolyte. The results of battery
evaluation are shown in Positive Electrode [3] Table 5.
TABLE-US-00017 POSITIVE ELECTRODE [3] TABLE 5 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
6.1 631 4.3 346 ple 13 trisiloxane Exam- trimethylsilyl 6.1 637 4.3
350 ple 14 methanesulfonate Exam- phenyldimethyl- 6.1 628 4.3 344
ple 15 fluorosilane Exam- LiPO.sub.2F.sub.2 6.1 645 4.3 354 ple 16
Compar- none 6.1 549 4.3 297 ative Exam- ple 4
[1459] The results given in Positive Electrode [3] Table 2 to
Positive Electrode [3] Table 5 show the following. With respect to
each of the positive electrodes, the incorporation of the specific
compounds in the electrolyte brought about improvements in output
and capacity retentivity. Furthermore, even after the cycle test,
the battery capacity and the output were sufficiently retained.
Positive Electrode [4]
Positive-Electrode Active Materials
[1460] The kinds and properties of the positive-electrode active
materials used in the following Examples and Comparative Examples
are as shown below.
TABLE-US-00018 POSITIVE ELECTRODE [4] TABLE 1 Positive- Average
Median electrode BET specific primary-particle diameter active
surface area diameter d.sub.50 Tap density material m.sup.2/g .mu.m
.mu.m g/cm.sup.3 A 1.1 0.9 7.0 2.1 B 3.2 0.2 0.8 1.0 C 1.1 0.9 7.0
2.1 D 1.0 0.5 8.0 2.3
[1461] With respect to the properties of the positive-electrode
active materials shown in Positive Electrode [4] Table 1, the BET
specific surface area, average primary-particle diameter (measured
with an SEM), median diameter d.sub.50, and tap density were
determined by the methods described above.
[Positive-Electrode Active Material A]
[1462] Positive-electrode active material A is a lithium-cobalt
composite oxide synthesized by the method shown below, and is
represented by the composition formula LiCoO.sub.2. LiOH and
Co(OH).sub.2 were weighed out as a lithium source and a cobalt
source, respectively, in such amounts as to result in an Li:Co
molar ratio of 1:1. Pure water was added thereto to prepare a
slurry. Using a circulating medium-stirring type wet bead mill, the
solid matters in the slurry were wet-pulverized with stirring to a
median diameter of 0.2 .mu.m.
[1463] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 9 .mu.m comprising the lithium source and the cobalt source.
The granulated particles were heat-treated at 880.degree. C. for 6
hours (heating/cooling rate, 5.degree. C./min) in an air stream.
The particles heat-treated were cooled to room temperature,
subsequently taken out and disaggregated, and then passed through a
sieve having an opening size of 45 .mu.m to obtain
positive-electrode active material A.
[Positive-Electrode Active Material B]
[1464] Positive-electrode active material B is a lithium-transition
metal composite oxide synthesized in the same manner as for
positive-electrode active material A, except that the spray drying
conditions were changed to obtain granulated particles having a
particle diameter of about 1 .mu.m and that the heat-treating
temperature was changed to 860.degree. C. This active material is
represented by the composition formula LiCoO.sub.2.
[Positive-Electrode Active Material C]
[1465] Positive-electrode active material C is a positive-electrode
active material synthesized by the following method and comprising
positive-electrode active material A and a sulfur compound and an
antimony compound which are adherent to the surface of the active
material A. While 96.7 parts by weight of positive-electrode active
material A was being stirred in a fluidizing vessel, an aqueous
solution of 1.3 parts by weight of lithium sulfate
(Li.sub.2SO.sub.4H.sub.2O) was sprayed over the active material. To
the mixture obtained was added 2.0 parts by weight of antimony
trioxide (Sb.sub.2O.sub.3; median diameter of the particles, 0.8
.mu.m). The ingredients were sufficiently mixed together. This
mixture was transferred to an alumina vessel and heat-treated at
680.degree. C. for 2 hours in an air atmosphere to obtain
positive-electrode active material C.
[Positive-Electrode Active Material D]
[1466] Positive-electrode active material D is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.04Mn.sub.1.84Al.sub.0.12O.sub.4. LiOH, Mn.sub.2O.sub.3,
and AlOOH were weighed out as a lithium source, manganese source,
and aluminum source, respectively, in such amounts as to result in
an Li:Mn:Al molar ratio of 1.04:1.84:0.12. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring to a median diameter of 0.5 .mu.m.
[1467] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 10 .mu.m comprising the lithium source, manganese source, and
aluminum source. These granulated particles were heat-treated at
900.degree. C. for 3 hours (heating rate, 5.degree. C./min) in a
nitrogen stream. Thereafter, the flowing gas was changed from
nitrogen to air, and the particles were further heat-treated at
900.degree. C. for 2 hours (cooling rate, 1.degree. C./min). The
particles were cooled to room temperature, subsequently taken out
and disaggregated, and then passed through a sieve having an
opening size of 45 .mu.m to obtain positive-electrode active
material D.
Positive Electrode [4]
Example 1
Production of Positive Electrode
[1468] Eighty-five percents by mass positive-electrode active
material A as a positive-electrode active material was mixed with
10% by mass acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 85 .mu.m. Pieces each having a positive-electrode
active-material layer size with a width of 100 mm and a length of
100 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, positive electrodes were
obtained. The positive-electrode active-material layers had a
density of 2.35 g/cm.sup.3, and the value of (thickness of the
positive-electrode active-material layer on one side)/(thickness of
the current collector) was 2.3.
<<Production of Negative Electrode>>
[1469] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having a
negative-electrode active-material layer size with a width of 104
mm and a length of 104 mm and including an uncoated part having a
width of 30 mm were cut out of the rolled foil. Thus, negative
electrodes were obtained.
<<Production of Nonaqueous Electrolytic Solution>>
[1470] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixed solvent composed of purified ethylene
carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl
carbonate (EMC) in a volume ratio of 3:3:4. Furthermore,
hexamethylcyclotrisiloxane was added thereto so as to result in a
concentration of 0.3% by mass.
<<Production of Battery>>
[1471] Thirty-one positive electrodes and 32 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of the nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a battery. The ratio of the total electrode area of
the positive electrodes to the overall surface area of the battery
case was 20.0.
<<Battery Evaluation>>
(Method of Measuring Battery Capacity)
[1472] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.2-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as initial capacity. The results of battery evaluation are shown in
Positive Electrode [4] Table 2.
(Method of Measuring Initial Output)
[1473] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W). The results of battery evaluation are shown in
Positive Electrode [4] Table 2.
(Cycle Test (Method of Measuring Battery Capacity after Durability
Test and Output after Durability Test))
[1474] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.2 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment at a current of 0.2
C. The 0.2-C discharge capacity thereof as measured in the third
cycle was taken as battery capacity after durability test.
Furthermore, the battery which had undergone the cycle test was
examined for output. This output was taken as output after
durability test. The results of battery evaluation are shown in
Positive Electrode [4] Table 2.
Positive Electrode [4]
Example 2
[1475] The same procedure as in Positive Electrode [4] Example 1
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [4] Table 2.
Positive Electrode [4]
Example 3
[1476] The same procedure as in Positive Electrode [4] Example 1
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 2.
Positive Electrode [4]
Example 4
[1477] The same procedure as in Positive Electrode [4] Example 1
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 2.
Positive Electrode [4]
Comparative Example 1
[1478] The same procedure as in Positive Electrode [4] Example 1
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive electrode [4] Table 2.
TABLE-US-00019 POSITIVE ELECTRODE [4] TABLE 2 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
5.9 530 4.2 296 ple 1 trisiloxane Exam- trimethylsilyl 5.9 535 4.2
299 ple 2 methanesulfonate Exam- phenyldimethyl- 5.9 524 4.2 292
ple 3 fluorosilane Exam- LiPO.sub.2F.sub.2 5.9 541 4.3 303 ple 4
Compar- none 5.9 460 4.2 249 ative Exam- ple 1
Positive Electrode [4]
Example 5
[1479] The same procedure as in Positive Electrode [4] Example 1
was conducted, except that positive-electrode active material B was
used as a positive-electrode active material to produce positive
electrodes. The results of battery evaluation are shown in Positive
Electrode [4] Table 3.
Positive Electrode [4]
Example 6
[1480] The same procedure as in Positive Electrode [4] Example 5
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [4] Table 3.
Positive Electrode [4]
Example 7
[1481] The same procedure as in Positive Electrode [4] Example 5
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 3.
Positive Electrode [4]
Example 8
[1482] The same procedure as in Positive Electrode [4] Example 5
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 3.
Positive Electrode [4]
Comparative Example 2
[1483] The same procedure as in Positive Electrode [4] Example 5
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive electrode [4] Table 3.
TABLE-US-00020 POSITIVE ELECTRODE [4] TABLE 3 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
5.9 584 3.6 276 ple 5 trisiloxane Exam- trimethylsilyl 5.9 590 3.6
277 ple 6 methanesulfonate Exam- phenyldimethyl- 5.9 579 3.6 272
ple 7 fluorosilane Exam- LiPO.sub.2F.sub.2 5.9 598 3.6 281 ple 8
Compar- none 5.9 509 3.6 236 ative Exam- ple 2
Positive Electrode [4]
Example 9
[1484] The same procedure as in Positive Electrode [4] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
A with positive-electrode active material B in a mass ratio of 2:1
was used to produce positive electrodes. This positive-electrode
active material obtained by mixing had a BET specific surface area
of 1.8 m.sup.2/g, average primary-particle diameter of 0.2 .mu.m,
median diameter d.sub.50 of 4.9 .mu.m, and tap density of 1.8
g/cm.sup.3. The results of battery evaluation are shown in Positive
Electrode [4] Table 4.
Positive Electrode [4]
Example 10
[1485] The same procedure as in Positive Electrode [4] Example 9
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [4] Table 4.
Positive Electrode [4]
Example 11
[1486] The same procedure as in Positive Electrode [4] Example 9
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 4.
Positive Electrode [4]
Example 12
[1487] The same procedure as in Positive Electrode [4] Example 9
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 4.
Positive Electrode [4]
Comparative Example 3
[1488] The same procedure as in Positive Electrode [4] Example 9
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive Electrode [4] Table 4.
TABLE-US-00021 POSITIVE ELECTRODE [4] TABLE 4 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
5.9 546 4.0 291 ple 9 trisiloxane Exam- trimethylsilyl 5.9 553 4.0
295 ple 10 methanesulfonate Exam- phenyldimethyl- 5.9 544 4.0 289
ple 11 fluorosilane Exam- LiPO.sub.2F.sub.2 5.9 560 4.0 299 ple 12
Compar- none 5.9 476 4.0 250 ative Exam- ple 3
Positive Electrode [4]
Example 13
[1489] The same procedure as in Positive Electrode [4] Example 1
was conducted, except that positive-electrode active material C was
used as a positive-electrode active material to produce positive
electrodes. The results of battery evaluation are shown in Positive
Electrode [4] Table 5.
Positive Electrode [4]
Example 14
[1490] The same procedure as in Positive Electrode [4] Example 13
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [4] Table 5.
Positive Electrode [4]
Example 15
[1491] The same procedure as in Positive Electrode [4] Example 13
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 5.
Positive Electrode [4]
Example 16
[1492] The same procedure as in Positive Electrode [4] Example 13
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 5.
Positive Electrode [4]
Comparative Example 4
[1493] The same procedure as in Positive Electrode [4] Example 13
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive electrode [4] Table 5.
TABLE-US-00022 POSITIVE ELECTRODE [4] TABLE 5 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
5.9 529 4.3 303 ple 13 trisiloxane Exam- trimethylsilyl 5.9 535 4.3
306 ple 14 methanesulfonate Exam- phenyldimethyl- 5.9 527 4.3 301
ple 15 fluorosilane Exam- LiPO.sub.2F.sub.2 5.9 541 4.3 310 ple 16
Compar- none 5.9 460 4.3 260 ative Exam- ple 4
Positive Electrode [4]
Example 17
[1494] The same procedure as in Positive Electrode [4] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
A with positive-electrode active material D in a mass ratio of 2:1
was used in positive-electrode production, that the coated foil was
rolled with a pressing machine to a thickness of 92 .mu.m, and that
30 positive electrodes thus produced and 31 negative electrodes
were used. The value of (thickness of the positive-electrode
active-material layer on one side of the positive
electrode)/(thickness of the positive-electrode current collector)
was 2.6. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
19.4. The positive-electrode active material obtained by mixing had
a BET specific surface area of 1.1 m.sup.2/g, average
primary-particle diameter of 0.6 .mu.m, median diameter d.sub.50 of
7.3 .mu.m, and tap density of 2.2 g/cm.sup.3. The results of
battery evaluation are shown in Positive Electrode [4] Table 6.
Positive Electrode [4]
Example 18
[1495] The same procedure as in Positive Electrode [4] Example 17
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [4] Table 6.
Positive Electrode [4]
Example 19
[1496] The same procedure as in Positive Electrode [4] Example 17
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 6.
Positive Electrode [4]
Example 20
[1497] The same procedure as in Positive Electrode [4] Example 17
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [4] Table 6.
Positive Electrode [4]
Comparative Example 5
[1498] The same procedure as in Positive Electrode [4] Example 17
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive Electrode [4] Table 6.
TABLE-US-00023 POSITIVE ELECTRODE [4] TABLE 6 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
5.7 624 3.9 333 ple 17 trisiloxane Exam- trimethylsilyl 5.7 629 3.9
335 ple 18 methanesulfonate Exam- phenyldimethyl- 5.7 621 3.9 330
ple 19 fluorosilane Exam- LiPO.sub.2F.sub.2 5.7 639 3.9 341 ple 20
Compar- none 5.7 543 3.9 285 ative Exam- ple 5
[1499] The results given in Positive Electrode [4] Table 2 to
Positive Electrode [4] Table 6 show the following. With respect to
each of the positive electrodes, the incorporation of the specific
compounds in the nonaqueous electrolyte brought about improvements
in initial output and in capacity retentivity. Furthermore, even
after the cycle test, the battery capacity and the output were
sufficiently retained.
Positive Electrode [5]
Positive-Electrode Active Materials
[1500] The kinds and properties of the positive-electrode active
materials used in the following Examples and Comparative Examples
are as shown below.
TABLE-US-00024 POSITIVE ELECTRODE [5] TABLE 1 Positive- Average
Median electrode BET specific primary-particle diameter active
surface area diameter d.sub.50 Tap density material m.sup.2/g .mu.m
.mu.m g/cm.sup.3 A 1.1 0.9 7.0 2.1 B 1.3 0.7 6.0 2.0 C 0.6 0.7 9.0
2.1 D 0.6 0.7 9.0 2.1 E 1.2 0.8 4.4 1.6 F 1.0 0.5 8.0 2.3
[1501] With respect to the properties of the positive-electrode
active materials shown in Positive Electrode [5] Table 1, the BET
specific surface area, average primary-particle diameter (measured
with an SEM), median diameter d.sub.50, and tap density were
determined by the methods described above.
[Positive-Electrode Active Material A]
[1502] Positive-electrode active material A is a lithium-cobalt
composite oxide synthesized by the method shown below, and is
represented by the composition formula LiCoO.sub.2. LiOH and
Co(OH).sub.2 were weighed out as a lithium source and a cobalt
source, respectively, in such amounts as to result in an Li:Co
molar ratio of 1:1. Pure water was added thereto to prepare a
slurry. Using a circulating medium-stirring type wet bead mill, the
solid matters in the slurry were wet-pulverized with stirring to a
median diameter of 0.2 .mu.m.
[1503] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 9 .mu.m comprising the lithium source and the cobalt source.
The granulated particles were heat-treated at 880.degree. C. for 6
hours (heating/cooling rate, 5.degree. C./min) in an air stream.
The particles heat-treated were cooled to room temperature,
subsequently taken out and disaggregated, and then passed through a
sieve having an opening size of 45 .mu.m to obtain
positive-electrode active material A.
[Positive-Electrode Active Material B]
[1504] Positive-electrode active material B is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.05Ni.sub.0.8Co.sub.0.2O.sub.2. NiO and Co(OH).sub.2 were
weighed out as a nickel source and a cobalt source, respectively,
in such amounts as to result in an Ni:Co molar ratio of 80:20. Pure
water was added thereto to prepare a slurry. Using a circulating
medium-stirring type wet bead mill, the solid matters in the slurry
were wet-pulverized with stirring to a median diameter of 0.25
.mu.m.
[1505] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 8 .mu.m consisting only of the nickel source and cobalt
source. An LiOH powder having a median diameter of 3 .mu.m was
added to the resultant granulated particles in such an amount that
the ratio of the number of moles of the Li to the total number of
moles of the Ni and Co became 1.05. The ingredients were mixed
together by means of a high-speed mixer to obtain a powder mixture
of the lithium source and the granulated particles of the nickel
source and cobalt source. This powder mixture was heat-treated at
740.degree. C. for 6 hours (heating/cooling rate, 5.degree. C./min)
in an oxygen stream, subsequently disaggregated, and passed through
a sieve having an opening size of 45 .mu.m to obtain
positive-electrode active material B.
[Positive-Electrode Active Material C]
[1506] Positive-electrode active material C is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.05Ni.sub.0.80Co.sub.0.15Al.sub.0.05O.sub.2. NiO,
Co(OH).sub.2, AlOOH were weighed out as a nickel source, cobalt
source, and aluminum source, respectively, in such amounts as to
result in an Ni:Co:Al molar ratio of 80:15:5. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring to a median diameter of 0.25
.mu.m.
[1507] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 10 .mu.m consisting only of the nickel source, cobalt source,
and aluminum source. An LiOH powder having a median diameter of 3
.mu.m was added to the resultant granulated particles in such an
amount that the ratio of the number of moles of the Li to the total
number of moles of the Ni, Co, and Al became 1.05. The ingredients
were mixed together by means of a high-speed mixer to obtain a
powder mixture of the lithium source and the granulated particles
of the nickel source, cobalt source, and aluminum source. This
powder mixture was heat-treated at 740.degree. C. for 6 hours
(heating/cooling rate, 5.degree. C./min) in an oxygen stream,
subsequently disaggregated, and passed through a sieve having an
opening size of 45 .mu.m to obtain positive-electrode active
material C.
[Positive-Electrode Active Material D]
[1508] Positive-electrode active material D is a positive-electrode
active material synthesized by the following method and comprising
positive-electrode active material C and a sulfur compound and an
antimony compound which are adherent to the surface of the active
material C. While 96.7 parts by weight of positive-electrode active
material C was being stirred in a fluidizing vessel, an aqueous
solution of 1.3 parts by weight of lithium sulfate
(Li.sub.2SO.sub.4H.sub.2O) was sprayed over the active material. To
the mixture obtained was added 2.0 parts by weight of antimony
trioxide (Sb.sub.2O.sub.3; median diameter of the particles, 0.8
.mu.m). The ingredients were sufficiently mixed together. This
mixture was transferred to an alumina vessel and heat-treated at
680.degree. C. for 2 hours in an air atmosphere to obtain
positive-electrode active material D.
[Positive-Electrode Active Material E]
[1509] Positive-electrode active material E is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
LiMn.sub.0.33Ni.sub.0.33Co.sub.0.33O.sub.2. Mn.sub.3O.sub.4, NiO,
and Co(OH).sub.2 were weighed out as a manganese source, nickel
source, and cobalt source, respectively, in such amounts as to
result in an Mn:Ni:Co molar ratio of 1:1:1. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring to a median diameter of 0.2 .mu.m.
[1510] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 5 .mu.m consisting only of the manganese source, nickel
source, and cobalt source. An LiOH powder having a median diameter
of 3 .mu.m was added to the resultant granulated particles in such
an amount that the ratio of the number of moles of the Li to the
total number of moles of the Mn, Ni, and Co became 1.05. The
ingredients were mixed together by means of a high-speed mixer to
obtain a powder mixture of the lithium source and the granulated
particles of the nickel source, cobalt source, and manganese
source. This powder mixture was heat-treated at 950.degree. C. for
12 hours (heating/cooling rate, 5.degree. C./min) in an air stream,
subsequently disaggregated, and passed through a sieve having an
opening size of 45 .mu.m to obtain positive-electrode active
material E.
[Positive-Electrode Active Material F]
[1511] Positive-electrode active material F is a lithium-transition
metal composite oxide synthesized by the method shown below, and is
represented by the composition formula
Li.sub.1.04Mn.sub.1.84Al.sub.0.12O.sub.4. LiOH, Mn.sub.2O.sub.3,
and AlOOH were weighed out as a lithium source, manganese source,
and aluminum source, respectively, in such amounts as to result in
an Li:Mn:Al molar ratio of 1.04:1.84:0.12. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring to a median diameter of 0.5 .mu.m.
[1512] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 10 .mu.m comprising the lithium source, manganese source, and
aluminum source. These granulated particles were heat-treated at
900.degree. C. for 3 hours (heating rate, 5.degree. C./min) in a
nitrogen stream. Thereafter, the flowing gas was changed from
nitrogen to air, and the particles were further heat-treated at
900.degree. C. for 2 hours (cooling rate, 1.degree. C./min). The
particles were cooled to room temperature, subsequently taken out
and disaggregated, and then passed through a sieve having an
opening size of 45 .mu.M to obtain positive-electrode active
material F.
Positive Electrode [5]
Example 1
Production of Positive Electrode
[1513] A positive-electrode active material obtained by
sufficiently mixing positive-electrode active material A with
positive-electrode active material B in a mass ratio of 1:1 was
used to produce positive electrodes. This positive-electrode active
material obtained by mixing had a BET specific surface area of 1.2
m.sup.2/g, average primary-particle diameter of 0.8 .mu.m, median
diameter d.sub.50 of 6.5 .mu.m, and tap density of 2.1
g/cm.sup.3.
[1514] Ninety percents by mass the positive-electrode active
material obtained by mixing was mixed with 5% by mass acetylene
black as a conductive material and 5% by mass poly(vinylidene
fluoride) (PVdF) as a binder in N-methylpyrrolidone solvent to
prepare a slurry. The slurry obtained was applied to each side of
an aluminum foil having a thickness of 15 .mu.m and dried. The
coated foil was rolled with a pressing machine to a thickness of 70
.mu.m. Pieces each having a positive-electrode active-material
layer size with a width of 100 mm and a length of 100 mm and
including an uncoated part having a width of 30 mm were cut out of
the rolled foil. Thus, positive electrodes were obtained. The
positive-electrode active-material layers had a density of 2.35
g/cm.sup.3, and the value of (thickness of the positive-electrode
active-material layer on one side)/(thickness of the current
collector) was 1.8.
<<Production of Negative Electrode>>
[1515] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having a
negative-electrode active-material layer size with a width of 104
mm and a length of 104 mm and including an uncoated part having a
width of 30 mm were cut out of the rolled foil. Thus, negative
electrodes were obtained.
<<Production of Nonaqueous Electrolytic Solution>>
[1516] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixed solvent composed of purified ethylene
carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl
carbonate (EMC) in a volume ratio of 3:3:4. Furthermore,
hexamethylcyclotrisiloxane was added thereto so as to result in a
concentration of 0.3% by mass.
<<Production of Battery>>
[1517] Thirty-three positive electrodes and 34 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of the nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a battery. The ratio of the total electrode area of
the positive electrodes to the overall surface area of the battery
case was 21.3.
<<Battery Evaluation>>
(Method of Measuring Battery Capacity)
[1518] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.2-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as initial capacity. The results of battery evaluation are shown in
Positive Electrode [5] Table 2.
(Method of Measuring Initial Output)
[1519] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W). The results of battery evaluation are shown in
Positive Electrode [5] Table 2.
(Cycle Test (Method of Measuring Battery Capacity after Durability
Test and Output after Durability Test))
[1520] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.2 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment at a current of 0.2
C. The 0.2-C discharge capacity thereof as measured in the third
cycle was taken as battery capacity after durability test.
Furthermore, the battery which had undergone the cycle test was
examined for output. This output was taken as output after
durability test. The results of battery evaluation are shown in
Positive Electrode [5] Table 2.
Positive Electrode [5]
Example 2
[1521] The same procedure as in Positive Electrode [5] Example 1
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [5] Table 2.
Positive Electrode [5]
Example 3
[1522] The same procedure as in Positive Electrode [5] Example 1
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 2.
Positive Electrode [5]
Example 4
[1523] The same procedure as in Positive Electrode [5] Example 1
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 2.
Positive Electrode [5]
Comparative Example 1
[1524] The same procedure as in Positive Electrode [5] Example 1
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive electrode [5] Table 2.
TABLE-US-00025 POSITIVE ELECTRODE [5] TABLE 2 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
6.3 539 4.6 309 ple 1 trisiloxane Exam- trimethylsilyl 6.3 544 4.6
311 ple 2 methanesulfonate Exam- phenyldimethyl- 6.3 534 4.6 305
ple 3 fluorosilane Exam- LiPO.sub.2F.sub.2 6.3 551 4.6 315 ple 4
Compar- none 6.3 468 4.6 264 ative Exam- ple 1
Positive Electrode [5]
Example 5
[1525] The same procedure as in Positive Electrode [5] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
A with positive-electrode active material E in a mass ratio of 1:1
was used in positive-electrode production, that the coated foil was
rolled with a pressing machine to a thickness of 78 .mu.m, and that
32 positive electrodes thus produced and 33 negative electrodes
were used. The value of (thickness of the positive-electrode
active-material layer on one side of the positive
electrode)/(thickness of the positive-electrode current collector)
was 2.1. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
20.6. The positive-electrode active material obtained by mixing had
a BET specific surface area of 1.2 m.sup.2/g, average
primary-particle diameter of 0.8 .mu.m, median diameter d.sub.50 of
5.7 .mu.m, and tap density of 2.0 g/cm.sup.3. The results of
battery evaluation are shown in Positive Electrode [5] Table 3.
Positive Electrode [5]
Example 6
[1526] The same procedure as in Positive Electrode [5] Example 5
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [5] Table 3.
Positive Electrode [5]
Example 7
[1527] The same procedure as in Positive Electrode [5] Example 5
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 3.
Positive Electrode [5]
Example 8
[1528] The same procedure as in Positive Electrode [5] Example 5
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 3.
Positive Electrode [5]
Comparative Example 2
[1529] The same procedure as in Positive Electrode [5] Example 5
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive Electrode [5] Table 3.
TABLE-US-00026 POSITIVE ELECTRODE [5] TABLE 3 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
6.1 589 5.0 377 ple 5 trisiloxane Exam- trimethylsilyl 6.1 595 5.0
379 ple 6 methanesulfonate Exam- phenyldimethyl- 6.1 584 5.0 370
ple 7 fluorosilane Exam- LiPO.sub.2F.sub.2 6.1 604 5.0 387 ple 8
Compar- none 6.1 513 4.7 305 ative Exam- ple 2
Positive Electrode [5]
Example 9
[1530] The same procedure as in Positive Electrode [5] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
A with positive-electrode active material F in a mass ratio of 1:1
was used in positive-electrode production, that the coated foil was
rolled with a pressing machine to a thickness of 91 .mu.m, and that
30 positive electrodes thus produced and 31 negative electrodes
were used. The value of (thickness of the positive-electrode
active-material layer on one side of the positive
electrode)/(thickness of the positive-electrode current collector)
was 2.5. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
19.4. The positive-electrode active material obtained by mixing had
a BET specific surface area of 1.0 m.sup.2/g, average
primary-particle diameter of 0.6 .mu.m, median diameter d.sub.50 of
7.5 .mu.m, and tap density of 2.2 g/cm.sup.3. The results of
battery evaluation are shown in Positive Electrode [5] Table 4.
Positive Electrode [5]
Example 10
[1531] The same procedure as in Positive Electrode [5] Example 9
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [5] Table 4.
Positive Electrode [5]
Example 11
[1532] The same procedure as in Positive Electrode [5] Example 9
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 4.
Positive Electrode [5]
Example 12
[1533] The same procedure as in Positive Electrode [5] Example 9
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 4.
Positive Electrode [5]
Comparative Example 3
[1534] The same procedure as in Positive Electrode [5] Example 9
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive Electrode [5] Table 4.
TABLE-US-00027 POSITIVE ELECTRODE [5] TABLE 4 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
5.7 679 4.2 390 ple 9 trisiloxane Exam- trimethylsilyl 5.7 687 4.2
397 ple 10 methanesulfonate Exam- phenyldimethyl- 5.7 675 4.2 387
ple 11 fluorosilane Exam- LiPO.sub.2F.sub.2 5.7 696 4.3 404 ple 12
Compar- none 5.7 592 3.9 313 ative Exam- ple 3
Positive Electrode [5]
Example 13
[1535] A battery was produced in the same manner as in Positive
Electrode [5] Example 1, except that a positive-electrode active
material obtained by sufficiently mixing positive-electrode active
material C with positive-electrode active material E in a mass
ratio of 1:1 was used in positive-electrode production, that the
coated foil was rolled with a pressing machine to a thickness of 72
.mu.m, and that 33 positive electrodes thus produced and 34
negative electrodes were used. The positive-electrode active
material obtained by mixing had a BET specific surface area of 0.9
m.sup.2/g, average primary-particle diameter of 0.7 .mu.m, median
diameter d.sub.50 of 6.7 .mu.m, and tap density of 2.0 g/cm.sup.3.
The value of (thickness of the positive-electrode active-material
layer on one side of the positive electrode)/(thickness of the
positive-electrode current collector) was 1.9. The ratio of the
total electrode area of the positive electrodes to the overall
surface area of the battery case was 21.3. This battery was
evaluated in the same manner as in Positive Electrode [5] Example
1, except that the voltage range in the capacity measurement was
changed to 3.0-4.1 V and the final upper voltage in the cycle test
was changed to 4.1 V. The results of battery evaluation are shown
in Positive Electrode [5] Table 5.
Positive Electrode [5]
Example 14
[1536] The same procedure as in Positive Electrode [5] Example 13
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [5] Table 5.
Positive Electrode [5]
Example 15
[1537] The same procedure as in Positive Electrode [5] Example 13
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 5.
Positive Electrode [5]
Example 16
[1538] The same procedure as in Positive Electrode [5] Example 13
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 5.
Positive Electrode [5]
Comparative Example 4
[1539] The same procedure as in Positive Electrode [5] Example 13
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive Electrode [5] Table 5.
TABLE-US-00028 POSITIVE ELECTRODE [5] TABLE 5 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
6.3 578 5.5 397 ple 13 trisiloxane Exam- trimethylsilyl 6.3 584 5.6
402 ple 14 methanesulfonate Exam- phenyldimethyl- 6.3 576 5.5 393
ple 15 fluorosilane Exam- LiPO.sub.2F.sub.2 6.3 591 5.6 410 ple 16
Compar- none 6.3 503 5.1 315 ative Exam- ple 4
Positive Electrode [5]
Example 17
[1540] The same procedure as in Positive Electrode [5] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
C with positive-electrode active material F in a mass ratio of 1:1
was used in positive-electrode production, that the coated foil was
rolled with a pressing machine to a thickness of 78 .mu.m, and that
32 positive electrodes thus produced and 33 negative electrodes
were used. The value of (thickness of the positive-electrode
active-material layer on one side of the positive
electrode)/(thickness of the positive-electrode current collector)
was 2.1. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
20.6. The positive-electrode active material obtained by mixing had
a BET specific surface area of 0.8 m.sup.2/g, average
primary-particle diameter of 0.6 .mu.m, median diameter d.sub.50 of
8.5 .mu.m, and tap density of 2.2 g/cm.sup.3. The results of
battery evaluation are shown in Positive Electrode [5] Table 6.
Positive Electrode [5]
Example 18
[1541] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [5] Table 6.
Positive Electrode [5]
Example 19
[1542] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 6.
Positive Electrode [5]
Example 20
[1543] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 6.
Positive Electrode [5]
Comparative Example 5
[1544] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive Electrode [5] Table 6.
TABLE-US-00029 POSITIVE ELECTRODE [5] TABLE 6 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
6.1 723 5.0 457 ple 17 trisiloxane Exam- trimethylsilyl 6.1 732 5.0
469 ple 18 methanesulfonate Exam- phenyldimethyl- 6.1 719 5.0 455
ple 19 fluorosilane Exam- LiPO.sub.2F.sub.2 6.1 741 5.0 477 ple 20
Compar- none 6.1 630 4.6 363 ative Exam- ple 5
Positive Electrode [5]
Example 21
[1545] A battery was produced in the same manner as in Positive
Electrode [5] Example 1, except that a positive-electrode active
material obtained by sufficiently mixing positive-electrode active
material D with positive-electrode active material E in a mass
ratio of 1:1 was used in positive-electrode production, that the
coated foil was rolled with a pressing machine to a thickness of 72
.mu.m, and that 33 positive electrodes thus produced and 34
negative electrodes were used. The positive-electrode active
material obtained by mixing had a BET specific surface area of 0.9
m.sup.2/g, average primary-particle diameter of 0.7 .mu.m, median
diameter d.sub.50 of 6.7 .mu.m, and tap density of 2.0 g/cm.sup.3.
The value of (thickness of the positive-electrode active-material
layer on one side of the positive electrode)/(thickness of the
positive-electrode current collector) was 1.9. The ratio of the
total electrode area of the positive electrodes to the overall
surface area of the battery case was 21.3. This battery was
evaluated in the same manner as in Positive Electrode [5] Example
1, except that the voltage range in the capacity measurement was
changed to 3.0-4.1 V and the final upper voltage in the cycle test
was changed to 4.1 V. The results of battery evaluation are shown
in Positive Electrode [5] Table 7.
Positive Electrode [5]
Example 22
[1546] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [5] Table 7.
Positive Electrode [5]
Example 23
[1547] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 7.
Positive Electrode [5]
Example 24
[1548] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 7.
Positive Electrode [5]
Comparative Example 6
[1549] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive Electrode [5] Table 7.
TABLE-US-00030 POSITIVE ELECTRODE [5] TABLE 7 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
6.3 578 5.6 402 ple 21 trisiloxane Exam- trimethylsilyl 6.3 584 5.6
406 ple 22 methanesulfonate Exam- phenyldimethyl- 6.3 576 5.6 397
ple 23 fluorosilane Exam- LiPO.sub.2F.sub.2 6.3 591 5.7 415 ple 24
Compar- none 6.3 503 5.2 319 ative Exam- ple 6
Positive Electrode [5]
Example 25
[1550] The same procedure as in Positive Electrode [5] Example 1
was conducted, except that a positive-electrode active material
obtained by sufficiently mixing positive-electrode active material
E with positive-electrode active material Fin a mass ratio of 1:1
was used in positive-electrode production, that the coated foil was
rolled with a pressing machine to a thickness of 86 .mu.m, and that
31 positive electrodes thus produced and 32 negative electrodes
were used. The value of (thickness of the positive-electrode
active-material layer on one side of the positive
electrode)/(thickness of the positive-electrode current collector)
was 2.4. The ratio of the total electrode area of the positive
electrodes to the overall surface area of the battery case was
20.0. The positive-electrode active material obtained by mixing had
a BET specific surface area of 1.1 m.sup.2/g, average
primary-particle diameter of 0.6 .mu.m, median diameter d.sub.50 of
6.2 .mu.m, and tap density of 2.0 g/cm.sup.3. The results of
battery evaluation are shown in Positive Electrode [5] Table 8.
Positive Electrode [5]
Example 26
[1551] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated into the nonaqueous electrolyte
in place of the hexamethylcyclotrisiloxane. The results of battery
evaluation are shown in Positive Electrode [5] Table 8.
Positive Electrode [5]
Example 27
[1552] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 8.
Positive Electrode [5]
Example 28
[1553] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that 0.3% by mass lithium difluorophosphate
was incorporated into the nonaqueous electrolyte in place of the
hexamethylcyclotrisiloxane. The results of battery evaluation are
shown in Positive Electrode [5] Table 8.
Positive Electrode [5]
Comparative Example 7
[1554] The same procedure as in Positive Electrode [5] Example 17
was conducted, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. The results of
battery evaluation are shown in Positive Electrode [5] Table 8.
TABLE-US-00031 POSITIVE ELECTRODE [5] TABLE 8 Battery capacity
Output after after Battery Initial durability durability Specific
capacity output test test compound Ah W Ah W Exam- hexamethylcyclo-
5.9 744 4.8 474 ple 25 trisiloxane Exam- trimethylsilyl 5.9 755 4.9
484 ple 26 methanesulfonate Exam- phenyldimethyl- 5.9 739 4.8 469
ple 27 fluorosilane Exam- LiPO.sub.2F.sub.2 5.9 763 4.9 493 ple 28
Compar- none 5.9 648 4.4 368 ative Exam- ple 7
[1555] The results given in Positive Electrode [5] Table 2 to
Positive Electrode [5] Table 8 show the following. With respect to
each of the positive electrodes, the incorporation of the specific
compounds in the nonaqueous electrolyte brought about improvements
in output and capacity retentivity. Furthermore, even after the
cycle test, the battery capacity and the output were sufficiently
retained.
Negative Electrode [1] [Production of Negative-Electrode Active
Materials]
(Negative-Electrode Active-Material Production 1)
[1556] A commercial natural-graphite powder as a particulate
carbonaceous substance was repeatedly passed through an ASTM
400-mesh sieve five times in order to prevent inclusion of coarse
particles. The negative-electrode material thus obtained is
referred to as carbonaceous substance (A).
(Negative-Electrode Active-Material Production 2)
[1557] A petroleum-derived heavy oil obtained in the thermal
cracking of naphtha was carbonized at 1,300.degree. C. in an inert
gas. Thereafter, the resultant sinter was classified to thereby
obtain carbonaceous substance (B). In the classification, the
sinter was repeatedly passed through an ASTM 400-mesh sieve five
times in order to prevent inclusion of coarse particles.
(Negative-Electrode Active-Material Production 3)
[1558] A mixture obtained by evenly mixing 95% by mass carbonaceous
substance (A) with 5% by mass carbonaceous substance (B) is
referred to as binary crystalline carbonaceous mixture (C).
(Negative-Electrode Active-Material Production 4)
[1559] Carbonaceous substance (A) was mixed with a
petroleum-derived heavy oil obtained in thermal naphtha cracking,
and this mixture was carbonized at 1,300.degree. C. in an inert
gas. Thereafter, the resultant sinter was classified to thereby
obtain composite carbonaceous substance (D) comprising particles of
the carbonaceous substance (A) and a carbonaceous substance having
different crystallinity adherent to the surface of the particles.
In the classification, the sinter was passed through an ASTM
400-mesh sieve five times in order to prevent inclusion of coarse
particles. It was ascertained from actual carbon ratio that the
negative-electrode active-material powder obtained comprised 95
parts by weight of a graphite and 5 parts by weight of a lowly
crystalline carbonaceous substance with which the graphite was
covered.
(Negative-Electrode Active-Material Production 5)
[1560] The same procedure as in Negative-Electrode Active-Material
Production 4 was conducted, except that the petroleum-derived heavy
oil obtained in thermal naphtha cracking was mixed in a reduced
amount. Thus, composite carbonaceous substance (E) was obtained
which comprised 99 parts by weight of a graphite and 1 part by
weight of a lowly crystalline carbonaceous substance with which the
graphite was covered.
(Negative-Electrode Active-Material Production 6)
[1561] The same procedure as in Negative-Electrode Active-Material
Production 4 was conducted, except that the petroleum-derived heavy
oil obtained in thermal naphtha cracking was mixed in an increased
amount. Thus, composite carbonaceous substance (F) was obtained
which comprised 90 parts by weight of a graphite and 10 parts by
weight of a lowly crystalline carbonaceous substance with which the
graphite was covered.
(Negative-Electrode Active-Material Production 7)
[1562] The same procedure as in Negative-Electrode Active-Material
Production 4 was conducted, except that the petroleum-derived heavy
oil obtained in thermal naphtha cracking was mixed in an increased
amount. Thus, composite carbonaceous substance (G) was obtained
which comprised 70 parts by weight of a graphite and 30 parts by
weight of a lowly crystalline carbonaceous substance with which the
graphite was covered.
(Negative-Electrode Active-Material Production 8)
[1563] The same procedure as in Negative-Electrode Active-Material
Production 4 was conducted, except that graphitization was further
conducted at 3,000.degree. C. in an inert gas. Thus, composite
carbonaceous substance (I) was obtained which comprised 95 parts by
weight of a graphite and 5 parts by weight of a lowly crystalline
carbonaceous substance with which the graphite was covered.
(Negative-Electrode Active-Material Production 9)
[1564] Carbonaceous substance (A) was mixed with a
phenol-formaldehyde solution, and this mixture was carbonized at
1,300.degree. C. in an inert gas. Thereafter, the resultant sinter
was classified to thereby obtain a composite carbonaceous powder
comprising graphite particles and a carbonaceous substance having
different crystallinity adherent to the surface of the particles.
In the classification, the sinter was repeatedly passed through an
ASTM 400-mesh sieve five times in order to prevent inclusion of
coarse particles. This composite powder is referred to as composite
carbonaceous substance (J). It was ascertained from actual carbon
ratio that the negative-electrode active-material powder obtained
comprised 95 parts by weight of a graphite and 5 parts by weight of
a lowly crystalline carbonaceous substance with which the graphite
was covered.
(Negative-Electrode Active-Material Production 10)
[1565] Coal tar pitch having a quinoline insoluble content of 0.05%
by mass or lower was heat-treated at 460.degree. C. for 10 hours in
a reaction furnace. The massive carbonaceous substance obtained was
ground with a mill (Orient Mill, manufactured by Seishin Enterprise
Co., Ltd.) and further pulverized with a pulverizer (Turbo Mill,
manufactured by Matsubo Corp.) to reduce the massive substance into
fine particles having a median diameter of 17 .mu.m. These
particles were packed into a metallic container and heat-treated at
540.degree. C. for 2 hours in a nitrogen gas stream in a box-shaped
electric furnace. The mass obtained was crushed with a crusher
(Roll Jaw Crusher, manufactured by Yoshida Seishakusho Co., Ltd.)
and further pulverized with a pulverizer (Turbo Mill, manufactured
by Matsubo Corp.). The powder obtained was put in a container and
heat-treated at 1,300.degree. C. for 1 hour in a nitrogen
atmosphere in an electric furnace. Thereafter, the resultant sinter
was classified to thereby obtain carbonaceous substance (K). In the
classification, the sinter was repeatedly passed through an ASTM
400-mesh sieve five times in order to prevent inclusion of coarse
particles.
(Negative-Electrode Active-Material Production 11)
[1566] Carbonaceous substance (K) was mixed with a
phenol-formaldehyde solution, and this mixture was carbonized at
1,300.degree. C. in an inert gas. Thereafter, the resultant sinter
was classified to thereby obtain a composite carbonaceous powder
comprising graphite particles and a carbonaceous substance having
different crystallinity adherent to the surface of the particles.
In the classification, the sinter was repeatedly passed through an
ASTM 400-mesh sieve five times in order to prevent inclusion of
coarse particles. This composite powder is referred to as composite
carbonaceous substance (L). It was ascertained from actual carbon
ratio that the negative-electrode active-material powder obtained
comprised 60 parts by weight of a graphite and 40 parts by weight
of a lowly crystalline carbonaceous substance with which the
graphite was covered.
(Negative-Electrode Active-Material Production 12)
[1567] Carbonaceous substance (K) was mixed with a
petroleum-derived heavy oil obtained in thermal naphtha cracking,
and this mixture was carbonized at 1,300.degree. C. in an inert
gas. Thereafter, the resultant sinter was classified to thereby
obtain a composite carbonaceous powder comprising graphite
particles and a carbonaceous substance having different
crystallinity adherent to the surface of the particles. In the
classification, the sinter was passed through an ASTM 400-mesh
sieve five times in order to prevent inclusion of coarse particles.
Thus, a composite carbonaceous substance (M) was obtained. It was
ascertained from actual carbon ratio that the negative-electrode
active-material powder obtained comprised 95 parts by weight of a
graphite and 5 parts by weight of a lowly crystalline carbonaceous
substance with which the graphite was covered.
(Negative-Electrode Active-Material Production 13)
[1568] Coal tar pitch having a quinoline insoluble content of 0.05%
by mass or lower was heat-treated at 460.degree. C. for 10 hours in
a reaction furnace. The massive carbonaceous substance obtained was
ground with a mill (Orient Mill, manufactured by Seishin Enterprise
Co., Ltd.) and further pulverized with a pulverizer (Turbo Mill,
manufactured by Matsubo Corp.) to reduce the massive substance into
fine particles having a median diameter of 17 .mu.m. These
particles were packed into a metallic container and heat-treated at
540.degree. C. for 2 hours in a nitrogen gas stream in a box-shaped
electric furnace. The mass obtained was crushed with a crusher
(Roll Jaw Crusher, manufactured by Yoshida Seishakusho Co., Ltd.)
and further pulverized with a pulverizer (Turbo Mill, manufactured
by Matsubo Corp.). The powder obtained was put in a container and
heat-treated at 1,000.degree. C. for 1 hour in a nitrogen
atmosphere in an electric furnace. Furthermore, the powder
heat-treated was transferred to a graphite crucible and heated with
a direct voltage application type electric furnace at 3,000.degree.
C. over 5 hours in an inert atmosphere to graphitize it. Thus,
carbonaceous substance (N) was obtained. This carbonaceous
substance (N) was mixed with a petroleum-derived heavy oil obtained
in thermal naphtha cracking, and this mixture was carbonized at
900.degree. C. in an inert gas. Thereafter, the resultant sinter
was classified to thereby obtain composite carbonaceous substance
(O) comprising graphite particles and a carbonaceous substance
having different crystallinity adherent to the surface of the
particles. In the classification, the sinter was passed through an
ASTM 400-mesh sieve five times in order to prevent inclusion of
coarse particles. It was ascertained from actual carbon ratio that
the negative-electrode active-material powder obtained comprised 95
parts by weight of a graphite and 5 parts by weight of a lowly
crystalline carbonaceous substance with which the graphite was
covered.
(Negative-Electrode Active-Material Production 14)
[1569] The same procedure as in Negative-Electrode Active-Material
Production 4 was conducted, except that natural flake graphite was
used in place of carbonaceous substance (A). Thus, composite
carbonaceous substance (P) was obtained which comprised 95 parts by
weight of a graphite and 5 parts by weight of a lowly crystalline
carbonaceous substance with which the graphite was covered.
(Negative-Electrode Active-Material Production 15)
[1570] The same procedure as in Negative-Electrode Active-Material
Production 4 was conducted, except that the petroleum-derived heavy
oil obtained in thermal naphtha cracking was mixed in a reduced
amount in Negative-Electrode Active-Material Production 14.
Furthermore, the powder obtained was transferred to a graphite
crucible and heated with a direct voltage application type electric
furnace at 3,000.degree. C. over 5 hours in an inert atmosphere to
graphitize it. Thereafter, the resultant sinter was classified to
thereby obtain composite carbonaceous substance (Q) comprising
particles and a carbonaceous substance having different
crystallinity adherent to the surface of the particles. In the
classification, the sinter was passed through an ASTM 400-mesh
sieve five times in order to prevent inclusion of coarse particles.
It was ascertained from actual carbon ratio that the composite
carbonaceous substance (Q) obtained comprised 99 parts by weight of
a graphite and 1 part by weight of a lowly crystalline carbonaceous
substance with which the graphite was covered.
(Negative-Electrode Active-Material Production 16)
[1571] The same procedure as in Negative-Electrode Active-Material
Production 4 was conducted, except that a natural graphite having a
volume-average particle diameter of 48 .mu.m was used in place of
carbonaceous substance (A). Thus, composite carbonaceous substance
(R) was obtained which comprised 95 parts by weight of a graphite
and 5 parts by weight of a lowly crystalline carbonaceous substance
with which the graphite was covered.
(Negative-Electrode Active-Material Production 17)
[1572] The same procedure as in Negative-Electrode Active-Material
Production 4 was conducted, except that a natural graphite which
had been lowly purified and had a residual ash content of 1% was
used in place of carbonaceous substance (A). Thus, composite
carbonaceous substance (S) was obtained which comprised 95 parts by
weight of a graphite and 5 parts by weight of a lowly crystalline
carbonaceous substance with which the graphite was covered.
[1573] Properties of the negative-electrode active materials
obtained are shown in Negative Electrode [1] Table 1. The methods
used for determining the properties are as described
hereinabove.
TABLE-US-00032 NEGATIVE ELECTRODE [1] TABLE 1 Negative- Specific
Raman electrode Particle Round- surface Raman half-value Ash
Micro-pore Total pore True Round- Aspect active diameter ness area
R value width d.sub.002 content distribution distribution density
ness ratio material .mu.m -- m.sup.2/g -- cm.sup.-1 nm mass % mL/g
mL/g g/cm.sup.3 -- -- Carbonaceous 14.2 0.93 10.5 0.28 26 0.335
0.07 0.125 0.760 2.26 0.93 1.4 substance (A) Carbonaceous 15.0 0.87
12.2 0.90 85 0.345 0.04 0.084 0.490 2.10 0.90 1.4 substance (B)
Binary 14.7 0.91 10.6 0.33 32 0.336 0.05 0.120 0.758 2.25 0.91 1.7
crystalline carbonaceous mixture (C) Composite 15.2 0.93 3.0 0.35
37 0.336 0.07 0.101 0.692 2.25 0.93 1.4 carbonaceous substance (D)
Composite 14.2 0.93 7.2 0.42 32 0.336 0.07 0.120 0.750 2.26 0.93
1.4 carbonaceous substance (E) Composite 15.7 0.93 3.0 0.42 44
0.336 0.06 0.090 0.683 2.24 0.93 1.4 carbonaceous substance (F)
Composite 14.2 0.91 2.5 0.68 65 0.338 0.06 0.086 0.625 2.21 0.91
1.6 carbonaceous substance (G) Composite 15.5 0.93 2.8 0.28 29
0.335 0.03 0.125 0.346 2.26 0.93 1.4 carbonaceous substance (I)
Composite 15.9 0.92 5.4 0.45 46 0.337 0.08 0.132 0.634 2.24 0.92
1.5 carbonaceous substance (J) Carbonaceous 18.0 0.91 7.5 1.00 86
0.345 0.05 0.086 0.489 2.10 0.87 2.3 substance (K) Composite 19.3
0.92 4.2 1.30 90 0.348 0.05 0.145 0.621 2.09 0.90 2.3 carbonaceous
substance (L) Composite 17.8 0.91 5.5 1.04 88 0.345 0.07 0.991
0.593 2.25 0.90 2.2 carbonaceous substance (M) Composite 18.3 0.91
1.7 0.22 22 0.336 0.03 0.058 0.345 2.25 0.92 2.0 carbonaceous
substance (O) Composite 19.4 0.80 3.4 0.25 27 0.336 0.08 0.125
0.693 2.25 0.80 3.1 carbonaceous substance (P) Composite 19.4 0.80
2.5 0.21 20 0.336 0.09 0.121 0.743 2.26 0.80 3.0 carbonaceous
substance (Q) Composite 53.2 0.90 2.2 0.38 40 0.336 0.08 0.124
0.789 2.25 0.92 1.4 carbonaceous substance (R) Composite 15.7 0.93
3.1 0.34 36 0.336 0.50 0.101 0.693 2.25 0.93 1.4 carbonaceous
substance (S)
Negative Electrode [1] [Production of Batteries]
<<Positive-Electrode Production 1>>
[1574] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained. The positive
electrodes thus obtained had an active-material density of 2.35
g/cm.sup.3.
<<Positive-Electrode Production 2>>
[1575] Positive electrodes were produced in the same manner as in
Positive-Electrode Production 1, except that the mass of the active
material to be applied per side was doubled.
<<Positive-Electrode Production 3>>
[1576] Ninety percents by mass
LiNi.sub.0.80Co.sub.0.15Al.sub.0.05O.sub.2 as a positive-electrode
active material was mixed with 5% by mass acetylene black as a
conductive material and 5% by mass poly(vinylidene fluoride) (PVdF)
as a binder in N-methylpyrrolidone solvent to prepare a slurry. The
slurry obtained was applied to each side of an aluminum foil having
a thickness of 15 .mu.m and dried. The coated foil was rolled with
a pressing machine to a thickness of 65 .mu.m. Pieces each having
an active-material layer size with a width of 100 mm and a length
of 100 mm and including an uncoated part having a width of 30 mm
were cut out of the rolled foil. Thus, positive electrodes were
obtained. The positive electrodes thus obtained had an
active-material density of 2.35 g/cm.sup.3.
<<Negative-Electrode Production 1>>
[1577] To 98 parts by weight of a negative-electrode active
material were added 100 parts by weight of an aqueous dispersion of
sodium carboxymethyl cellulose (concentration of sodium
carboxymethyl cellulose, 1% by mass) as a thickener and 2 parts by
weight of an aqueous dispersion of a styrene/butadiene rubber
(concentration of styrene/butadiene rubber, 50% by mass) as a
binder. The ingredients were mixed together by means of a disperser
to obtain a slurry. The slurry obtained was applied to each side of
a rolled copper foil having a thickness of 10 .mu.m and dried. The
coated foil was rolled with a pressing machine to a thickness of 75
.mu.m. Pieces each having an active-material layer size with a
width of 104 mm and a length of 104 mm and including an uncoated
part having a width of 30 mm were cut out of the rolled foil. Thus,
negative electrodes were obtained. The negative electrodes thus
obtained had an active-material density of 1.35 g/cm.sup.3.
<<Negative-Electrode Production 2>>
[1578] Negative electrodes were produced in the same manner as in
Negative-Electrode Production 1, except that the weight of the
active material to be applied per side was doubled.
<<Negative-Electrode Production 3>>
[1579] Negative electrodes were produced in the same manner as in
Negative-Electrode Production 1, except that the active-material
density on one side was 1.70 g/cm.sup.3.
<<Negative-Electrode Production 4>>
[1580] To 95 parts by weight of composite carbonaceous substance A
were added 100 parts by weight of an aqueous dispersion of sodium
carboxymethyl cellulose (concentration of sodium carboxymethyl
cellulose, 1% by mass) as a thickener and 8 parts by weight of an
aqueous dispersion of a styrene/butadiene rubber (concentration of
styrene/butadiene rubber, 50% by mass) as a binder. The ingredients
were mixed together by means of a disperser to obtain a slurry. The
slurry obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained. The negative electrodes thus obtained had an
active-material density of 1.35 g/cm.sup.3.
<<Electrolytic-Solution Production 1>>
[1581] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, lithium difluorophosphate (LiPO.sub.2F.sub.2)
was added thereto so as to result in a concentration of 0.3% by
mass.
<<Electrolytic-Solution Production 2>>
[1582] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, trimethylsilyl methanesulfonate was added
thereto so as to result in a concentration of 0.3% by mass.
<<Electrolytic-Solution Production 3>>
[1583] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, hexamethylcyclotrisiloxane was added thereto
so as to result in a concentration of 0.3% by mass.
<<Electrolytic-Solution Production 4>>
[1584] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4).
<<Battery Production 1>>
[1585] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery had a rated
discharge capacity of about 6 ampere-hours (Ah) and a
direct-current resistance as measured by the 10-kHz
alternating-current method of about 5 milliohms (m.OMEGA.).
<<Battery Production 2>>
[1586] Sixteen positive electrodes and 17 negative electrodes were
alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery had a rated
discharge capacity of about 6 ampere-hours (Ah) and a
direct-current resistance as measured by the 10-kHz
alternating-current method of about 7 milliohms (m.OMEGA.).
Negative Electrode [1]
Example 1
[1587] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Electrode Production 1>> in
which binary crystalline carbonaceous mixture (C) was used as the
negative-electrode active material and further using the positive
electrodes produced in <<Positive-Electrode Production
1>> and the electrolyte produced in
<<Electrolytic-Solution Production 1>>. This battery
was examined by the methods which will be described under
<<Battery Evaluation>> given later and by the
measurement methods described above.
Negative Electrode [1]
Example 2
[1588] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (D) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 3
[1589] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (E) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 4
[1590] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (F) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 5
[1591] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (G) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 6
[1592] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (J) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 7
[1593] A battery was produced in the same manner as in Example 1,
except that composite carbonaceous substance (I) was used as the
negative-electrode active material in <<Negative-Electrode
Production 1>>. The battery evaluation described under
<<Battery Evaluation>> was conducted.
Negative Electrode [1]
Example 8
[1594] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (M) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 9
[1595] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (R) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 10
[1596] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (L) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 11
[1597] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (S) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 12
[1598] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (O) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 13
[1599] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that composite carbonaceous
substance (P) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 14
[1600] A battery was produced in the same manner as in Example 1,
except that composite carbonaceous substance (Q) was used as the
negative-electrode active material in <<Negative-Electrode
Production 1>>. The battery evaluation described under
<<Battery Evaluation>> was conducted.
Negative Electrode [1]
Example 15
[1601] A battery was produced by the method described under
<<Battery Production 2>> using the negative electrodes
produced in <<Negative-Electrode Production 2>> in
which composite carbonaceous substance (D) was used as the
negative-electrode active material and further using the positive
electrodes produced in <<Positive-Electrode Production
2>> and the electrolyte produced in
<<Electrolytic-Solution Production 1>>. Except these,
battery evaluation was conducted in the same manner as in Negative
Electrode [1] Example 1.
Negative Electrode [1]
Example 16
[1602] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Electrode Production 3>> in
which composite carbonaceous substance (D) was used as the
negative-electrode active material and further using the positive
electrodes produced in <<Positive-Electrode Production
1>> and the electrolyte produced in
<<Electrolytic-Solution Production 1>>. Except these,
battery evaluation was conducted in the same manner as in Negative
Electrode [1] Example 1.
Negative Electrode [1]
Example 17
[1603] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Flectrode Production 4>> in
which composite carbonaceous substance (D) was used as the
negative-electrode active material and further using the positive
electrodes produced in <<Positive-Electrode Production
1>> and the electrolyte produced in
<<Electrolytic-Solution Production 1>>. Except these,
battery evaluation was conducted in the same manner as in Negative
Electrode [1] Example 1.
[1604] The results of evaluation in Negative Electrode [1] Example
1 to Negative Electrode [1] Example 17 are shown in Negative
Electrode [1] Table 2.
Negative Electrode [1]
Examples 18 to 34
[1605] Battery evaluation was conducted in the same manner as in
Negative Example [1] Examples 1 to 17, except that the electrolyte
used in these Examples was replaced by the electrolyte produced in
<<Electrolytic-Solution Production 2>>. The results of
evaluation in Negative Electrode [1] Example 18 to Negative
Electrode [1] Example 34 are shown in Negative Electrode [1] Table
3.
Negative Electrode [1]
Examples 35 to 51
[1606] Battery evaluation was conducted in the same manner as in
Negative Example [1] Examples 1 to 17, except that the electrolyte
used in these Examples was replaced by the electrolyte produced in
<<Electrolytic-Solution Production 3>>. The results of
evaluation in Negative Electrode [1] Example 35 to Negative
Electrode [1] Example 51 are shown in Negative Electrode [1] Table
4.
Negative Electrode [1]
Comparative Example 1
[1607] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that carbonaceous substance (A) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Comparative Example 2
[1608] A battery was produced in the same manner as in Negative
Electrode [1] Comparative Example 1, except that the electrolyte
used in the Comparative Example was replaced by the electrolyte
produced in <<Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [1]
Comparative Example 3
[1609] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that carbonaceous substance (B) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Comparative Example 4
[1610] A battery was produced in the same manner as in Negative
Electrode [1] Comparative Example 3, except that the electrolyte
used in the Negative Electrode [1] Comparative Example 3 was
replaced by the electrolyte produced in
<<Electrolytic-Solution Production 4>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Comparative Example 5
[1611] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that the electrolyte used in the
Negative Electrode [1] Example 1 was replaced by the electrolyte
produced in <<Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [1]
Comparative Example 6
[1612] A battery was produced in the same manner as in Negative
Electrode [1] Example 2, except that the electrolyte used in the
Example was replaced by the electrolyte produced in
<<Electrolytic-Solution Production 4>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Comparative Example 7
[1613] A battery was produced in the same manner as in Negative
Electrode [1] Example 1, except that carbonaceous substance (K) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Comparative Example 8
[1614] A battery was produced in the same manner as in Negative
Electrode [1] Comparative Example 7, except that the electrolyte
used in the Comparative Example was replaced by the electrolyte
produced in <<Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [1]
Comparative Examples 9 to 11
[1615] Battery evaluation was conducted in the same manner as in
Negative Electrode [1] Comparative Examples 1, 3, and 7, except
that the electrolyte used in these Comparative Examples was
replaced by the electrolyte produced in
<<Electrolytic-Solution Production 2>>.
Negative Electrode [1]
Comparative Examples 12 to 14
[1616] Battery evaluation was conducted in the same manner as in
Negative Electrode [1] Comparative Examples 1, 3, and 7, except
that the electrolyte used in these Comparative Examples was
replaced by the electrolyte produced in
<<Electrolytic-Solution Production 3>>.
[1617] The results of evaluation in Negative Electrode [1]
Comparative Example 1 to Negative Electrode [1] Comparative Example
14 are shown in Negative Example [1] Table 5.
Negative Electrode [1]
Example 52
[1618] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Electrode Production 1>> in
which composite carbonaceous substance (D) was used as the
negative-electrode active material and further using the positive
electrodes produced in <<Positive-Electrode Production
3>> and the electrolyte produced in
<<Electrolytic-Solution Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [1]
Example 53
[1619] A battery was produced in the same manner as in Example 52,
except that the electrolyte used in Example 52 was replaced by the
electrolyte produced in <<Electrolytic-Solution Production
2>>. The battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [1]
Example 54
[1620] A battery was produced in the same manner as in Example 52,
except that the electrolyte used in Example 52 was replaced by the
electrolyte produced in <<Electrolytic-Solution Production
3>>. The battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [1]
Comparative Example 15
[1621] A battery was produced in the same manner as in Example 52,
except that the electrolyte used in Example 52 was replaced by the
electrolyte produced in <<Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted.
[1622] The results of evaluation in Examples 52 to 54 and
Comparative Example 15 are shown in Negative Example [1] Table
6.
Negative Electrode [1]
Battery Evaluation
(Capacity Measurement)
[1623] The battery which had undergone no charge/discharge cycling
was subjected to 5 cycles of initial charge/discharge at 25.degree.
C. in a voltage range of 4.2-3.0 V at a current of 0.2 C (the value
of current at which the rated capacity based on 1-hour-rate
discharge capacity is discharged over 1 hour is taken as 1 C; the
same applies hereinafter). The 0.2-C discharge capacity thereof as
measured in the fifth cycle in this operation was taken as initial
capacity. An output measurement shown below was then made.
(Output Measurement)
[1624] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W).
(Cycle Test)
[1625] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.2 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment. The 0.2-C
discharge capacity thereof as measured in the third cycle was taken
as capacity after cycling. Using the following calculation formula,
the capacity retention through cycling was determined from the
initial capacity measured prior to the cycling and the capacity
after cycling measured after completion of the cycle test.
Capacity retention through cycling (%)=100.times.(capacity after
cycling)/(initial capacity)
[1626] The battery which had undergone the cycle test was subjected
to the output measurement described under (Output Measurement).
From the output measured before the cycle test and from the output
thus determined after completion of the cycle test, the output
retention represented by the following formula was calculated.
Output retention (%)=100.times.(output after cycle test)/(output
before cycling)
[1627] Impedance Rct and double-layer capacity Cdl in Negative
Electrode [1] Table 2 are parameters contributing to output. The
smaller the value of impedance Rct and the larger the value of
double-layer capacity Cdl, the more the output tends to improve.
Incidentally, "Impedance Rct" and "Double-layer capacity Cdl" were
determined by the methods described hereinabove under
"Impedance".
TABLE-US-00033 NEGATIVE ELECTRODE [1] TABLE 2 Capacity Negative-
Direct retention Output electrode Impedance Impedance Initial
current through Initial after Output active Rct Cdl capacity
resistance cycling output cycling retention No. material .OMEGA.
.times.10.sup.-4 F mAh m.OMEGA. % W W % Ex. 1 Binary 22 1.1 6001
5.2 80 574 420 73 crystalline carbonaceous mixture (C) Ex. 2
Composite 25 0.9 6010 4.9 84 595 450 76 carbonaceous substance (D)
Ex. 3 Composite 11 1.9 6005 5.2 82 602 432 72 carbonaceous
substance (E) Ex. 4 Composite 28 1.2 6000 5.2 83 581 420 72
carbonaceous substance (F) Ex. 5 Composite 30 0.8 5890 5.6 81 567
396 70 carbonaceous substance (G) Ex. 6 Composite 24 1.2 5987 5.1
82 588 414 70 carbonaceous substance (J) Ex. 7 Composite 35 1.0
6010 5.3 84 574 414 72 carbonaceous substance (I) Ex. 8 Composite
75 0.7 5609 5.4 79 560 390 70 carbonaceous substance (M) Ex. 9
Composite 25 0.9 6007 4.9 78 574 408 71 carbonaceous substance (R)
Ex. 10 Composite 60 0.7 5045 5.5 76 532 342 64 carbonaceous
substance (L) Ex. 11 Composite 22 1.0 6002 5.8 81 574 402 70
carbonaceous substance (S) Ex. 12 Composite 150 0.1 5928 5.4 80 539
366 68 carbonaceous substance (O) Ex. 13 Composite 42 0.7 5920 5.4
82 539 366 68 carbonaceous substance (P) Ex. 14 Composite 45 0.9
5902 5.2 79 553 372 67 carbonaceous substance (Q) Ex. 15 Composite
14 0.8 5989 5.1 76 560 366 65 carbonaceous substance (D) Ex. 16
Composite 15 1.0 5989 4.8 79 588 396 67 carbonaceous substance (D)
Ex. 17 Composite 50 0.4 5982 5.5 78 602 390 65 carbonaceous
substance (D)
[1628] In Negative Electrode [1] Table 2, the electrolyte is one
produced in <<Electrolytic-Solution Production 1>> and
containing 0.3% by mass lithium difluorophosphate
(LiPO.sub.2F.sub.2).
TABLE-US-00034 NEGATIVE ELECTRODE [1] TABLE 3 Capacity Negative-
Direct retention Output electrode Impedance Impedance Initial
current through Initial after Output active Rct Cdl capacity
resistance cycling output cycling retention No. material .OMEGA.
.times.10.sup.-4 F mAh m.OMEGA. % W W % Ex. 18 Binary 23 1.1 5821
5.3 78 557 400 72 crystalline carbonaceous mixture (C) Ex. 19
Composite 27 0.9 5830 5.6 82 577 426 74 carbonaceous substance (D)
Ex. 20 Composite 12 1.8 5825 4.9 80 584 415 71 carbonaceous
substance (E) Ex. 21 Composite 30 1.2 5820 4.6 81 564 391 69
carbonaceous substance (F) Ex. 22 Composite 32 0.7 5713 5.5 79 550
375 68 carbonaceous substance (G) Ex. 23 Composite 25 1.2 5807 5.2
80 570 396 69 carbonaceous substance (J) Ex. 24 Composite 37 0.9
5830 5.3 82 557 394 71 carbonaceous substance (I) Ex. 25 Composite
80 0.7 5441 5.1 77 543 368 68 carbonaceous substance (M) Ex. 26
Composite 27 0.9 5827 5.3 76 557 389 70 carbonaceous substance (R)
Ex. 27 Composite 64 0.7 4894 4.8 74 516 318 62 carbonaceous
substance (L) Ex. 28 Composite 23 0.9 5822 4.7 79 557 302 60
carbonaceous substance (S) Ex. 29 Composite 159 0.1 5750 4.8 78 523
348 67 carbonaceous substance (O) Ex. 30 Composite 45 0.6 5742 5.2
80 523 336 64 carbonaceous substance (P) Ex. 31 Composite 48 0.8
5725 5.2 77 536 345 64 carbonaceous substance (Q) Ex. 32 Composite
15 0.8 5809 5.0 74 543 335 62 carbonaceous substance (D) Ex. 33
Composite 16 1.0 5809 4.7 77 570 372 65 carbonaceous substance (D)
Ex. 34 Composite 53 0.4 5803 4.9 76 584 365 63 carbonaceous
substance (D)
[1629] In Negative Electrode [1] Table 3, the electrolyte is one
produced in <<Electrolytic-Solution Production 2>> and
containing 0.3% by mass trimethylsilyl methanesulfonate.
TABLE-US-00035 NEGATIVE ELECTRODE [1] TABLE 4 Capacity Negative-
Direct retention Output electrode Impedance Impedance Initial
current through Initial after Output active Rct Cdl capacity
resistance cycling output cycling retention No. material .OMEGA.
.times.10.sup.-4 F mAh m.OMEGA. % W W % Ex. 35 Binary 26 1.3 6061
4.8 78 557 391 70 crystalline carbonaceous mixture (C) Ex. 36
Composite 30 1.0 6070 5.2 81 577 419 73 carbonaceous substance (D)
Ex. 37 Composite 13 2.0 6065 5.1 80 584 402 69 carbonaceous
substance (E) Ex. 38 Composite 34 1.3 6060 5.0 81 564 391 69
carbonaceous substance (F) Ex. 39 Composite 36 0.8 5949 5.2 79 550
369 67 carbonaceous substance (G) Ex. 40 Composite 29 1.3 6047 4.9
80 570 386 68 carbonaceous substance (J) Ex. 41 Composite 42 1.1
6070 4.7 81 557 386 69 carbonaceous substance (I) Ex. 42 Composite
90 0.8 5665 5.1 77 543 363 67 carbonaceous substance (M) Ex. 43
Composite 30 1.0 6067 5.0 76 557 377 68 carbonaceous substance (R)
Ex. 44 Composite 72 0.8 5095 4.8 74 516 314 61 carbonaceous
substance (L) Ex. 45 Composite 26 1.1 6062 5.3 79 557 358 64
carbonaceous substance (S) Ex. 46 Composite 180 0.1 5987 5.1 78 523
341 65 carbonaceous substance (O) Ex. 47 Composite 50 0.7 5979 4.9
80 523 341 65 carbonaceous substance (P) Ex. 48 Composite 54 1.0
5961 5.1 77 536 354 66 carbonaceous substance (Q) Ex. 49 Composite
17 0.9 6049 5.0 74 543 345 63 carbonaceous substance (D) Ex. 50
Composite 18 1.1 6049 4.6 77 570 352 62 carbonaceous substance (D)
Ex. 51 Composite 60 0.4 6042 5.4 76 584 350 60 carbonaceous
substance (D)
[1630] In Negative Electrode [1] Table 4, the electrolyte is one
produced in <<Electrolytic-Solution Production 3>> and
containing 0.3% by mass hexamethylcyclotrisiloxane.
TABLE-US-00036 Negative Electrode [1] Table 5 Capacity Negative-
Direct retention Output electrode Impedance Impedance Initial
current through Initial after Output active Specific Rct Cdl
capacity resistance cycling output cycling retention No. material
compound .OMEGA. .times.10.sup.-4 F mAh m.OMEGA. % W W % Comp.
Carbonaceous lithium 9 1.4 6006 4.7 75 525 342 65 Ex. 1 substance
(A) difluorophosphate Comp. Carbonaceous -- 9 1.4 6003 4.9 72 504
312 62 Ex. 2 substance (A) Comp. Carbonaceous lithium 50 0.5 5102
5.1 75 483 318 66 Ex. 3 substance (B) difluorophosphate Comp.
Carbonaceous -- 50 0.5 5034 5.2 73 455 288 63 Ex. 4 substance (B)
Comp. Binary -- 22 1.1 5989 5.4 75 511 330 65 Ex. 5 crystalline
carbonaceous mixture (C) Comp. Composite -- 9 1.4 5996 4.9 78 518
330 64 Ex. 6 carbonaceous substance (D) Comp. Carbonaceous lithium
60 0.5 5042 4.9 75 511 282 55 Ex. 7 substance (K) difluorophosphate
Comp. Carbonaceous -- 60 0.5 5111 5.0 73 497 276 56 Ex. 8 substance
(K) Comp. Carbonaceous trimethylsilyl 10 1.4 5886 4.6 74 515 331 64
Ex. 9 substance (A) methanesulfonate Comp. Carbonaceous
trimethylsilyl 53 0.5 4898 4.9 74 473 312 66 Ex. 10 substance (B)
methanesulfonate Comp. Carbonaceous trimethylsilyl 64 0.5 4992 4.8
74 501 266 53 Ex. 11 substance (K) methanesulfonate Comp.
Carbonaceous hexamethylcyclo- 11 1.5 6186 4.9 72 504 315 63 Ex. 12
substance (A) trisiloxane Comp. Carbonaceous hexamethylcyclo- 59
0.5 5204 5.3 72 464 298 64 Ex. 13 substance (B) trisiloxane Comp.
Carbonaceous hexamethylcyclo- 74 0.6 5395 5.4 72 491 244 50 Ex. 14
substance (K) trisiloxane Negative Electrode [1] Table 6 Capacity
Direct retention Output Additive for Initial current through
Initial after Output Electrolytic electrolyte capacity resistance
cycling % output cycling retention No. solution solution mAh
m.OMEGA. % W W % Ex. 52 Electrolytic- lithium 5780 4.6 88 634 465
73 Solution difluorophosphate Production 1 Ex. 53 Electrolytic-
trimethylsilyl 5830 5.5 88 632 457 72 Solution methanesulfonate
Production 2 Ex. 54 Electrolytic- hexamethylcyclo- 5760 4.9 88 619
452 73 Solution trisiloxane Production 3 Comp. Electrolytic- none
5810 4.7 82 543 364 67 Ex. 15 Solution Production 4
[1631] The results given in Negative Electrode [1] Tables 2 to 6
show the following. The output retention through cycling was
greatly improved by the combination of an electrolyte containing
lithium difluorophosphate, trimethylsilyl methanesulfonate, or
hexamethylcyclotrisiloxane and a negative-electrode active material
comprising two or more carbonaceous substances differing in
crystallinity.
Negative Electrode [2]
Production of Negative-Electrode Active Materials
(Negative-Electrode Active-Material Production 1)
[1632] Coal tar pitch having a quinoline insoluble content of 0.05%
by mass or lower was heat-treated at 460.degree. C. for 10 hours in
a reaction furnace. The massive carbonaceous substance obtained was
ground with a mill (Orient Mill, manufactured by Seishin Enterprise
Co., Ltd.) and further pulverized with a pulverizer (Turbo Mill,
manufactured by Matsubo Corp.) to reduce the massive substance into
fine particles having a median diameter of 18 .mu.m. These
particles were packed into a metallic container and heat-treated at
540.degree. C. for 2 hours in a nitrogen gas stream in a box-shaped
electric furnace. The mass obtained was crushed with a crusher
(Roll Jaw Crusher, manufactured by Yoshida Seishakusho Co., Ltd.)
and further pulverized with a pulverizer (Turbo Mill, manufactured
by Matsubo Corp.). The powder obtained was put in a container and
heat-treated at 1,000.degree. C. for 1 hour in a nitrogen
atmosphere in an electric furnace. Thereafter, the resultant sinter
was classified to thereby obtain amorphous carbonaceous substance
(A). In the classification, the sinter was repeatedly passed
through an ASTM 400-mesh sieve five times in order to prevent
inclusion of coarse particles.
(Negative-Electrode Active-Material Production 2)
[1633] Coal tar pitch having a quinoline insoluble content of 0.05%
by mass or lower was heat-treated at 460.degree. C. for 10 hours in
a reaction furnace. The massive carbonaceous substance obtained was
packed into a metallic container and heat-treated at 1,000.degree.
C. for 2 hours in a nitrogen gas stream in a box-shaped electric
furnace. The amorphous mass obtained was crushed with a crusher
(Roll Jaw Crusher, manufactured by Yoshida Seisakusho Co., Ltd.)
and further pulverized with a pulverizer (Turbo Mill, manufactured
by Matsubo Corp.) to obtain an amorphous powder. The powder
obtained was repeatedly passed through an ASTM 400-mesh sieve five
times in order to prevent inclusion of coarse particles. The
negative-electrode active material thus obtained is referred to as
amorphous carbonaceous substance (B).
(Negative-Electric Active-Material Production 3)
[1634] The amorphous mass obtained in (Negative-Electrode
Active-Material Production 2) was transferred to a graphite
crucible and heat-treated with a direct voltage application type
electric furnace at 2,200.degree. C. over 5 hours in an inert
atmosphere. The mass obtained was crushed with a crusher (Roll Jaw
Crusher, manufactured by Yoshida Seisakusho Co., Ltd.) and further
pulverized with a pulverizer (Turbo Mill, manufactured by Matsubo
Corp.). The powder obtained was repeatedly passed through an ASTM
400-mesh sieve five times in order to prevent inclusion of coarse
particles. The negative-electrode active material thus obtained is
referred to as amorphous carbonaceous substance (C).
(Negative-Electric Active-Material Production 4)
[1635] The amorphous mass obtained in (Negative-Electrode
Active-Material Production 2) was transferred to a graphite
crucible and heated with a direct voltage application type electric
furnace at 3,000.degree. C. over 5 hours in an inert atmosphere to
graphitize it. The mass obtained was crushed with a crusher (Roll
Jaw Crusher, manufactured by Yoshida Seisakusho Co., Ltd.) and
further pulverized with a pulverizer (Turbo Mill, manufactured by
Matsubo Corp.). The powder obtained was repeatedly passed through
an ASTM 400-mesh sieve five times in order to prevent inclusion of
coarse particles. The negative-electrode active material thus
obtained is referred to as graphitic carbonaceous substance
(D).
(Negative-Electrode Active-Material Production 5)
[1636] A commercial powder of natural flake graphite was repeatedly
passed through an ASTM 400-mesh sieve five times in order to
prevent inclusion of coarse particles. The negative-electrode
active material thus obtained is referred to as graphitic
carbonaceous substance (E).
[1637] The negative-electrode active materials obtained in
Negative-Electrode Active-Material Productions 1 to 5 were examined
for properties, shape, etc. by the methods described above. The
results obtained are shown in Negative Electrode [2] Table 1.
TABLE-US-00037 NEGATIVE ELECTRODE [2] TABLE 1 BET Volume- Negative-
Raman specific average electrode Raman half-value True surface Tap
Ash particle Production active d002 Lc R value width density area
density content diameter No. material nm nm -- cm.sup.-1 g/cm.sup.3
m.sup.2/g g/cm.sup.3 % .mu.m 1 Amorphous 0.344 2.4 0.98 83.0 1.93
1.5 1.1 0.05 12.4 carbonaceous substance (A) 2 Amorphous 0.343 2.2
1.06 86.0 1.97 5.4 0.9 0.05 14.6 carbonaceous substance (B) 3
Amorphous 0.340 32.9 0.21 27.8 2.21 7.1 0.3 0.03 14.7 carbonaceous
substance (C) 4 Graphitic 0.336 100< 0.11 25.9 2.26 8.6 0.4 0.02
15.3 carbonaceous substance (D) 5 Graphitic 0.335 100< 0.16 25.8
2.26 10.4 0.7 0.04 11.6 carbonaceous substance (E)
Negative Electrode [2]
Production of Batteries
<<Positive-Electrode Production 1>>
[1638] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained. The positive
electrodes thus obtained had an active-material density of 2.35
g/cm.sup.3.
<<Negative-Electrode Production 1>>
[1639] To 98 parts by weight of a negative-electrode active
material were added 100 parts by weight of an aqueous dispersion of
sodium carboxymethyl cellulose (concentration of sodium
carboxymethyl cellulose, 1% by mass) as a thickener and 2 parts by
weight of an aqueous dispersion of a styrene/butadiene rubber
(concentration of styrene/butadiene rubber, 50% by mass) as a
binder. The ingredients were mixed together by means of a disperser
to obtain a slurry. The slurry obtained was applied to each side of
a rolled copper foil having a thickness of 10 .mu.m and dried. The
coated foil was rolled with a pressing machine to a thickness of 75
.mu.m. Pieces each having an active-material layer size with a
width of 104 mm and a length of 104 mm and including an uncoated
part having a width of 30 mm were cut out of the rolled foil. Thus,
negative electrodes were obtained. The negative electrodes thus
obtained had an active-material density of 1.35 g/cm.sup.3.
<<Nonaqueous-Electrolytic-Solution Production 1>>
[1640] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, lithium difluorophosphate (LiPO.sub.2F.sub.2)
was added thereto so as to result in a concentration of 0.3% by
mass.
<<Nonaqueous-Electrolytic-Solution Production 2>>
[1641] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, trimethylsilyl methanesulfonate was added
thereto so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 3>>
[1642] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, hexamethylcyclotrisiloxane was added thereto
so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 4>>
[1643] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4).
<<Battery Production 1>>
[1644] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery had a rated
discharge capacity of about 6 ampere-hours (Ah) and a
direct-current resistance component as measured by the 10-kHz
alternating-current method of about 5 milliohms (m.OMEGA.). The
ratio of the total electrode area of the positive electrodes to the
overall surface area of the battery case was 20.6.
Negative Electrode [2]
Example 1
[1645] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Electrode Production 1>> in
which amorphous carbonaceous substance (A) was used as the
negative-electrode active material and further using the positive
electrodes produced in <<Positive-Electrode Production
1>> and the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 1>>. This
battery was subjected to the battery evaluation described under
<<Battery Evaluation>> given later. The results are
shown in Negative Electrode [2] Table 2.
Negative Electrode [2]
Example 2
[1646] A battery was produced in the same manner as in Negative
Electrode [2] Example 1, except that amorphous carbonaceous
substance (B) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [2] Table
2.
Negative Electrode [2]
Example 3
[1647] A battery was produced in the same manner as in Negative
Electrode [2] Example 1, except that amorphous carbonaceous
substance (C) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [2] Table
2.
Negative Electrode [2]
Examples 4 to 6
[1648] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [2] Examples 1 to 2, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrode [2] Table 2.
Negative Electrode [2]
Examples 7 to 9
[1649] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [2] Examples 1 to 3, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrode [2] Table 2.
Negative Electrode [2]
Comparative Examples 1 to 3
[1650] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [2] Examples 1 to 3, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
results are shown in Negative Electrode [2] Table 2.
Negative Electrode [2]
Comparative Example 4
[1651] A battery was produced in the same manner as in Negative
Electrode [2] Example 1, except that graphitic carbonaceous
substance (D) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [2]
[1652] Table 2.
Negative Electrode [2]
Comparative Example 5
[1653] A battery was produced in the same manner as in Negative
Electrode [2] Example 1, except that graphitic carbonaceous
substance (E) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [2] Table
2.
Negative Electrode [2]
Comparative Examples 6 and 7
[1654] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [2] Comparative Examples 4
and 5, except that the nonaqueous electrolyte used in these
Comparative Examples was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
2>>. The results are shown in Negative Electrode [2] Table
2.
Negative Electrode [2]
Comparative Examples 8 and 9
[1655] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [2] Comparative Examples 4
and 5, except that the nonaqueous electrolyte used in these
Comparative Examples was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
3>>. The results are shown in Negative Electrode [2] Table
2.
Negative Electrode [2]
Comparative Examples 10 and 11
[1656] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [2] Comparative Examples 4
and 5, except that the nonaqueous electrolyte used in these
Comparative Examples was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The results are shown in Negative Electrode [2] Table
2.
Negative Electrode [2]
Battery Evaluation
(Capacity Measurement)
[1657] The fresh battery which had undergone no charge/discharge
was subjected to 5 cycles of initial charge/discharge at 25.degree.
C. in a voltage range of 4.1-3.0 V at 25.degree. C. (voltage range,
4.1-3.0 V). The 0.2-C (the value of current at which the rated
capacity based on 1-hour-rate discharge capacity is discharged over
1 hour is taken as 1 C; the same applies hereinafter) discharge
capacity thereof as measured in the fifth cycle in this operation
was taken as initial capacity.
(Test for Determining Short-Time High-Current-Density
Charge/Discharge Characteristics)
[1658] The battery which had undergone the capacity measurement was
charged at a constant current of 0.2 C for 150 minutes in a
25.degree. C. room-temperature atmosphere. A high-load current of
10 C was caused to flow in the charge direction and the discharge
direction each for about 10 seconds, with the voltage in that 0.2-C
charge as the center. This charge/discharge operation as one cycle
which took 35 seconds including a pause period was continuously
repeated. At the time when the battery had undergone 100,000
cycles, it was taken out. This battery was discharged to 3 V at a
current of 0.2 C and subjected to 1 cycle in the same manner as in
(Capacity Measurement). The capacity as measured in this cycle was
taken as capacity after cycling. The short-time
high-current-density charge/discharge characteristics were
calculated using the following formula.
[Short-time high-current-density charge/discharge
characteristics]=100.times.[capacity after cycling]/[initial
capacity]
TABLE-US-00038 NEGATIVE ELECTRODE [2] TABLE 2 Short-time
high-current-density Initial charge/discharge Negative-electrode
capacity characteristics No. active material Specific compound mAh
% Ex. 1 Amorphous carbonaceous lithium 6020 93 substance (A)
difluorophosphate Ex. 2 Amorphous carbonaceous lithium 6035 94
substance (B) difluorophosphate Ex. 3 Amorphous carbonaceous
lithium 6001 90 substance (C) difluorophosphate Ex. 4 Amorphous
carbonaceous trimethylsilyl 6021 91 substance (A) methanesulfonate
Ex. 5 Amorphous carbonaceous trimethylsilyl 6023 92 substance (B)
methanesulfonate Ex. 6 Amorphous carbonaceous trimethylsilyl 6015
88 substance (C) methanesulfonate Ex. 7 Amorphous carbonaceous
hexamethylcyclo- 6004 91 substance (A) trisiloxane Ex. 8 Amorphous
carbonaceous hexamethylcyclo- 5997 90 substance (B) trisiloxane Ex.
9 Amorphous carbonaceous hexamethylcyclo- 5987 92 substance (C)
trisiloxane Comp. Amorphous carbonaceous -- 6012 88 Ex. 1 substance
(A) Comp. Amorphous carbonaceous -- 6029 89 Ex. 2 substance (B)
Comp. Amorphous carbonaceous -- 6009 85 Ex. 3 substance (C) Comp.
Graphitic carbonaceous lithium 5998 87 Ex. 4 substance (D)
difluorophosphate Comp. Graphitic carbonaceous lithium 6012 78 Ex.
5 substance (E) difluorophosphate Comp. Graphitic carbonaceous
trimethylsilyl 6001 86 Ex. 6 substance (D) methanesulfonate Comp.
Graphitic carbonaceous trimethylsilyl 6003 79 Ex. 7 substance (E)
methanesulfonate Comp. Graphitic carbonaceous hexamethylcyclo- 6032
87 Ex. 8 substance (D) trisiloxane Comp. Graphitic carbonaceous
hexamethylcyclo- 6002 78 Ex. 9 substance (E) trisiloxane Comp.
Graphitic carbonaceous -- 6004 85 Ex. 10 substance (D) Comp.
Graphitic carbonaceous -- 5990 77 Ex. 11 substance (E)
[1659] The results given in Negative Electrode [2] Table 2 show the
following. The short-time high-current-density charge/discharge
characteristics were greatly improved by the use of an electrolyte
containing lithium difluorophosphate, trimethylsilyl
methanesulfonate, or hexamethylcyclotrisiloxane and of an amorphous
carbonaceous substance contained as a negative-electrode active
material and having an interplanar spacing (d002) for the (002)
planes and an Lc, both determined by wide-angle X-ray
diffractometry, of 0.337 nm or larger and 80 nm or smaller,
respectively, and a Raman R value of 0.2 or higher.
Negative Electrode [3]
Production of Negative-Electrode Active Materials
(Negative-Electrode Active-Material Production 1)
[1660] Commercial Li.sub.1.33Ti.sub.1.66O.sub.4 having a
volume-average particle diameter of 23 .mu.m was repeatedly passed
through an ASTM 400-mesh sieve five times in order to prevent
inclusion of coarse particles. Thus, lithium-titanium composite
oxide (A) was obtained.
(Negative-Electrode Active-Material Production 2)
[1661] Commercial Li.sub.1.33Ti.sub.1.66O.sub.4 having a
volume-average particle diameter of 1.0 .mu.m was repeatedly passed
through an ASTM 400-mesh sieve five times in order to prevent
inclusion of coarse particles. Thus, lithium-titanium composite
oxide (B) was obtained.
(Negative-Electrode Active-Material Production 3)
[1662] Commercial Li.sub.1.33Ti.sub.1.66O.sub.4 having a
volume-average particle diameter of 0.1 .mu.m was repeatedly passed
through an ASTM 400-mesh sieve five times in order to prevent
inclusion of coarse particles. Thus, lithium-titanium composite
oxide (C) was obtained.
(Negative-Electrode Active-Material Production 4)
[1663] A commercial powder of natural flake graphite was repeatedly
passed through an ASTM 400-mesh sieve five times in order to
prevent inclusion of coarse particles. The negative-electrode
active material thus obtained is referred to as graphitic
carbonaceous substance (D).
[1664] The composition, structure, shape, properties, etc. of each
of the negative-electrode active materials are summarized in
Negative Electrode [3] Table 1.
TABLE-US-00039 NEGATIVE ELECTRODE [3] TABLE 1 Volume-average
particle BET specific Production Negative-electrode diameter
surface area Tap density Aspect No. active material Composition
Structure .mu.m m.sup.2/g g/cm.sup.3 Roundness ratio 1
Lithium-titanium Li.sub.1.33Ti.sub.1.66O.sub.4 spinel 23 3.2 1.5
0.99 1.0 composite oxide (A) structure 2 Lithium-titanium
Li.sub.1.33Ti.sub.1.66O.sub.4 spinel 1.0 10.5 1.2 0.99 1.0
composite oxide (B) structure 3 Lithium-titanium
Li.sub.1.33Ti.sub.1.66O.sub.4 spinel 0.1 38.9 0.5 0.97 1.0
composite oxide (C) structure 4 Graphitic C flake 11.6 10.4 0.7
0.90 1.8 carbonaceous graphite substance (D)
Negative Electrode [3]
Production of Batteries
<<Positive-Electrode Production 1>>
[1665] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to one
side of an aluminum foil having a thickness of 15 .mu.m and dried.
The coated foil was rolled with a pressing machine to a thickness
of 80 .mu.m. Pieces each having an active-material layer size with
a width of 100 mm and a length of 100 mm and including an uncoated
part having a width of 30 mm were cut out of the rolled foil. Thus,
positive electrodes were obtained. The positive electrodes thus
obtained had an active-material density of 2.35 g/cm.sup.3.
<<Negative-Electrode Production 1>>
[1666] Ninety parts by weight of a negative-electrode active
material was mixed with 5% by mass acetylene black as a conductive
material and 5% by mass poly(vinylidene fluoride) (PVdF) as a
binder in N-methylpyrrolidone solvent to prepare a slurry. The
slurry obtained was applied to one side of a rolled copper foil
having a thickness of 10 .mu.m and dried. The coated foil was
rolled with a pressing machine to a thickness of 90 .mu.m. Pieces
each having an active-material layer size with a width of 104 mm
and a length of 104 mm and including an uncoated part having a
width of 30 mm were cut out of the rolled foil. Thus, negative
electrodes were obtained.
<<Nonaqueous-Electrolytic-Solution Production 1>>
[1667] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, lithium difluorophosphate (LiPO.sub.2F.sub.2)
was added thereto so as to result in a concentration of 0.3% by
mass.
<<Nonaqueous-Electrolytic-Solution Production 2>>
[1668] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, trimethylsilyl methanesulfonate was added
thereto so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 3>>
[1669] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, hexamethylcyclotrisiloxane was added thereto
so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 4>>
[1670] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4).
(Battery Production 1)
[1671] A negative electrode and a positive electrode each having a
size of 12.5 mm.phi. were punched out and vacuum-dried at
110.degree. C. Thereafter, the electrodes were transferred to a
gloved box. In an argon atmosphere, the positive electrode and the
negative electrode were disposed face to face through a
polyethylene separator which had been punched out in a size of 14
mm.phi.. A nonaqueous electrolyte described under
Nonaqueous-Electrolytic-Solution Production was added thereto to
produce a 2032 type coin battery (lithium secondary battery).
Negative Electrode [3]
Example 1
[1672] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrode
produced in <<Negative-Electrode Production 1>> in
which lithium-titanium composite oxide (A) was used as the
negative-electrode active material and further using the positive
electrode produced in <<Positive-Electrode Production
1>> and the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 1>>. This
battery was subjected to the battery evaluation described under
<<Battery Evaluation>> given later. The results are
shown in Negative Electrode [3] Table 2.
Negative Electrode [3]
Example 2
[1673] A battery was produced in the same manner as in Negative
Electrode [3] Example 1, except that lithium-titanium composite
oxide (B) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [3] Table
2.
Negative Electrode [3]
Example 3
[1674] A battery was produced in the same manner as in Negative
Electrode [3] Example 1, except that lithium-titanium composite
oxide (C) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [3] Table
2.
Negative Electrode [3]
Examples 4 to 6
[1675] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [3] Examples 1 to 3, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrode [3] Table 2.
Negative Electrode [3]
Examples 7 to 9
[1676] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [3] Examples 1 to 3, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrode [3] Table 2.
Negative Electrode [3]
Comparative Examples 1 to 3
[1677] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [3] Examples 1 to 3, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
results are shown in Negative Electrode [3] Table 2.
Negative Electrode [3]
Comparative Example 4
[1678] A battery was produced in the same manner as in Negative
Electrode [3] Example 1, except that graphitic carbonaceous
substance (D) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [3] Table
2.
Negative Electrode [3]
Comparative Example 5
[1679] A battery was produced and evaluated in the same manner as
in Negative Electrode [3] Comparative Example 4, except that the
nonaqueous electrolyte used in the Comparative Example was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrode [3] Table 2.
Negative Electrode [3]
Comparative Example 6
[1680] A battery was produced and evaluated in the same manner as
in Negative Electrode [3] Comparative Example 4, except that the
nonaqueous electrolyte used in the Comparative Example was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrode [3] Table 2.
Negative Electrode [3]
Comparative Example 7
[1681] A battery was produced and evaluated in the same manner as
in Negative Electrode [3] Comparative Examples 4, except that the
nonaqueous electrolyte used in the Comparative Example was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
results are shown in Negative Electrode [3] Table 2.
Negative Electrode [3]
Battery Evaluation
(Capacity Measurement)
[1682] With respect to each fresh battery which had undergone no
charge/discharge, the battery capacity was calculated from the
amount of the active material present on the copper foil; the
capacity of each lithium-titanium composite oxide and that of the
graphitic carbonaceous substance were taken as 175 mAh/g and 350
mAh/g, respectively. Each battery employing a lithium-titanium
composite oxide was subjected to 5 cycles of initial
charge/discharge at 25.degree. C. in a voltage range of 2.7-1.9 V
at 0.2 C based on that battery capacity (the value of current at
which the rated capacity based on 1-hour-rate discharge capacity is
discharged over 1 hour is taken as 1 C; the same applies
hereinafter). Each battery employing the graphitic carbonaceous
substance was subjected in the same manner to initial
charge/discharge at 25.degree. C. in a voltage range of 4.1-3.0 V.
The 0.2-C discharge capacity as measured in the fifth cycle in this
operation was taken as initial capacity.
(Measurement of Output Resistance)
[1683] In a 25.degree. C. environment, each battery was charged for
150 minutes at a constant current of 0.2 C. In a -30.degree. C.
environment, the battery was discharged for 10 seconds at each of
0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The voltage was measured at
10 seconds after the discharge initiation. The inclination of the
resultant current-voltage straight line was taken as output
resistance (.OMEGA.). The results obtained are shown in Negative
Electrode [3] Table 2.
TABLE-US-00040 NEGATIVE ELECTRODE [3] TABLE 2 Output Output
resistance Negative-electrode Specific resistance reduction No.
active material compound .OMEGA. % Ex. 1 Lithium-titanium lithium
75 34 composite oxide (A) difluorophosphate Ex. 2 Lithium-titanium
lithium 69 31 composite oxide (B) difluorophosphate Ex. 3
Lithium-titanium lithium 65 28 composite oxide (C)
difluorophosphate Ex. 4 Lithium-titanium trimethylsilyl 68 40
composite oxide (A) methanesulfonate Ex. 5 Lithium-titanium
trimethylsilyl 71 29 composite oxide (B) methanesulfonate Ex. 6
Lithium-titanium trimethylsilyl 64 29 composite oxide (C)
methanesulfonate Ex. 7 Lithium-titanium hexamethylcyclo- 71 38
composite oxide (A) trisiloxane Ex. 8 Lithium-titanium
hexamethylcyclo- 66 34 composite oxide (B) trisiloxane Ex. 9
Lithium-titanium hexamethylcyclo- 63 30 composite oxide (C)
trisiloxane Comp. Lithium-titanium -- 114 -- Ex. 1 composite oxide
(A) Comp. Lithium-titanium -- 100 -- Ex. 2 composite oxide (B)
Comp. Lithium-titanium -- 90 -- Ex. 3 composite oxide (C) Comp.
Graphitic lithium 130 17 Ex. 4 carbonaceous difluorophosphate
substance (D) Comp. Graphitic trimethylsilyl 137 13 Ex. 5
carbonaceous methanesulfonate substance (D) Comp. Graphitic
hexamethylcyclo- 135 14 Ex. 6 carbonaceous trisiloxane substance
(D) Comp. Graphitic -- 157 -- Ex. 7 carbonaceous substance (D)
[1684] In Negative Electrode [3] Table 2, "Output resistance
reduction" means the reduction in output resistance (%) which is
determined from a comparison with the corresponding battery
containing no specific compound.
[1685] The results given in Negative Electrode [3] Table 2 show the
following. The output resistance is remarkably reduced by using an
electrolyte containing lithium difluorophosphate, trimethylsilyl
methanesulfonate, or hexamethylcyclotrisiloxane and using a
negative-electrode active material comprising a titanium-containing
metal oxide capable of occluding and releasing lithium.
Negative Electrode [4]
Production of Negative-Electrode Active Materials
(Negative-Electrode Active-Material Production 1)
[1686] Highly purified natural flake graphite (ash content, 0.05%
by weight) having a median diameter of about 150 .mu.m was
subjected to a rounding treatment with a rounding apparatus
(Hybridization System, manufactured by Nara Machinery Co., Ltd.) at
a rotation speed of 6,500 rpm for 5 minutes. Furthermore, this
graphite was treated with an air classifier (OMC-100, manufactured
by Seishin Enterprise Co., Ltd.) to remove 45% by weight fine
particles therefrom. Thus, rounded natural graphite (C) was
obtained.
(Negative-Electrode Active-Material Production 2)
[1687] A powder of the rounded natural graphite (C) obtained
through classification was packed into a graphite crucible. This
powder was heat-treated with a direct voltage application type
electric furnace at 3,000.degree. C. for 5 hours in an inert
atmosphere to obtain carbonaceous substance (D).
(Negative-Electrode Active-Material Production 3)
[1688] The same procedure as in (Negative-Electrode Active-Material
Production 2) was conducted, except that the heat treatment
temperature was changed to 2,000.degree. C. Thus, carbonaceous
substance (E) was obtained.
(Negative-Electrode Active-Material Production 4)
[1689] The same procedure as in (Negative-Electrode Active-Material
Production 2) was conducted, except that the heat treatment
temperature was changed to 1,600.degree. C. Thus, carbonaceous
substance (F) was obtained.
(Negative-Electrode Active-Material Production 5)
[1690] The same procedure as in (Negative-Electrode Active-Material
Production 2) was conducted, except that the heat treatment
temperature was changed to 1,200.degree. C. Thus, carbonaceous
substance (G) was obtained.
(Negative-Electrode Active-Material Production 5)
[1691] Highly purified natural flake graphite (ash content, 0.1% by
weight) having a median diameter of 17 .mu.m, tap density of 0.5
g/cm.sup.3, and BET specific surface area of 6 m.sup.2/g was
heat-treated in the same manner as in (Negative-Electrode
Active-Material Production 2) without being subjected to a rounding
treatment. Thus, heat-treated natural graphite (H) was
obtained.
(Negative-Electrode Active-Material Production 6)
[1692] A highly purified natural graphite (ash content, 0.5% by
weight) having a median diameter of 20 .mu.m, tap density of 0.75
g/cm.sup.3, and BET specific surface area of 3 m.sup.2/g was
heat-treated in the same manner as in (Negative-Electrode
Active-Material Production 2) without being subjected to a rounding
treatment. Thus, carbonaceous substance (I) was obtained.
[1693] The rounded natural graphite (C), carbonaceous substance
(D), carbonaceous substance (E), carbonaceous substance (F),
carbonaceous substance (G), heat-treated natural graphite (H), and
carbonaceous substance (I) produced were examined for shape and
properties by the methods described above. The results obtained are
shown in Negative Electrode [4] Table 1.
TABLE-US-00041 NEGATIVE ELECTRODE [4] TABLE 1 Negative- electrode
BET active- specific Raman material Tap surface Raman half-value
O/C production Negative-electrode density area R value width value
d.sub.50 d.sub.90/d.sub.10 d002 Lc004 Roundness No. active material
g/cm.sup.3 m.sup.2/g -- cm.sup.-1 -- .mu.m -- nm nm -- 1 Rounded
natural 1.0 7.5 0.27 23.5 0.032 17 2.5 0.3354 100< 0.94 graphite
(C) 2 Carbonaceous 1.0 5.4 0.03 20.5 <0.001 17 2.5 0.3354
100< 0.94 substance (D) 3 Carbonaceous 1.0 5.7 0.04 21.0
<0.001 17 2.5 0.3354 100< 0.94 substance (E) 4 Carbonaceous
1.0 6.0 0.08 21.4 <0.001 17 2.5 0.3354 100< 0.94 substance
(F) 5 Carbonaceous 1.0 6.5 0.14 22.5 <0.001 17 2.5 0.3354
100< 0.94 substance (G) 6 Heat-treated natural 0.3 4.7 0.04 25.0
<0.001 17 4.5 0.3354 100< 0.82 graphite (H) 7 Carbonaceous
0.7 4.0 0.03 22.9 <0.001 20 7.7 0.3354 100< 0.86 substance
(I)
Negative Electrode [4]
Production of Batteries
<<Positive-Electrode Production 1>>
[1694] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained. The positive
electrodes thus obtained had an active-material density of 2.35
g/cm.sup.3.
<<Negative-Electrode Production 1>>
[1695] To 98 parts by weight of a negative-electrode active
material were added 100 parts by weight of an aqueous dispersion of
sodium carboxymethyl cellulose (concentration of sodium
carboxymethyl cellulose, 1% by mass) as a thickener and 2 parts by
weight of an aqueous dispersion of a styrene/butadiene rubber
(concentration of styrene/butadiene rubber, 50% by mass) as a
binder. The ingredients were mixed together by means of a disperser
to obtain a slurry. The slurry obtained was applied to each side of
a rolled copper foil having a thickness of 10 .mu.m and dried. The
coated foil was rolled with a pressing machine to a thickness of 75
.mu.m. Pieces each having an active-material layer size with a
width of 104 mm and a length of 104 mm and including an uncoated
part having a width of 30 mm were cut out of the rolled foil. Thus,
negative electrodes were obtained. The negative electrodes thus
obtained had an active-material density of 1.35 g/cm.sup.3.
<<Nonaqueous-Electrolytic-Solution Production 1>>
[1696] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, lithium difluorophosphate (LiPO.sub.2F.sub.2)
was added thereto so as to result in a concentration of 0.3% by
mass.
<<Nonaqueous-Electrolytic-Solution Production 2>>
[1697] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, trimethylsilyl methanesulfonate was added
thereto so as to result in a concentration of 0.3% by mass.
[1698] <<Nonaqueous-Electrolytic-Solution Production
3>>
[1699] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, hexamethylcyclotrisiloxane was added thereto
so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 4>>
[1700] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4).
<<Battery Production 1>>
[1701] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery has a rated
discharge capacity of about 6 ampere-hours (Ah) and a
direct-current resistance component as measured by the 10-kHz
alternating-current method of about 5 milliohms (m.OMEGA.). The
ratio of the total electrode area of the positive electrodes to the
overall surface area of the battery case was 20.6.
Negative Electrode [4]
Example 1
[1702] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Electrode Production 1>> in
which carbonaceous substance (D) was used as the negative-electrode
active material and further using the positive electrodes produced
in <<Positive-Electrode Production 1>> and the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 1>>. This
battery was examined by the methods which will be described under
<<Battery Evaluation>> given later and by the
measurement methods described above. The results are shown in
Negative Electrode [4] Table 2.
Negative Electrode [4]
Example 2
[1703] A battery was produced in the same manner as in Negative
Electrode [4] Example 1, except that carbonaceous substance (E) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [4] Table
2.
Negative Electrode [4]
Example 3
[1704] A battery was produced in the same manner as in Negative
Electrode [4] Example 1, except that carbonaceous substance (F) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [4] Table
2.
Negative Electrode [4]
Example 4
[1705] A battery was produced in the same manner as in Negative
Electrode [4] Example 1, except that carbonaceous substance (G) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [4] Table
2.
Negative Electrode [4]
Example 5
[1706] A battery was produced in the same manner as in Negative
Electrode [4] Example 1, except that carbonaceous substance (I) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [4] Table
2.
Negative Electrode [4]
Examples 6 to 10
[1707] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [4] Examples 1 to 5, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Table 2 (Negative Electrode [4] Table 2).
Negative Electrode [4]
Examples 11 to 15
[1708] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [4] Examples 1 to 5, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrode [4] Table 2.
Negative Electrode [4]
Comparative Example 1
[1709] A battery was produced in the same manner as in Negative
Electrode [4] Example 1, except that rounded natural graphite (C)
was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [4] Table
2.
Negative Electrode [4]
Comparative Example 2
[1710] A battery was produced in the same manner as in Negative
Electrode [4] Comparative Example 1, except that the nonaqueous
electrolyte used in the Comparative Example was replaced by the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [4] Table 2.
Negative Electrode [4]
Comparative Example 3
[1711] A battery was produced in the same manner as in Negative
Electrode [4] Example 1, except that heat-treated natural graphite
(H) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [4] Table
2.
Negative Electrode [4]
Comparative Example 4
[1712] A battery was produced in the same manner as in Negative
Electrode [4] Comparative Example 3, except that the nonaqueous
electrolyte used in the Comparative Example was replaced by the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [4] Table 2.
Negative Electrode [4]
Comparative Example 5
[1713] A battery was produced in the same manner as in Negative
Electrode [4] Example 5, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [4] Table 2.
Negative Electrode [4]
Comparative Example 6
[1714] A battery was produced in the same manner as in Negative
Electrode [4] Example 1, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [4] Table 2.
Negative Electrode [4]
Comparative Examples 7 and 8
[1715] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [4] Comparative Examples 1
and 3, except that the nonaqueous electrolyte used in these
Comparative Examples was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
2>>. The results are shown in Negative Electrode [4] Table
2.
Negative Electrode [4]
Comparative Examples 9 and 10
[1716] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [4] Comparative Examples 1
and 3, except that the nonaqueous electrolyte used in these
Comparative Examples was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
3>>. The results are shown in Negative Electrode [4] Table
2.
TABLE-US-00042 NEGATIVE ELECTRODE [4] TABLE 2 Capacity after
Recovery after Initial storage in low state storage in low state
Negative-electrode active capacity of charge of charge No. material
Specific compound mAh mAh % Example 1 Carbonaceous substance (D)
lithium difluorophosphate 6034 5504 91.2 Example 2 Carbonaceous
substance (E) lithium difluorophosphate 6015 5509 91.6 Example 3
Carbonaceous substance (F) lithium difluorophosphate 6025 5302 88.0
Example 4 Carbonaceous substance (G) lithium difluorophosphate 6009
5109 85.0 Example 5 Carbonaceous substance (I) lithium
difluorophosphate 6032 5023 83.3 Example 6 Carbonaceous substance
(D) trimethylsilyl methanesulfonate 6028 5405 89.7 Example 7
Carbonaceous substance (E) trimethylsilyl methanesulfonate 6019
5398 89.7 Example 8 Carbonaceous substance (F) trimethylsilyl
methanesulfonate 6015 5411 90.0 Example 9 Carbonaceous substance
(G) trimethylsilyl methanesulfonate 6022 5202 86.4 Example 10
Carbonaceous substance (I) trimethylsilyl methanesulfonate 5999
5111 85.2 Example 11 Carbonaceous substance (D)
hexamethylcyclotrisiloxane 6002 5468 91.1 Example 12 Carbonaceous
substance (E) hexamethylcyclotrisiloxane 6003 5389 89.8 Example 13
Carbonaceous substance (F) hexamethylcyclotrisiloxane 6034 5201
86.2 Example 14 Carbonaceous substance (G)
hexamethylcyclotrisiloxane 6012 5109 85.0 Example 15 Carbonaceous
substance (I) hexamethylcyclotrisiloxane 6034 4987 82.6 Comparative
Rounded natural graphite lithium difluorophosphate 6021 4877 81.0
Example 1 (C) Comparative Rounded natural graphite -- 6009 4523
75.3 Example 2 (C) Comparative Heat-treated natural lithium
difluorophosphate 6002 4700 78.3 Example 3 graphite (H) Comparative
Heat-treated natural -- 6001 4690 78.2 Example 4 graphite (H)
Comparative Carbonaceous substance (I) -- 6001 4567 76.1 Example 5
Comparative Carbonaceous substance (D) -- 6009 4776 79.5 Example 6
Comparative Rounded natural graphite trimethylsilyl
methanesulfonate 6001 4809 80.1 Example 7 (C) Comparative
Heat-treated natural trimethylsilyl methanesulfonate 6012 4798 79.8
Example 8 graphite (H) Comparative Rounded natural graphite
hexamethylcyclotrisiloxane 6021 4877 81.0 Example 9 (C) Comparative
Heat-treated natural hexamethylcyclotrisiloxane 5999 4897 81.6
Example 10 graphite (H)
Negative Electrode [4]
Battery Evaluation
(Capacity Measurement)
[1717] The battery which had undergone no charge/discharge cycling
was subjected to 5 cycles of initial charge/discharge at 25.degree.
C. in a voltage range of 4.2-3.0 V at a current of 0.2 C (the value
of current at which the rated capacity based on 1-hour-rate
discharge capacity is discharged over 1 hour is taken as 1 C; the
same applies hereinafter). The 0.2-C discharge capacity thereof as
measured in the fifth cycle in this operation was taken as initial
capacity.
(Storage Test)
[1718] The battery was stored in a high-temperature environment of
60.degree. C., which is regarded as the upper-limit temperature for
the actual use of lithium secondary batteries. The battery was
charged by the constant-current constant-voltage method at 0.2 C to
a final charge voltage of 4.2 V to 20% of the initial capacity
measured in Capacity Measurement. Thereafter, this battery was
stored in a 60.degree. C. high-temperature environment for 1 week.
The battery after the storage was discharged at 0.2 C to 3 V in a
25.degree. C. environment and then subjected to 3 cycles of
charge/discharge under the same conditions as in (Capacity
Measurement). The 0.2-C discharge capacity thereof as measured in
the third cycle was taken as capacity after storage in low state of
charge. The capacity retention through cycling was determined using
the following calculation formula from the initial capacity
measured before the storage test and the capacity after storage in
low state of charge measured after completion of the storage
test.
Recovery after storage in low state of charge
(%)=100.times.(capacity after storage in low state of
charge)/(initial capacity)
[1719] The results given above show the following. The recovery
after storage in low state of charge, as determined after storage
in a low state of charge, was greatly improved by using an
electrolyte containing lithium difluorophosphate, trimethylsilyl
methanesulfonate, or hexamethylcyclotrisiloxane and by using a
negative electrode containing as a negative-electrode active
material a carbonaceous substance having a roundness of 0.85 or
higher and a surface functional-group amount O/C value of from 0 to
0.01.
Negative Electrode [5]
Production of Negative-Electrode Materials
[Negative-Electrode Material Production 1]
[1720] Coal tar pitch having a quinoline insoluble content of 0.05%
by mass or lower was heat-treated at 460.degree. C. for 10 hours in
a reaction furnace to obtain a massive heat-treated graphite
crystal precursor which was meltable and had a softening point of
385.degree. C. The massive heat-treated graphite crystal precursor
obtained was ground with an intermediate pulverizer (Orient Mill,
manufactured by Seishin Enterprise Co., Ltd.) and further
pulverized with a pulverizer (Turbo Mill, manufactured by Matsubo
Corp.) to reduce the massive precursor into fine particles. Thus,
graphite crystal precursor powder (E) was obtained, which had a
median diameter of 17 .mu.m.
[1721] A natural graphite having a median diameter of 17 .mu.m,
aspect ratio of 1.4, tap density of 1.0 g/cm.sup.3, BET specific
surface area of 6.5 g/cm.sup.3, and roundness of 0.92 was mixed
with the fine graphite crystal precursor powder (E) in an amount of
50% by mass based on the total weight of the fine graphite crystal
precursor powder and the natural graphite. Thus, a powder mixture
was obtained.
[1722] This powder mixture of the heat-treated graphite crystal
precursor was packed into a metallic container and subjected to
heat treatment A at 540.degree. C. for 2 hours in a nitrogen gas
stream in a box-shaped electric furnace. During the heat treatment
A, the fine graphite crystal precursor powder melted and was evenly
combined with the natural graphite. As a result, a massive mixture
of the heat-treated graphite crystal precursor was obtained.
[1723] This solidified mass of the mixture of the heat-treated
graphite crystal precursor was crushed with a crusher (Roll Jaw
Crusher, manufactured by Yoshida Seisakusho Co., Ltd.) and further
pulverized with a pulverizer (Turbo Mill, manufactured by Matsubo
Corp.) to obtain a powder having a median diameter of 18.5
.mu.m.
[1724] The powder obtained was put in a container and heat-treated
with an electric furnace at 1,000.degree. C. for 1 hour in a
nitrogen atmosphere. The powder obtained through the heat-treating
(precursor mixture (F) before heat treatment B) remained in a
powder state, and appeared to have undergone almost no melting or
fusion bonding.
[1725] Furthermore, the powder heat-treated was transferred to a
graphite crucible and heated with a direct voltage application type
electric furnace at 3,000.degree. C. over 5 hours in an inert
atmosphere to graphitize it. This powder was repeatedly passed
through an ASTM 400-mesh sieve five times in order to prevent
inclusion of coarse particles. Thus, orientation-differing-carbon
composite (G) was obtained.
[Negative-Electrode Material Production 2]
[1726] The natural graphite used in [Negative-Electrode Material
Production 1] and a binder pitch having a softening point of
88.degree. C. as a graphitizable binder were mixed with each other
in a weight ratio of 100:30. This mixture was introduced into a
kneader having a matiscator type stirring blade and preheated at
128.degree. C., and was kneaded for 20 minutes.
[1727] The mixture sufficiently kneaded was packed into that mold
of a mold press which had been preheated at 108.degree. C. After
the mixture in the mold was allowed to stand for 5 minutes to make
the temperature of the mixture stable, the plunger was pushed to
apply a pressure of 2 kgf/cm.sup.2 (0.20 MPa) and mold the mixture.
That pressure was maintained for 1 minute and the operation was
then stopped. After the resultant pressure decrease ended, the
molded object comprising the natural graphite and a graphite
crystal precursor powder combined therewith was taken out.
[1728] The molded object obtained was placed in a metallic saggar
as a refractory container and the space was filled with a graphitic
breeze. In an electric furnace, this molded object was heated from
room temperature to 1,000.degree. C. over 48 hours and held at
1,000.degree. C. for 3 hours to conduct volatile
removal/heat-treating. Subsequently, the molded object was placed
in a graphite crucible and the space was filled with a graphitic
breeze. Using a direct voltage application type electric furnace,
this molded object was heated at 3,000.degree. C. for 4 hours in an
inert atmosphere to conduct graphitization.
[1729] The graphitic molded object obtained was crushed with a jaw
crusher and then pulverized with a mill having a pulverization
blade rotation speed set at 4,000 rpm. Furthermore, sieving with an
ASTM 400-mesh sieve was repeatedly conducted five times as a
classification treatment in order to prevent inclusion of coarse
particles. Thus, orientation-differing-carbon composite (H) was
obtained.
[Negative-Electrode Material Production 3]
[1730] The same procedure as in [Negative-Electrode Material
Production 2] was conducted, except that the heat treatment with a
direct voltage application type electric furnace was conducted at
2,200.degree. C. Thus, orientation-differing-carbon composite (I)
was obtained.
[Negative-Electrode Material Production 4]
[1731] The same procedure as in [Negative-Electrode Material
Production 2] was conducted, except that the natural graphite was
replaced by a coke having a median diameter of 10 .mu.m, aspect
ratio of 2.3, tap density of 0.64 g/cm.sup.3, BET specific surface
area of 9.5 m.sup.2/g, and roundness of 0.83. Thus,
orientation-differing-carbon composite (J) was obtained.
[Negative-Electrode Material Production 5]
[1732] The same procedure as in [Negative-Electrode Material
Production 2] was conducted, except that the coke used in
[Negative-Electrode Material Production 4], silicon carbide as a
graphitization catalyst, and a binder pitch having a softening
point of 88.degree. C. as a graphitizable binder were mixed
together in a mass ratio of 100:10:30. Thus,
orientation-differing-carbon composite (K) was obtained.
[Negative-Electrode Material Production 6]
[1733] The same procedure as in [Negative-Electrode Material
Production 2] was conducted, except that the natural graphite was
replaced by natural flake graphite having a median diameter of 19.8
.mu.m, aspect ratio of 3.2, tap density of 0.47 g/cm.sup.3, BET
specific surface area of 5.9 m.sup.2/g, and roundness of 0.81.
Thus, orientation-differing-carbon composite (L) was obtained.
[Negative-Electrode Material Production 7]
[1734] The same procedure as in [Negative-Electrode Material
Production 2] was conducted, except that the natural graphite was
replaced by a natural graphite having a median diameter of 35
.mu.m, aspect ratio of 1.4, tap density of 1.02 g/cm.sup.3, BET
specific surface area of 3.9 m.sup.2/g, and roundness of 0.90.
Thus, orientation-differing-carbon composite (M) was obtained.
[Negative-Electrode Material Production 8]
[1735] The same procedure as in [Negative-Electrode Material
Production 2] was conducted, except that the pulverization blade
rotation speed was set at 1,500 rpm. Thus,
orientation-differing-carbon composite (N) was obtained.
[Negative-Electrode Material Production 9]
[1736] The same procedure as in [Negative-Electrode Material
Production 2] was conducted, except that the natural graphite was
replaced by a natural graphite having a median diameter of 6 .mu.m,
aspect ratio of 1.5, and tap density of 0.15 g/cm.sup.3. Thus,
orientation-differing-carbon composite (O) was obtained.
[Negative-Electrode Material Production 10]
[1737] The graphite crystal precursor powder (E) obtained in
[Negative-Electrode Material Production 1] was packed into a
metallic container and subjected to heat treatment A at 540.degree.
C. for 2 hours in a nitrogen gas stream in a box-shaped electric
furnace. The graphite crystal precursor powder (E) melted during
the heat treatment A to give a mass.
[1738] This solidified mass of the heat-treated graphite crystal
precursor was crushed with a crusher (Roll Jaw Crusher,
manufactured by Yoshida Seisakusho Co., Ltd.) and further
pulverized with a pulverizer (Turbo Mill, manufactured by Matsubo
Corp.) to obtain a powder having a median diameter of 18.5
.mu.m.
[1739] The powder obtained was put in a container and heat-treated
with an electric furnace at 1,000.degree. C. for 1 hour in a
nitrogen atmosphere. The powder obtained through the heat-treating
remained in a powder state, and appeared to have undergone almost
no melting or fusion bonding.
[1740] Furthermore, the powder heat-treated was transferred to a
graphite crucible and heated with a direct voltage application type
electric furnace at 3,000.degree. C. over 5 hours in an inert
atmosphere to graphitize it. This powder was repeatedly passed
through an ASTM 400-mesh sieve five times in order to prevent
inclusion of coarse particles. Thus, carbonaceous substance (P) was
obtained.
[Negative-Electrode Material Production 11]
[1741] The natural graphite used in [Negative-Electrode Material
Production 1] was packed into a metallic container and subjected to
heat treatment A at 540.degree. C. for 2 hours in a nitrogen gas
stream in a box-shaped electric furnace. After the heat treatment
A, the natural graphite appeared to have undergone no
melting/fusion bonding. The powder obtained was put in a container
and heat-treated with an electric furnace at 1,000.degree. C. for 1
hour in a nitrogen atmosphere. The powder obtained through the
heat-treating remained in a powder state and appeared to have
undergone no melting or fusion bonding.
[1742] Furthermore, the powder heat-treated was transferred to a
graphite crucible and heated with a direct voltage application type
electric furnace at 3,000.degree. C. over 5 hours in an inert
atmosphere to graphitize it. This powder was repeatedly passed
through an ASTM 400-mesh sieve five times in order to prevent
inclusion of coarse particles. Thus, carbonaceous substance (Q) was
obtained.
[Negative-Electrode Material Production 12]
[1743] Carbonaceous substance (P) and carbonaceous substance (Q)
were evenly mixed together each in an amount of 50% by mass. Thus,
carbonaceous substance mixture (R) was obtained.
Negative Electrode [5]
Production of Batteries
<<Positive-Electrode Production 1>>
[1744] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained. The positive
electrodes thus obtained had an active-material density of 2.35
g/cm.sup.3.
<<Negative-Electrode Production 1>>
[1745] To 98 parts by weight of a negative-electrode active
material were added 100 parts by weight of an aqueous dispersion of
sodium carboxymethyl cellulose (concentration of sodium
carboxymethyl cellulose, 1% by mass) as a thickener and 2 parts by
weight of an aqueous dispersion of a styrene/butadiene rubber
(concentration of styrene/butadiene rubber, 50% by mass) as a
binder. The ingredients were mixed together by means of a disperser
to obtain a slurry. The slurry obtained was applied to each side of
a rolled copper foil having a thickness of 10 .mu.m and dried. The
coated foil was rolled with a pressing machine to a thickness of 75
.mu.m. Pieces each having an active-material layer size with a
width of 104 mm and a length of 104 mm and including an uncoated
part having a width of 30 mm were cut out of the rolled foil. Thus,
negative electrodes were obtained. The negative electrodes thus
obtained had an active-material density of 1.35 g/cm.sup.3.
<<Nonaqueous-Electrolytic-Solution Production 1>>
[1746] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, lithium difluorophosphate (LiPO.sub.2F.sub.2)
was added thereto so as to result in a concentration of 0.3% by
mass.
<<Nonaqueous-Electrolytic-Solution Production 2>>
[1747] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, trimethylsilyl methanesulfonate was added
thereto so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 3>>
[1748] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, hexamethylcyclotrisiloxane was added thereto
so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 4>>
[1749] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4).
<<Battery Production 1>>
[1750] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery has a rated
discharge capacity of about 6 ampere-hours (Ah) and a
direct-current resistance as measured by the 10-kHz
alternating-current method of about 5 milliohms (m.OMEGA.).
Negative Electrode [5]
Example 1
[1751] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Electrode Production 1>> in
which orientation-differing-carbon composite (G) was used as the
negative-electrode active material and further using the positive
electrodes produced in <<Positive-Electrode Production
1>> and the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 1>>. This
battery was examined by the methods which will be described under
<<Battery Evaluation>> given later and by the
measurement methods described above.
Negative Electrode [5]
Example 2
[1752] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that orientation-differing-carbon
composite (H) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Example 3
[1753] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that orientation-differing-carbon
composite (I) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Example 4
[1754] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that orientation-differing-carbon
composite (J) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Example 5
[1755] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that orientation-differing-carbon
composite (K) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Example 6
[1756] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that orientation-differing-carbon
composite (L) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Example 7
[1757] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that orientation-differing-carbon
composite (M) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Example 8
[1758] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that orientation-differing-carbon
composite (N) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Example 9
[1759] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that orientation-differing-carbon
composite (O) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Example 10 to 18
[1760] Batteries were produced in the same manners as in Negative
Electrode [5] Examples 1 to 9, except that the nonaqueous
electrolyte used in these Examples was replaced by the nonaqueous
electrolyte produced in <<Nonaqueous-Electrolytic-Solution
Production 2>>. The batteries were evaluated by the same
methods.
Negative Electrode [5]
Example 19 to 27
[1761] Batteries were produced in the same manners as in Negative
Electrode [5] Examples 1 to 9, except that the nonaqueous
electrolyte used in these Examples was replaced by the nonaqueous
electrolyte produced in <<Nonaqueous-Electrolytic-Solution
Production 3>>. The batteries were evaluated by the same
methods.
Negative Electrode [5]
Comparative Example 1
[1762] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that carbonaceous substance (P) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Comparative Example 2
[1763] A battery was produced in the same manner as in Negative
Electrode [5] Comparative Example 1, except that the nonaqueous
electrolyte used in the Comparative Example was replaced by the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [5]
Comparative Example 3
[1764] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that carbonaceous substance (Q) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Comparative Example 4
[1765] A battery was produced in the same manner as in Negative
Electrode [5] Comparative Example 3, except that the nonaqueous
electrolyte used in the Comparative Example was replaced by the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [5]
Comparative Example 5
[1766] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that carbonaceous substance mixture
(R) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted.
Negative Electrode [5]
Comparative Example 6
[1767] A battery was produced in the same manner as in Negative
Electrode [5] Comparative Example 5, except that the nonaqueous
electrolyte used in the Comparative Example was replaced by the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [5]
Comparative Example 7
[1768] A battery was produced in the same manner as in Negative
Electrode [5] Example 1, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [5]
Comparative Example 8
[1769] A battery was produced in the same manner as in Negative
Electrode [5] Example 2, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [5]
Comparative Example 9
[1770] A battery was produced in the same manner as in Negative
Electrode [5] Example 4, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [5]
Comparative Example 10
[1771] A battery was produced in the same manner as in Negative
Electrode [5] Example 5, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted.
Negative Electrode [5]
Comparative Example 11 to 13
[1772] Batteries were produced in the same manners as in Negative
Electrode [5] Comparative Examples 1, 3, and 5, except that the
nonaqueous electrolyte used in these Comparative Examples was
replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
batteries were evaluated by the same methods.
Negative Electrode [5]
Comparative Example 14 to 16
[1773] Batteries were produced in the same manners as in Negative
Electrode [5] Comparative Examples 1, 3, and 5, except that the
nonaqueous electrolyte used in these Comparative Examples was
replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
batteries were evaluated by the same methods.
Negative Electrode [5]
Battery Evaluation
(Capacity Measurement)
[1774] The battery which had undergone no charge/discharge cycling
was subjected to 5 cycles of initial charge/discharge at 25.degree.
C. in a voltage range of 4.2-3.0 V at a current of 0.2 C (the value
of current at which the rated capacity based on 1-hour-rate
discharge capacity is discharged over 1 hour is taken as 1 C; the
same applies hereinafter). The 0.2-C discharge capacity thereof as
measured in the fifth cycle in this operation was taken as initial
capacity. The output measurement shown below was then made.
(Low-State-Of-Charge Cycle Test)
[1775] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.2 V
to 20% of the initial capacity measured in Capacity Measurement and
then discharged at a constant current of 2 C to a final discharge
voltage of 3.0 V. This charge/discharge operation as one cycle was
repeated to conduct 500 cycles in total.
[1776] The battery which had undergone the cycle test was subjected
to 3 cycles of charge/discharge in a 25.degree. C. environment. The
0.2-C discharge capacity thereof as measured in the third cycle was
taken as capacity after low-state-of-charge cycling. Using the
following calculation formula, the capacity retention through
cycling was determined from the initial capacity measured prior to
the cycling and the capacity after low-state-of-charge cycling
measured after completion of the cycle test.
Capacity retention through cycling (%)=100.times.(capacity after
low-state-of-charge cycling)/(initial capacity)
[1777] The negative-electrode active materials used in the Negative
Electrode [5] Examples and Negative Electrode [5] Comparative
Examples are listed in Negative Electrode [5] Table 1, and the
results of battery evaluation are shown in Negative Electrode [5]
Table 2 and Negative Electrode [5] Table 3. The results given in
Negative Electrode [5] Table 2 and Negative Electrode [5] Table 3
show the following. The capacity retention through a
low-state-of-charge cycle test (capacity retention through cycling)
was greatly improved by using an electrolyte containing lithium
difluorophosphate, trimethylsilyl methanesulfonate, or
hexamethylcyclotrisiloxane in combination with an
orientation-differing-carbon composite as a negative-electrode
active material.
TABLE-US-00043 NEGATIVE ELECTRODE [5] TABLE 1 Raman Micro- Total
Negative- Specific half- pore pore electrode Particle Round-
surface Raman R value Tap Ash distri- distri- True Aspect active
diameter ness area value width density d002 Lc content bution
bution density ratio material .mu.m -- m.sup.2/g -- cm.sup.-1
g/cm.sup.3 nm nm mass % mL/g mL/g g/cm.sup.3 -- Orientation- 17.5
0.92 2.3 0.06 20.1 1.20 0.3357 >100 0.02 0.042 0.448 2.26 1.3
differing- carbon composite (G) Orientation- 18.0 0.93 2.8 0.12
22.1 1.17 0.3358 >100 0.02 0.022 0.467 2.26 1.2 differing-
carbon composite (H) Orientation- 18.5 0.91 3.0 0.17 24.5 1.10
0.3610 >100 0.05 0.030 0.489 2.24 1.4 differing- carbon
composite (I) Orientation- 21.4 0.88 2.3 0.12 21.4 0.75 0.3359
>100 0.03 0.067 0.569 2.25 1.5 differing- carbon composite (J)
Orientation- 20.7 0.89 3.0 0.08 21.9 0.73 0.3357 >100 0.05 0.079
0.656 2.26 1.5 differing- carbon composite (K) Orientation- 21.4
0.82 2.5 0.13 23.5 0.63 0.3357 >100 0.03 0.033 0.556 2.26 2.5
differing- carbon composite (L) Orientation- 35.2 0.91 1.4 0.05
21.1 1.11 0.3356 >100 0.04 0.018 0.405 2.26 1.3 differing-
carbon composite (M) Orientation- 22.0 0.92 0.9 0.06 22.1 1.25
0.3356 >100 0.05 0.020 0.455 2.26 1.2 differing- carbon
composite (N) Orientation- 16.5 0.90 3.8 0.09 24.3 0.50 0.3359
>100 0.03 0.102 0.764 2.25 1.7 differing- carbon composite (O)
Carbonaceous 18.9 0.86 1.5 0.11 27.8 1.06 0.3357 >100 0.04 0.024
0.459 2.25 2.3 substance (P) Carbonaceous 17.7 0.93 5.4 0.03 20.5
1.00 0.3354 >100 0.03 0.112 0.701 2.26 1.3 substance (Q)
Carbonaceous 18.8 0.90 3.4 0.06 23.8 1.02 0.3355 >100 0.03 0.090
0.590 2.26 1.8 substance mixture (R)
TABLE-US-00044 NEGATIVE ELECTRODE [5] TABLE 2 Capacity after
Initial low-state-of-charge Capacity retention Negative-electrode
capacity cycling through cycling No. active material Specific
compound mAh mAh % Example 1 Orientation-differing-carbon composite
(G) lithium difluorophosphate 6045 5592 92.5 Example 2
Orientation-differing-carbon composite (H) lithium
difluorophosphate 6032 5640 93.5 Example 3
Orientation-differing-carbon composite (I) lithium
difluorophosphate 5780 5162 89.3 Example 4
Orientation-differing-carbon composite (J) lithium
difluorophosphate 6013 5478 91.1 Example 5
Orientation-differing-carbon composite (K) lithium
difluorophosphate 6003 5427 90.4 Example 6
Orientation-differing-carbon composite (L) lithium
difluorophosphate 6009 5426 90.3 Example 7
Orientation-differing-carbon composite (M) lithium
difluorophosphate 6001 5383 89.7 Example 8
Orientation-differing-carbon composite (N) lithium
difluorophosphate 5914 5246 88.7 Example 9
Orientation-differing-carbon composite (O) lithium
difluorophosphate 5959 5375 90.2 Example 10
Orientation-differing-carbon composite (G) trimethylsilyl
methanesulfonate 6037 5410 89.6 Example 11
Orientation-differing-carbon composite (H) trimethylsilyl
methanesulfonate 6035 5441 90.2 Example 12
Orientation-differing-carbon composite (I) trimethylsilyl
methanesulfonate 5806 5058 87.1 Example 13
Orientation-differing-carbon composite (J) trimethylsilyl
methanesulfonate 6019 5346 88.8 Example 14
Orientation-differing-carbon composite (K) trimethylsilyl
methanesulfonate 5997 5241 87.4 Example 15
Orientation-differing-carbon composite (L) trimethylsilyl
methanesulfonate 6011 5300 88.2 Example 16
Orientation-differing-carbon composite (M) trimethylsilyl
methanesulfonate 6006 5233 87.1 Example 17
Orientation-differing-carbon composite (N) trimethylsilyl
methanesulfonate 5921 5104 86.2 Example 18
Orientation-differing-carbon composite (O) trimethylsilyl
methanesulfonate 5968 5101 85.5 Example 19
Orientation-differing-carbon composite (G)
hexamethylcyclotrisiloxane 6039 5767 95.5 Example 20
Orientation-differing-carbon composite (H)
hexamethylcyclotrisiloxane 6023 5580 92.6 Example 21
Orientation-differing-carbon composite (I)
hexamethylcyclotrisiloxane 5793 5208 89.9 Example 22
Orientation-differing-carbon composite (J)
hexamethylcyclotrisiloxane 6009 5364 89.3 Example 23
Orientation-differing-carbon composite (K)
hexamethylcyclotrisiloxane 6003 5349 89.1 Example 24
Orientation-differing-carbon composite (L)
hexamethylcyclotrisiloxane 6010 5306 88.3 Example 25
Orientation-differing-carbon composite (M)
hexamethylcyclotrisiloxane 5996 5413 90.3 Example 26
Orientation-differing-carbon composite (N)
hexamethylcyclotrisiloxane 5917 5275 89.1 Example 27
Orientation-differing-carbon composite (O)
hexamethylcyclotrisiloxane 5961 5500 92.3
TABLE-US-00045 NEGATIVE ELECTRODE [5] TABLE 3 Capacity after
Initial low-state-of-charge Capacity retention Negative-electrode
capacity cycling through cycling No. active material Specific
compound mAh mAh % Comparative Carbonaceous lithium
difluorophosphate 5890 4671 79.3 Example 1 substance (P)
Comparative Carbonaceous -- 5873 4552 77.5 Example 2 substance (P)
Comparative Carbonaceous lithium difluorophosphate 6058 4919 81.2
Example 3 substance (Q) Comparative Carbonaceous -- 6067 4866 80.2
Example 4 substance (Q) Comparative Carbonaceous substance lithium
difluorophosphate 5890 4836 82.1 Example 5 mixture (R) Comparative
Carbonaceous substance -- 5887 4739 80.5 Example 6 mixture (R)
Comparative Orientation-differing- -- 6031 4915 81.5 Example 7
carbon composite (G) Comparative Orientation-differing- -- 6045
4975 82.3 Example 8 carbon composite (H) Comparative
Orientation-differing- -- 5790 4713 81.4 Example 9 carbon composite
(J) Comparative Orientation-differing- -- 5813 4604 79.2 Example 10
carbon composite (K) Comparative Carbonaceous trimethylsilyl
methanesulfonate 5903 4728 80.1 Example 11 substance (P)
Comparative Carbonaceous trimethylsilyl methanesulfonate 6038 4921
81.5 Example 12 substance (Q) Comparative Carbonaceous substance
trimethylsilyl methanesulfonate 5900 4832 81.9 Example 13 mixture
(R) Comparative Carbonaceous hexamethylcyclotrisiloxane 5940 4764
80.2 Example 14 substance (P) Comparative Carbonaceous
hexamethylcyclotrisiloxane 6002 4874 81.2 Example 15 substance (Q)
Comparative Carbonaceous substance hexamethylcyclotrisiloxane 5970
4782 80.1 Example 16 mixture (R)
Negative Electrode [6]
Production of Negative-Electrode Active Materials
(Negative-Electrode Active-Material Production 1)
[1778] A commercial natural-graphite powder (A) was repeatedly
passed through an ASTM 400-mesh sieve five times in order to
prevent inclusion of coarse particles. The negative-electrode
active material thus obtained is referred to as carbonaceous
substance (B).
(Negative-Electrode Active-Material Production 2)
[1779] A commercial natural-graphite powder (C) (d002, 0.336 nm;
Lc, 100 nm or larger; Raman R value, 0.11; tap density, 0.46
g/cm.sup.3; true density, 2.27 g/cm.sup.3; volume-average particle
diameter, 28.7 .mu.m) was treated with a pulverizer (Turbo Mill,
manufactured by Matsubo Corp.). The resultant powder was repeatedly
passed through an ASTM 400-mesh sieve five times in order to
prevent inclusion of coarse particles. Thus, carbonaceous substance
(D) was prepared.
(Negative-Electrode Active-Material Production 3)
[1780] The natural-graphite powder (C) was treated with
Hybridization System (Hybridization System Type NHS-1, manufactured
by Nara Machinery Co., Ltd.) under the conditions of a treatment
amount of 90 g, rotor peripheral speed of 60 m/s, and treatment
period of 3 minutes to thereby round the particles. Furthermore,
this powder was repeatedly passed through an ASTM 400-mesh sieve
five times in order to prevent inclusion of coarse particles. The
negative-electrode active material thus obtained is referred to as
carbonaceous substance (E). This operation was repeated to thereby
obtain the active material in an amount necessary for battery
production.
(Negative-Electrode Active-Material Production 4)
[1781] A commercial natural-graphite powder (F) (d002, 0.336 nm;
Lc, 100 nm or larger; Raman R value, 0.09; tap density, 0.57
g/cm.sup.3; true density, 2.26 g/cm.sup.3; volume-average particle
diameter, 85.4 .mu.m) was rounded with Hybridization System under
the conditions of a treatment amount of 90 g, rotor peripheral
speed of 30 m/s, and treatment period of 1 minute. Furthermore,
this powder was repeatedly passed through an ASTM 400-mesh sieve
five times in order to prevent inclusion of coarse particles. The
negative-electrode active material thus obtained is referred to as
carbonaceous substance (G). This operation was repeated to thereby
obtain the active material in an amount necessary for battery
production.
(Negative-Electrode Active-Material Production 5)
[1782] Coal tar pitch having a quinoline insoluble content of 0.05%
by mass or lower was heat-treated at 460.degree. C. for 10 hours in
a reaction furnace to obtain a massive carbonized precursor which
was meltable and had a softening point of 385.degree. C. The
massive carbonized precursor obtained was packed into a metallic
container and heat-treated at 1,000.degree. C. for 2 hours in a
nitrogen gas stream in a box-shaped electric furnace. The amorphous
mass obtained was crushed with a crusher (Roll Jaw Crusher,
manufactured by Yoshida Seisakusho Co., Ltd.) and further
pulverized with a pulverizer (Turbo Mill, manufactured by Matsubo
Corp.). Thus, an amorphous powder having a volume-average particle
diameter of 18 .mu.m was obtained. The powder obtained was
repeatedly passed through an ASTM 400-mesh sieve five times in
order to prevent inclusion of coarse particles. The
negative-electrode active material thus obtained is referred to as
carbonaceous substance (H).
(Negative-Electrode Active-Material Production 6)
[1783] Carbonaceous substance (H) obtained in (Negative-Electrode
Active-Material Production 5) was transferred to a graphite
crucible and heated with a direct voltage application type electric
furnace at 3,000.degree. C. over 5 hours in an inert atmosphere to
graphitize it. The powder obtained was repeatedly passed through an
ASTM 400-mesh sieve five times in order to prevent inclusion of
coarse particles. The negative-electrode active material thus
obtained is referred to as carbonaceous substance (I).
(Negative-Electrode Active-Material Production 7)
[1784] Carbonaceous substance (I) obtained in (Negative-Electrode
Active-Material Production 6) was rounded with Hybridization System
under the conditions of a treatment amount of 90 g, rotor
peripheral speed of 30 m/s, and treatment period of 1 minute.
Furthermore, this powder was repeatedly passed through an ASTM
400-mesh sieve five times in order to prevent inclusion of coarse
particles. The negative-electrode active material thus obtained is
referred to as carbonaceous substance (J). This operation was
repeated to thereby obtain the active material in an amount
necessary for battery production.
(Negative-Electrode Active-Material Production 8)
[1785] A natural-graphite powder (K) (d002, 0.336 nm; Lc, 100 nm or
larger; Raman R value, 0.10; tap density, 0.49 g/cm.sup.3; true
density, 2.27 g/cm.sup.3; volume-average particle diameter, 27.3
.mu.m; ash content, 0.5% by mass) having a lower purity than the
natural-graphite powder (A) was rounded and sieved under the same
conditions as in (Negative-Electrode Active-Material Production 3).
Thus, carbonaceous substance (L) was prepared. This operation was
repeated to thereby obtain the active material in an amount
necessary for battery production.
(Negative-Electrode Active-Material Production 9)
[1786] A commercial powder of natural flake graphite (M) was
repeatedly passed through an ASTM 400-mesh sieve five times in
order to prevent inclusion of coarse particles. The
negative-electrode active material thus obtained is referred to as
carbonaceous substance (N).
[1787] Carbonaceous substances (B), (D), (E), (G), (H), (I), (J),
(L), and (N) were examined for shape and properties by the methods
described above. The results obtained are shown in Negative
Electrode [6] Table 1.
TABLE-US-00046 Negative Electrode [6] Table 1 Negative Negative
electrode electrode BET Raman Micro- Total active active specific
Raman half- pore pore material material Particle Round- surface R
value Tap Ash distri- distri- True Aspect production Carbonaceous
diameter ness area value width density d002 Lc content bution
bution density ratio No. substance .mu.m -- m.sup.2/g -- cm.sup.-1
g/cm.sup.3 nm nm mass % mL/g mL/g g/cm.sup.3 -- 1 (B) 14.0 0.82 4.9
0.09 19 0.54 0.335 >100 0.1 0.110 0.692 2.26 1.7 2 (D) 9.1 0.93
12.0 0.42 40 0.85 0.336 >100 0.1 0.124 0.799 2.26 1.2 3 (E) 13.6
0.94 10.6 0.33 30 0.92 0.335 >100 0.1 0.119 0.859 2.26 1.2 4 (G)
35.4 0.90 4.6 0.18 24 0.78 0.336 >100 0.1 0.098 0.629 2.26 1.6 5
(H) 15.4 0.90 4.6 1.03 121 0.94 0.345 30 0.1 0.048 0.662 1.92 2.3 6
(I) 17.5 0.90 1.7 0.11 24 1.00 0.335 >100 0.0 0.079 0.610 2.26
2.0 7 (J) 12.5 0.92 2.7 0.15 26 1.05 0.335 >100 0.0 0.087 0.680
2.26 1.8 8 (L) 15.7 0.93 10.8 0.34 36 0.91 0.336 >100 0.5 0.110
0.789 2.25 1.3 9 (N) 17.0 0.83 8.9 0.15 22 0.49 0.336 >100 0.0
0.125 0.693 2.26 2.4
Negative Electrode [6]
Production of Batteries
<<Positive-Electrode Production 1>>
[1788] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained. The positive
electrodes thus obtained had an active-material density of 2.35
g/cm.sup.3.
<<Negative-Electrode Production 1>>
[1789] To 98 parts by weight of a negative-electrode active
material were added 100 parts by weight of an aqueous dispersion of
sodium carboxymethyl cellulose (concentration of sodium
carboxymethyl cellulose, 1% by mass) as a thickener and 2 parts by
weight of an aqueous dispersion of a styrene/butadiene rubber
(concentration of styrene/butadiene rubber, 50% by mass) as a
binder. The ingredients were mixed together by means of a disperser
to obtain a slurry. The slurry obtained was applied to each side of
a rolled copper foil having a thickness of 10 .mu.m and dried. The
coated foil was rolled with a pressing machine to a thickness of 75
.mu.m. Pieces each having an active-material layer size with a
width of 104 mm and a length of 104 mm and including an uncoated
part having a width of 30 mm were cut out of the rolled foil. Thus,
negative electrodes were obtained. The negative electrodes thus
obtained had an active-material density of 1.35 g/cm.sup.3.
[1790] <<Nonaqueous-Electrolytic-Solution Production
1>>
[1791] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, lithium difluorophosphate (LiPO.sub.2F.sub.2)
was added thereto so as to result in a concentration of 0.3% by
mass.
<<Nonaqueous-Electrolytic-Solution Production 2>>
[1792] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, trimethylsilyl methanesulfonate was added
thereto so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 3>>
[1793] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, hexamethylcyclotrisiloxane was added thereto
so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 4>>
[1794] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4).
<<Battery Production 1>>
[1795] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery has a rated
discharge capacity of about 6 ampere-hours (Ah) and a
direct-current resistance component as measured by the 10-kHz
alternating-current method of about 5 milliohms (m.OMEGA.). The
ratio of the total electrode area of the positive electrodes to the
overall surface area of the battery case was 20.6.
Negative Electrode [6]
Example 1
[1796] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Electrode Production 1>> in
which carbonaceous substance (D) was used as the negative-electrode
active material and further using the positive electrodes produced
in <<Positive-Electrode Production 1>> and the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 1>>. This
battery was examined by the methods which will be described under
<<Battery Evaluation>> given later and by the
measurement methods described above. The results are shown in Table
2 (Negative Electrode [6] Table 2).
Negative Electrode [6]
Example 2
[1797] A battery was produced in the same manner as in Negative
Electrode [6] Example 1, except that carbonaceous substance (E) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Example 3
[1798] A battery was produced in the same manner as in Negative
Electrode [6] Example 1, except that carbonaceous substance (G) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Example 4
[1799] A battery was produced in the same manner as in Negative.
Electrode [6] Example 1, except that carbonaceous substance (J) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Example 5
[1800] A battery was produced in the same manner as in Negative
Electrode [6] Example 1, except that carbonaceous substance (L) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Examples 6 to 10
[1801] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [6] Examples 1 to 5, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrode [6] Table 2.
Negative Electrode [6]
Examples 11 to 15
[1802] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [6] Examples 1 to 5, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrode [6] Table 2.
Negative Electrode [6]
Comparative Example 1
[1803] A battery was produced in the same manner as in Negative
Electrode [6] Example 1, except that carbonaceous substance (B) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Comparative Example 2
[1804] A battery was produced in the same manner as in Negative
Electrode [6] Comparative Example 1, except that the nonaqueous
electrolyte used in the Comparative Example was replaced by the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [6] Table 2.
Negative Electrode [6]
Comparative Example 3
[1805] A battery was produced in the same manner as in Negative
Electrode [6] Example 2, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [6] Table 2.
Negative Electrode [6]
Comparative Example 4
[1806] A battery was produced in the same manner as in Negative
Electrode [6] Example 1, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [6] Table 2.
Negative Electrode [6]
Comparative Example 5
[1807] A battery was produced in the same manner as in Negative
Electrode [6] Example 1, except that carbonaceous substance (H) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Comparative Example 6
[1808] A battery was produced in the same manner as in Negative
Electrode [6] Comparative Example 5, except that the nonaqueous
electrolyte used in the Comparative Example was replaced by the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [6] Table 2.
Negative Electrode [6]
Comparative Example 7
[1809] A battery was produced in the same manner as in Negative
Electrode [6] Example 1, except that carbonaceous substance (N) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Comparative Example 8
[1810] A battery was produced in the same manner as in Negative
Electrode [6] Comparative Example 7, except that the nonaqueous
electrolyte used in the Comparative Example was replaced by the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [6] Table 2.
Negative Electrode [6]
Comparative Example 9
[1811] A battery was produced in the same manner as in Negative
Electrode [6] Example 1, except that carbonaceous substance (I) was
used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Comparative Example 10
[1812] A battery was produced in the same manner as in Negative
Electrode [6] Comparative Example 9, except that the nonaqueous
electrolyte used in the Comparative Example was replaced by the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [6] Table 2.
Negative Electrode [6]
Comparative Examples 11 to 14
[1813] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [6] Comparative Examples 1,
5, 7, and 9, except that the nonaqueous electrolyte used in these
Comparative Examples was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
2>>. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Comparative Examples 15 to 18
[1814] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [6] Comparative Examples 1,
5, 7, and 9, except that the nonaqueous electrolyte used in these
Comparative Examples was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
3>>. The results are shown in Negative Electrode [6] Table
2.
Negative Electrode [6]
Battery Evaluation
(Capacity Measurement)
[1815] The fresh battery which had undergone no charge/discharge
was subjected to 5 cycles of initial charge/discharge at 25.degree.
C. in a voltage range of 4.1-3.0 V at 25.degree. C. (voltage range,
4.1-3.0 V). The 0.2-C (the value of current at which the rated
capacity based on 1-hour-rate discharge capacity is discharged over
1 hour is taken as 1 C; the same applies hereinafter) discharge
capacity thereof as measured in the fifth cycle in this operation
was taken as initial capacity.
(Output Measurement 1)
[1816] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C. This battery was
allowed to stand in a -30.degree. C. environment for 3 hours and
then discharged for 10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0
C, and 5.0 C. The voltage was measured at 10 seconds after the
discharge initiation. The area of the triangle surrounded by the
resultant current-voltage straight line and the final lower voltage
(3 V) was taken as initial low-temperature output (W).
(Output Measurement 2)
[1817] After Output Measurement 1, 4.1-V low-voltage charge was
conducted for 1 hour. Thereafter, the battery was transferred to a
25.degree. C. environment, allowed to stand for 15 minutes, and
then discharged for 10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0
C, and 5.0 C. The voltage was measured at 10 seconds after the
discharge initiation. The area of the triangle surrounded by the
resultant current-voltage straight line and the final lower voltage
(3 V) was taken as output during heating up (W).
[1818] Using the following calculation formula, the percentage
improvement in temperature adaptation output (%) was calculated
from the results of Output Measurement 1 and Output Measurement
2.
Percentage improvement in temperature adaptation output
(%)=[(output during heating up (W)/initial low-temperature output
(W))-1].times.100
[1819] Impedance Rct and double-layer capacity Cdl in Negative
Electrode [6] Table 2 are parameters contributing to output. The
smaller the value of impedance Rct and the larger the value of
double-layer capacity Cdl, the more the output tends to improve.
Incidentally, "Impedance Rct" and "Double-layer capacity Cdl" were
determined by the methods described hereinabove under
"Impedance".
TABLE-US-00047 Negative Electrode [6] Table 2 Negative- Percentage
Double- electrode Initial Output improvement in layer active
material low- during temperature capacity Carbonaceous temperature
heating adaptation Impedance Cdl .times. No. substance Specific
compound output W up W output % Rct .OMEGA. 10.sup.-4 F Example 1
(D) lithium difluorophosphate 21.7 29.0 33 6 0.8 Example 2 (E)
lithium difluorophosphate 22.2 31.0 40 8 0.6 Example 3 (G) lithium
difluorophosphate 20.0 24.9 24 13 0.5 Example 4 (J) lithium
difluorophosphate 21.5 27.6 28 100 0 Example 5 (L) lithium
difluorophosphate 21.9 29.6 35 8 0.5 Example 6 (D) trimethylsilyl
methanesulfonate 21.6 27.7 28 9 1 Example 7 (E) trimethylsilyl
methanesulfonate 21.9 29.1 33 11 0.9 Example 8 (G) trimethylsilyl
methanesulfonate 20.2 24.5 21 22 0.6 Example 9 (J) trimethylsilyl
methanesulfonate 21.2 25.9 22 135 0 Example 10 (L) trimethylsilyl
methanesulfonate 21.9 28.7 31 10 1 Example 11 (D)
hexamethylcyclotrisiloxane 22.0 27.9 27 9 0.9 Example 12 (E)
hexamethylcyclotrisiloxane 22.1 30.2 37 9 0.6 Example 13 (G)
hexamethylcyclotrisiloxane 20.1 24.1 20 20 0.5 Example 14 (J)
hexamethylcyclotrisiloxane 21.3 25.8 21 121 0 Example 15 (L)
hexamethylcyclotrisiloxane 21.5 29.2 36 9 0.7 Comparative (B)
lithium difluorophosphate 21.1 24.7 17 7 1 Example 1 Comparative
(B) -- 20.0 21.1 5 13 1.2 Example 2 Comparative (E) -- 20.5 22.7 11
10 0.9 Example 3 Comparative (D) -- 21.5 23.4 9 10 1 Example 4
Comparative (H) lithium difluorophosphate 21.3 23.0 8 36 0.2
Example 5 Comparative (H) -- 20.7 21.2 2 50 0.3 Example 6
Comparative (N) lithium difluorophosphate 19.4 21.4 10 11 0.8
Example 7 Comparative (N) -- 19.0 20.1 6 16 0.7 Example 8
Comparative (I) lithium difluorophosphate 17.5 19.5 11 125 0
Example 9 Comparative (I) -- 17.1 18.3 7 160 0 Example 10
Comparative (B) trimethylsilyl methanesulfonate 21.1 24.7 14 7 1.1
Example 11 Comparative (H) trimethylsilyl methanesulfonate 21.3
23.0 5 44 0.3 Example 12 Comparative (N) trimethylsilyl
methanesulfonate 19.4 21.4 7 13 0.7 Example 13 Comparative (I)
trimethylsilyl methanesulfonate 17.5 19.4 9 131 0 Example 14
Comparative (B) hexamethylcyclotrisiloxane 21.1 24.7 15 9 1.4
Example 15 Comparative (H) hexamethylcyclotrisiloxane 21.3 23.0 6
42 0.4 Example 16 Comparative (N) hexamethylcyclotrisiloxane 19.4
21.4 9 13 0.4 Example 17 Comparative (I) hexamethylcyclotrisiloxane
17.5 19.4 10 142 0 Example 18
[1820] The results given in Negative Electrode [6] Table 2 show the
following. The output recovery from a -30.degree. C.
low-temperature state through heating up was greatly accelerated by
using an electrolyte containing lithium difluorophosphate,
trimethylsilyl methanesulfonate, or hexamethylcyclotrisiloxane and
using a negative electrode containing, as a negative-electrode
active material, graphitic carbon particles having a roundness of
0.85 or higher, an interplanar spacing (d002) for the (002) planes
as determined by wide-angle X-ray diffractometry of smaller than
0.337 nm, and a Raman R value of from 0.12 to 0.8.
Negative Electrode [7]
Production of Negative Electrodes
(Negative-Electrode Production 1)
[1821] A mixture of Si and C (disk having an Si/C areal ratio of
about 100:9) was used as a target material to conduct deposition of
an active-material thin film for 45 minutes on an electrolytic
copper foil having an average surface roughness (Ra) of 0.2 .mu.m,
tensile strength of 280 N/mm.sup.2, 0.2% proof stress of 220
N/mm.sup.2, and thickness of 18 .mu.m as a current collector base
with a direct-current sputtering apparatus ("HSM-52" manufactured
by Shimadzu Corp.). Thus, a thin-film negative electrode (1) was
obtained.
[1822] In this operation, the current collector base was attached
to a water-cooled holder and kept at about 25.degree. C. The
chamber was evacuated to 4.times.10.sup.-4 Pa beforehand.
Thereafter, high-purity argon gas was caused to flow through the
chamber at 40 sccm, and the pressure of the internal atmosphere was
adjusted to 1.6 Pa by regulating the degree of opening of the main
valve. Film deposition was then conducted at a power density of 4.7
W/cm.sup.2 and a deposition rate (film formation rate) of about 1.8
nm/sec (0.108 .mu.m/min). The sputtering gas had an oxygen
concentration of 0.0010%. Prior to the thin-film formation, reverse
sputtering was conducted to etch the surface of the electrolytic
copper foil for the purpose of removing the oxide film present on
the base surface.
[1823] From an examination with a scanning electron microscope
(SEM) of a section of the thin-film negative electrode (1)
obtained, it was found that the thickness of the thin film
deposited was 6 .mu.m (see FIG. 1 (a)). Furthermore, the thin film
was analyzed for composition by XPS by the method shown below. As a
result, the thin film was found to contain the element C in an
amount of 24 at. %, and the C concentration ratio Q(C) to the
element C concentration in the SiC corresponded to 0.49. The value
of Si/C/O in terms of atomic concentration ratio was
1.00/0.33/0.04. Raman values of the thin film were determined by
the following method. As a result, RC=0.05; no peak "sc" concerning
RSC was detected; and RS=0.55. Furthermore, the thin film was
examined by X-ray diffractometry by the following method. As a
result, no clear peak attributable to SiC was detected and
XIsz=0.38. The results obtained are shown in Negative Electrode [7]
Table 1.
[1824] Furthermore, the thin film was examined for the mass
concentration distribution of Si in the film thickness direction
with an electron probe microanalyzer (EPMA) by the method shown
below. As a result, the difference (absolute value) between the
maximum value or minimum value of Si and the average was found to
be within 25%, indicating that Si had been deposited substantially
continuously from the current collector as shown in FIG. 1 (b).
Moreover, the thin film was examined for element C distribution. As
a result, the element C was found to be evenly distributed so as to
have a size of 1 .mu.m or smaller in the thin Si film as shown in
FIG. 1 (c).
(Negative-Electrode Production 2)
[1825] An active-material thin film was deposited in the same
manner as in (Negative-Electrode Production 1), except that Si was
used as a target material. Thus, a thin-film negative electrode (2)
was produced.
[1826] From an examination with a scanning electron microscope
(SEM) of a section of the thin-film negative electrode (2)
obtained, the thin film deposited was found to have a thickness of
6 .mu.m. The thin film was analyzed for composition. As a result,
the thin film was found to contain neither the element C nor the
element N and have an atomic concentration ratio Si/O of 1.00/0.02.
Furthermore, Raman values of the thin film were determined. As a
result, no peak "c" concerning RC was detected, no peak "sc"
concerning RSC was detected, and RS=0.30. The results obtained are
shown in Negative Electrode [7] Table 1.
<XPS>
[1827] In analysis by X-ray photoelectron spectroscopy, an X-ray
photoelectron spectroscope ("ESCA" manufactured by ULVAC-PHI, INC.)
was used. The thin-film negative electrode was placed on the sample
table so that the surface of the electrode was flat, and a depth
profile examination was made while conducting Ar sputtering using
an aluminum K.alpha. line as an X-ray source. A spectrum including
Si2p (90-110 eV), C1s (280-300 eV), and O1s (525-545 eV) was
obtained for a depth (e.g., 200 nm) where concentrations had become
constant. Charge correction was made, with the top of the C1s peak
obtained being at 284.5 eV. The areas of the Si2p, C1s, and O1s
peaks in the spectrum were determined and multiplied by the
apparatus sensitivity coefficient to calculate the atomic
concentrations of Si, C, and O. From the Si, C, and O atomic
concentrations obtained, the atomic concentration ratio Si/C/O (Si
atom concentration/C atom concentration/O atom concentration) is
calculated. This ratio is defined as the composition value Si/C/O
of the thin film.
<Raman Spectroscopy>
[1828] In Raman spectroscopy, a Raman spectrometer ("Raman
Spectrometer" manufactured by Japan Spectroscopic Co., Ltd.) was
used. The thin-film negative electrode was set in a measuring cell
and was examined while irradiating the surface of the sample in the
cell with argon ion laser light.
[1829] Conditions for this Raman spectroscopy are as follows.
[1830] Wavelength of argon ion laser: 514.5 nm [1831] Laser power
on sample: 15-40 mW [1832] Resolution: 10-20 cm.sup.-1 [1833]
Examination range: 200 cm.sup.1 to 1,900 cm.sup.1 [1834] Smoothing:
simple average; convolution, 15 points
<X-Ray Diffractometry>
[1835] In X-ray diffractometry, "RINT 2000PC", manufactured by
Rigaku Corp., was used. The thin-film negative electrode was set in
a measuring cell and examined by the out-of-plane method in the
range of 2.theta.=10-70.degree.. Background correction was made by
connecting a point around 2.theta.=15-20.degree. to a point around
2.theta.=40-45.degree. with a straight line.
<EPMA Examination>
[1836] For EPMA analysis for a film-thickness-direction mass
concentration distribution or a distribution in a thin-film
section, use was made of an electron probe microanalyzer
("JXA-8100" manufactured by JEOL Ltd.). A section of the thin-film
negative electrode was prepared with a microtome without conducting
resin embedding, and the section ranging from the current collector
to the surface of the thin film was subjected to elemental
analysis. In determining a film-thickness-direction mass
concentration distribution, the sum of the found element amounts
was converted to 100% and this value was used to determine the
film-thickness-direction mass concentration distribution of Si.
TABLE-US-00048 Negative Electrode [7] Table 1 Composition of
Negative- Z SiZ.sub.xM.sub.y Atomic electrode concentration
concentration Raman values XIsz of SiZ.sub.xM.sub.y active Kind of
ratio ratio of SiZ.sub.xM.sub.y by X-ray material element Z Q(Z) Si
Z O RC RSC RS diffractometry Thin-film C 0.49 1.00 0.33 0.04 0.05
nopeak 0.55 0.38 negative electrode (1) Thin-film (Si) (0.00) 1.00
(0.00) 0.02 nopeak nopeak 0.30 -- negative electrode (2)
[1837] In Negative Electrode [7] Table 1, (Si) does not fall under
element Z.
Negative Electrode [7]
Production of Positive Electrode
(Positive-Electrode Production 1)
[1838] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to one
side of an aluminum foil having a thickness of 15 .mu.m and dried.
The coated foil was rolled with a pressing machine to a thickness
of 80 .mu.m.
Negative Electrode [7]
Production of Nonaqueous Electrolytic Solutions
(Nonaqueous-Electrolytic-Solution Production 1)
[1839] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, lithium difluorophosphate (LiPO.sub.2F.sub.2)
was added thereto so as to result in a concentration of 0.3% by
mass.
(Nonaqueous-Electrolytic-Solution Production 2)
[1840] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, trimethylsilyl methanesulfonate was added
thereto so as to result in a concentration of 0.3% by mass.
(Nonaqueous-Electrolytic-Solution Production 3)
[1841] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, hexamethylcyclotrisiloxane was added thereto
so as to result in a concentration of 0.3% by mass.
(Nonaqueous-Electrolytic-Solution Production 4)
[1842] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4).
Negative Electrode [7]
Production of Lithium Secondary Battery
[1843] A thin-film negative electrode and a positive electrode each
having a size of 10 mm.phi. were punched out and vacuum-dried at
110.degree. C. Thereafter, the electrodes were transferred to a
gloved box. In an argon atmosphere, the positive electrode and the
negative electrode were disposed face to face through a
polyethylene separator which had been punched out in a size of 14
mm.phi.. A nonaqueous electrolyte was added thereto to produce a
2032 type coin battery (lithium secondary battery).
Negative Electrode [7]
Example 1
[1844] A coin battery was produced by the method described under
<Production of Lithium Secondary Battery> using the negative
electrode produced in (Negative-Electrode Production 1), the
positive electrode produced in (Positive-Electrode Production), and
the nonaqueous electrolyte produced in
(Nonaqueous-Electrolytic-Solution Production 1). This battery was
examined for battery characteristics by the methods which will be
described under <Battery Evaluation> given later.
Negative Electrode [7]
Example 2
[1845] A battery was produced in the same manner as in Negative
Electrode [7] Example 1, except that the nonaqueous electrolyte
used in Negative Electrode [7] Example 1 was replaced by the
nonaqueous electrolyte produced in
(Nonaqueous-Electrolytic-Solution Production 2). Battery evaluation
was conducted by the methods described under <Battery
Evaluation>. The results are shown in Negative Electrode [7]
Table 2.
Negative Electrode [7]
Example 3
[1846] A battery was produced in the same manner as in Negative
Electrode [7] Example 1, except that the nonaqueous electrolyte
used in Negative Electrode [7] Example 1 was replaced by the
nonaqueous electrolyte produced in
(Nonaqueous-Electrolytic-Solution Production 3). Battery evaluation
was conducted by the methods described under <Battery
Evaluation>. The results are shown in Negative Electrode [7]
Table 2.
Negative Electrode [7]
Comparative Example 1
[1847] A battery was produced in the same manner as in Negative
Electrode [7] Example 1, except that the nonaqueous electrolyte
used in Negative Electrode [7] Example 1 was replaced by the
nonaqueous electrolyte produced in
(Nonaqueous-Electrolytic-Solution Production 4). Battery evaluation
was conducted by the methods described under <Battery
Evaluation>. The results are shown in Negative Electrode [7]
Table 2.
Negative Electrode [7]
Comparative Example 2
[1848] A battery was produced in the same manner as in Negative
Electrode [7] Example 1, except that the negative electrode used in
Negative Electrode [7] Example 1 was replaced by the negative
electrode produced in (Negative-Electrode Production 2). Battery
evaluation was conducted by the methods described under <Battery
Evaluation>. The results are shown in Negative Electrode [7]
Table 2.
Negative Electrode [7]
Comparative Example 3
[1849] A battery was produced in the same manner as in Negative
Electrode [7] Comparative Example 2, except that the nonaqueous
electrolyte used in Negative Electrode [7] Comparative Example 2
was replaced by the nonaqueous electrolyte produced in
(Nonaqueous-Electrolytic-Solution Production 2). Battery evaluation
was conducted by the methods described under <Battery
Evaluation>. The results are shown in Negative Electrode [7]
Table 2.
Negative Electrode [7]
Comparative Example 4
[1850] A battery was produced in the same manner as in Negative
Electrode [7] Comparative Example 2, except that the nonaqueous
electrolyte used in Negative Electrode [7] Comparative Example 2
was replaced by the nonaqueous electrolyte produced in
(Nonaqueous-Electrolytic-Solution Production 3). Battery evaluation
was conducted by the methods described under <Battery
Evaluation>. The results are shown in Negative Electrode [7]
Table 2.
Negative Electrode [7]
Comparative Example 5
[1851] A battery was produced in the same manner as in Negative
Electrode [7] Comparative Example 2, except that the nonaqueous
electrolyte used in Negative Electrode [7] Comparative Example 2
was replaced by the nonaqueous electrolyte produced in
(Nonaqueous-Electrolytic-Solution Production 4). Battery evaluation
was conducted by the methods described under <Battery
Evaluation>. The results are shown in Negative Electrode [7]
Table 2.
Negative Electrode [7]
Battery Evaluation
[1852] The coin battery produced in <Production of Lithium
Secondary Battery> was evaluated for discharge capacity and
examined for suitability for charge by the following methods.
<<Evaluation of Discharge Capacity>>
[1853] The lithium counter electrode was charged to 4.2 V at a
current density of 1.23 mA/cm.sup.2 and further charged at a
constant voltage of 4.2 V until the current value reached 0.123 mA
to dope the negative electrode with lithium. Thereafter, the
lithium counter electrode was discharged to 2.5 V at a current
density of 1.23 mA/cm.sup.2. This charge/discharge cycle was
repeated five times, and the average of the discharge capacity
values for the third to fifth cycles was taken as discharge
capacity. In the case of determining discharge capacity per unit
mass, the mass of the active material was determined by subtracting
the mass of the copper foil punched out so as to have the same area
as the negative electrode from the mass of the negative electrode
and that discharge capacity was calculated using the following
formula.
Discharge capacity (mAh/g)=[average discharge capacity for 3rd to
5th cycles (mAh)]/[mass of active material (g)]
Mass of active material (g)=[mass of negative electrode (g)]-[mass
of copper foil having the same area (g)]
<<Examination of Suitability for Charge>>
[1854] The capacity as measured in a 25.degree. C. environment in
charge at 0.2 C (the value of current at which the rated capacity
based on 1-hour-rate discharge capacity is discharged over 1 hour
is taken as 1 C; the same applies hereinafter) to 4.2 V was taken
as 0.2-C charge capacity. This battery was discharged at 0.2 C to
2.5 V and then charged at 1 C to 4.2 V; the capacity as measured in
this charge was taken as 1-C charge capacity. From the results
obtained, suitability for charge was determined using the following
formula.
Suitability for charge (%)=100.times.[1-C charge capacity]/[0.2-C
charge capacity]
TABLE-US-00049 Negative Electrode [7] Table 2 Battery
characteristics Nonaqueous Discharge 0.2-C charge 1-C charge
Suitability electrolyte Negative-electrode capacity capacity
capacity for charge No. Specific compound active material (mAh/g)
(mAh/g) (mAh/g) (%) Remarks Example 1 lithium Thin-film negative
2614 2680 2567 95.8 within the invention difluorophosphate
electrode (1) Example 2 trimethylsilyl Thin-film negative 2598 2690
2566 95.4 within the invention methanesulfonate electrode (1)
Example 3 hexamethylcyclo- Thin-film negative 2621 2670 2488 93.2
within the invention trisiloxane electrode (1) Comparative --
Thin-film negative 2614 2690 2405 89.4 no specific compound Example
1 electrode (1) Comparative lithium Thin-film negative 3920 4012
3406 84.9 no element Z Example 2 difluorophosphate electrode (2)
Comparative trimethylsilyl Thin-film negative 3896 4002 3458 86.4
no element Z Example 3 methanesulfonate electrode (2) Comparative
hexamethylcyclo- Thin-film negative 3945 3989 3486 87.4 no element
Z Example 4 trisiloxane electrode (2) Comparative -- Thin-film
negative 3930 3998 3406 85.2 no element Z Example 5 electrode
(2)
[1855] The results given in Negative Electrode [7] Table 2 show the
following. The lithium secondary batteries which employed a
nonaqueous electrolyte containing lithium difluorophosphate,
trimethylsilyl methanesulfonate, or hexamethylcyclotrisiloxane and
further employed as a negative-electrode active material a
multielement-containing negative-electrode active material (C)
containing at least one of the lithium-occluding metal (A) and/or
lithium-occluding alloy (B) and further containing at least one
element (element Z) selected from C and N had greatly improved
suitability for charge.
Negative Electrode [8]
Production of Negative-Electrode Active Materials
(Negative-Electrode Active-Material Production 1)
[1856] A natural-graphite powder (d002, 0.336 nm; Lc, 100 nm or
larger; Raman R value, 0.11; tap density, 0.46 g/cm.sup.3; true
density, 2.27 g/cm.sup.3; volume-average particle diameter, 35.4
.mu.m) was treated with Hybridization System (Hybridization System
Type NHS-1, manufactured by Nara Machinery Co., Ltd.) under the
conditions of a treatment amount of 90 g, rotor peripheral speed of
60 m/s, and treatment period of 3 minutes to thereby round the
particles. Furthermore, this powder was repeatedly passed through
an ASTM 400-mesh sieve five times in order to prevent inclusion of
coarse particles. The negative-electrode active material thus
obtained is referred to as carbonaceous substance (A). This
operation was repeated to thereby obtain the active material in an
amount necessary for battery production.
(Negative-Electrode Active-Material Production 2)
[1857] A commercial powder of natural flake graphite was treated
with an air classifier to remove fine particles. The powder
obtained was repeatedly passed through an ASTM 400-mesh sieve five
times in order to prevent inclusion of coarse particles. The
negative-electrode active material thus obtained is referred to as
carbonaceous substance (B).
(Negative-Electrode Active-Material Production 3)
[1858] Coal tar pitch having a quinoline insoluble content of 0.05%
by mass or lower was heat-treated at 460.degree. C. for 10 hours in
a reaction furnace to obtain a massive carbonized precursor which
was meltable and had a softening point of 385.degree. C. The
massive carbonized precursor obtained was packed into a metallic
container and heat-treated at 1,000.degree. C. for 2 hours in a
nitrogen gas stream in a box-shaped electric furnace. The amorphous
mass obtained was crushed with a crusher (Roll Jaw Crusher,
manufactured by Yoshida Seisakusho Co., Ltd.) and further
pulverized with a pulverizer (Turbo Mill, manufactured by Matsubo
Corp.). Thereafter, the powder was treated with an air classifier
to remove fine particles. The powder obtained was repeatedly passed
through an ASTM 400-mesh sieve five times in order to prevent
inclusion of coarse particles. Thus, an amorphous powder having a
volume-average diameter of 9 .mu.m was obtained. The
negative-electrode active material thus obtained is referred to as
carbonaceous substance (C).
(Negative-Electrode Active-Material Production 4)
[1859] Carbonaceous substance (A) was mixed with a
petroleum-derived heavy oil obtained in thermal naphtha cracking,
and this mixture was carbonized at 1,300.degree. C. in an inert
gas. Thereafter, the resultant sinter was classified to thereby
obtain a carbonaceous substance comprising particles of the
carbonaceous substance (A) and a carbonaceous substance having
different crystallinity adherent to the surface of the particles.
In the classification, the sinter was passed through an ASTM
400-mesh sieve five times in order to prevent inclusion of coarse
particles. Thus, carbonaceous substance (D) was obtained. It was
ascertained from actual carbon ratio that the negative-electrode
active-material powder obtained comprised 95 parts by weight of a
graphite and 5 parts by weight of a carbonaceous substance which
was derived from the petroleum-derived heavy oil and with which the
graphite was covered.
(Negative-Electrode Active-Material Production 5)
[1860] Eighty percents by mass carbonaceous substance (A) was mixed
with 20% by mass carbonaceous substance (B) until the mixture
became homogeneous. Thus, carbonaceous-substance mixture (E) was
obtained.
(Negative-Electrode Active-Material Production 6)
[1861] Ninety-five percents by mass carbonaceous substance (A) was
mixed with 5% by mass carbonaceous substance (C) until the mixture
became homogeneous. Thus, carbonaceous-substance mixture (F) was
obtained.
(Negative-Electrode Active-Material Production 7)
[1862] Eighty percents by mass carbonaceous substance (D) was mixed
with 20% by mass carbonaceous substance (A) until the mixture
became homogeneous. Thus, carbonaceous-substance mixture (G) was
obtained.
[1863] Carbonaceous substances (A), (B), and (C) and
carbonaceous-substance mixtures (E), (F), and (G), which were
produced in Negative-Electrode Active-Material Productions 1 to 7,
were examined for properties, shape, etc. by the methods described
above. The results obtained are summarized in Negative Electrode
[8] Table 1.
TABLE-US-00050 Negative Electrode [8] Table 1 BET Inter- Negative-
specific Raman planar electrode Median Mode surface Raman R
half-value Tap spacing Ash True Aspect active diameter diameter Z
Roundness area value width density d002 Lc content density ratio
material .mu.m .mu.m .mu.m -- m.sup.2/g -- cm.sup.-1 g/cm.sup.3 nm
nm mass % g/cm.sup.3 -- Carbonaceous 14.7 14.8 0.1 0.93 10.9 0.32
28 0.95 0.335 >100 0.07 2.26 1.2 substance (A) Carbonaceous 12.2
12.4 0.2 0.90 7.5 0.12 20 0.22 0.335 >100 0.03 2.26 1.8
substance (B) Carbonaceous 9.7 9.8 0.1 0.89 8.5 1.05 90 0.68 0.343
2.2 0.05 2.10 1.9 substance (C) Carbonaceous 15.2 15.4 0.2 0.93 3.0
0.35 37 0.98 0.336 >100 0.07 2.25 1.4 substance (D)
Carbonaceous- 13.8 14.2 0.4 0.93 10.1 0.29 26 0.80 0.335 >100
0.06 2.26 1.3 substance mixture (E) Carbonaceous- 14.3 14.6 0.3
0.93 10.7 0.37 28 0.91 0.336 >100 0.07 2.25 1.2 substance
mixture (F) Carbonaceous- 13.8 14.8 1.0 0.92 3.7 0.31 36 0.85 0.336
>100 0.06 2.25 1.5 substance mixture (G)
Negative Electrode [8]
Production of Batteries
<<Positive-Electrode Production 1>>
[1864] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained. The positive
electrodes thus obtained had an active-material density of 2.35
g/cm.sup.3.
<<Negative-Electrode Production 1>>
[1865] To 98 parts by weight of a negative-electrode active
material were added 100 parts by weight of an aqueous dispersion of
sodium carboxymethyl cellulose (concentration of sodium
carboxymethyl cellulose, 1% by mass) as a thickener and 2 parts by
weight of an aqueous dispersion of a styrene/butadiene rubber
(concentration of styrene/butadiene rubber, 50% by mass) as a
binder. The ingredients were mixed together by means of a disperser
to obtain a slurry. The slurry obtained was applied to each side of
a rolled copper foil having a thickness of 10 .mu.m and dried. The
coated foil was rolled with a pressing machine to a thickness of 75
.mu.m. Pieces each having an active-material layer size with a
width of 104 mm and a length of 104 mm and including an uncoated
part having a width of 30 mm were cut out of the rolled foil. Thus,
negative electrodes were obtained. The negative electrodes thus
obtained had an active-material density of 1.35 g/cm.sup.3.
<<Nonaqueous-Electrolytic-Solution Production 1>>
[1866] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, lithium difluorophosphate (LiPO.sub.2F.sub.2)
was added thereto so as to result in a concentration of 0.3% by
mass.
<<Nonaqueous-Electrolytic-Solution Production 2>>
[1867] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, trimethylsilyl methanesulfonate was added
thereto so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 3>>
[1868] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, hexamethylcyclotrisiloxane was added thereto
so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 4>>
[1869] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4).
<<Battery Production 1>>
[1870] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery has a rated
discharge capacity of about 6 ampere-hours (Ah) and a
direct-current resistance component as measured by the 10-kHz
alternating-current method of about 5 milliohms (m.OMEGA.). The
ratio of the total electrode area of the positive electrodes to the
overall surface area of the battery case was 20.6.
Negative Electrode [8]
Example 1
[1871] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Electrode Production 1>> in
which carbonaceous-substance mixture (E) was used as the
negative-electrode active material and further using the positive
electrodes produced in <<Positive-Electrode Production
1>> and the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 1>>. This
battery was subjected to the battery evaluation described under
<<Battery Evaluation>> given later. The results are
shown in Negative Electrode [8] Table 2.
Negative Electrode [8]
Example 2
[1872] A battery was produced in the same manner as in Negative
Electrode [8] Example 1, except that carbonaceous-substance mixture
(F) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [8] Table
2.
Negative Electrode [8]
Example 3
[1873] A battery was produced in the same manner as in Negative
Electrode [8] Example 1, except that carbonaceous-substance mixture
(G) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [8] Table
2.
Negative Electrode [8]
Examples 4 to 6
[1874] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [8] Examples 1 to 3, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrode [8] Table 2.
Negative Electrode [8]
Examples 7 to 9
[1875] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [8] Examples 1 to 3, except
that the nonaqueous electrolyte used in these Examples was replaced
by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrode [8] Table 2.
Negative Electrode [8]
Comparative Example 1
[1876] A battery was produced in the same manner as in Negative
Electrode [8] Example 1, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [8] Table 2.
Negative Electrode [8]
Comparative Example 2
[1877] A battery was produced in the same manner as in Negative
Electrode [8] Example 2, except that the nonaqueous electrolyte
used in the Example was replaced by the nonaqueous electrolyte
produced in <<Nonaqueous-Electrolytic-Solution Production
4>>. The battery evaluation described under <<Battery
Evaluation>> was conducted. The results are shown in Negative
Electrode [8] Table 2.
Negative Electrode [8]
Comparative Example 3
[1878] A battery was produced in the same manner as in Negative
Electrode [8] Comparative Example 1, except that carbonaceous
substance (A) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [8] Table
2.
Negative Electrode [8]
Comparative Example 4
[1879] A battery was produced in the same manner as in Negative
Electrode [8] Comparative Example 1, except that carbonaceous
substance (B) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [8] Table
2.
Negative Electrode [8]
Comparative Example 5
[1880] A battery was produced in the same manner as in Negative
Electrode [8] Comparative Example 1, except that carbonaceous
substance (C) was used as the negative-electrode active material in
<<Negative-Electrode Production 1>>. The battery
evaluation described under <<Battery Evaluation>> was
conducted. The results are shown in Negative Electrode [8] Table
2.
Negative Electrode [8]
Comparative Examples 6 to 8
[1881] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [8] Comparative Examples 3 to
5, except that the nonaqueous electrolyte used in these Comparative
Examples was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 1>>. The
results are shown in Negative Electrode [8] Table 2.
Negative Electrode [8]
Comparative Examples 9 to 11
[1882] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [8] Comparative Examples 3 to
5, except that the nonaqueous electrolyte used in these Comparative
Examples was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrode [8] Table 2.
Negative Electrode [8]
Comparative Examples 12 to 14
[1883] Batteries were produced and evaluated respectively in the
same manners as in Negative Electrode [8] Comparative Examples 3 to
5, except that the nonaqueous electrolyte used in these Comparative
Examples was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrode [8] Table 2.
Negative Electrode [8]
Battery Evaluation
(Capacity Measurement)
[1884] The fresh battery which had undergone no charge/discharge
was subjected to 5 cycles of initial charge/discharge at 25.degree.
C. in a voltage range of 4.1-3.0 Vat 25.degree. C. (voltage range,
4.1-3.0 V). The 0.2-C (the value of current at which the rated
capacity based on 1-hour-rate discharge capacity is discharged over
1 hour is taken as 1 C; the same applies hereinafter) discharge
capacity thereof as measured in the fifth cycle in this operation
was taken as initial capacity.
(Output Measurement 1)
[1885] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C. This battery was
allowed to stand in a -30.degree. C. environment for 3 hours and
then discharged for 10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0
C, and 5.0 C. The voltage was measured at 10 seconds after the
discharge initiation. The area of the triangle surrounded by the
resultant current-voltage straight line and the final lower voltage
(3 V) was taken as low-temperature output (W).
(Cycle Test)
[1886] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.2 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment. The 0.2-C
discharge capacity thereof as measured in the third cycle was taken
as capacity after cycling. Using the following calculation formula,
the capacity retention through cycling was determined from the
initial capacity measured prior to the cycling and the capacity
after cycling measured after completion of the cycle test.
Capacity retention through cycling (%)=100.times.(capacity after
cycling)/(initial capacity)
TABLE-US-00051 NEGATIVE ELECTRODE [8] TABLE 2 Capacity Low-
retention Negative- temperature through electrode Specific output
cycling No. active material compound W % Ex. 1 Carbonaceous-
lithium 26.2 81 substance difluorophosphate mixture (E) Ex. 2
Carbonaceous- lithium 27.4 80 substance difluorophosphate mixture
(F) Ex. 3 Carbonaceous- lithium 29.4 85 substance difluorophosphate
mixture (G) Ex. 4 Carbonaceous- trimethylsilyl 24.7 80 substance
methanesulfonate mixture (E) Ex. 5 Carbonaceous- trimethylsilyl
28.2 78 substance methanesulfonate mixture (F) Ex. 6 Carbonaceous-
trimethylsilyl 29.7 84 substance methanesulfonate mixture (G) Ex. 7
Carbonaceous- hexamethylcyclo- 26.1 80 substance trisiloxane
mixture (E) Ex. 8 Carbonaceous- hexamethylcyclo- 29.1 77 substance
trisiloxane mixture (F) Ex. 9 Carbonaceous- hexamethylcyclo- 28.8
84 substance trisiloxane mixture (G) Comp. Carbonaceous- -- 21.1 78
Ex. 1 substance mixture (E) Comp. Carbonaceous- -- 23.2 75 Ex. 2
substance mixture (F) Comp. Carbonaceous -- 20.5 77 Ex. 3 substance
(A) Comp. Carbonaceous -- 24.5 71 Ex. 4 substance (B) Comp.
Carbonaceous -- 27.5 69 Ex. 5 substance (C) Comp. Carbonaceous
lithium 22.2 79 Ex. 6 substance (A) difluorophosphate Comp.
Carbonaceous lithium 25.8 72 Ex. 7 substance (B) difluorophosphate
Comp. Carbonaceous lithium 29.4 70 Ex. 8 substance (C)
difluorophosphate Comp. Carbonaceous trimethylsilyl 21.7 80 Ex. 9
substance (A) methanesulfonate Comp. Carbonaceous trimethylsilyl
25.2 72 Ex. 10 substance (B) methanesulfonate Comp. Carbonaceous
trimethylsilyl 28.8 70 Ex. 11 substance (C) methanesulfonate Comp.
Carbonaceous hexamethylcyclo- 21.7 78 Ex. 12 substance (A)
trisiloxane Comp. Carbonaceous hexamethylcyclo- 24.5 72 Ex. 13
substance (B) trisiloxane Comp. Carbonaceous hexamethylcyclo- 28.2
69 Ex. 14 substance (C) trisiloxane
[1887] The results given in Negative Electrode [8] Table 2 show the
following. Both satisfactory cycle characteristics and satisfactory
low-temperature output are imparted by using an electrolyte
containing lithium difluorophosphate, trimethylsilyl
methanesulfonate, or hexamethylcyclotrisiloxane and using a
negative electrode containing two or more negative-electrode active
materials differing in property.
Negative Electrodes [9] and [10]
Production of Negative-Electrode Active Materials
(Negative-Electrode Active-Material Production 1)
[1888] Highly purified natural flake graphite having a median
diameter of about 150 .mu.m (d002, 0.336 nm; Lc, 100 nm or larger;
Raman R value, 0.11; true density, 2.27 g/cm.sup.3; ash content,
0.05% by mass) was subjected to a rounding treatment with a
rounding apparatus (Hybridization System, manufactured by Nara
Machinery Co., Ltd.) at a rotation speed of 6,500 rpm for 5
minutes. Furthermore, this graphite was treated with an air
classifier (OMC-100, manufactured by Seishin Enterprise Co., Ltd.)
to remove 45% by weight fine particles therefrom. Thus,
carbonaceous substance (A) was obtained.
(Negative-Electrode Active-Material Production 2)
[1889] Carbonaceous substance (A) produced in (Negative-Electrode
Active-Material Production 1) was packed into a graphite crucible
and heat-treated with a direct voltage application type electric
furnace at 3,000.degree. C. for 5 hours in an inert atmosphere to
obtain carbonaceous substance (B).
(Negative-Electrode Active-Material Production 3)
[1890] Carbonaceous substance (A) produced in (Negative-Electrode
Active-Material Production 1) was mixed with a petroleum-derived
heavy oil obtained in thermal naphtha cracking, and this mixture
was carbonized at 1,300.degree. C. in an inert gas. Thereafter, the
resultant sinter was classified to thereby obtain a carbonaceous
substance comprising particles of the carbonaceous substance (A)
and a carbonaceous substance having different crystallinity
adherent to the surface of the particles. In the classification,
the sinter was passed through an ASTM 400-mesh sieve five times in
order to prevent inclusion of coarse particles. Thus, carbonaceous
substance (C) was obtained. It was ascertained from actual carbon
ratio that the negative-electrode active-material powder obtained
comprised 95 parts by weight of a graphite and 5 parts by weight of
a carbonaceous substance which was derived trom the
petroleum-derived heavy oil and with which the graphite was
covered.
(Negative-Electrode Active-Material Production 4)
[1891] Coal tar pitch having a quinoline insoluble content of 0.05%
by mass or lower was heat-treated at 460.degree. C. for 10 hours in
a reaction furnace to obtain a massive carbonized precursor which
was meltable and had a softening point of 385.degree. C. The
massive carbonized precursor obtained was packed into a metallic
container and heat-treated at 1,000.degree. C. for 2 hours in a
nitrogen gas stream in a box-shaped electric furnace. The amorphous
mass obtained was crushed with a crusher (Roll Jaw Crusher,
manufactured by Yoshida Seisakusho Co., Ltd.) and further
pulverized with a pulverizer (Turbo Mill, manufactured by Matsubo
Corp.). Thus, an amorphous powder having a volume-average particle
diameter of 17 .mu.m was obtained. The powder obtained was
repeatedly passed through an ASTM 400-mesh sieve five times in
order to prevent inclusion of coarse particles. The
negative-electrode active material thus obtained is referred to as
carbonaceous substance (D).
[1892] Carbonaceous substance (A), carbonaceous substance (B),
carbonaceous substance (C), and carbonaceous substance (D) produced
above were examined for shape and properties by the methods
described above. The results obtained are shown in Negative
Electrodes [9] and [10] Table 1.
TABLE-US-00052 Negative Electrodes [9] and [10] Table 1 Volume- BET
Raman Inter- Negative- Total average specific Raman half- planar
electrode Tap Pore pore particle surface R value spacing
Crystallite True Aspect active density volume volume diameter
Roundness area value width d002 size Lc Ash density ratio material
g/cm.sup.3 mL/g mL/g .mu.m -- m.sup.2/g -- cm.sup.-1 nm nm content
% g/cm.sup.3 -- Carbonaceous 0.99 0.228 0.78 16.5 0.94 7.5 0.27
24.0 0.335 >100 0.05 2.26 1.1 substance (A) Carbonaceous 1.00
0.215 0.75 17.0 0.94 5.4 0.03 20.5 0.335 >100 0.02 2.26 1.1
substance (B) Carbonaceous 1.08 0.203 0.68 16.5 0.93 2.7 0.35 34.0
0.336 >100 0.07 2.25 1.2 substance (C) Carbonaceous 0.91 0.079
0.45 16.6 0.89 5.9 1.05 91.0 0.343 2.3 0.05 1.91 1.9 substance
(D)
Negative Electrodes [9] and [10]
Production of Batteries
<<Positive-Electrode Production 1>>
[1893] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having a positive-electrode
active-material layer size with a width of 100 mm and a length of
100 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, positive electrodes were
obtained. In the positive electrodes thus obtained, the
positive-electrode active-material layers had a density of 2.35
g/cm.sup.3.
<<Negative-Electrode Production 1>>
[1894] To 98 parts by weight of a negative-electrode active
material were added 100 parts by weight of an aqueous dispersion of
sodium carboxymethyl cellulose (concentration of sodium
carboxymethyl cellulose, 1% by mass) as a thickener and 2 parts by
weight of an aqueous dispersion of a styrene/butadiene rubber
(concentration of styrene/butadiene rubber, 50% by mass) as a
binder. The ingredients were mixed together by means of a disperser
to obtain a slurry. The slurry obtained was applied to each side of
a rolled copper foil having a thickness of 10 .mu.m as a current
collector and dried. The coated foil was rolled with a pressing
machine to a thickness of 75 .mu.m. Pieces each having a
negative-electrode active-material layer size with a width of 104
mm and a length of 104 mm and including an uncoated part having a
width of 30 mm were cut out of the rolled foil. Thus, negative
electrodes were obtained.
[1895] In the negative electrodes thus obtained, the
negative-electrode active-material layers had a density in the
range of 1.33-1.36 g/cm.sup.3 (shown in the rightmost column in
Negative Electrodes [9] and [10] Table 2). As apparent from the
above, the content of the styrene/butadiene rubber as a binder for
the negative-electrode active-material layers is 1% by mass based
on the whole negative-electrode active-material layers.
Furthermore, the value of (thickness of the negative-electrode
active-material layer on one side)/(thickness of the current
collector) is (75 .mu.m)/(10 .mu.m)=7.5.
<<Nonaqueous-Electrolytic-Solution Production 1>>
[1896] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, lithium difluorophosphate (LiPO.sub.2F.sub.2)
was added thereto so as to result in a concentration of 0.3% by
mass.
<<Nonaqueous-Electrolytic-Solution Production 2>>
[1897] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, trimethylsilyl methanesulfonate was added
thereto so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 3>>
[1898] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4). Furthermore, hexamethylcyclotrisiloxane was added thereto
so as to result in a concentration of 0.3% by mass.
<<Nonaqueous-Electrolytic-Solution Production 4>>
[1899] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4).
<<Battery Production 1>>
[1900] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery has a rated
discharge capacity of about 6 ampere-hours (Ah) and a
direct-current resistance component as measured by the 10-kHz
alternating-current method of about 5 milliohms (m.OMEGA.). The
ratio of the total electrode area of the positive electrodes to the
overall surface area of the battery case was 20.6.
Negative Electrodes [9] and [10]
Example 1
[1901] A battery was produced by the method described under
<<Battery Production 1>> using the negative electrodes
produced in <<Negative-Electrode Production 1>> in
which carbonaceous substance (A) was used as the negative-electrode
active material and further using the positive electrodes produced
in <<Positive-Electrode Production 1>> and the
nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 1>>. This
battery was examined by the methods which will be described under
<<Battery Evaluation>> given later. The results are
shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 2
[1902] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 1, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 1 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 3
[1903] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 1, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 1 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Comparative Example 1
[1904] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 1, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 1 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 4
[1905] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 1, except that
carbonaceous substance (B) was used as the negative-electrode
active material in <<Negative-Electrode Production 1>>
in Negative Electrodes [9] and [10] Example 1. The results are
shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 5
[1906] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 4, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 4 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrodes [9] and [10].
Negative Electrodes [9] and [10]
Example 6
[1907] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 4, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 4 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Comparative Example 2
[1908] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 4, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 4 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 7
[1909] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 1, except that
carbonaceous substance (C) was used as the negative-electrode
active material in <<Negative-Electrode Production 1>>
in Negative Electrodes [9] and [10] Example 1. The results are
shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 8
[1910] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 7, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 7 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 9
[1911] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 7, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 7 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Comparative Example 3
[1912] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 7, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 7 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 10
[1913] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 1, except that
carbonaceous substance (D) was used as the negative-electrode
active material in <<Negative-Electrode Production 1>>
in Negative Electrodes [9] and [10] Example 1. The results are
shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 11
[1914] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 10, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 10 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 2>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Example 12
[1915] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 10, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 10 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 3>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Comparative Example 4
[1916] A battery was produced and evaluated in the same manner as
in Negative Electrodes [9] and [10] Example 10, except that the
nonaqueous electrolyte used in Negative Electrodes [9] and [10]
Example 10 was replaced by the nonaqueous electrolyte produced in
<<Nonaqueous-Electrolytic-Solution Production 4>>. The
results are shown in Negative Electrodes [9] and [10] Table 2.
Negative Electrodes [9] and [10]
Battery Evaluation
(Capacity Measurement)
[1917] The battery which had undergone no charge/discharge cycling
was subjected to 5 cycles of initial charge/discharge at 25.degree.
C. in a voltage range of 4.2-3.0 V at a current of 0.2 C (the value
of current at which the rated capacity based on 1-hour-rate
discharge capacity is discharged over 1 hour is taken as 1 C; the
same applies hereinafter). The 0.2-C discharge capacity thereof as
measured in the fifth cycle in this operation was taken as initial
capacity. An output measurement shown below was then made.
(Output Measurement)
[1918] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W). The output as measured before the cycle test
is referred to as "initial output".
(Cycle Test)
[1919] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.2 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment. The 0.2-C
discharge capacity thereof as measured in the third cycle was taken
as capacity after cycle test. Using the following calculation
formula, the capacity retention through cycling was determined from
the initial capacity measured prior to the cycle test and the
capacity after cycle test measured after completion of the cycle
test.
Capacity retention through cycling (%)=100.times.(capacity after
cycle test)/(initial capacity)
[1920] Impedance Rct and double-layer capacity Cdl in Negative
Electrodes [9] and [10] Table 2 are parameters contributing to
output. The smaller the value of impedance Rct and the larger the
value of double-layer capacity Cdl, the more the output tends to
improve. Incidentally, "Impedance Rct" and "Double-layer capacity
Cdl" were determined by the methods described hereinabove under
"Impedance".
[1921] The battery which had undergone the cycle test was subjected
to the output measurement described under (Output Measurement), and
the value obtained is referred to as "output after cycle test".
With respect to the lithium secondary batteries obtained in the
Negative Electrodes [9] and [10] Examples and Negative Electrodes
[9] and [10] Comparative Examples given above, the results of the
output measurement, results of the capacity measurement, capacity
retentions through cycling, and results of reaction resistance and
double-layer capacity measurement in the opposing impedance
measurement are summarized in Negative Electrodes [9] and [10]
Table 2.
TABLE-US-00053 Negative Electrodes [9] and [10] Table 2 Density of
negative- Impedance electrode Capacity Cdl Double- active-
Negative- Initial Output retention Impedance Rct layer material
electrode capacity Initial after through Reaction capacity .times.
layer No. active material Specific compound mAh output W cycling W
cycling % resistance .OMEGA. 10.sup.-4 F g/cm.sup.3 Ex. 1
Carbonaceous lithium 6021 558 398 82 9.0 1.5 1.36 substance (A)
difluorophosphate Ex. 2 Carbonaceous trimethylsilyl 6022 545 384 82
9.5 1.4 1.36 substance (A) methanesulfonate Ex. 3 Carbonaceous
hexamethylcyclo- 6020 531 375 81 10.8 1.4 1.36 substance (A)
trisiloxane Comp. Carbonaceous -- 6099 486 290 78 12.1 1.3 1.36 Ex.
1 substance (A) Ex. 4 Carbonaceous lithium 6032 551 350 81 10.4 1.5
1.35 substance (B) difluorophosphate Ex. 5 Carbonaceous
trimethylsilyl 6021 525 332 79 10.8 1.4 1.35 substance (B)
methanesulfonate Ex. 6 Carbonaceous hexamethylcyclo- 6112 524 315
78 10.8 1.4 1.35 substance (B) trisiloxane Comp. Carbonaceous --
6099 469 275 71 12.3 1.5 1.35 Ex. 2 substance (B) Ex. 7
Carbonaceous lithium 5998 570 445 85 16.4 1.2 1.36 substance (C)
difluorophosphate Ex. 8 Carbonaceous trimethylsilyl 6021 564 423 83
17.8 1.1 1.36 substance (C) methanesulfonate Ex. 9 Carbonaceous
hexamethylcyclo- 6070 554 418 82 15.6 1.2 1.36 substance (C)
trisiloxane Comp. Carbonaceous -- 5989 507 340 79 19.4 1.1 1.36 Ex.
3 substance (C) Ex. 10 Carbonaceous lithium 5991 546 346 77 32.0
0.9 1.33 substance (D) difluorophosphate Ex. 11 Carbonaceous
trimethylsilyl 5987 557 337 76 41.0 0.8 1.33 substance (D)
methanesulfonate Ex. 12 Carbonaceous hexamethylcyclo- 5982 541 335
76 34.2 0.9 1.33 substance (D) trisiloxane Comp. Carbonaceous --
5889 521 287 73 48.0 0.7 1.33 Ex. 4 substance (D)
[1922] The results given in Negative Electrodes [9] and [10] Table
2 show the following. A lithium secondary battery which, even when
large, can attain a high output after cycling and a high capacity
retention through cycling and has both a high output and a long
life can be provided by using a nonaqueous electrolyte containing
the specific compound, e.g., lithium difluorophosphate,
trimethylsilyl methanesulfonate, or hexamethylcyclotrisiloxane, and
by using a negative electrode containing a negative-electrode
active material which has a tap density of 0.1 or higher and a pore
volume, in terms of the volume of pores in the range of from 0.01
.mu.m to 1 .mu.m as determined by mercury porosimetry, of 0.01 mL/g
or larger.
[1923] It was further found that the lithium secondary batteries
which employ a nonaqueous electrolyte containing the specific
compound and satisfy the requirement that the reaction resistance
in a negative-electrode opposing cell is 500.OMEGA. or lower can
attain a high output after cycling and a high capacity retention
through cycling and have both a high output and a satisfactory
life, even when large.
Electrolytic Solution [1]
Secondary-Battery Production
[Production of Positive Electrode]
[1924] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained.
[Production of Negative Electrode]
[1925] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained.
[Battery Fabrication]
[1926] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery has a rated
discharge capacity of about 6 Ah and a direct-current resistance as
measured by the 10-kHz alternating-current method of about 5
milliohms
[Battery Evaluation]
(Method of Measuring Initial Capacity)
[1927] The fresh battery which had undergone no charge/discharge
was subjected to 5 cycles of initial charge/discharge at 25.degree.
C. in a voltage range of 4.1-3.0 Vat 25.degree. C. (voltage range,
4.1-3.0 V). The 0.2-C (the value of current at which the rated
capacity based on 1-hour-rate discharge capacity is discharged over
1 hour is taken as 1 C; the same applies hereinafter) discharge
capacity thereof as measured in the fifth cycle in this operation
was taken as initial capacity.
(Method of Measuring Low-Temperature Output)
[1928] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C. This battery was
discharged in a -30.degree. C. environment for 10 seconds at each
of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 5.0 C. The voltage was measured
at 10 seconds after the discharge initiation. The area of the
triangle surrounded by the resultant current-voltage straight line
and the final lower voltage (3 V) was taken as output (W).
Electrolytic Solution [1]
Example 1
[1929] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio,
15:85) in such an amount as to result in a concentration of 1
mol/L. Hexamethylcyclotrisiloxane was mixed with this solution in
such an amount as to result in a concentration of 0.3% by mass.
Thus, a nonaqueous electrolyte was prepared. This nonaqueous
electrolyte was used to produce a battery by the method described
above. The battery was examined for low-temperature output. The
results are shown in Electrolytic Solution [1] Table 1.
Electrolytic Solution [1]
Example 2
[1930] In Electrolytic Solution [1] Example 1, a nonaqueous
electrolyte was prepared using EC and EMC in a volume ratio of
20:80 and this nonaqueous electrolyte was used to produce a
battery. This battery was examined for low-temperature output. The
results are shown in Electrolytic Solution [1] Table 1.
Electrolytic Solution [1]
Example 3
[1931] In Electrolytic Solution [1] Example 1, a nonaqueous
electrolyte was prepared using a mixture of EC, dimethyl carbonate
(DMC), and EMC (volume ratio, 15:40:45) in place of that nonaqueous
solvent and this nonaqueous electrolyte was used to produce a
battery. This battery was examined for low-temperature output. The
results are shown in Electrolytic Solution [1] Table 1.
Electrolytic Solution [1]
Example 4
[1932] In Electrolytic Solution [1] Example 1,
phenyldimethylfluorosilane was mixed in place of the
hexamethylcyclotrisiloxane and the nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Example 5
[1933] In Electrolytic Solution [1] Example 1, hexamethyldisiloxane
was mixed in place of the hexamethylcyclotrisiloxane and the
nonaqueous electrolyte thus prepared was used to produce a battery.
This battery was examined for low-temperature output. The results
are shown in Electrolytic Solution [1] Table 1.
Electrolytic Solution [1]
Example 6
[1934] In Electrolytic Solution [1] Example 1, trimethylsilyl
methanesulfonate was mixed in place of the
hexamethylcyclotrisiloxane and the nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Example 7
[1935] In Electrolytic Solution [1] Example 1, methyl
fluorosulfonate was mixed in place of the
hexamethylcyclotrisiloxane and the nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Example 8
[1936] In Electrolytic Solution [1] Example 1, lithium nitrate was
mixed in place of the hexamethylcyclotrisiloxane and the nonaqueous
electrolyte thus prepared was stored for 3 days and then used to
produce a battery. This battery was examined for low-temperature
output. The results are shown in Electrolytic Solution [1] Table
1.
Electrolytic Solution [1]
Example 9
[1937] In Electrolytic Solution [1] Example 1, a nonaqueous
electrolyte containing lithium difluorophosphate produced by the
method described in Inorganic Nuclear Chemistry Letters (1969),
5(7), pp. 581-582 in place of the hexamethylcyclotrisiloxane was
used to produce a battery. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Example 10
[1938] In Electrolytic Solution [1] Example 9, the amount of the
lithium difluorophosphate to be mixed was changed to 0.08% by mass
based on the solution prepared by mixing the nonaqueous solvent and
the lithium hexafluorophosphate. The nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Example 11
[1939] In Electrolytic Solution [1] Example 1, lithium acetate was
mixed in place of the hexamethylcyclotrisiloxane and the nonaqueous
electrolyte thus prepared was stored for 3 days and then used to
produce a battery. This battery was examined for low-temperature
output. The results are shown in Electrolytic Solution [1] Table
1.
Electrolytic Solution [1]
Example 12
Secondary-Battery Production 2
[Production of Positive Electrode]
[1940] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 20 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. A strip having a width of 52 mm and a length
of 830 mm was cut out of the rolled foil to obtain a positive
electrode. This positive electrode had a 50-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides, and the active-material layer had a length of 780 mm.
[Production of Negative Electrode]
[1941] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was evenly applied to each side of a copper foil having a
thickness of 18 .mu.m as a negative-electrode current collector and
dried. The coated foil was rolled with a pressing machine to a
thickness of 85 .mu.m. A strip having a width of 56 mm and a length
of 850 mm was cut out of the rolled foil to obtain a negative
electrode. This negative electrode had a 30-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides.
[Production of Electrolytic Solution]
[1942] The same nonaqueous electrolyte as in Electrolytic Solution
[1] Example 1 was produced.
[Battery Fabrication]
[1943] The positive electrode and the negative electrode were
superposed and wound together with polyethylene separators so as to
prevent the positive electrode and the negative electrode from
coming into direct contact with each other to thereby obtain an
electrode roll. This electrode roll was packed into a battery can
so that the terminal of the positive electrode and that of the
negative electrode protruded outward.
[1944] Subsequently, 5 mL of the electrolyte described later was
introduced into the battery can, which was then caulked. Thus, a
18650 type cylindrical battery was produced. This battery had a
rated discharge capacity of about 0.7 ampere-hours (Ah) and a
direct-current resistance as measured by the 10-kHz
alternating-current method of about 35 milliohms (m.OMEGA.). The
overall electrode area of the positive electrode is 19.4 times the
surface area of the case of this secondary battery. This battery
was examined for low-temperature output in the same manner as in
Electrolytic Solution [1] Example 1. The results are shown in
Electrolytic Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 1
[1945] In Electrolytic Solution [1] Example 1, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for low-temperature output. The results are
shown in Electrolytic Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 2
[1946] In Electrolytic Solution [1] Example 2, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for low-temperature output. The results are
shown in Electrolytic Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 3
[1947] In Electrolytic Solution [1] Example 3, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for low-temperature output. The results are
shown in Electrolytic Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 4
[1948] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate and ethyl methyl carbonate (volume ratio, 4:6) in such an
amount as to result in a concentration of 1 mol/L.
Hexamethylcyclotrisiloxane was mixed with this solution in such an
amount as to result in a concentration of 0.3% by mass. Thus, a
nonaqueous electrolyte was prepared.
[1949] This nonaqueous electrolyte was used to produce a battery.
The battery was examined for low-temperature output. The results
are shown in Electrolytic Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 5
[1950] In Electrolytic Solution [1] Comparative Example 4,
phenyldimethylfluorosilane was mixed in place of the
hexamethylcyclotrisiloxane and the nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 6
[1951] In Electrolytic Solution [1] Comparative Example 4,
hexamethyldisiloxane was mixed in place of the
hexamethylcyclotrisiloxane and the nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 7
[1952] In Electrolytic Solution [1] Comparative Example 4,
trimethylsilyl methanesulfonate was mixed in place of the
hexamethylcyclotrisiloxane and the nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 8
[1953] In Electrolytic Solution [1] Comparative Example 4, methyl
fluorosulfonate was mixed in place of the
hexamethylcyclotrisiloxane and the nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 9
[1954] In Electrolytic Solution [1] Comparative Example 4, lithium
nitrate was mixed in place of the hexamethylcyclotrisiloxane and
the nonaqueous electrolyte thus prepared was stored for 3 days and
then used to produce a battery. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 10
[1955] In Electrolytic Solution [1] Comparative Example 4, lithium
difluorophosphate was mixed in place of the
hexamethylcyclotrisiloxane and the nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 11
[1956] In Electrolytic Solution [1] Comparative Example 4, lithium
acetate was mixed in place of the hexamethylcyclotrisiloxane and
the nonaqueous electrolyte thus prepared was stored for 3 days and
then used to produce a battery. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
Electrolytic Solution [1]
Comparative Example 12
[1957] In Electrolytic Solution [1] Comparative Example 4, a
nonaqueous electrolyte was prepared without mixing
hexamethylcyclotrisiloxane and this nonaqueous electrolyte was used
to produce a battery. This battery was examined for low-temperature
output. The results are shown in Electrolytic Solution [1] Table
1.
Electrolytic Solution [1]
Comparative Example 13
[1958] In Electrolytic Solution [1] Example 12, the same nonaqueous
electrolyte as in Electrolytic Solution [1] Comparative Example 1
was used to produce a battery. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [1] Table 1.
TABLE-US-00054 ELECTROLYTIC SOLUTION [1] TABLE 1 Proportion
Low-temperature of EC output Solvent (vol %) Compound contained (W)
Remarks Example 1 EC:EMC 15 haxamethylcyclotrisiloxane 20.1 26.4%
increase based on Comparative Example 1 Example 2 EC:EMC 20
hexamethylcyclotrisiloxane 19.9 25.9% increase based on Comparative
Example 2 Example 3 EC:DMC:EMC 15 hexamethylcyclotrisiloxane 20.5
27.3% increase based on Comparative Example 3 Example 4 EC:EMC 15
phenyldimethylfluorosilane 19.8 24.5% increase based on Comparative
Example 1 Example 5 EC:EMC 15 hexamethyldisiloxane 20.2 27.0%
increase based on Comparative Example 1 Example 6 EC:EMC 15
trinethylsilyl methanesulfonate 20.4 28.3% increase based on
Comparative Example 1 Example 7 EC:EMC 15 methyl fluorosulfonate
19.8 24.5% increase based on Comparative Example 1 Example 8 EC:EMC
15 lithium nitrate 20.0 25.8% increase based on Comparative Example
1 Example 9 EC:EMC 15 lithium difluorophosphate (0.3%) 20.8 30.8%
increase based on Comparative Example 1 Example 10 EC:EMC 15
lithium difluorophosphate (0.08%) 19.0 19.5% increase based on
Comparative Example 1 Example 11 EC:EMC 15 lithium acetate 19.9
25.2% increase based on Comparative Example 1 Example 12 EC:EMC 15
hexamethylcyclotrisiloxane 1.86 20.8% increase based on Comparative
Example 13 Comparative Example 1 EC:EMC 15 none 15.9 Comparative
Example 2 EC:EMC 20 none 15.8 Comparative Example 3 EC:DMC:EMC 15
none 16.1 Comparative Example 4 EC:EMC 40
hexamethylcyclotrisiloxane 17.6 15.0% increase based on Comparative
Example 12 Comparative Example 5 EC:EMC 40
phenyldimethylfluorosilane 17.5 14.4% increase based on Comparative
Example 12 Comparative Example 6 EC:EMC 40 hexamethyldisiloxane
17.6 15.0% increase based on Comparative Example 12 Comparative
Example 7 EC:EMC 40 trinethylsilyl methanesulfonate 17.8 16.3%
increase based on Comparative Example 12 Comparative Example 8
EC:EMC 40 methyl fluorosulfonate 17.5 14.4% increase based on
Comparative Example 12 Comparative Example 9 EC:EMC 40 lithium
nitrate 17.7 15.7% increase based on Comparative Example 12
Comparative Example 10 EC:EMC 40 lithium difluorophosphate 17.9
17.0% increase based on Comparative Example 12 Comparative Example
11 EC:EMC 40 lithium acetate 17.6 15.0% increase based on
Comparative Example 12 Comparative Example 12 EC:EMC 40 none 15.3
Comparative Example 13 EC:EMC 15 none 1.54
[1959] Electrolytic Solution [1] Table 1 shows the following. The
lithium secondary batteries of Electrolytic Solution [1] Examples 1
to 11, in each of which the nonaqueous electrolyte contains a
specific amount of EC and the specific compound, have improved
low-temperature output characteristics as compared not only with
the lithium secondary battery of Electrolytic Solution [1]
Comparative Example 12, in which the electrolyte contains excess EC
and does not contain the specific compound, but also with the
lithium secondary batteries of Electrolytic Solution [1]
Comparative Examples 1 to 3, in which the electrolytes contain EC
in an amount in a specific range but do not contain the specific
compound, and the lithium secondary batteries of Electrolytic
Solution [1] Comparative Examples 4 to 11, in which the
electrolytes contain the specific compound but contain excess C. It
was further found that the lithium secondary battery of
Electrolytic Solution [1] Example 12, in which the nonaqueous
electrolyte contains a specific amount of EC and the specific
compound, has improved low-temperature output characteristics as
compared with the lithium secondary battery of Electrolytic
Solution [1] Comparative Example 13, in which the electrolyte has
an EC amount in the specific range but does not contain the
specific compound.
[1960] That effect is not a mere addition of effects to be brought
about by the two requirements. When both of the two requirements
are satisfied, the effects thereof are clearly amplified.
Electrolytic Solution [1] Table 1 includes the percentage increase
in low-temperature output of each battery employing a nonaqueous
electrolyte containing the specific compound based on a battery
employing a nonaqueous electrolyte which does not contain the
specific compound (and which, except this, has the same
composition). The values of that percentage increase for
Electrolytic Solution [1] Examples 1 to 12 are larger than for
Electrolytic Solution [1] Comparative Examples 4 to 11, in which
the EC amount is excessively large, indicating that the effect of
this invention is high.
[1961] The percentage increase in output for Electrolytic Solution
[1] Example 12 based on the battery of Electrolytic Solution [1]
Comparative Example 13 is 20.8%. On the other hand, in the case of
Electrolytic Solution [1] Example 1 and Comparative Example 1,
which have a different battery structure although employing the
same materials, the percentage increase in output for Electrolytic
Solution [1] Example 1 based on the battery of Electrolytic
Solution [1] Comparative Example 1 is 26.4%. This indicates that
battery structure influences the degree of the effect of the
nonaqueous electrolyte according to the invention. Namely, the
effect of this invention is high especially in high-capacity
batteries and in batteries having a low direct-current
resistance.
[1962] Furthermore, although not shown in the table, a battery
employing an electrolyte having an EC content lower than 1% by
volume had a slightly lower initial capacity than the battery of
Electrolytic Solution [1] Example 1. This battery was poor in
output characteristics at ordinary temperature and in cycle
characteristics.
[1963] As described above, it has become possible to impart
exceedingly high low-temperature output characteristics by using
the nonaqueous electrolyte according to this invention, i.e., a
nonaqueous electrolyte for secondary battery which is characterized
by employing as the nonaqueous solvent a mixed solvent at least
containing ethylene carbonate, the proportion of the ethylene
carbonate to the whole nonaqueous solvent being from 1% by volume
to 25% by volume, and by containing at least one compound selected
from the group consisting of cyclic siloxane compounds represented
by general formula (1), fluorosilane compounds represented by
general formula (2), compounds represented by general formula (3),
compounds having an S--F bond in the molecule, nitric acid salts,
nitrous acid salts, monofluorophosphoric acid salts,
difluorophosphoric acid salts, acetic acid salts, and propionic
acid salts in an amount of 10 ppm or more of the whole nonaqueous
electrolyte.
[1964] Furthermore, it has become possible that such effect can be
more remarkably produced in battery structures such as that of
Electrolytic Solution [1] Example 1, i.e., high-capacity batteries
and batteries having a low direct-current resistance, than in
battery structures such as that of Electrolytic Solution [1]
Example 12.
Electrolytic Solution [2]
Secondary-Battery Production 1
[Production of Positive Electrode]
[1965] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum toil having a thickness of 20 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. A strip having a width of 52 mm and a length
of 830 mm was cut out of the rolled foil to obtain a positive
electrode. This positive electrode had a 50-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides, and the active-material layer had a length of 780 mm.
[Production of Negative Electrode]
[1966] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was evenly applied to each side of a copper foil having a
thickness of 18 .mu.m as a negative-electrode current collector and
dried. The coated foil was rolled with a pressing machine to a
thickness of 85 .mu.m. A strip having a width of 56 mm and a length
of 850 mm was cut out of the rolled foil to obtain a negative
electrode. This negative electrode had a 30-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides.
[Battery Fabrication]
[1967] The positive electrode and the negative electrode were
superposed and wound together with polyethylene separators so as to
prevent the positive electrode and the negative electrode from
coming into direct contact with each other to thereby obtain an
electrode roll. This electrode roll was packed into a battery can
so that the terminal of the positive electrode and that of the
negative electrode protruded outward. Subsequently, 5 mL of the
electrolyte which will be described later was introduced into the
battery can, which was then caulked. Thus, a 18650 type cylindrical
battery was produced. The electric capacity of the battery elements
held in one battery case of this secondary battery, i.e., the rated
discharge capacity of this battery, was about 0.7 ampere-hours
(Ah). This battery had a direct-current resistance component as
measured by the 10-kHz alternating-current method of about 35
milliohms (m.OMEGA.).
[Battery Evaluation]
(Initial Charge/Discharge)
[1968] At 25.degree. C., the battery produced was charged to 4.2 V
by the constant-current constant-voltage charging method at 0.2 C
and then discharged to 3.0 V at a constant current of 0.2 C. This
operation as one cycle was repeated to conducted 5 cycles and
thereby stabilize the battery. The discharge capacity thereof as
measured in the fifth cycle was taken as initial capacity.
Incidentally, the value of current at which the rated capacity
based on 1-hour-rate discharge capacity is discharged over 1 hour
is taken as 1 C.
(Cycle Test)
[1969] At 60.degree. C., the battery which had undergone the
initial charge/discharge was charged to 4.2 V by the
constant-current constant-voltage method at 1 C and then discharged
to 3.0 V at a constant current of 1 C. This charge/discharge
operation as one cycle was repeated to conduct 500 cycles. The
proportion of the discharge capacity of the battery as measured in
the 500th cycle to the discharge capacity thereof as measured in
the first cycle was taken as capacity retention through
cycling.
(Low-Temperature Test)
[1970] The battery which had undergone the initial charge/discharge
was charged at 25.degree. C. to 4.2 V by the constant-current
constant-voltage charging method at 0.2 C and then discharged at
-30.degree. C. at a constant current of 0.2 C. The discharge
capacity thereof as measured in this discharge was taken as initial
low-temperature capacity. The proportion of the initial
low-temperature capacity to the initial capacity was taken as
proportion of initial low-temperature discharge.
[1971] Furthermore, at 25.degree. C., the battery which had
undergone the cycle test was charged to 4.2 V by the
constant-current constant-voltage charging method at 0.2 C and then
discharged to 3.0 V at a constant current of 0.2 C. This operation
as one cycle was repeated to conduct 3 cycles. The discharge
capacity thereof as measured in the third cycle was taken as
capacity after cycling. Thereafter, the same battery was charged at
25.degree. C. to 4.2 V by the constant-current constant-voltage
charging method at 0.2 C and then discharged at -30.degree. C. at a
constant current of 0.2 C. The discharge capacity thereof as
measured in this discharge was taken as low-temperature capacity
after cycling. The proportion of the low-temperature capacity after
cycling to the capacity after cycling was taken as proportion of
low-temperature discharge after cycling.
Electrolytic Solution [2]
Example 1
[1972] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate
(DEC) (volume ratio, 2:4:4) in such an amount as to result in a
concentration of 0.9 mol/L. Lithium difluorophosphate produced by
the method described in Inorganic Nuclear Chemistry Letters (1969),
5(7), pp. 581-582 was incorporated into that solution in such an
amount as to result in a concentration of 0.3% by mass. Thus, a
nonaqueous electrolyte was prepared. This nonaqueous electrolyte
was used to produce a 18650 type cylindrical battery. The battery
was examined for capacity retention through cycling and proportions
of low-temperature discharge. The results are shown in Electrolytic
Solution [2] Table 1. Incidentally, the electrolyte was recovered
with a centrifugal separator from the battery which had undergone
the measurement of low-temperature capacity after cycling, and
subjected to transesterification reaction. The amount of the
dimethyl carbonate (DMC) yielded by the reaction was determined by
gas chromatography (GC) and, as a result, was found to be 0.5% by
mass based on the electrolyte.
Electrolytic Solution [2]
Comparative Example 1
[1973] A 18650 type cylindrical battery was produced in the same
manner as in Electrolytic Solution [2] Example 1, except that
lithium difluorophosphate was not incorporated. This battery was
examined for capacity retention through cycling and proportions of
low-temperature discharge in the same manners. The results are
shown in Electrolytic Solution [2] Table 1. Incidentally, the
electrolyte was recovered with a centrifugal separator from the
battery which had undergone the measurement of low-temperature
capacity after cycling, and subjected to transesterification
reaction. The amount oi the dimethyl carbonate (DMC) yielded by the
reaction was determined by gas chromatography (GC) and, as a
result, was found to be 9.7% by mass based on the electrolyte.
Electrolytic Solution [2]
Comparative Example 2
[1974] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate (EC) and diethyl carbonate (DEC) (volume ratio, 2:8) in
such an amount as to result in a concentration of 0.9 mol/L.
Lithium difluorophosphate was incorporated into this solution in
such an amount as to result in a concentration of 0.3% by mass.
Thus, a nonaqueous electrolyte was prepared. This nonaqueous
electrolyte was used to produce a 18650 type cylindrical battery.
The battery was examined for capacity retention through cycling and
proportions of low-temperature discharge. The results are shown in
Electrolytic Solution [2] Table 1.
Electrolytic Solution [2]
Example 2
[1975] A 18650 type cylindrical battery was produced in the same
manner as in Electrolytic Solution [2] Example 1, except that
methyl n-propyl carbonate was used in place of the ethyl methyl
carbonate. This battery was examined for capacity retention through
cycling and proportions of low-temperature discharge in the same
manners. The results are shown in Electrolytic Solution [2] Table
1.
Electrolytic Solution [2]
Comparative Example 3
[1976] A 18650 type cylindrical battery was produced in the same
manner as in Electrolytic Solution [2] Example 2, except that
lithium difluorophosphate was not incorporated. This battery was
examined for capacity retention through cycling and proportions of
low-temperature discharge in the same manners. The results are
shown in Electrolytic Solution [2] Table 1.
Electrolytic Solution [2]
Secondary-Battery Production 2
<Production of Positive Electrode>
[1977] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained.
<Production of Negative Electrode>
[1978] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained.
<Battery Fabrication>
[1979] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of the nonaqueous electrolyte which will be
described later was introduced into the battery can packed with the
electrode group and sufficiently infiltrated into the electrodes.
This can was sealed to fabricate a prismatic battery. The electric
capacity of the battery elements held in one battery case of this
secondary battery, i.e., the rated discharge capacity of this
battery, was about 6 ampere-hours (Ah). This battery had a
direct-current resistance component as measured by the 10-kHz
alternating-current method of about 5 milliohms (m.OMEGA.).
Electrolytic Solution [2]
Example 3
[1980] The electrolyte used in Electrolytic Solution [2] Example 1
was used to produce a prismatic battery. This battery was subjected
to the cycle test and the low-temperature test in the same manners
as in Electrolytic Solution [2] Example 1 to determine the capacity
retention through cycling and the proportions of low-temperature
discharge thereof. The results are shown in Electrolytic Solution
[2] Table 1.
Electrolytic Solution [2]
Comparative Example 4
[1981] The electrolyte used in Electrolytic Solution [2]
Comparative Example 1 was used to produce a prismatic battery. This
battery was subjected to the cycle test and the low-temperature
test to determine the capacity retention through cycling and the
proportions of low-temperature discharge thereof. The results are
shown in Electrolytic Solution [2] Table 1.
Electrolytic Solution [2]
Comparative Example 5
[1982] The electrolyte used in Electrolytic Solution [2]
Comparative Example 2 was used to produce a prismatic battery. This
battery was subjected to the cycle test and the low-temperature
test to determine the capacity retention through cycling and the
proportions of low-temperature discharge thereof. The results are
shown in Electrolytic Solution [2] Table 1.
TABLE-US-00055 ELECTROLYTIC SOLUTION [2] TABLE 1 Capacity
Proportion of Proportion of low- retention initial low- temperature
through temperature discharge after cycling discharge cycling No.
Battery (%) (%) (%) Example 1 cylindrical 64 68 64 Comparative
cylindrical 58 59 50 Example 1 Comparative cylindrical 60 60 50
Example 2 Example 2 cylindrical 63 65 63 Comparative cylindrical 55
57 45 Example 3 Example 3 prismatic 73 74 71 Comparative prismatic
68 62 53 Example 4 Comparative prismatic 68 63 59 Example 5
[1983] As apparent from Electrolytic Solution [2] Table 1, a
comparison among the cylindrical batteries (Electrolytic Solution
[2] Examples 1 and 2 and Electrolytic Solution [2] Comparative
Examples 1 to 3) and a separate comparison among the prismatic
batteries (Electrolytic Solution [2] Example 3 and Electrolytic
Solution [2] Comparative Examples 4 and 5) show the following. The
lithium secondary batteries of the Electrolytic Solution [2]
Examples, in each of which the nonaqueous electrolyte contains both
an asymmetric chain carbonate and a difluorophosphoric acid salt,
have been improved in both of capacity retention through cycling
and the proportions of low-temperature discharge as compared with
the lithium secondary batteries of the Electrolytic Solution [2]
Comparative Examples, in each of which the nonaqueous electrolyte
does not contain either of these.
[1984] As described above, the cylindrical batteries of the
Examples and the Comparative Examples each have a rated discharge
capacity lower than 3 ampere-hours (Ah) and a direct-current
resistance component higher than 10 milliohms (m.OMEGA.). On the
other hand, the prismatic batteries of the Example and the
Comparative Examples each have a rated discharge capacity of 3
ampere-hours (Ah) or higher and a direct-current resistance
component of 10 milliohms (m.OMEGA.) or lower. Namely, the
prismatic batteries of the Example and the Comparative Examples
have a lower resistance and a higher electric capacity than the
cylindrical batteries. Furthermore, the degree of improvement in
low-temperature characteristics of Electrolytic Solution [2]
Example 3 on Electrolytic Solution [2] Comparative Example 4 is
higher than the degree of improvement in low-temperature
characteristics of Electrolytic Solution [2] Example 1 on
Electrolytic Solution [2] Comparative Example 1. It was thus found
that the effect of this invention is higher in secondary batteries
having a high electric capacity and secondary batteries having a
low direct-current resistance.
Electrolytic Solution [3]
Secondary-Battery Production 1
[Production of Positive Electrode]
[1985] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum toil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained.
[Production of Negative Electrode]
[1986] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained.
[Battery Fabrication]
[1987] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm).
[1988] Thereafter, 20 mL of the nonaqueous electrolyte shown in
each of the Electrolytic Solution [3] Examples and Electrolytic
Solution [3] Comparative Examples which will be given later was
introduced into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. This battery had a rated
discharge capacity of about 6 ampere-hours (Ah) and a
direct-current resistance as measured by the 10-kHz
alternating-current method of about 5 milliohms (m.OMEGA.). The
ratio of the total electrode area of the positive electrodes to the
surface area of the case of this secondary battery is 20.65.
[Battery Evaluation]
(Capacity Measurement)
[1989] The fresh battery which had undergone no charge/discharge
was subjected to 5 cycles of initial charge/discharge at 25.degree.
C. in a voltage range of 4.1-3.0 V at 25.degree. C. (voltage range,
4.1-3.0 V). The 0.2-C (the value of current at which the rated
capacity based on 1-hour-rate discharge capacity is discharged over
1 hour is taken as 1 C; the same applies hereinafter) discharge
capacity thereof as measured in the fifth cycle in this operation
was taken as initial capacity.
(Output Measurement)
[1990] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C. This battery was
discharged in a -30.degree. C. environment for 10 seconds at each
of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 5.0 C. The voltage was measured
at 10 seconds after the discharge initiation. The area of the
triangle surrounded by the resultant current-voltage straight line
and the final lower voltage (3 V) was taken as output (W).
Electrolytic Solution [3]
Example 1
[1991] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate (EC), ethyl methyl carbonate (EMC), and methyl propionate
(MP) (volume ratio, 3:6:1) in such an amount as to result in a
concentration of 1 mol/L. Hexamethylcyclotrisiloxane was mixed with
this solution in such an amount as to result in a concentration of
0.3% by mass. Thus, a nonaqueous electrolyte was prepared. This
nonaqueous electrolyte was used to produce a battery by the method
described above. The battery was examined for low-temperature
output. The results are shown in Electrolytic Solution [3] Table
1.
Electrolytic Solution [3]
Example 2
[1992] In Electrolytic Solution [3] Example 1, a nonaqueous
electrolyte was prepared using ethyl acetate (EA), in place of the
methyl propionate, in the same amount and this nonaqueous
electrolyte was used to produce a battery. This battery was
examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Example 3
[1993] In Electrolytic Solution [3] Example 1, a nonaqueous
electrolyte was prepared using methyl acetate (MA), in place of the
methyl propionate, in the same amount and this nonaqueous
electrolyte was used to produce a battery. This battery was
examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Example 4
[1994] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate (EC), ethyl methyl carbonate (EMC), methyl propionate
(MP), and methyl acetate (MA) (volume ratio, 30:60:5:5) in such an
amount as to result in a concentration of 1 mol/L.
Hexamethylcyclotrisiloxane was mixed with this solution in such an
amount as to result in a concentration of 0.3% by mass. Thus, a
nonaqueous electrolyte was prepared. This nonaqueous electrolyte
was used to produce a battery by the method described above. The
battery was examined for low-temperature output. The results are
shown in Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Example 5
[1995] In Electrolytic Solution [3] Example 1, a nonaqueous
electrolyte was prepared using a mixture of ethylene carbonate
(EC), ethyl methyl carbonate (EMC), and methyl propionate (MP)
having a volume ratio changed to 3:4:3, and this nonaqueous
electrolyte was used to produce a battery by the method described
above. This battery was examined for low-temperature output. The
results are shown in Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Example 6
[1996] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate (EC) and methyl propionate (MP) (volume ratio, 3:7) in
such an amount as to result in a concentration of 1 mol/L.
Hexamethylcyclotrisiloxane was mixed with this solution in such an
amount as to result in a concentration of 0.3% by mass. Thus, a
nonaqueous electrolyte was prepared. This nonaqueous electrolyte
was used to produce a battery by the method described above. The
battery was examined for low-temperature output. The results are
shown in Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Example 7
[1997] In Electrolytic Solution [3] Example 1,
phenyldimethylfluorosilane was mixed, in place of the
hexamethylcyclotrisiloxane, in the same amount to prepare a
nonaqueous electrolyte and this nonaqueous electrolyte was used to
produce a battery. This battery was examined for low-temperature
output. The results are shown in Electrolytic Solution [3] Table
1.
Electrolytic Solution [3]
Example 8
[1998] In Electrolytic Solution [3] Example 1, hexamethyldisiloxane
was mixed, in place of the hexamethylcyclotrisiloxane, in the same
amount to prepare a nonaqueous electrolyte and this nonaqueous
electrolyte was used to produce a battery. This battery was
examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Example 9
[1999] In Electrolytic Solution [3] Example 1, trimethylsilyl
methanesulfonate was mixed, in place of the
hexamethylcyclotrisiloxane, in the same amount to prepare a
nonaqueous electrolyte and this nonaqueous electrolyte was used to
produce a battery. This battery was examined for low-temperature
output. The results are shown in Electrolytic Solution [3] Table
1.
Electrolytic Solution [3]
Example 10
[2000] In Electrolytic Solution [3] Example 1, methyl
fluorosulfonate was mixed, in place of the
hexamethylcyclotrisiloxane, in the same amount to prepare a
nonaqueous electrolyte and this nonaqueous electrolyte was used to
produce a battery. This battery was examined for low-temperature
output. The results are shown in Electrolytic Solution [3] Table
1.
Electrolytic Solution [3]
Example 11
[2001] In Electrolytic Solution [3] Example 1, lithium nitrate was
mixed, in place of the hexamethylcyclotrisiloxane, in the same
amount to prepare a nonaqueous electrolyte and this nonaqueous
electrolyte was stored for 3 days and then used to produce a
battery. This battery was examined for low-temperature output. The
results are shown in Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Example 12
[2002] In Electrolytic Solution [3] Example 1, lithium
difluorophosphate produced by the method described in Inorganic
Nuclear Chemistry Letters (1969), 5 (7), pp. 581-582 was used, in
place of the hexamethylcyclotrisiloxane, in the same amount (amount
resulting in a concentration of 0.3% by mass based on the solution
prepared by mixing the nonaqueous solvent and the lithium
hexafluorophosphate) to prepare a nonaqueous electrolyte and this
nonaqueous electrolyte was used to produce a battery. This battery
was examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Example 13
[2003] In Electrolytic Solution [3] Example 12, the amount of the
lithium difluorophosphate to be mixed was changed to 0.08% by mass
based on the solution prepared by mixing the nonaqueous solvent and
the lithium hexafluorophosphate. The nonaqueous electrolyte thus
prepared was used to produce a battery. This battery was examined
for low-temperature output. The results are shown in Electrolytic
Solution [3] Table 1.
Electrolytic Solution [3]
Example 14
Secondary-Battery Production 2
<Production of Positive Electrode>
[2004] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 20 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. A strip having a width of 52 mm and a length
of 830 mm was cut out of the rolled foil to obtain a positive
electrode. This positive electrode had a 50-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides, and the active-material layer had a length of 780 mm.
<Production of Negative Electrode>
[2005] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was evenly applied to each side of a copper foil having a
thickness of 18 .mu.m as a negative-electrode current collector and
dried. The coated foil was rolled with a pressing machine to a
thickness of 85 .mu.m. A strip having a width of 56 mm and a length
of 850 mm was cut out of the rolled foil to obtain a negative
electrode. This negative electrode had a 30-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides.
<Production of Electrolytic Solution>
[2006] The same nonaqueous electrolyte as in Electrolytic Solution
[3] Example 1 was produced.
<Battery Fabrication>
[2007] The positive electrode and the negative electrode were
superposed and wound together with polyethylene separators so as to
prevent the positive electrode and the negative electrode from
coming into direct contact with each other to thereby obtain an
electrode roll. This electrode roll was packed into a battery can
so that the terminal of the positive electrode and that of the
negative electrode protruded outward. Subsequently, 5 mL of the
electrolyte described later was introduced into the battery can,
which was then caulked. Thus, a 18650 type cylindrical battery was
produced. This battery had a rated discharge capacity of about 0.7
ampere-hours (Ah) and a direct-current resistance as measured by
the 10-kHz alternating-current method of about 35 milliohms
(m.OMEGA.). The overall electrode area of the positive electrode is
19.4 times the surface area of the case of this secondary battery.
This battery was examined for low-temperature output in the same
manner as in Electrolytic Solution [3] Example 1. The results are
shown in Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 1
[2008] In Electrolytic Solution [3] Example 1, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery by
the method described above. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 2
[2009] In Electrolytic Solution [3] Example 2, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery by
the method described above. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 3
[2010] In Electrolytic Solution [3] Example 3, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery by
the method described above. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 4
[2011] In Electrolytic Solution [3] Example 4, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery by
the method described above. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 5
[2012] In Electrolytic Solution [3] Example 5, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery by
the method described above. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 6
[2013] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate and ethyl methyl carbonate (volume ratio, 3:7) in such an
amount as to result in a concentration of 1 mol/L. Thus, a
nonaqueous electrolyte was prepared. This nonaqueous electrolyte
was used to produce a battery by the method described above. The
battery was examined for low-temperature output. The results are
shown in Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 7
[2014] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate and ethyl methyl carbonate (volume ratio, 3:7) in such an
amount as to result in a concentration of 1 mol/L.
Hexamethylcyclotrisiloxane was mixed with this solution in such an
amount as to result in a concentration of 0.3% by mass. Thus, a
nonaqueous electrolyte was prepared. This nonaqueous electrolyte
was used to produce a battery by the method described above. The
battery was examined for low-temperature output. The results are
shown in Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 8
[2015] In Electrolytic Solution [3] Comparative Example 7,
phenyldimethylfluorosilane was mixed, in place of the
hexamethylcyclotrisiloxane, in the same amount to prepare a
nonaqueous electrolyte and this nonaqueous electrolyte was used to
produce a battery by the method described above. This battery was
examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 9
[2016] In Electrolytic Solution [3] Comparative Example 7,
hexamethyldisiloxane was mixed, in place of the
hexamethylcyclotrisiloxane, in the same amount to prepare a
nonaqueous electrolyte and this nonaqueous electrolyte was used to
produce a battery by the method described above. This battery was
examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 10
[2017] In Electrolytic Solution [3] Comparative Example 7,
trimethylsilyl methanesulfonate was mixed, in place of the
hexamethylcyclotrisiloxane, in the same amount to prepare a
nonaqueous electrolyte and this nonaqueous electrolyte was used to
produce a battery by the method described above. This battery was
examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 11
[2018] In Electrolytic Solution [3] Comparative Example 7, methyl
fluorosulfonate was mixed, in place of the
hexamethylcyclotrisiloxane, in the same amount to prepare a
nonaqueous electrolyte and this nonaqueous electrolyte was used to
produce a battery by the method described above. This battery was
examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 12
[2019] In Electrolytic Solution [3] Comparative Example 7, lithium
nitrate was mixed, in place of the hexamethylcyclotrisiloxane, in
the same amount to prepare a nonaqueous electrolyte and this
nonaqueous electrolyte was stored for 3 days and then used to
produce a battery by the method described above. This battery was
examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 13
[2020] In Electrolytic Solution [3] Comparative Example 7, lithium
difluorophosphate was mixed, in place of the
hexamethylcyclotrisiloxane, in the same amount to prepare a
nonaqueous electrolyte and this nonaqueous electrolyte was used to
produce a battery by the method described above. This battery was
examined for low-temperature output. The results are shown in
Electrolytic Solution [3] Table 1.
Electrolytic Solution [3]
Comparative Example 14
[2021] In Electrolytic Solution [3] Example 14, a nonaqueous
electrolyte prepared in the same manner as in Electrolytic Solution
[3] Comparative Example 1 was used to produce a battery by the
method described above. This battery was examined for
low-temperature output. The results are shown in Electrolytic
Solution [3] Table 1.
TABLE-US-00056 ELECTROLYTIC SOLUTION [3] TABLE 1 Low-temperature
Nonaqueous output No. solvent Specific compound (mass %) (W)
Example 1 EC:EMC:MP hexamethylcyclotrisiloxane (0.3%) 20.3 Example
2 EC:EMC:EA hexamethylcyclotrisiloxane (0.3%) 20.5 Example 3
EC:EMC:MA hexamethylcyclotrisiloxane (0.3%) 20.6 Example 4
EC:EMC:MA:MP hexamethylcyclotrisiloxane (0.3%) 20.5 Example 5
EC:EMC:MP hexamethylcyclotrisiloxane (0.3%) 20.0 Example 6 EC:MP
hexamethylcyclotrisiloxane (0.3%) 19.1 Example 7 EC:EMC:MP
phenyldimethylfluorosilane (0.3%) 20.0 Example 8 EC:EMC:MP
hexamethyldisiloxane (0.3%) 20.5 Example 9 EC:EMC:MP trimethylsilyl
methanesulfonate (0.3%) 20.6 Example 10 EC:EMC:MP methyl
fluorosulfonate (0.3%) 20.0 Example 11 EC:EMC:MP lithium nitrate
(0.3%) 20.2 Example 12 EC:EMC:MP lithium difluorophosphate (0.3%)
21.2 Example 13 EC:EMC:MP lithium difluorophosphate (0.08%) 19.0
Example 14 EC:EMC:MP hexamethylcyclotrisiloxane (0.3%) 1.84
Comparative EC:EMC:MP none 15.7 Example 1 Comparative EC:EMC:EA
none 15.8 Example 2 Comparative EC:EMC:MA none 16.1 Example 3
Comparative EC:EMC:MA:MP none 16.0 Example 4 Comparative EC:MP none
15.7 Example 5 Comparative EC:EMC none 15.5 Example 6 Comparative
EC:EMC hexamethylcyclotrisiloxane (0.3%) 18.3 Example 7 Comparative
EC:EMC phenyldimethylfluorosilane (0.3%) 18.0 Example 8 Comparative
EC:EMC hexamethyldisiloxane (0.3%) 18.4 Example 9 Comparative
EC:EMC trimethylsilyl methanesulfonate (0.3%) 18.5 Example 10
Comparative EC:EMC methyl fluorosulfonate (0.3%) 18.1 Example 11
Comparative EC:EMC lithium nitrate (0.3%) 18.4 Example 12
Comparative EC:EMC lithium difluorophosphate (0.3%) 18.6 Example 13
Comparative EC:EMC:MP none 1.52 Example 14
[2022] Electrolytic Solution [3] Table 1 shows the following. The
lithium secondary batteries of Electrolytic Solution [3] Examples 1
to 13, in each of which the nonaqueous electrolyte contains both a
chain carboxylic acid ester and the specific compound, have
improved low-temperature output characteristics as compared not
only with the lithium secondary battery of Electrolytic Solution
[3] Comparative Example 6, in which the electrolyte contains
neither of these, but also with the lithium secondary batteries of
Electrolytic Solution [3] Comparative Examples 1 to 5, in which the
electrolytes do not contain the specific compound, and the lithium
secondary batteries of Electrolytic Solution [3] Comparative
Examples 7 to 13, in which the electrolytes do not contain a chain
carboxylic acid ester. This effect is not a mere addition of
effects to be brought about by the two ingredients. When the two
ingredients are used in combination, the effects thereof are
clearly amplified.
[2023] The percentage increase in output for Electrolytic Solution
[3] Example 14 based on the battery of Electrolytic Solution [3]
Comparative Example 14 is about 21%. On the other hand, in the case
of Electrolytic Solution [3] Example 1 and Comparative Example 1,
which have a different battery structure although employing the
same materials, the percentage increase in output for Electrolytic
Solution [3] Example 1 based on the battery of Electrolytic
Solution [3] Comparative Example 1 is 29%. This indicates that
battery structure influences the degree of the effect of the
nonaqueous electrolyte according to the invention.
[2024] As described above, it has become possible to impart
exceedingly high low-temperature output characteristics by using
the nonaqueous electrolyte according to this invention, i.e., a
nonaqueous electrolyte for secondary battery which is characterized
by containing at least one chain carboxylic acid ester and further
containing at least one compound selected from the group consisting
of cyclic siloxane compounds represented by general formula (1),
fluorosilane compounds represented by general formula (2),
compounds represented by general tormula (3), compounds having an
S--F bond in the molecule, nitric acid salts, nitrous acid salts,
monofluorophosphoric acid salts, difluorophosphoric acid salts,
acetic acid salts, and propionic acid salts in an amount of 10 ppm
or more of the whole nonaqueous electrolyte.
Electrolytic Solution [4]
Secondary-Battery Production 1 (Production of Prismatic
Battery)
[Production of Positive Electrode]
[2025] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained.
[Production of Negative Electrode]
[2026] Two kilograms of artificial graphite powder KS-44 (trade
name; manufactured by Timcal) was mixed with 1 kg of petroleum
pitch. Using a batch heating oven, the slurry mixture obtained was
heated to 1,100.degree. C. over 2 hours and held at that
temperature for 2 hours in an inert atmosphere.
[2027] The mixture heat-treated was pulverized and the particles
were passed through vibrating sieves to regulate the particle
diameters to 18-22 .mu.m. Finally, an "amorphous-substance-coated
graphitic carbonaceous substance" was obtained which comprised the
graphite and 7% by mass amorphous carbon with which the surface of
the graphite was coated. This "amorphous-substance-coated graphitic
carbonaceous substance" was used as a negative-electrode active
material. To 98 parts by weight of this "amorphous-substance-coated
graphitic carbonaceous substance" were added 100 parts by weight of
an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained.
[Battery Fabrication]
[2028] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm) equipped with a gas release valve. Thereafter, 20 mL of the
nonaqueous electrolyte which will be described later was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. The electric capacity of the
battery elements held in one battery case of this secondary
battery, i.e., the rated discharge capacity of this battery, was
about 6 Ah. This battery had a direct-current resistance component
as measured by the 10-kHz alternating-current method of about 5
milliohms.
<<Battery Evaluation>>
(Capacity Measurement)
[2029] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.2-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as initial capacity.
(Output Measurement)
[2030] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W).
Electrolytic Solution [4]
Example 1
[2031] In a dry argon atmosphere, LiPF.sub.6 and LiBF.sub.4 were
added to and dissolved in a mixture of ethylene carbonate (EC;
flash point, 143.degree. C.), .gamma.-butyrolactone (GBL; flash
point, 101.degree. C.), and diethyl carbonate (DEC; flash point,
25.degree. C.) (volume ratio, 2:7:1) in such amounts as to result
in an LiPF.sub.6 concentration of 0.3 mol/L and an LiBF.sub.4
concentration of 0.7 mol/L. Vinylene carbonate and
hexamethylcyclotrisiloxane were incorporated into the resultant
solution in such amounts as to result in concentrations of 1% by
mass and 0.3% by mass, respectively. Thus, a nonaqueous electrolyte
was prepared. This nonaqueous electrolyte had a flash point of
61.degree. C. This nonaqueous electrolyte was used to produce a
prismatic battery by the method described above. This battery was
examined for output. The results are shown in Electrolytic Solution
[4] Table 1.
Electrolytic Solution [4]
Example 2
[2032] A prismatic battery was produced in the same manner as in
Electrolytic Solution [4] Example 1, except that
phenyldimethylfluorosilane was used in place of the
hexamethylcyclotrisiloxane in preparing a nonaqueous electrolyte.
This battery was examined for output in the same manner. The
results are shown in Electrolytic Solution [4] Table 1.
Electrolytic Solution [4]
Example 3
[2033] A prismatic battery was produced in the same manner as in
Electrolytic Solution [4] Example 1, except that methyl
fluorosulfonate was used in place of the hexamethylcyclotrisiloxane
in preparing a nonaqueous electrolyte. This battery was examined
for output in the same manner. The results are shown in
Electrolytic Solution [4] Table 1.
Electrolytic Solution [4]
Example 4
[2034] A prismatic battery was produced in the same manner as in
Electrolytic Solution [4] Example 1, except that lithium
difluorophosphate produced by the method described in Inorganic
Nuclear Chemistry Letters (1969), 5(7), pp. 581-582 was used in
place of the hexamethylcyclotrisiloxane in preparing a nonaqueous
electrolyte. This battery was examined for output in the same
manner. The results are shown in Electrolytic Solution [4] Table
1.
Electrolytic Solution [4]
Comparative Example 1
[2035] A prismatic battery was produced in the same manner as in
Electrolytic Solution [4] Example 1, except that
hexamethylcyclotrisiloxane was not incorporated in preparing a
nonaqueous electrolyte. This battery was examined for output in the
same manner. The results are shown in Electrolytic Solution [4]
Table 1.
Electrolytic Solution [4]
Secondary-Battery Production 2 (Production of Cylindrical
Battery)
[Production of Positive Electrode]
[2036] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 20 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. A strip having a width of 52 mm and a length
of 830 mm was cut out of the rolled foil to obtain a positive
electrode. This positive electrode had a 50-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides, and the active-material layer had a length of 780 mm.
[Production of Negative Electrode]
[2037] Two kilograms of artificial graphite powder KS-44 (trade
name; manufactured by Timcal) was mixed with 1 kg of petroleum
pitch. Using a batch heating oven, the slurry mixture obtained was
heated to 1,100.degree. C. over 2 hours and held at that
temperature for 2 hours in an inert atmosphere.
[2038] The mixture heat-treated was pulverized and the particles
were passed through vibrating sieves to regulate the particle
diameters to 18-22 Finally, an "amorphous-substance-coated
graphitic carbonaceous substance" was obtained which comprised the
graphite and 7% by mass amorphous carbon with which the surface of
the graphite was coated. This "amorphous-substance-coated graphitic
carbonaceous substance" was used as a negative-electrode active
material. To 98 parts by weight of this "amorphous-substance-coated
graphitic carbonaceous substance" were added 100 parts by weight of
an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was evenly applied to each side of a copper foil having a
thickness of 18 .mu.m as a negative-electrode current collector and
dried. The coated foil was rolled with a pressing machine to a
thickness of 85 .mu.m. A strip having a width of 56 mm and a length
of 850 mm was cut out of the rolled foil to obtain a negative
electrode. This negative electrode had a 30-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides.
[Battery Fabrication]
[2039] The positive electrode and the negative electrode were
superposed and wound together with polyethylene separators so as to
prevent the positive electrode and the negative electrode from
coming into direct contact with each other to thereby obtain an
electrode roll. This electrode roll was packed into a battery can
so that the terminal of the positive electrode and that of the
negative electrode protruded outward. Subsequently, 5 mL of the
nonaqueous electrolyte which will be described later was introduced
into the battery can, which was then caulked. Thus, a 18650 type
cylindrical battery was produced. The electric capacity of the
battery elements held in one battery case of this secondary
battery, i.e., the rated discharge capacity of this battery, was
about 0.7 ampere-hours (Ah). This battery had a direct-current
resistance component as measured by the 10-kHz alternating-current
method of about 35 milliohms (m.OMEGA.).
Electrolytic Solution [4]
Example 5
[2040] A cylindrical battery was produced using the nonaqueous
electrolyte used in Electrolytic Solution [4] Example 1. This
battery was examined for output. The results are shown in
Electrolytic Solution [4] Table 1.
Electrolytic Solution [4]
Comparative Example 2
[2041] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [4] Example 5, except that the nonaqueous
electrolyte used in Electrolytic Solution [4] Comparative Example 1
was used. This battery was examined for output in the same manner.
The results are shown in Electrolytic Solution [4] Table 1.
TABLE-US-00057 ELECTROLYTIC SOLUTION [4] TABLE 1 No. Specific
compound Output (W) Example 1 hexamethylcyclotrisiloxane 500
Example 2 phenyldimethylfluorosilane 510 Example 3 methyl
fluorosulfonate 510 Example 4 lithium difluorophosphate 525 Example
5 hexamethylcyclotrisiloxane 45 Comparative Example 1 -- 390
Comparative Example 2 -- 37
[2042] As apparent from Electrolytic Solution [4] Table 1, a
comparison among the prismatic batteries (Electrolytic Solution [4]
Examples 1 to 4 and Electrolytic Solution [4] Comparative Example
1) and a separate comparison between the cylindrical batteries
(Electrolytic Solution [4] Example 5 and Electrolytic Solution [4]
Comparative Example 2) show that the lithium secondary batteries of
the Examples have been improved in output.
[2043] As described above, the prismatic batteries of the Examples
and the Comparative Example each have a rated discharge capacity of
3 ampere-hours (Ah) or higher and a direct-current resistance
component of 10 milliohms (m.OMEGA.) or lower. On the other hand,
the cylindrical batteries of the Example and the Comparative
Example each have a rated discharge capacity lower than 3
ampere-hours (Ah) and a direct-current resistance component higher
than 10 milliohms (m.OMEGA.). Namely, the prismatic batteries of
the Examples and the Comparative Example have a lower resistance
and a higher electric capacity than the cylindrical batteries.
Furthermore, the degree of improvement in output of Electrolytic
Solution [4] Example 1 on Electrolytic Solution [4] Comparative
Example 1 is higher than the degree of improvement in output of
Electrolytic Solution [4] Example 5 on Electrolytic Solution [4]
Comparative Example 2. It was thus found that the effect of this
invention is higher in secondary batteries having a high electric
capacity and secondary batteries having a low direct-current
resistance.
Electrolytic Solution [5]
Secondary-Battery Production
[Production of Positive Electrode]
[2044] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained.
[Production of Negative Electrode]
[2045] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained.
[Battery Fabrication]
[2046] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm) equipped with a gas release valve. Thereafter, 20 mL of the
nonaqueous electrolyte which will be described later was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery. The electric capacity of the
battery elements held in one battery case of this secondary
battery, i.e., the rated discharge capacity of this battery, was
about 6 ampere-hours (Ah). This battery had a direct-current
resistance component as measured by the 10-kHz alternating-current
method of about 5 milliohms (m.OMEGA.).
[Battery Evaluation]
(Capacity Measurement)
[2047] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.2-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as initial capacity.
(Output Measurement)
[2048] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W).
(Cycle Test)
[2049] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.2 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total.
(Storage Test)
[2050] A storage test was conducted in a high-temperature
environment of 60.degree. C. The battery which had been charged
beforehand to a final charge voltage of 4.2 V in a 25.degree. C.
environment by the constant-current constant-voltage method was
stored at 60.degree. C. for 1 month.
[2051] The battery which had undergone the capacity measurement,
battery which had undergone the cycle test, and battery which had
undergone the storage test were subjected to an output measurement
to determine initial output, output after cycling, and output after
storage, respectively.
Electrolytic Solution [5]
Example 1
[2052] In a dry argon atmosphere, LiPF.sub.6 and
LiN(CF.sub.3SO.sub.2).sub.2 were added to and dissolved in a
mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC),
and dimethyl carbonate (DMC) (volume ratio, 2:5:3) in such amounts
as to result in an LiPF.sub.6 concentration of 1 mol/L and an
LiN(CF.sub.3SO.sub.2).sub.2 concentration of 0.01 mol/L.
Hexamethylcyclotrisiloxane was incorporated into the resultant
solution in such an amount as to result in a concentration of 0.3%
by mass. Thus, a nonaqueous electrolyte was prepared. This
nonaqueous electrolyte was used to produce a battery by the method
described above. This battery was examined for initial output,
output after cycling, and output after storage. The results are
shown in Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Example 2
[2053] A battery was produced in the same manner as in Electrolytic
Solution [5] Example 1, except that phenyldimethylfluorosilane was
used in place of the hexamethylcyclotrisiloxane in preparing a
nonaqueous electrolyte. This battery was examined for initial
output, output after cycling, and output after storage in the same
manners. The results are shown in Electrolytic Solution [5] Table
1.
Electrolytic Solution [5]
Example 3
[2054] A battery was produced in the same manner as in Electrolytic
Solution [5] Example 1, except that methyl fluorosulfonate was used
in place of the hexamethylcyclotrisiloxane in preparing a
nonaqueous electrolyte. This battery was examined for initial
output, output after cycling, and output after storage in the same
manners. The results are shown in Electrolytic Solution [5] Table
1.
Electrolytic Solution [5]
Example 4
[2055] A battery was produced in the same manner as in Electrolytic
Solution [5] Example 1, except that lithium difluorophosphate
produced by the method described in Inorganic Nuclear Chemistry
Letters (1969), 5(7), pp. 581-582 was used in place of the
hexamethylcyclotrisiloxane in preparing a nonaqueous electrolyte.
This battery was examined for initial output, output after cycling,
and output after storage in the same manners. The results are shown
in Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Example 5
[2056] A battery was produced in the same manner as in Electrolytic
Solution [5] Example 1, except that lithium bis(oxalato)borate
(LiBOB) was used in place of the LiN(CF.sub.3SO.sub.2).sub.2 in
preparing a nonaqueous electrolyte. This battery was examined for
initial output, output after cycling, and output after storage in
the same manners. The results are shown in Electrolytic Solution
[5] Table 1.
Electrolytic Solution [5]
Example 6
[2057] A battery was produced in the same manner as in Electrolytic
Solution [5] Example 4, except that lithium bis(oxalato)borate
(LiBOB) was used in place of the LiN(CF.sub.3SO.sub.2).sub.2 in
preparing a nonaqueous electrolyte. This battery was examined for
initial output, output after cycling, and output after storage in
the same manners. The results are shown in Electrolytic Solution
[5] Table 1.
Electrolytic Solution [5]
Comparative Example 1
[2058] A battery was produced in the same manner as in Example 1,
except that LiN(CF.sub.3SO.sub.2).sub.2 was not dissolved in
preparing a nonaqueous electrolyte. This battery was examined for
initial output, output after cycling, and output after storage in
the same manners. The results are shown in Electrolytic Solution
[5] Table 1.
Electrolytic Solution [5]
Comparative Example 2
[2059] A battery was produced in the same manner as in Electrolytic
Solution [5] Example 1, except that hexamethylcyclotrisiloxane was
not incorporated in preparing a nonaqueous electrolyte. This
battery was examined for initial output, output after cycling, and
output after storage in the same manners. The results are shown in
Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Comparative Example 3
[2060] A battery was produced in the same manner as in Electrolytic
Solution [5] Example 1, except that neither
LiN(CF.sub.3SO.sub.2).sub.2 nor hexamethylcyclotrisiloxane was
incorporated in preparing a nonaqueous electrolyte. This battery
was examined for initial output, output after cycling, and output
after storage in the same manners. The results are shown in
Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Example 7
Secondary-Battery Production 2
[Production of Positive Electrode]
[2061] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 20 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. A strip having a width of 52 mm and a length
of 830 mm was cut out of the rolled foil to obtain a positive
electrode. This positive electrode had a 50-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides, and the active-material layer had a length of 780 mm.
[Production of Negative Electrode]
[2062] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was evenly applied to each side of a copper foil having a
thickness of 18 .mu.m as a negative-electrode current collector and
dried. The coated foil was rolled with a pressing machine to a
thickness of 85 .mu.m. A strip having a width of 56 mm and a length
of 850 mm was cut out of the rolled foil to obtain a negative
electrode. This negative electrode had a 30-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides.
[Battery Fabrication]
[2063] The positive electrode and the negative electrode were
superposed and wound together with polyethylene separators so as to
prevent the positive electrode and the negative electrode from
coming into direct contact with each other to thereby obtain an
electrode roll. This electrode roll was packed into a battery can
so that the terminal of the positive electrode and that of the
negative electrode protruded outward. Subsequently, 5 mL of the
nonaqueous electrolyte which will be described later was introduced
into the battery can, which was then caulked. Thus, a 18650 type
cylindrical battery was produced. The electric capacity of the
battery elements held in one battery case of this secondary
battery, i.e., the rated discharge capacity of this battery, was
about 0.7 ampere-hours (Ah). This battery had a direct-current
resistance component as measured by the 10-kHz alternating-current
method of about 35 milliohms (m.OMEGA.).
[2064] The nonaqueous electrolyte used in Electrolytic Solution [5]
Example 1 was used to produce a cylindrical battery. This battery
was examined for initial output, output after cycling, and output
after storage. The results are shown in Electrolytic Solution [5]
Table 1.
Electrolytic Solution [5]
Example 8
[2065] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [5] Example 7, except that the nonaqueous
electrolyte used in Electrolytic Solution [5] Example 2 was used.
This battery was examined for initial output, output after cycling,
and output after storage in the same manners. The results are shown
in Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Example 9
[2066] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [5] Example 7, except that the nonaqueous
electrolyte used in Electrolytic Solution [5] Example 3 was used.
This battery was examined for initial output, output after cycling,
and output after storage in the same manners. The results are shown
in Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Example 10
[2067] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [5] Example 7, except that the nonaqueous
electrolyte used in Electrolytic Solution [5] Example 4 was used.
This battery was examined for initial output, output after cycling,
and output after storage in the same manners. The results are shown
in Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Example 11
[2068] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [5] Example 7, except that the nonaqueous
electrolyte used in Electrolytic Solution [5] Example 5 was used.
This battery was examined for initial output, output after cycling,
and output after storage in the same manners. The results are shown
in Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Example 12
[2069] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [5] Example 7, except that the nonaqueous
electrolyte used in Electrolytic Solution [5] Example 6 was used.
This battery was examined for initial output, output after cycling,
and output after storage in the same manners. The results are shown
in Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Comparative Example 4
[2070] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [5] Example 7, except that the nonaqueous
electrolyte used in Electrolytic Solution [5] Comparative Example 1
was used. This battery was examined for initial output, output
after cycling, and output after storage in the same manners. The
results are shown in Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Comparative Example 5
[2071] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [5] Example 7, except that the nonaqueous
electrolyte used in Electrolytic Solution [5] Comparative Example 2
was used. This battery was examined for initial output, output
after cycling, and output after storage in the same manners. The
results are shown in Electrolytic Solution [5] Table 1.
Electrolytic Solution [5]
Comparative Example 6
[2072] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [5] Example 7, except that the nonaqueous
electrolyte used in Electrolytic Solution [5] Comparative Example 3
was used. This battery was examined for initial output, output
after cycling, and output after storage in the same manners. The
results are shown in Electrolytic Solution [5] Table 1.
TABLE-US-00058 ELECTROLYTIC SOLUTION [5] TABLE 1 Output Output
Specific Initial after after lithium output cycling storage No.
salt Specific compound (W) (W) (W) Example 1
LiN(CF.sub.3SO.sub.2).sub.2 hexamethyltrisiloxane 535 350 410
Example 2 LiN(CF.sub.3SO.sub.2).sub.2 phenyldimethylfluorosilane
520 320 405 Example 3 LiN(CF.sub.3SO.sub.2).sub.2 methyl
fluorosulfonate 515 330 400 Example 4 LiN(CF.sub.3SO.sub.2).sub.2
lithium difluorophosphate 545 360 435 Example 5 LiBOB
hexamethyltrisiloxane 515 325 400 Example 6 LiBOB lithium
difluorophosphate 520 355 420 Comparative -- hexamethyltrisiloxane
500 285 345 Example 1 Comparative LiN(CF.sub.3SO.sub.2).sub.2 --
450 240 340 Example 2 Comparative -- -- 440 225 330 Example 3
Example 7 LiN(CF.sub.3SO.sub.2).sub.2 hexamethyltrisiloxane 48 29
35 Example 8 LiN(CF.sub.3SO.sub.2).sub.2 phenyldimethylfluorosilane
47 28 34 Example 9 LiN(CF.sub.3SO.sub.2).sub.2 methyl
fluorosulfonate 47 28 34 Example 10 LiN(CF.sub.3SO.sub.2).sub.2
lithium difluorophosphate 49 31 36 Example 11 LiBOB
hexamethyltrisiloxane 48 29 33 Example 12 LiBOB lithium
difluorophosphate 48 31 34 Comparative -- hexamethyltrisiloxane 45
25 32 Example 4 Comparative LiN(CF.sub.3SO.sub.2).sub.2 -- 41 23 30
Example 5 Comparative -- -- 40 21 29 Example 6
[2073] As apparent from Electrolytic Solution [5] Table 1, a
comparison among the prismatic batteries (Electrolytic Solution [5]
Examples 1 to 6 and Electrolytic Solution [5] Comparative Examples
1 to 3) and a separate comparison among the cylindrical batteries
(Electrolytic Solution [5] Examples 7 to 12 and Electrolytic
Solution [5] Comparative Examples 4 to 6) show the following. The
lithium secondary batteries of the Electrolytic Solution [5]
Examples, in each of which the nonaqueous electrolyte contains
LiN(C.sub.nF.sub.2n+1SO.sub.2).sub.2 (wherein n is an integer of
1-4) and/or lithium bis(oxalato)borate and further contains the
specific compound, have been improved in output as compared with
the lithium secondary batteries of the Electrolytic Solution [5]
Comparative Examples, in each of which the nonaqueous electrolyte
does not contain either of these.
[2074] As described above, the prismatic batteries of the Examples
and the Comparative Examples each have a rated discharge capacity
of 3 ampere-hours (Ah) or higher and a direct-current resistance
component of 10 milliohms (m.OMEGA.) or lower. On the other hand,
the cylindrical batteries of the Examples and the Comparative
Examples each have a rated discharge capacity lower than 3
ampere-hours (Ah) and a direct-current resistance component higher
than 10 milliohms (m.OMEGA.). Namely, the prismatic batteries of
the Examples and the Comparative Examples have a lower resistance
and a higher electric capacity than the cylindrical batteries.
Furthermore, the degree of improvement in output of Electrolytic
Solution [5] Example 1 on Electrolytic Solution [5] Comparative
Example 1 is higher than the degree of improvement in output of
Electrolytic Solution [5] Example 7 on Electrolytic Solution [5]
Comparative Example 4. In particular, there is a large difference
in the degree of improvement in output after cycling and in the
degree of improvement in output after storage. It was thus found
that the effect of this invention is higher in secondary batteries
having a high electric capacity and secondary batteries having a
low direct-current resistance.
Electrolytic Solution [6]
Preparation of Nonaqueous Solvent
[2075] Commercial ethylene carbonate (EC) was subjected to an
adsorption treatment with molecular sieve 4A (50.degree. C.; LHSV,
1/hr). On the other hand, dimethyl carbonate and ethyl methyl
carbonate each were sufficiently subjected to precision
distillation under the conditions of a reflux ratio of 1 and a
theoretical-plate number of 30 and then subjected to an adsorption
treatment with molecular sieve 4A (25.degree. C.; LHSV, 1/hr).
Thereafter, these solvents were mixed together in an EC:DMC:EMC
ratio of 3:3:4 by volume. Furthermore, this mixture was subjected
to an adsorption treatment with molecular sieve 4A (25.degree. C.;
LHSV, 1/hr) to prepare a mixed nonaqueous solvent. Neither water
nor any alcohol was detected in this nonaqueous solvent.
Electrolytic Solution [6]
Secondary-Battery Production
[Production of Positive Electrode]
[2076] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained.
[Production of Negative Electrode]
[2077] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained.
[Battery Fabrication]
[2078] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm) equipped with a gas release valve. Thereafter, 20 mL of a
nonaqueous electrolyte was introduced into the battery can packed
with the electrode group and sufficiently infiltrated into the
electrodes. This can was sealed to fabricate a prismatic battery.
This battery has a rated discharge capacity as high as about 6 Ah,
and has a direct-current resistance component as measured by the
10-kHz alternating-current method of about 5 milliohms
(m.OMEGA.).
[Battery Evaluation]
(Capacity Measurement)
[2079] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.2-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as initial capacity.
(Output Measurement)
[2080] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W).
(Cycle Test)
[2081] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.2 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment. The 0.2-C
discharge capacity thereof as measured in the third cycle was taken
as capacity after cycling. The proportion of this capacity after
cycling to the initial capacity was taken as capacity
retention.
Electrolytic Solution [6]
Example 1
[2082] In a dry argon atmosphere, methanol was mixed with the mixed
nonaqueous solvent described above in such an amount as to result
in a concentration of 10 ppm. Lithium hexafluorophosphate
(LiPF.sub.6) was added thereto and dissolved therein in such an
amount as to result in a concentration of 0.8 mol/L. After one day,
the amount of hydrogen fluoride (HF) in the solution was 12 ppm.
After 2 weeks, the amount thereof was measured again and was found
to be 14 ppm. Hexamethylcyclotrisiloxane was mixed with this
mixture solution (2 weeks after the mixing) in such an amount as to
result in a concentration of 0.3% by mass. Thus, a nonaqueous
electrolyte was prepared. This nonaqueous electrolyte was used to
produce a battery by the method described above. This battery was
examined for output and capacity retention. The results are shown
in Electrolytic Solution [6] Table 1.
Electrolytic Solution [6]
Example 2
[2083] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 1, except that the amount of
the methanol to be mixed was changed to 20 ppm of the mixed
nonaqueous solvent. This nonaqueous electrolyte was used to produce
a battery in the same manner as in Electrolytic Solution [6]
Example 1. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1. Incidentally, at one day after the mixing of LiPF.sub.6, the
amount of hydrogen fluoride (HF) in the mixture solution was 16
ppm. After 2 weeks, the amount thereof was measured again and was
found to be 19 ppm.
Electrolytic Solution [6]
Example 3
[2084] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 1, except that the amount of
the methanol to be mixed was changed to 35 ppm of the mixed
nonaqueous solvent. This nonaqueous electrolyte was used to produce
a battery in the same manner as in Electrolytic Solution [6]
Example 1. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1. Incidentally, at one day after the mixing of LiPF.sub.6, the
amount of hydrogen fluoride (HF) in the mixture solution was 22
ppm. After 2 weeks, the amount thereof was measured again and was
found to be 27 ppm.
Electrolytic Solution [6]
Example 4
[2085] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 1, except that ethylene glycol
(EG) was mixed in place of the methanol in an amount of 15 ppm of
the mixed nonaqueous solvent. This nonaqueous electrolyte was used
to produce a battery in the same manner as in Electrolytic Solution
[6] Example 1. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1. Incidentally, at one day after the mixing of LiPF.sub.6, the
amount of hydrogen fluoride (HF) in the mixture solution was 14
ppm. After 2 weeks, the amount thereof was measured again and was
found to be 16 ppm.
Electrolytic Solution [6]
Example 5
[2086] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 1, except that ethylene glycol
was mixed in place of the methanol in an amount of 35 ppm of the
mixed nonaqueous solvent. This nonaqueous electrolyte was used to
produce a battery in the same manner as in Electrolytic Solution
[6] Example 1. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1. Incidentally, at one day after the mixing of LiPF.sub.6, the
amount of hydrogen fluoride (HF) in the mixture solution was 23
ppm. After 2 weeks, the amount thereof was measured again and was
found to be 27 ppm.
Electrolytic Solution [6]
Example 6
[2087] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 1, except that methanol and
ethylene glycol were mixed with the mixed nonaqueous solvent
respectively in amounts of 25 ppm and 25 ppm of the mixed
nonaqueous solvent. This nonaqueous electrolyte was used to produce
a battery in the same manner as in Electrolytic Solution [6]
Example 1. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1. Incidentally, at one day after the mixing of LiPF.sub.6, the
amount of hydrogen fluoride (HF) in the mixture solution was 31
ppm. After 2 weeks, the amount thereof was measured again and was
found to be 36 ppm.
Electrolytic Solution [6]
Example 7
[2088] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 1, except that lithium
difluorophosphate (produced by the method described in Inorganic
Nuclear Chemistry Letters (1969), 5(7), pp. 581-582) was mixed, in
place of the hexamethylcyclotrisiloxane, in an amount of 0.3% by
mass based on the mixture solution. This nonaqueous electrolyte was
used to produce a battery in the same manner as in Electrolytic
Solution [6] Example 1. This battery was examined for output and
capacity retention. The results are shown in Electrolytic Solution
[6] Table 1.
Electrolytic Solution [6]
Example 8
[2089] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 3, except that lithium
difluorophosphate was mixed, in place of the
hexamethylcyclotrisiloxane, in an amount of 0.3% by mass based on
the mixture solution. This nonaqueous electrolyte was used to
produce a battery in the same manner as in Electrolytic Solution
[6] Example 3. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1.
Electrolytic Solution [6]
Example 9
[2090] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 4, except that lithium
difluorophosphate was mixed, in place of the
hexamethylcyclotrisiloxane, in an amount of 0.3% by mass based on
the mixture solution. This nonaqueous electrolyte was used to
produce a battery in the same manner as in Electrolytic Solution
[6] Example 4. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1.
Electrolytic Solution [6]
Example 10
[2091] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 6, except that lithium
difluorophosphate was mixed, in place of the
hexamethylcyclotrisiloxane, in an amount of 0.3% by mass based on
the mixture solution. This nonaqueous electrolyte was used to
produce a battery in the same manner as in Electrolytic Solution
[6] Example 6. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1.
Electrolytic Solution [6]
Example 11
[2092] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 6, except that trimethylsilyl
methanesulfonate was mixed, in place of the
hexamethylcyclotrisiloxane, in an amount of 0.3% by mass based on
the mixture solution. This nonaqueous electrolyte was used to
produce a battery in the same manner as in Electrolytic Solution
[6] Example 6. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1.
Electrolytic Solution [6]
Example 12
Secondary-Battery Production 2
[Production of Positive Electrode]
[2093] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 20 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. A strip having a width of 52 mm and a length
of 830 mm was cut out of the rolled foil to obtain a positive
electrode. This positive electrode had a 50-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides, and the active-material layer had a length of 780 mm.
[Production of Negative Electrode]
[2094] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was evenly applied to each side of a copper foil having a
thickness of 18 .mu.m as a negative-electrode current collector and
dried. The coated foil was rolled with a pressing machine to a
thickness of 85 m. A strip having a width of 56 mm and a length of
850 mm was cut out of the rolled foil to obtain a negative
electrode. This negative electrode had a 30-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides.
[Production of Nonaqueous Electrolytic Solution]
[2095] The same nonaqueous electrolyte as in Electrolytic Solution
[6] Example 6 was produced.
[Battery Fabrication]
[2096] The positive electrode and the negative electrode were
superposed and wound together with polyethylene separators so as to
prevent the positive electrode and the negative electrode from
coming into direct contact with each other to thereby obtain an
electrode roll. This electrode roll was packed into a battery can
so that the terminal of the positive electrode and that of the
negative electrode protruded outward. Subsequently, 5 mL of the
nonaqueous electrolyte described later was introduced into the
battery can, which was then caulked. Thus, a 18650 type cylindrical
battery was produced. This battery had a rated discharge capacity
of about 0.7 ampere-hours (Ah), and a direct-current resistance as
measured by the 10-kHz alternating-current method of about 35
milliohms (m.OMEGA.). This battery was examined for output and
capacity retention in the same manners as in Electrolytic Solution
[6] Example 6. The results are shown in Electrolytic Solution [6]
Table 1.
Electrolytic Solution [6]
Comparative Example 1
[2097] In Electrolytic Solution [6] Example 3, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for output and capacity retention. The results
are shown in Electrolytic Solution [6] Table 1.
Electrolytic Solution [6]
Comparative Example 2
[2098] In Electrolytic Solution [6] Example 5, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for output and capacity retention. The results
are shown in Electrolytic Solution [6] Table 1.
Electrolytic Solution [6]
Comparative Example 3
[2099] In Electrolytic Solution [6] Example 6, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for output and capacity retention. The results
are shown in Electrolytic Solution [6] Table 1.
Electrolytic Solution [6]
Comparative Example 4
[2100] In Electrolytic Solution [6] Example 1, a nonaqueous
electrolyte was prepared without mixing methanol with the mixed
nonaqueous solvent and this nonaqueous electrolyte was used to
produce a battery. This battery was examined for output and
capacity retention. The results are shown in Electrolytic Solution
[6] Table 1. Incidentally, at one day after the mixing of
LiPF.sub.6, the amount of hydrogen fluoride (HF) in the solution
was 9 ppm. After 2 weeks, the amount thereof was measured again and
was found to remain at 9 ppm.
Electrolytic Solution [6]
Comparative Example 5
[2101] In Electrolytic Solution [6] Comparative Example 4, 0.3% by
mass lithium difluorophosphate was mixed in place of the
hexamethylcyclotrisiloxane to prepare a nonaqueous electrolyte and
this nonaqueous electrolyte was used to produce a battery. This
battery was examined for output and capacity retention. The results
are shown in Electrolytic Solution [6] Table 1.
Electrolytic Solution [6]
Comparative Example 6
[2102] In Electrolytic Solution [6] Comparative Example 4, 0.3% by
mass trimethylsilyl methanesulfonate was mixed in place of the
hexamethylcyclotrisiloxane to prepare a nonaqueous electrolyte and
this nonaqueous electrolyte was used to produce a battery. This
battery was examined for output and capacity retention. The results
are shown in Electrolytic Solution [6] Table 1.
Electrolytic Solution [6]
Comparative Example 7
[2103] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 1, except that the amount of
the methanol to be mixed was changed to 700 ppm of the mixed
nonaqueous solvent. This nonaqueous electrolyte was used to produce
a battery in the same manner as in Electrolytic Solution [6]
Example 1. This battery was examined for output and capacity
retention. The results are shown in Electrolytic Solution [6] Table
1. Incidentally, at one day after the mixing of LiPF.sub.6, the
amount of hydrogen fluoride (HF) in the solution was 321 ppm. After
2 weeks, the amount thereof was measured again and was found to be
403 ppm.
Electrolytic Solution [6]
Comparative Example 8
[2104] In Electrolytic Solution [6] Example 1, neither methanol nor
hexamethylcyclotrisiloxane was mixed with the mixed nonaqueous
solvent to prepare a nonaqueous electrolyte and this nonaqueous
electrolyte was used to produce a battery. This battery was
examined for output and capacity retention. The results are shown
in Electrolytic Solution [6] Table 1.
Electrolytic Solution [6]
Comparative Example 9
[2105] A nonaqueous electrolyte was prepared in the same manner as
in Electrolytic Solution [6] Example 12, except that
hexamethylcyclotrisiloxane was not mixed. This nonaqueous
electrolyte was used to produce a battery in the same manner as in
Electrolytic Solution [6] Example 12. This battery was examined for
output and capacity retention. The results are shown in
Electrolytic Solution [6] Table 1.
TABLE-US-00059 ELECTROLYTIC SOLUTION [6] TABLE 1 Amount Capacity of
HF Output retention Alcohol (ppm) Compound contained (W) (%)
Example 1 methanol 10 ppm 14 hexamethylcyclotrisiloxane 481 71.0
Example 2 methanol 20 ppm 19 hexamethylcyclotrisiloxane 489 71.0
Example 3 methanol 35 ppm 27 hexamethylcyclotrisiloxane 498 70.9
Example 4 EG 15 ppm 16 hexamethylcyclotrisiloxane 483 71.0 Example
5 EG 35 ppm 27 hexamethylcyclotrisiloxane 498 70.9 Example 6
methanol 25 ppm 36 hexamethylcyclotrisiloxane 507 70.9 EG 25 ppm
Example 7 methanol 10 ppm 14 lithium difluorophosphate 492 71.3
Example 8 methanol 35 ppm 27 lithium difluorophosphate 506 71.1
Example 9 EG 15 ppm 16 lithium difluorophosphate 494 71.3 Example
10 methanol 25 ppm 36 lithium difluorophosphate 519 71.0 EG 25 ppm
Example 11 methanol 25 ppm 36 trimethylsilyl methanesulfonate 509
70.9 EG 25 ppm Example 12 methanol 25 ppm 36
hexamethylcyclotrisiloxane 45.7 70.4 EG 25 ppm Comparative methanol
35 ppm 27 none 395 69.5 Example 1 Comparative EG 35 ppm 27 none 395
69.6 Example 2 Comparative methanol 25 ppm 36 none 393 68.8 Example
3 EG 25 ppm Comparative none 9 hexamethylcyclotrisiloxane 462 71.0
Example 4 Comparative none 9 lithium difluorophosphate 472 71.3
Example 5 Comparative none 9 trimethylsilyl methanesulfonate 467
71.0 Example 6 Comparative methanol 700 ppm 403
hexamethylcyclotrisiloxane 458 59.6 Example 7 Comparative none 9
none 401 70.7 Example 8 Comparative methanol 25 ppm 36 none 38.4
68.5 Example 9 EG 25 ppm
[2106] Electrolytic Solution [6] Table 1 shows the following. The
lithium secondary batteries of Electrolytic Solution [6]
Comparative Examples 1 to 3, which employed an electrolyte
containing a larger amount of hydrogen fluoride (HF), were inferior
to the lithium secondary battery of Electrolytic Solution [6]
Comparative Example 8 in both output characteristics and cycle
characteristics. However, the lithium secondary batteries of
Electrolytic Solution [6] Examples 1 to 11, which each employed the
electrolyte further containing the specific compound, had been
improved in both output characteristics and cycle characteristics.
Furthermore, with respect to output, the lithium secondary
batteries of Electrolytic Solution [6] Examples 1 to 11 had higher
performance than the lithium secondary batteries of Electrolytic
Solution [6] Comparative Examples 4 to 6, which had a low hydrogen
fluoride (HF) content although containing the specific compound.
Namely, the results obtained surprisingly showed that the
output-improving effect of the specific compound is heightened by
the presence of hydrogen fluoride (HF). It should, however, be
noted that the lithium secondary battery of Electrolytic Solution
[6] Comparative Example 7, which contained excess hydrogen fluoride
(HF), was inferior especially in cycle characteristics.
[2107] With respect to the batteries employing a battery structure
having a low capacity and a high direct-current resistance, the
percentage improvement in output for Electrolytic Solution [6]
Example 12 based on the battery of Electrolytic Solution [6]
Comparative Example 9 was as low as about 19%. In contrast,
percentage improvements in output in the batteries employing a
battery structure having a high capacity and a low direct-current
resistance are as follows. The percentage improvement in output for
Electrolytic Solution [6] Example 6 based on the battery of
Electrolytic Solution [6] Comparative Example 3 (the two batteries
each contained 25 ppm methanol and 25 ppm ethylene glycol) was as
high as about 29%. The percentage improvement in output for
Electrolytic Solution [6] Example 5 based on the battery of
Electrolytic Solution [6] Comparative Example 2 (the two batteries
each contained 35 ppm ethylene glycol) was as high as about 26%.
Furthermore, the percentage improvement in output for Electrolytic
Solution [6] Example 3 based on the battery of Electrolytic
Solution [6] Comparative Example 1 (the two batteries each
contained 35 ppm methanol) was as high as about 26%. It was found
from these results that the effect of this invention is high
specifically in battery structures having a high capacity and a low
direct-current resistance.
[2108] As described above, it has become possible to obtain high
output characteristics without impairing cycle characteristics, by
using the nonaqueous electrolyte according to this invention, i.e.,
a nonaqueous electrolyte for secondary battery which comprises a
nonaqueous solvent and a fluorine-containing lithium salt mixed
therewith and is characterized in that the nonaqueous electrolyte
contains hydrogen fluoride (HF) in an amount of from 10 ppm to 300
ppm and further contains at least one compound selected from the
group consisting of cyclic siloxane compounds represented by
general formula (1), fluorosilane compounds represented by general
formula (2), compounds represented by general formula (3),
compounds having an S--F bond in the molecule, nitric acid salts,
nitrous acid salts, monofluorophosphoric acid salts,
difluorophosphoric acid salts, acetic acid salts, and propionic
acid salts in an amount of 10 ppm or more of the whole nonaqueous
electrolyte.
[2109] In the Examples given above, an alcohol is added to a
purified nonaqueous solvent to thereby produce the effect. However,
in the case of using a nonaqueous solvent originally containing
water or an alcohol, the same effect can be obtained, without
purposely adding an alcohol, by regulating conditions for the
purification of this nonaqueous solvent. In general, the
purification of nonaqueous solvents is troublesome and is
industrially causative of a cost increase. In this invention,
however, a high-performance nonaqueous electrolyte can be prepared
without necessitating excessive purification and this is
exceedingly valuable.
Electrolytic Solution [7]
Battery Production 1
[Production of Positive Electrode]
[2110] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 20 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. A strip having a width of 52 mm and a length
of 830 mm was cut out of the rolled foil to obtain a positive
electrode. This positive electrode had a 50-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides, and the active-material layer had a length of 780 mm.
[Production of Negative Electrode]
[2111] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was evenly applied to each side of a copper foil having a
thickness of 18 .mu.m as a negative-electrode current collector and
dried. The coated foil was rolled with a pressing machine to a
thickness of 85 .mu.m. A strip having a width of 56 mm and a length
of 850 mm was cut out of the rolled foil to obtain a negative
electrode. This negative electrode had a 30-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides.
[Production of Electrolytic Solution]
[2112] In a dry argon atmosphere, sufficient dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixed solvent composed of purified ethylene
carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl
carbonate (EMC) in a volume ratio of 3:3:4 to prepare a nonaqueous
electrolyte (1). Furthermore, lithium difluorophosphate produced by
the method described in Inorganic Nuclear Chemistry Letters (1969),
5(7), pp. 581-582 was incorporated into the nonaqueous electrolyte
(1) in such an amount as to result in a concentration of 0.5% by
mass. Thus, a nonaqueous electrolyte (2) was prepared.
[Battery Fabrication]
[2113] The positive electrode and the negative electrode were
superposed and wound together with polyethylene separators so as to
prevent the positive electrode and the negative electrode from
coming into direct contact with each other to thereby obtain an
electrode roll. This electrode roll was packed into a battery can
so that the terminal of the positive electrode and that of the
negative electrode protruded outward. Subsequently, 5 mL of the
electrolyte described later was introduced into the battery can,
which was then caulked. Thus, a 18650 type cylindrical battery was
produced.
[Battery Evaluation]
(Initial Charge/Discharge)
[2114] At 25.degree. C., the cylindrical battery produced was
charged to 4.2 V by the constant-current constant-voltage charging
method at 0.2 C and then discharged to 3.0 V at a constant current
of 0.2 C. This operation as one cycle was repeated to conducted 5
cycles and thereby stabilize the battery. The discharge capacity
thereof as measured in the fifth cycle was taken as initial
capacity. Incidentally, the value of current at which the rated
capacity based on 1-hour-rate discharge capacity is discharged over
1 hour is taken as 1 C.
(Cycle Test)
[2115] At 60.degree. C., the battery which had undergone the
initial charge/discharge was charged to 4.2 V by the
constant-current constant-voltage method at 1 C and then discharged
to 3.0 V at a constant current of 1 C. This charge/discharge
operation as one cycle was repeated to conduct 500 cycles. The
proportion of the discharge capacity of the battery as measured in
the 500th cycle to the discharge capacity thereof as measured in
the first cycle was taken as capacity retention through
cycling.
(Low-Temperature Test)
[2116] The battery which had undergone the initial charge/discharge
was charged at 25.degree. C. to 4.2 V by the constant-current
constant-voltage charging method at 0.2 C and then discharged at
-30.degree. C. at a constant current of 0.2 C. The discharge
capacity thereof as measured in this discharge was taken as initial
low-temperature capacity. The proportion of the initial
low-temperature capacity to the initial capacity was taken as
proportion of initial low-temperature discharge.
[2117] Furthermore, at 25.degree. C., the battery which had
undergone the cycle test was charged to 4.2 V by the
constant-current constant-voltage charging method at 0.2 C and then
discharged to 3.0 V at a constant current of 0.2 C. This operation
as one cycle was repeated to conduct 3 cycles. The discharge
capacity thereof as measured in the third cycle was taken as
capacity after cycling. Thereafter, the same battery was charged at
25.degree. C. to 4.2 V by the constant-current constant-voltage
charging method at 0.2 C and then discharged at -30.degree. C. at a
constant current of 0.2 C. The discharge capacity thereof as
measured in this discharge was taken as low-temperature capacity
after cycling. The proportion of the low-temperature capacity after
cycling to the capacity after cycling was taken as proportion of
low-temperature discharge after cycling.
Electrolytic Solution [7]
Example 1
[2118] Vinylene carbonate (hereinafter abbreviated to "VC") was
mixed with the nonaqueous electrolyte (2) in an amount of 1% by
mass based on the whole electrolyte to prepare a nonaqueous
electrolyte (3). This nonaqueous electrolyte (3) was used to
produce a 18650 type battery, which was examined for capacity
retention through cycling and proportion of low-temperature
discharge. The results are shown in Electrolytic Solution [7] Table
1. Incidentally, the electrolyte was recovered with a centrifugal
separator from the battery which had undergone the initial
charge/discharge, and examined for VC amount. As a result, the
amount of VC was found to be 0.40% by mass.
Electrolytic Solution [7]
Comparative Example 1
[2119] The nonaqueous electrolyte (1) was used to produce a 18650
type battery in the same manner as in Electrolytic Solution [7]
Example 1. This battery was examined for capacity retention through
cycling and proportion of low-temperature discharge. The results
are shown in Electrolytic Solution [7] Table 1.
Electrolytic Solution [7]
Comparative Example 2
[2120] VC was mixed with the nonaqueous electrolyte (1) in an
amount of 1% by mass based on the whole electrolyte to prepare a
nonaqueous electrolyte (4). This nonaqueous electrolyte (4) was
used to produce a 18650 type battery in the same manner as in
Electrolytic Solution [7] Example 1. This battery was examined for
capacity retention through cycling and proportion of
low-temperature discharge. The results are shown in Electrolytic
Solution [7] Table 1. Incidentally, the electrolyte was recovered
with a centrifugal separator from the battery which had undergone
the initial charge/discharge, and examined for VC amount. As a
result, the amount of VC was found to be 0.22% by mass.
Electrolytic Solution [7]
Comparative Example 3
[2121] A 18650 type battery was produced in the same manner as in
Electrolytic Solution [7] Example 1, except that the nonaqueous
electrolyte (2) was used and VC was not mixed. This battery was
examined for capacity retention through cycling and proportion of
low-temperature discharge. The results are shown in Electrolytic
Solution [7] Table 1.
Electrolytic Solution [7]
Comparative Example 4
[2122] VC was mixed with the nonaqueous electrolyte (2) in an
amount of 5% by mass based on the whole electrolyte to prepare a
nonaqueous electrolyte (5). In Electrolytic Solution [7] Example 1,
this nonaqueous electrolyte (5) was used to produce a 18650 type
battery. This battery was examined for capacity retention through
cycling and proportion of low-temperature discharge. The results
are shown in Electrolytic Solution [7] Table 1.
Electrolytic Solution [7]
Example 2
Battery Production 2
[Production of Positive Electrode]
[2123] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained.
[Production of Negative Electrode]
[2124] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained.
[Battery Fabrication]
[2125] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm). Thereafter, 20 mL of the nonaqueous electrolyte (3) was
introduced into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery.
[2126] The prismatic battery thus produced was subjected to the
cycle test and the low-temperature test in the same manners as in
Electrolytic Solution [7] Example 1 to determine the capacity
retention through cycling and the proportion of low-temperature
discharge thereof. The results are shown in Electrolytic Solution
[7] Table 1.
Electrolytic Solution [7]
Comparative Example 5
[2127] In Electrolytic Solution [7] Example 2, the nonaqueous
electrolyte (1) was used in place of the nonaqueous electrolyte (3)
to produce a prismatic battery. This battery was examined for
capacity retention through cycling and proportion of
low-temperature discharge. The results are shown in Electrolytic
Solution [7] Table 1.
Electrolytic Solution [7]
Comparative Example 6
[2128] In Electrolytic Solution [7] Example 2, the nonaqueous
electrolyte (4) was used in place of the nonaqueous electrolyte (3)
to produce a prismatic battery. This battery was examined for
capacity retention through cycling and proportion of
low-temperature discharge. The results are shown in Electrolytic
Solution [7] Table 1.
Electrolytic Solution [7]
Comparative Example 7
[2129] In Electrolytic Solution [7] Example 2, the nonaqueous
electrolyte (2) was used in place of the nonaqueous electrolyte (3)
to produce a prismatic battery. This battery was examined for
capacity retention through cycling and proportion of
low-temperature discharge. The results are shown in Electrolytic
Solution [7] Table 1.
TABLE-US-00060 Electrolytic Solution [7] Table 1 Proportion of
Proportion of low- Direct- Capacity initial low- temperature
Initial current retention temperature discharge after capacity
resistance through cycling discharge cycling No. Battery VC
LiPO.sub.2F.sub.2 (mAh) (m.OMEGA.) (%) (%) (%) Example 1
cylindrical 1% present 702 35 79 68 69 Comparative cylindrical Nil
absent 700 35 64 61 54 Example 1 Comparative cylindrical 1% absent
701 35 74 59 56 Example 2 Comparative cylindrical Nil present 700
35 65 68 64 Example 3 Comparative cylindrical 5% present 699 35 84
37 29 Example 4 Example 2 prismatic 1% present 6010 5 85 73 75
Comparative prismatic Nil absent 6005 5 70 63 57 Example 5
Comparative prismatic 1% absent 6003 5 78 60 57 Example 6
Comparative prismatic Nil present 6006 5 72 72 69 Example 7
[2130] As apparent from Electrolytic Solution [7] Table 1, the
lithium secondary battery of Electrolytic Solution [7] Example 1,
in which the nonaqueous electrolyte contained vinylene carbonate
and a difluorophosphoric acid salt, was found to have been improved
in both capacity retention through cycling and the proportion of
low-temperature discharge as compared with the lithium secondary
battery of Electrolytic Solution [7] Comparative Example 1, which
had the same constitution as the battery of the Example 1 except
that the nonaqueous electrolyte contained neither of these.
[2131] It was also found that even when compared with the lithium
secondary battery of Electrolytic Solution [7] Comparative Example
2, which had the same constitution except that the nonaqueous
electrolyte contained VC only, the battery of the Example 1 had
been improved in both capacity retention through cycling and the
proportion of low-temperature discharge. Although the presence or
absence of VC considerably influences capacity retention through
cycling, the effect of VC is enhanced by the addition of a
difluorophosphoric acid salt thereto. The amount of VC detected in
the battery of the Example after the initial charge/discharge was
about two times the amount thereof in the battery of the
Comparative Example after the initial charge/discharge, and the VC
thus remained is thought to have inhibited battery deterioration
during the cycle test.
[2132] As apparent from Electrolytic Solution [7] Example 1 and
Electrolytic Solution [7] Comparative Example 3, the incorporation
of a difluorophosphoric acid salt alone is not considered to bring
about a sufficient improvement in capacity retention through
cycling. Furthermore, as compared with the lithium secondary
battery of Electrolytic Solution [7] Comparative Example 3, the
lithium secondary battery of Electrolytic Solution [7] Example 1
not only has greatly improved capacity retention through cycling
but has an improved proportion of low-temperature discharge after
cycling, although equal in the proportion of initial
low-temperature discharge. It has become clear that VC and the
difluorophosphoric acid salt produce a synergistic effect to
inhibit the internal battery resistance from increasing through the
cycle test.
[2133] The same applies to a comparison between Electrolytic
Solution [7] Example 2 and each of Electrolytic Solution [7]
Comparative Examples 5 to 7.
[2134] From a comparison between the lithium secondary battery of
Electrolytic Solution [7] Example 1 and the lithium secondary
battery of Electrolytic Solution [7] Comparative Example 4, which
contains a large amount of VC, it was found that the lithium
secondary battery of Electrolytic Solution [7] Comparative Example
4 has too poor low-temperature discharge characteristics to
withstand use in low-temperature environments although highly
satisfactory in capacity retention through cycling.
[2135] The table shows values of direct-current resistance measured
by the 10-kHz alternating-current method. The prismatic batteries
in this experiment have a lower resistance and a higher electric
capacity than the cylindrical batteries. The degree of improvement
in low-temperature characteristics of Electrolytic Solution [7]
Example 2 on Electrolytic Solution [7] Comparative Example 5 is
higher than the degree of improvement in low-temperature
characteristics of Electrolytic Solution [7] Example 1 on
Electrolytic Solution [7] Comparative Example 1. It was thus found
that the nonaqueous electrolyte according to this invention is
especially suitable for use in secondary batteries having a high
electric capacity and secondary batteries having a low
direct-current resistance.
Electrolytic Solution [8]
Secondary-Battery Production
[Production of Positive Electrode]
[2136] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained.
[Production of Negative Electrode]
[2137] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained.
[Battery Fabrication]
[2138] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (thickness, 25 .mu.m) sandwiched
between the electrodes. In this operation, each positive electrode
was disposed so that the positive-electrode active-material surface
was within the range of the opposed negative-electrode
active-material surface. The uncoated parts of the positive
electrodes were welded together and those of the negative
electrodes were welded together. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can (outer dimensions: 120.times.110.times.10
mm) equipped with a gas release valve. Thereafter, 20 mL of the
nonaqueous electrolyte which will be described later was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery.
[2139] The electric capacity of the battery elements held in one
battery case of this secondary battery, i.e., the rated discharge
capacity of this battery, was about 6 ampere-hours (Ah). This
battery had a direct-current resistance component as measured by
the 10-kHz alternating-current method of about 5 milliohms
(m.OMEGA.).
Electrolytic Solution [8]
Battery Evaluation
[Capacity Retention Through Cycling]
(Initial Charge/Discharge)
[2140] At 25.degree. C., the battery was charged to 4.2 V by the
constant-current constant-voltage charging method at 0.2 C and then
discharged to 3.0 V at a constant current of 0.2 C. This operation
as one cycle was repeated to conducted 5 cycles and thereby
stabilize the battery. The discharge capacity thereof as measured
in the fifth cycle was taken as initial capacity. Incidentally, the
value of current at which the rated capacity is discharged over 1
hour is taken as 1 C.
(Cycle Test)
[2141] At 60.degree. C., the battery which had undergone the
initial charge/discharge was charged to 4.2 V by the
constant-current constant-voltage method at 1 C and then discharged
to 3.0 V at a constant current of 1 C. This charge/discharge
operation as one cycle was repeated to conduct 500 cycles. The
proportion of the discharge capacity of the battery as measured in
the 500th cycle to the discharge capacity thereof as measured in
the first cycle was taken as capacity retention through
cycling.
[Proportion of Initial Low-Temperature Discharge]
(Low-Temperature Test)
[2142] The battery which had undergone the initial charge/discharge
was charged at 25.degree. C. to 4.2 V by the constant-current
constant-voltage charging method at 0.2 C and then discharged at
-30.degree. C. at a constant current of 0.2 C. The discharge
capacity thereof as measured in this discharge was taken as initial
low-temperature capacity. The proportion of the initial
low-temperature capacity to the initial capacity was taken as
proportion of initial low-temperature discharge.
[Proportion of Low-Temperature Discharge after Cycling]
[2143] Furthermore, at 25.degree. C., the battery which had
undergone the cycle test was charged to 4.2 V by the
constant-current constant-voltage charging method at 0.2 C and then
discharged to 3.0 V at a constant current of 0.2 C. This operation
as one cycle was repeated to conduct 3 cycles. The discharge
capacity thereof as measured in the third cycle was taken as
capacity after cycling. Thereafter, the same battery was charged at
25.degree. C. to 4.2 V by the constant-current constant-voltage
charging method at 0.2 C and then discharged at -30.degree. C. at a
constant current of 0.2 C. The discharge capacity thereof as
measured in this discharge was taken as low-temperature capacity
after cycling. The proportion of the low-temperature capacity after
cycling to the capacity after cycling was taken as proportion of
low-temperature discharge after cycling.
Electrolytic Solution [8]
Example 1
[2144] In a dry argon atmosphere, LiPF.sub.6 was added to and
dissolved in a mixture of ethylene carbonate (EC), ethyl methyl
carbonate (EMC), and dimethyl carbonate (DMC) (volume ratio, 2:5:3)
in such an amount as to result in a concentration of 1 mol/L.
Hexamethylcyclotrisiloxane and vinylethylene carbonate were
incorporated into this solution in such amounts as to result in
concentrations of 0.3% by mass and 1% by mass, respectively. Thus,
a nonaqueous electrolyte was prepared. This nonaqueous electrolyte
was used to produce a battery by the method described above. This
battery was examined for capacity retention through cycling,
proportion of initial low-temperature discharge, and proportion of
low-temperature discharge after cycling. The results are shown in
Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Example 2
[2145] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 1, except that phenyldimethylfluorosilane was
used in place of the hexamethylcyclotrisiloxane in preparing a
nonaqueous electrolyte. This battery was examined for capacity
retention through cycling, proportion of initial low-temperature
discharge, and proportion of low-temperature discharge after
cycling. The results are shown in Electrolytic Solution [8] Table
1.
Electrolytic Solution [8]
Example 3
[2146] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 1, except that methyl fluorosulfonate was used
in place of the hexamethylcyclotrisiloxane in preparing a
nonaqueous electrolyte. This battery was examined for capacity
retention through cycling, proportion of initial low-temperature
discharge, and proportion of low-temperature discharge after
cycling. The results are shown in Electrolytic Solution [8] Table
1.
Electrolytic Solution [8]
Example 4
[2147] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 1, except that lithium difluorophosphate
produced by the method described in Inorganic Nuclear Chemistry
Letters (1969), 5(7), pp. 581-582 was used in place of the
hexamethylcyclotrisiloxane in preparing a nonaqueous electrolyte.
This battery was examined for capacity retention through cycling,
proportion of initial low-temperature discharge, and proportion of
low-temperature discharge after cycling. The results are shown in
Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Example 5
[2148] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 1, except that 1,3-propanesultone was used in
place of the vinylethylene carbonate in preparing a nonaqueous
electrolyte. This battery was examined for capacity retention
through cycling, proportion of initial low-temperature discharge,
and proportion of low-temperature discharge after cycling. The
results are shown in Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Example 6
[2149] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 1, except that .gamma.-butyrolactone was used
in place of the vinylethylene carbonate in preparing a nonaqueous
electrolyte. This battery was examined for capacity retention
through cycling, proportion of initial low-temperature discharge,
and proportion of low-temperature discharge after cycling. The
results are shown in Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Example 7
[2150] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 1, except that fluoroethylene carbonate was
used in place of the vinylethylene carbonate in preparing a
nonaqueous electrolyte. This battery was examined for capacity
retention through cycling, proportion of initial low-temperature
discharge, and proportion of low-temperature discharge after
cycling. The results are shown in Electrolytic Solution [8] Table
1.
Electrolytic Solution [8]
Example 8
[2151] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 4, except that 1,3-propanesultone was used in
place of the vinylethylene carbonate in preparing a nonaqueous
electrolyte. This battery was examined for capacity retention
through cycling, proportion of initial low-temperature discharge,
and proportion of low-temperature discharge after cycling. The
results are shown in Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Example 9
[2152] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 4, except that .gamma.-butyrolactone was used
in place of the vinylethylene carbonate in preparing a nonaqueous
electrolyte. This battery was examined for capacity retention
through cycling, proportion of initial low-temperature discharge,
and proportion of low-temperature discharge after cycling. The
results are shown in Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Example 10
[2153] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 4, except that fluoroethylene carbonate was
used in place of the vinylethylene carbonate in preparing a
nonaqueous electrolyte. This battery was examined for capacity
retention through cycling, proportion of initial low-temperature
discharge, and proportion of low-temperature discharge after
cycling. The results are shown in Electrolytic Solution [8] Table
1.
Electrolytic Solution [8]
Comparative Example 1
[2154] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 1, except that hexamethylcyclotrisiloxane was
not incorporated in preparing a nonaqueous electrolyte. This
battery was examined for capacity retention through cycling,
proportion of initial low-temperature discharge, and proportion of
low-temperature discharge after cycling. The results are shown in
Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Comparative Example 2
[2155] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 1, except that vinylethylene carbonate was not
incorporated in preparing a nonaqueous electrolyte. This battery
was examined for capacity retention through cycling, proportion of
initial low-temperature discharge, and proportion of
low-temperature discharge after cycling. The results are shown in
Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Comparative Example 3
[2156] A battery was produced in the same manner as in Electrolytic
Solution [8] Example 1, except that neither
hexamethylcyclotrisiloxane nor vinylethylene carbonate was
incorporated in preparing a nonaqueous electrolyte. This battery
was examined for capacity retention through cycling, proportion of
initial low-temperature discharge, and proportion of
low-temperature discharge after cycling. The results are shown in
Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Example 11
Secondary-Battery Production 2
[Production of Positive Electrode]
[2157] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 20 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. A strip having a width of 52 mm and a length
of 830 mm was cut out of the rolled foil to obtain a positive
electrode. This positive electrode had a 50-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides, and the active-material layer had a length of 780 mm.
[Production of Negative Electrode]
[2158] To 98 parts by weight of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was evenly applied to each side of a copper foil having a
thickness of 18 .mu.m as a negative-electrode current collector and
dried. The coated foil was rolled with a pressing machine to a
thickness of 85 .mu.m. A strip having a width of 56 mm and a length
of 850 mm was cut out of the rolled foil to obtain a negative
electrode. This negative electrode had a 30-mm uncoated part
extending in the lengthwise direction on each of the front and back
sides.
[Battery Fabrication]
[2159] The positive electrode and the negative electrode were
superposed and wound together with polyethylene separators so as to
prevent the positive electrode and the negative electrode from
coming into direct contact with each other to thereby obtain an
electrode roll. This electrode roll was packed into a battery can
so that the terminal of the positive electrode and that of the
negative electrode protruded outward. Subsequently, 5 mL of the
electrolyte which will be described later was introduced into the
battery can, which was then caulked. Thus, a 18650 type cylindrical
battery was produced. The electric capacity of the battery elements
held in one battery case of this secondary battery, i.e., the rated
discharge capacity of this battery, was about 0.7 ampere-hours
(Ah). This battery had a direct-current resistance component as
measured by the 10-kHz alternating-current method of about 35
milliohms (m.OMEGA.).
[2160] The nonaqueous electrolyte used in Electrolytic Solution [8]
Example 1 was used to produce a cylindrical battery. This battery
was examined for capacity retention through cycling, proportion of
initial low-temperature discharge, and proportion of
low-temperature discharge after cycling. The results are shown in
Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Example 12
[2161] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [8] Example 11, except that the nonaqueous
electrolyte used in Electrolytic Solution [8] Example 4 was used.
This battery was examined for capacity retention through cycling,
proportion of initial low-temperature discharge, and proportion of
low-temperature discharge after cycling. The results are shown in
Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Comparative Example 4
[2162] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [8] Example 11, except that the nonaqueous
electrolyte used in Electrolytic Solution [8] Comparative Example 1
was used. This battery was examined for capacity retention through
cycling, proportion of initial low-temperature discharge, and
proportion of low-temperature discharge after cycling. The results
are shown in Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Comparative Example 5
[2163] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [8] Example 11, except that the nonaqueous
electrolyte used in Electrolytic Solution [8] Comparative Example 2
was used. This battery was examined for capacity retention through
cycling, proportion of initial low-temperature discharge, and
proportion of low-temperature discharge after cycling. The results
are shown in Electrolytic Solution [8] Table 1.
Electrolytic Solution [8]
Comparative Example 6
[2164] A cylindrical battery was produced in the same manner as in
Electrolytic Solution [8] Example 11, except that the nonaqueous
electrolyte used in Electrolytic Solution [8] Comparative Example 3
was used. This battery was examined for capacity retention through
cycling, proportion of initial low-temperature discharge, and
proportion of low-temperature discharge after cycling. The results
are shown in Electrolytic Solution [8] Table 1.
TABLE-US-00061 ELECTROLYTIC SOLUTION [8] TABLE 1 Proportion
Proportion of low- Capacity of initial temperature retention low-
discharge through temperature after Specific cycling discharge
cycling No. compound A Specific compound B (%) (%) (%) Example 1
hexamethylcyclo- vinylethylene carbonate 83 74 73 trisiloxane
Example 2 phenyldimethyl- vinylethylene carbonate 81 71 69
fluorosilane Example 3 methylfluorosulfonate vinylethylene
carbonate 80 70 69 Example 4 lithium di- vinylethylene carbonate 84
76 75 fluorophosphate Example 5 hexamethylcyclo- 1,3-propanesultone
82 72 69 trisiloxane Example 6 hexamethylcyclo-
.gamma.-butyrolactone 79 73 66 trisiloxane Example 7
hexamethylcyclo- fluoroethylene 83 73 70 trisiloxane carbonate
Example 8 lithium di- 1,3-propanesultone 83 73 72 fluorophosphate
Example 9 lithium di- .gamma.-butyrolactone 80 73 69
fluorophosphate Example 10 lithium di- fluoroethylene 84 74 73
fluorophosphate carbonate Comparative -- vinylethylene carbonate 76
58 51 Example 1 Comparative hexamethylcyclo- -- 72 73 66 Example 2
trisiloxane Comparative -- -- 68 65 54 Example 3 Example 11
hexamethylcyclo- vinylethylene carbonate 69 67 65 trisiloxane
Example 12 lithium di- vinylethylene carbonate 70 69 67
fluorophosphate Comparative -- vinylethylene carbonate 65 54 49
Example 4 Comparative hexamethylcyclo- -- 60 66 60 Example 5
trisiloxane Comparative -- -- 58 60 52 Example 6
[2165] As apparent from Electrolytic Solution [8] Table 1, a
comparison among the prismatic batteries (used in Electrolytic
Solution [8] Examples 1 to 10 and Electrolytic Solution [8]
Comparative Examples 1 to 3) and a separate comparison among the
cylindrical batteries (used in Electrolytic Solution [8] Examples
11 and 12 and Electrolytic solution [8] Comparative Examples 4 to
6) show the following. The lithium secondary batteries of the
Electrolytic Solution [8] Examples, in each of which the nonaqueous
electrolyte contains both at least one specific compound A and at
least one specific compound B, have been improved in capacity
retention through cycling, proportion of initial low-temperature
discharge, and proportion of low-temperature discharge after
cycling as compared with the lithium secondary batteries of the
Electrolytic Solution [8] Comparative Examples, in each of which
the nonaqueous electrolyte does not contain either of these.
[2166] As described above, the prismatic batteries of the Examples
and the Comparative Examples each have a rated discharge capacity
of 3 ampere-hours (Ah) or higher and a direct-current resistance
component of 10 milliohms (m.OMEGA.) or lower. On the other hand,
the cylindrical batteries of the Examples and the Comparative
Examples each have a rated discharge capacity lower than 3
ampere-hours (Ah) and a direct-current resistance component higher
than 10 milliohms (m.OMEGA.). Namely, the prismatic batteries of
the Examples and the Comparative Examples have a lower resistance
and a higher electric capacity than the cylindrical batteries.
Furthermore, the degree of improvement in output of Electrolytic
Solution [8] Example 1 on Electrolytic Solution [8] Comparative
Example is higher than the degree of improvement in output of
Electrolytic Solution [8] Example 11 on Electrolytic Solution [8]
Comparative Example 4. It was thus found that the effect of this
invention is higher in secondary batteries having a high electric
capacity and secondary batteries having a low direct-current
resistance.
Electrolytic Solution [9]
Secondary-Battery Production
[Production of Positive Electrode]
[2167] Ninety percents by mass lithium cobalt oxide (LiCoO.sub.2)
as a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to prepare a slurry. The slurry obtained was applied to
each side of an aluminum foil having a thickness of 15 .mu.m and
dried. The coated foil was rolled with a pressing machine to a
thickness of 80 .mu.m. Pieces each having an active-material layer
size with a width of 100 mm and a length of 100 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, positive electrodes were obtained.
[Production of Negative Electrode]
[2168] To 98 parts by mass of artificial graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by mass
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by mass of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to each side of a copper foil having a
thickness of 10 .mu.m and dried. The coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m. Pieces each having an
active-material layer size with a width of 104 mm and a length of
104 mm and including an uncoated part having a width of 30 mm were
cut out of the rolled foil. Thus, negative electrodes were
obtained.
[Battery Fabrication]
[2169] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with porous
polyethylene sheet separators (25 .mu.m) sandwiched between the
electrodes. In this operation, each positive electrode was disposed
so that the positive-electrode active-material surface was within
the range of the opposed negative-electrode active-material
surface. The uncoated parts of the positive electrodes were welded
together and those of the negative electrodes were welded together.
Thus, current collector tabs were produced to obtain an electrode
group. This electrode group was packed into a battery can (outer
dimensions: 120.times.110.times.10 mm) equipped with a gas release
valve. Thereafter, 20 mL of a nonaqueous electrolyte was introduced
into the battery can packed with the electrode group and
sufficiently infiltrated into the electrodes. This can was sealed
to fabricate a prismatic battery.
[Electric Capacity and Direct-Current Resistance Component]
[2170] The rated discharge capacity of this battery (electric
capacity of the battery elements held in one battery case) was as
high as about 6 Ah, and the direct-current resistance component
thereof as measured by the 10-kHz alternating-current method was
about 5 milliohms.
[Battery Evaluation]
(Method of Capacity Measurement)
[2171] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.2-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as capacity.
(5-C Discharge Capacity after Storage)
[2172] A storage test was conducted in a high-temperature
environment of 60.degree. C. The battery which had been charged
beforehand to a final charge voltage of 4.2 V in a 25.degree. C.
environment by the constant-current constant-voltage method was
stored at 60.degree. C. for 1 month. This battery after the storage
was subjected to a rate test in a 25.degree. C. environment.
Namely, the battery which had been charged beforehand to a final
charge voltage of 4.2 V in a 25.degree. C. environment by the
constant-current constant-voltage method was discharged at a
constant current corresponding to 5 C, and the capacity as measured
in this discharge was taken as 5-C discharge capacity after
storage.
(Overcharge Test)
[2173] An overcharge test was conducted in a 25.degree. C.
environment. The battery in a discharged state (3V) was charged at
a constant current of 3 C and the behavior thereof was examined.
The term "valve working" represents the phenomenon in which the gas
release valve works to release electrolyte ingredients, while
"rupture" represents the phenomenon in which the battery can breaks
violently and the contents are forcedly released.
Electrolytic Solution [9]
Example 1
[2174] In a dry argon atmosphere, lithium hexafluorophosphate
(LiPF.sub.6) was added to and dissolved in a mixture of ethylene
carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl
carbonate (EMC) (volume ratio, 3:3:4) in such an amount as to
result in a concentration of 1 mol/L. Cyclohexylbenzene (CHB) and
hexamethylcyclotrisiloxane were mixed with the solution in such
amounts as to result in concentrations of 1% by mass and 0.3% by
mass, respectively, to prepare a nonaqueous electrolyte. This
nonaqueous electrolyte was used to produce a battery by the method
described above. This battery was examined for 5-C discharge
capacity after storage. The results are shown in Electrolytic
Solution [9] Table 1.
Electrolytic Solution [9]
Example 2
[2175] In Electrolytic Solution [9] Example 1,
phenyldimethylfluorosilane was mixed in place of the
hexamethylcyclotrisiloxane to prepare a nonaqueous electrolyte and
this nonaqueous electrolyte was used to produce a battery. This
battery was examined for 5-C discharge capacity after storage. The
results are shown in Electrolytic Solution [9] Table 1.
Electrolytic Solution [9]
Example 3
[2176] In Electrolytic Solution [9] Example 1, trimethylsilyl
methanesulfonate was mixed in place of the
hexamethylcyclotrisiloxane to prepare a nonaqueous electrolyte and
this nonaqueous electrolyte was used to produce a battery. This
battery was examined for 5-C discharge capacity after storage. The
results are shown in Electrolytic Solution [9] Table 1.
Electrolytic Solution [9]
Example 4
[2177] In Electrolytic Solution [9] Example 1, lithium
difluorophosphate produced by the method described in Inorganic
Nuclear Chemistry Letters (1969), 5 (7), pp. 581-582 was mixed in
place of the hexamethylcyclotrisiloxane to prepare a nonaqueous
electrolyte and this nonaqueous electrolyte was used to produce a
battery. This battery was examined for 5-C discharge capacity after
storage. The results are shown in Electrolytic Solution [9] Table
1.
Electrolytic Solution [9]
Example 5
[2178] In Electrolytic Solution [9] Example 1, biphenyl was mixed
in place of the cyclohexylbenzene to prepare a nonaqueous
electrolyte and this nonaqueous electrolyte was used to produce a
battery. This battery was examined for 5-C discharge capacity after
storage. The results are shown in Electrolytic Solution [9] Table
1.
Electrolytic Solution [9]
Example 6
[2179] In Electrolytic Solution [9] Example 5,
phenyldimethylfluorosilane was mixed in place of the
hexamethylcyclotrisiloxane to prepare a nonaqueous electrolyte and
this nonaqueous electrolyte was used to produce a battery. This
battery was examined for 5-C discharge capacity after storage. The
results are shown in Electrolytic Solution [9] Table 1.
Electrolytic Solution [9]
Example 7
[2180] In Electrolytic Solution [9] Example 5, trimethylsilyl
methanesulfonate was mixed in place of the
hexamethylcyclotrisiloxane to prepare a nonaqueous electrolyte and
this nonaqueous electrolyte was used to produce a battery. This
battery was examined for 5-C discharge capacity after storage. The
results are shown in Electrolytic Solution [9] Table 1.
Electrolytic Solution [9]
Example 8
[2181] In Electrolytic Solution [9] Example 5, lithium
difluorophosphate produced by the method described in Inorganic
Nuclear Chemistry Letters (1969), 5 (7), pp. 581-582 was mixed in
place of the hexamethylcyclotrisiloxane to prepare a nonaqueous
electrolyte and this nonaqueous electrolyte was used to produce a
battery. This battery was examined for 5-C discharge capacity after
storage. The results are shown in Electrolytic Solution [9] Table
1.
Electrolytic Solution [9]
Example 9
[2182] In Electrolytic Solution [9] Example 1, t-amylbenzene was
mixed in place of the cyclohexylbenzene to prepare a nonaqueous
electrolyte and this nonaqueous electrolyte was used to produce a
battery. This battery was examined for 5-C discharge capacity after
storage. The results are shown in Electrolytic Solution [9] Table
1.
Electrolytic Solution [9]
Example 10
[2183] In Electrolytic Solution [9] Example 1, "a product of the
partial hydrogenation of m-terphenyl" having an m-terphenyl content
of 3.7% by mass and a degree of partial hydrogenation of 42% was
mixed in place of the cyclohexylbenzene to prepare a nonaqueous
electrolyte and this nonaqueous electrolyte was used to produce a
battery. This battery was examined for 5-C discharge capacity after
storage. The results are shown in Electrolytic Solution [9] Table
1. The product of partial m-terphenyl hydrogenation used in this
Electrolytic Solution [9] Example was one obtained by reacting
m-terphenyl as a raw material with hydrogen gas under
high-temperature high-pressure conditions in the presence of a
platinum, palladium, or nickel catalyst. The degree of partial
hydrogenation was determined as an average value from that
proportion of components of the product of partial m-terphenyl
hydrogenation which was determined by analysis by gas
chromatography. The content of m-terphenyl also was determined from
the results of the analysis by gas chromatography.
Electrolytic Solution [9]
Comparative Example 1
[2184] In Electrolytic Solution [9] Example 1, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for 5-C discharge capacity. The results are
shown in Electrolytic Solution [9] Table 1.
Electrolytic Solution [9]
Comparative Example 2
[2185] In Electrolytic Solution [9] Example 5, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for 5-C discharge capacity. The results are
shown in Electrolytic Solution [9] Table 1.
Electrolytic Solution [9]
Comparative Example 3
[2186] In Electrolytic Solution [9] Example 9, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for 5-C discharge capacity. The results are
shown in Electrolytic Solution [9] Table 1.
Electrolytic Solution [9]
Comparative Example 4
[2187] In Electrolytic Solution [9] Example 10, a nonaqueous
electrolyte was prepared without mixing hexamethylcyclotrisiloxane
and this nonaqueous electrolyte was used to produce a battery. This
battery was examined for 5-C discharge capacity. The results are
shown in Electrolytic Solution [9] Table 1.
Electrolytic Solution [9]
Comparative Example 5
[2188] In Electrolytic Solution [9] Example 1, neither
hexamethylcyclotrisiloxane nor CHB was mixed in preparing a
nonaqueous electrolyte and this nonaqueous electrolyte was used to
produce a battery. This battery was examined for 5-C discharge
capacity. The results are shown in Electrolytic Solution [9] Table
1.
TABLE-US-00062 ELECTROLYTIC SOLUTION [9] TABLE 1 5-C discharge
capacity after storage Over-charge No. Overcharge inhibitor
Specific compound (mAh) test Example 1 cyclohexylbenzene
hexamethylcyclotri-siloxane 4016 valve working Example 2
cyclohexylbenzene phenyldimethylfluoro-silane 3967 valve working
Example 3 cyclohexylbenzene trimethylsilylmethanesulfonate 4023
valve working Example 4 cyclohexylbenzene lithiumdifluorophosphate
4062 valve working Example 5 biphenyl hexamethylcyclotri-siloxane
3810 valve working Example 6 biphenyl phenyldimethylfluoro-silane
3772 valve working Example 7 biphenyl
trimethylsilylmethanesulfonate 3806 valve working Example 8
biphenyl lithiumdifluorophosphate 3855 valve working Example 9
t-amylbenzene hexamethylcyclotri-siloxane 4004 valve working
Example 10 product of partial hexamethylcyclotri-siloxane 3959
valve working hydrogenation of m-terphenyl Comparative
cyclohexylbenzene none 3522 valve working Example 1 Comparative
biphenyl none 3217 valve working Example 2 Comparative
t-amylbenzene none 3511 valve working Example 3 Comparative product
of partial none 3403 valve working Example 4 hydrogenation of
m-terphenyl Comparative none none 4091 rupture Example 5
[2189] Electrolytic Solution [9] Table 1 shows the following. The
lithium secondary batteries of Electrolytic Solution [9] Examples 1
to 10, in each of the nonaqueous electrolyte contained an
overcharge inhibitor and the specific compound, not only were
prevented from rupturing in the overcharge test but also showed
better characteristics in the high-current (5-C) discharge test
after storage than the lithium secondary batteries of Electrolytic
Solution [9] Comparative Examples 1 to 4. The characteristics
thereof were almost comparable to those of the lithium secondary
battery of Electrolytic Solution [9] Comparative Example 5, which
contained no overcharge inhibitor. It was thus found that the
batteries of the Examples have improved overcharge safety and an
exceedingly high practical value.
[2190] As described above, it has become possible to provide a
lithium secondary battery which simultaneously satisfies high
high-current discharge characteristics and overcharge safety, by
using a nonaqueous electrolyte for lithium secondary batteries
which comprises a nonaqueous solvent and one or more lithium salts
mixed therewith and is characterized by containing an overcharge
inhibitor and further containing at least one compound selected
from the group consisting of cyclic siloxane compounds represented
by general formula (1), fluorosilane compounds represented by
general formula (2), compounds represented by general formula (3),
compounds having an S--F bond in the molecule, nitric acid salts,
nitrous acid salts, monofluorophosphoric acid salts,
difluorophosphoric acid salts, acetic acid salts, and propionic
acid salts in an amount of 10 ppm or more of the whole nonaqueous
electrolyte. In particular, it has become possible to impart high
suitability for practical use to batteries which have been
difficult to simultaneously satisfy those two requirements, such as
batteries in which the electric capacity of the battery elements
held in one battery case is 3 ampere-hours (Ah) or higher.
Structures [1] to [5]
Example 1
Production of Nonaqueous Electrolytic Solution
[2191] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixed solvent composed of purified ethylene
carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl
carbonate (EMC) in a volume ratio of 3:3:4. Furthermore,
hexamethylcyclotrisiloxane was added thereto so as to result in a
concentration of 0.3% by mass.
<<Production of Positive Electrode>>
[2192] As a positive-electrode active material was used a
lithium-transition metal composite oxide synthesized by the method
shown below and represented by the composition formula
LiMn.sub.0.33Ni.sub.0.33Co.sub.0.33O.sub.2. Mn.sub.3O.sub.4, NiO,
and Co(OH).sub.2 were weighed out as a manganese source, nickel
source, and cobalt source, respectively, in such amounts as to
result in an Mn:Ni:Co molar ratio of 1:1:1. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring so as to result in a median diameter
of 0.2 .mu.m.
[2193] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 5 .mu.m consisting only of the manganese source, nickel
source, and cobalt source. An LiOH powder having a median diameter
of 3 .mu.m was added to the resultant granulated particles in such
an amount that the ratio of the number of moles of the Li to the
total number of moles of the Mn, Ni, and Co became 1.05. The
ingredients were mixed together by means of a high-speed mixer to
obtain a powder mixture of the lithium source and the granulated
particles of the nickel source, cobalt source, and manganese
source. This powder mixture was heat-treated at 950.degree. C. for
12 hours (heating/cooling rate, 5.degree. C./min) in an air stream,
subsequently disaggregated, and passed through a sieve having an
opening size of 45 .mu.m to obtain a positive-electrode active
material. This positive-electrode active material had a BET
specific surface area of 1.2 m.sup.2/g, average primary-particle
diameter of 0.8 .mu.M, median diameter d.sub.50 of 4.4 .mu.m, and
tap density of 1.6 g/cm.sup.3.
[2194] Ninety percents by mass the positive-electrode active
material described above was mixed with 5% by mass acetylene black
as a conductive material and 5% by mass poly(vinylidene fluoride)
(PVdF) as a binder in N-methylpyrrolidone solvent to prepare a
slurry. The slurry obtained was applied to each side of an aluminum
foil having a thickness of 15 .mu.m and dried. The coated foil was
rolled with a pressing machine to a thickness of 81 .mu.m. Pieces
each having a positive-electrode active-material layer size with a
width of 100 mm and a length of 100 mm and including an uncoated
part having a width of 30 mm were cut out of the rolled foil. Thus,
positive electrodes were obtained. The positive-electrode
active-material layers had a density of 2.35 g/cm.sup.3. The value
of (thickness of the positive-electrode active-material layer on
one side)/(thickness of the current collector) was 2.2, and
L/(2.times.S.sub.2) was 0.2.
<<Production of Negative Electrode>>
[2195] A commercial natural-graphite powder as a particulate
carbonaceous substance was repeatedly passed through an ASTM
400-mesh sieve five times in order to prevent inclusion of coarse
particles. The negative-electrode material thus obtained is
referred to as carbonaceous substance (A).
[2196] Carbonaceous substance (A) was mixed with a
petroleum-derived heavy oil obtained in thermal naphtha cracking,
and this mixture was carbonized at 1,300.degree. C. in an inert
gas. Thereafter, the resultant sinter was classified to thereby
obtain as a negative-electrode active material a composite
carbonaceous material comprising particles of the carbonaceous
substance (A) and a carbonaceous substance having different
crystallinity adherent to the surface of the particles. In the
classification, the sinter was passed through an ASTM 400-mesh
sieve five times in order to prevent inclusion of coarse particles.
It was ascertained from actual carbon ratio that the
negative-electrode active-material powder obtained comprised 95
parts by weight of a graphite and 5 parts by weight of a lowly
crystalline carbonaceous substance with which the graphite was
covered. This negative-electrode active material had the properties
shown in Structures [1] to [5] Table 1.
TABLE-US-00063 Structures [1] to [5] Table 1 Raman Micro- Total
Specific Raman half- pore pore Particle Round- surface R value Ash
distri- distri- True Round- Aspect diameter ness area value width
d.sub.002 content bution bution density ness ratio .mu.m --
m.sup.2/g -- cm.sup.-1 nm mass % mL/g mL/g g/cm.sup.3 -- --
Negative- 15.2 0.93 3.0 0.35 37 0.336 0.07 0.101 0.692 2.25 0.93
1.4 electrode active material
[2197] To 98 parts by weight of the negative-electrode active
material described above were added 100 parts by weight of an
aqueous dispersion of sodium carboxymethyl cellulose (concentration
of sodium carboxymethyl cellulose, 1% by mass) as a thickener and 2
parts by weight of an aqueous dispersion of a styrene/butadiene
rubber (concentration of styrene/butadiene rubber, 50% by mass) as
a binder. The ingredients were mixed together by means of a
disperser to obtain a slurry. The slurry obtained was applied to
each side of a rolled copper foil having a thickness of 10 .mu.m
and dried. The coated foil was rolled with a pressing machine to a
thickness of 75 .mu.m. Pieces each having an active-material layer
size with a width of 104 mm and a length of 104 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, negative electrodes were obtained. The negative
electrodes thus obtained had an active-material density of 1.35
g/cm.sup.3, and L/(2.times.S.sub.2) was 0.19.
<<Production of Battery>>
[2198] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with microporous
film separators comprising a porous polyethylene sheet (thickness,
25 .mu.m) sandwiched between the electrodes. In this operation,
each positive electrode was disposed so that the positive-electrode
active-material surface was within the range of the opposed
negative-electrode active-material surface. The uncoated parts of
the positive electrodes were bunched and welded together by spot
welding and those of the negative electrodes were bunched and
welded together by spot welding. Thus, current collector tabs were
produced to obtain an electrode group. This electrode group was
packed into a battery can made of aluminum (outer dimensions:
120.times.110.times.10 mm). The battery can used was one in which
the lid part was equipped with current collector terminals for the
positive and negative electrodes, a pressure release valve, and a
nonaqueous-electrolyte introduction opening. Each current collector
tab was connected to the corresponding current collector terminal
by spot welding. Thereafter, 20 mL of the nonaqueous electrolyte
was introduced into the battery can packed with the electrode group
and sufficiently infiltrated into the electrodes. The introduction
opening was closed to produce a battery. The ratio of the total
electrode area of the positive electrodes to the overall surface
area of the battery case was 20.5. The value of 2.times.S.sub.1/T
was 264, and the electrode group proportion was 0.54.
<<Battery Evaluation>>
(Method of Measuring Battery Capacity)
[2199] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.1-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as initial capacity. The results of battery evaluation are shown in
Structures [1] to [5] Table 2.
(Method of Measuring Initial Output)
[2200] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as output (W). The results of battery evaluation are shown in
Structures [1] to [5] Table 2.
(Method of Measuring Initial Series Resistance)
[2201] In a 25.degree. C. environment, the battery was charged at a
constant current of 0.2 C for 150 minutes and a 10-kHz alternating
current was applied thereto to measure the impedance, which was
taken as series resistance. The results of battery evaluation are
shown in Structures [1] to [5] Table 2.
(Cycle Test (Method of Measuring Battery Capacity after Durability
Test and Output after Durability Test))
[2202] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.1 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment at a current of 0.2
C. The 0.2-C discharge capacity thereof as measured in the third
cycle was taken as battery capacity after durability test.
Furthermore, the battery which had undergone the cycle test was
examined for output, which was taken as output after durability
test, and further examined for series resistance, which was taken
as series resistance after durability test. The results of battery
evaluation are shown in Structures [1] to [5] Table 2.
(Overcharge Test)
[2203] An overcharge test was conducted in a 25.degree. C.
environment. The battery in a discharged state (3V) was charged at
a constant current of 3 C and the behavior thereof was examined.
The term "valve working" represents the phenomenon in which the gas
release valve works to release electrolyte ingredients, while
"rupture" represents the phenomenon in which the battery can breaks
violently and the contents are forcedly released. The results of
battery evaluation are shown in Structures [1] to [5] Table 2.
Structures [1] to [5]
Example 2
[2204] A battery was produced in the same manner as in Structures
[1] to [5] Example 1, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated in place of the
hexamethylcyclotrisiloxane into the nonaqueous electrolyte. This
battery was evaluated in the same manners. The results also are
shown in Structures [1] to [5] Table 2.
Structures [1] to [5]
Example 3
[2205] A battery was produced in the same manner as in Structures
[1] to [5] Example 1, except that 0.3% by mass
phenyldimethylfluorosilane was incorporated in place of the
hexamethylcyclotrisiloxane into the nonaqueous electrolyte. This
battery was evaluated in the same manners. The results also are
shown in Structures [1] to [5] Table 2.
Structures [1] to [5]
Example 4
[2206] A battery was produced in the same manner as in Structures
[1] to [5] Example 1, except that 0.3% by mass lithium
difluorophosphate was incorporated in place of the
hexamethylcyclotrisiloxane into the nonaqueous electrolyte. This
battery was evaluated in the same manners. The results also are
shown in Structures [1] to [5] Table 2.
Structures [1] to [5]
Comparative Example 1
[2207] A battery was produced in the same manner as in Structures
[1] to [5] Example 1, except that hexamethylcyclotrisiloxane was
not incorporated into the nonaqueous electrolyte. This battery was
evaluated in the same manners. The results also are shown in
Structures [1] to [5] Table 2.
TABLE-US-00064 Structures [1] to [5] Table 2 Battery capacity
Direct-current Output Direct- after resistance after Battery
current Initial durability after durability capacity resistance
output test durability test test Over-charge Specific compound Ah
m.OMEGA. W Ah m.OMEGA. W test Example 1 hexamethylcyclotrisiloxane
6.1 4.8 577 5.4 5.0 395 valve opening Example 2 trimethylsilyl
methanesulfonate 6.1 4.9 579 5.4 5.1 403 valve opening Example 3
phenyldimethylfluorosilane 6.1 4.9 572 5.4 5.1 393 valve opening
Example 4 lithium difluorophosphate 6.1 4.8 588 5.5 5.0 410 valve
opening Comparative None 6.1 5.0 500 5.1 5.5 321 rupture Example
1
Structures [1] to [5]
Example 5
Production of Positive Electrode
[2208] The same positive-electrode active material as in Structures
[1] to [5] Example 1 was used to prepare a slurry in the same
manner as in Structures [1] to [5] Example 1. The slurry obtained
was applied to each side of an aluminum foil having a thickness of
15 .mu.m and dried. The coated foil was rolled with a pressing
machine to a thickness of 81 .mu.m. A strip having a
positive-electrode active-material layer size with a width of 100
mm and a length of 3,200 mm and including uncoated parts having a
width of 6 mm located at intervals of 20 mm in the length direction
was cut out of the rolled foil. Thus, a positive electrode was
obtained. The positive-electrode active-material layers had a
density of 2.35 g/cm.sup.3, and the value of (thickness of the
positive-electrode active-material layer on one side)/(thickness of
the current collector) was 2.2. The proportion of the
width-direction dimension to the length-direction dimension in the
positive electrode was 32.
<<Production of Negative Electrode>>
[2209] The same negative-electrode active material as in Structures
[1] to [5] Example 1 was used to prepare a slurry in the same
manner as in Structures [1] to [5] Example 1. The slurry obtained
was applied to each side of a rolled copper foil having a thickness
of 10 .mu.m and dried. The coated foil was rolled with a pressing
machine to a thickness of 75 .mu.m. A strip having an
active-material layer size with a width of 104 mm and a length of
3,300 mm and including uncoated parts having a width of 6 mm
located at intervals of 20 mm in the length direction was cut out
of the rolled foil. Thus, a negative electrode was obtained. This
negative electrode had an active-material density of 1.35
g/cm.sup.3.
<<Production of Battery>>
[2210] The positive electrode and the negative electrode were
superposed and spirally wound together with microporous film
separators comprising a porous polyethylene sheet (thickness, 25
.mu.m) sandwiched between the electrodes so as to prevent the
positive electrode and the negative electrode from coming into
direct contact with each other and so that the uncoated parts of
one electrode were disposed on the side opposite to the uncoated
parts of the other electrode. Thus, an electrode roll was obtained.
With respect to each of the positive electrode and the negative
electrode, the uncoated parts were bunched and spot-welded together
at the center of the roll to produce a current collector tab and
thereby obtain an electrode group. This electrode group was
disposed in a battery can made of aluminum (outer dimensions: 36
mm.phi..times.120 mm) so that the negative-electrode current
collector tab was on the bottom. The negative-electrode current
collector tab was spot-welded to the can bottom to constitute a
structure in which the battery can served as a negative-electrode
current collector terminal. A battery can lid equipped with a
positive-electrode current collector terminal and a pressure
release valve was prepared, and the positive-electrode current
collector tab was connected to the positive-electrode current
collector terminal by spot welding. Thereafter, 20 mL of a
nonaqueous electrolyte was introduced into the battery can packed
with the electrode group and sufficiently infiltrated into the
electrodes. The battery can lid was caulked to seal the can. Thus,
a cylindrical battery was produced. The ratio of the total
electrode area of the positive electrode to the overall surface
area of the battery case was 41.0, and the electrode group
proportion was 0.58.
<<Battery Evaluation>>
[2211] Battery evaluation was conducted in the same manners as in
Structures [1] to [5] Example 1, except that the cylindrical
battery described above was used. The results of battery evaluation
are shown in Structures [1] to [5] Table 3.
Structures [1] to [5]
Example 6
[2212] A battery was produced in the same manner as in Structures
[1] to [5] Example 5, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated in place of the
hexamethylcyclotrisiloxane into the nonaqueous electrolyte. This
battery was evaluated in the same manners. The results also are
shown in Structures [1] to [5] Table 3.
Structures [1] to [5]
Example 7
[2213] A battery was produced in the same manner as in Structures
[1] to [5] Example 5, except that 0.3% by mass
phenyldimethylfluorosilane was incorporated in place of the
hexamethylcyclotrisiloxane into the nonaqueous electrolyte. This
battery was evaluated in the same manners. The results also are
shown in Structures [1] to [5] Table 3.
Structures [1] to [5]
Example 8
[2214] A battery was produced in the same manner as in Structures
[1] to [5] Example 5, except that 0.3% by mass lithium
difluorophosphate was incorporated in place of the
hexamethylcyclotrisiloxane into the nonaqueous electrolyte. This
battery was evaluated in the same manners. The results also are
shown in Structures [1] to [5] Table 3.
Structures [1] to [5]
Comparative Example 2
[2215] A battery was produced in the same manner as in Structures
[1] to [5] Example 5, except that hexamethylcyclotrisiloxane was
not incorporated into the nonaqueous electrolyte. This battery was
evaluated in the same manners. The results also are shown in
Structures [1] to [5] Table 3.
TABLE-US-00065 Structures [1] to [5] Table 3 Direct- Battery
current capacity resistance Output Direct- after after after
Battery current Initial durability durability durability capacity
resistance output test test test Over-charge Specific compound Ah
m.OMEGA. W Ah m.OMEGA. W test Example 5 hexamethylcyclotrisiloxane
6.1 5.7 569 5.3 5.9 386 valve opening Example 6 trimethylsilyl
methanesulfonate 6.1 5.6 572 5.3 5.8 389 valve opening Example 7
phenyldimethylfluorosilane 6.1 5.7 564 5.3 5.9 379 valve opening
Example 8 lithium difluorophosphate 6.1 5.6 582 5.3 5.8 399 valve
opening Comparative None 6.1 5.9 493 5.0 6.3 303 rupture Example
2
[2216] The following was found from the results given in Structures
[1] to [5] Table 2 and Structures [1] to [5] Table 3. Due to the
nonaqueous electrolyte containing the specific compound, each
battery has been improved in output, capacity retention, and safety
and, even after the cycle test, has a small increase in
direct-current resistance component to sufficiently retain the
battery capacity and output.
Structure [6]
Example 1
Production of Nonaqueous Electrolytic Solution
[2217] In a dry argon atmosphere, sufficiently dried lithium
hexafluorophosphate (LiPF.sub.6) was dissolved in a concentration
of 1 mol/L in a mixed solvent composed of purified ethylene
carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl
carbonate (EMC) in a volume ratio of 3:3:4. Furthermore,
hexamethylcyclotrisiloxane was added thereto so as to result in a
concentration of 0.3% by mass.
<<Production of Positive Electrode>>
[2218] As a positive-electrode active material was used a
lithium-transition metal composite oxide synthesized by the method
shown below and represented by the composition formula
LiMn.sub.0.33Ni.sub.0.32Co.sub.0.33O.sub.2. Mn.sub.3O.sub.4, NiO,
and Co(OH).sub.2 were weighed out as a manganese source, nickel
source, and cobalt source, respectively, in such amounts as to
result in an Mn:Ni:Co molar ratio of 1:1:1. Pure water was added
thereto to prepare a slurry. Using a circulating medium-stirring
type wet bead mill, the solid matters in the slurry were
wet-pulverized with stirring so as to result in a median diameter
of 0.2 .mu.m.
[2219] The slurry was spray-dried with a spray dryer to obtain
nearly spherical granulated particles having a particle diameter of
about 5 .mu.m consisting only of the manganese source, nickel
source, and cobalt source. An LiOH powder having a median diameter
of 3 .mu.m was added to the resultant granulated particles in such
an amount that the ratio of the number of moles of the Li to the
total number of moles of the Mn, Ni, and Co became 1.05. The
ingredients were mixed together by means of a high-speed mixer to
obtain a powder mixture of the lithium source and the granulated
particles of the nickel source, cobalt source, and manganese
source. This powder mixture was heat-treated at 950.degree. C. for
12 hours (heating/cooling rate, 5.degree. C./min) in an air stream,
subsequently disaggregated, and passed through a sieve having an
opening size of 45 .mu.m to obtain a positive-electrode active
material. This positive-electrode active material had a BET
specific surface area of 1.2 m.sup.2/g, average primary-particle
diameter of 0.8 .mu.m, median diameter d.sub.50 of 4.4 .mu.m, and
tap density of 1.6 g/cm.sup.3.
[2220] Ninety percents by mass the positive-electrode active
material described above was mixed with 5% by mass acetylene black
as a conductive material and 5% by mass poly(vinylidene fluoride)
(PVdF) as a binder in N-methylpyrrolidone solvent to prepare a
slurry. The slurry obtained was applied to each side of an aluminum
foil having a thickness of 15 .mu.m and dried. The coated foil was
rolled with a pressing machine to a thickness of 81 .mu.m. Pieces
each having a positive-electrode active-material layer size with a
width of 100 mm and a length of 100 mm and including an uncoated
part having a width of 30 mm were cut out of the rolled foil. Thus,
positive electrodes were obtained. The positive-electrode
active-material layers had a density of 2.35 g/cm.sup.3. The value
of (thickness of the positive-electrode active-material layer on
one side)/(thickness of the current collector) was 2.2, and
L/(2.times.S.sub.2) was 0.2.
<<Production of Negative Electrode>>
[2221] A commercial natural-graphite powder as a particulate
carbonaceous substance was repeatedly passed through an ASTM
400-mesh sieve five times in order to prevent inclusion of coarse
particles. The negative-electrode material thus obtained is
referred to as carbonaceous substance (A).
[2222] Carbonaceous substance (A) was mixed with a
petroleum-derived heavy oil obtained in thermal naphtha cracking,
and this mixture was carbonized at 1,300.degree. C. in an inert
gas. Thereafter, the resultant sinter was classified to thereby
obtain as a negative-electrode active material a composite
carbonaceous material comprising particles of the carbonaceous
substance (A) and a carbonaceous substance having different
crystallinity adherent to the surface of the particles. In the
classification, the sinter was passed through an ASTM 400-mesh
sieve five times in order to prevent inclusion of coarse particles.
It was ascertained from actual carbon ratio that the
negative-electrode active-material powder obtained comprised 95
parts by weight of a graphite and 5 parts by weight of a lowly
crystalline carbonaceous substance with which the graphite was
covered. This negative-electrode active material had the properties
shown in Structure [6] Table 1.
TABLE-US-00066 Structure [6] Table 1 Raman Micro- Total Specific
Raman half- pore pore Particle Round- surface R value Ash distri-
distri- True Round- Aspect diameter ness area value width d.sub.002
content bution bution density ness ratio .mu.m -- M.sup.2/g --
cm.sup.-1 nm mass % mL/g mL/g g/cm.sup.3 -- -- Negative- 15.2 0.93
3.0 0.35 37 0.336 0.07 0.101 0.692 2.25 0.93 1.4 electrode active
material
[2223] To 98 parts by weight of the negative-electrode active
material described above were added 100 parts by weight of an
aqueous dispersion of sodium carboxymethyl cellulose (concentration
of sodium carboxymethyl cellulose, 1% by mass) as a thickener and 2
parts by weight of an aqueous dispersion of a styrene/butadiene
rubber (concentration of styrene/butadiene rubber, 50% by mass) as
a binder. The ingredients were mixed together by means of a
disperser to obtain a slurry. The slurry obtained was applied to
each side of a rolled copper foil having a thickness of 10 .mu.m
and dried. The coated foil was rolled with a pressing machine to a
thickness of 75 .mu.m. Pieces each having an active-material layer
size with a width of 104 mm and a length of 104 mm and including an
uncoated part having a width of 30 mm were cut out of the rolled
foil. Thus, negative electrodes were obtained. The negative
electrodes thus obtained had an active-material density of 1.35
g/cm.sup.3, and L/(2.times.S.sub.2) was 0.19.
<<Production of Battery>>
[2224] Thirty-two positive electrodes and 33 negative electrodes
were alternately disposed and superposed together with microporous
film separators comprising a porous polyethylene sheet (thickness,
25 .mu.m) sandwiched between the electrodes. In this operation,
each positive electrode was disposed so that the positive-electrode
active-material surface was within the range of the opposed
negative-electrode active-material surface. With respect to each of
the positive electrodes and the negative electrodes, the uncoated
parts were bunched and connected to each other by spot welding
together with a metal piece serving as a current collector
terminal. Thus, current collector tabs were produced to obtain an
electrode group. The porous polyethylene sheet was one having the
property of closing the pores at a temperature of 135.degree. C. or
higher.
[2225] As a battery casing material, use was made of a laminated
sheet comprising a polypropylene film, an aluminum foil having a
thickness of 0.04 mm, and a nylon film in this order (total
thickness, 0.1 mm). This casing material was formed into a
rectangular cup so that the polypropylene film faced inward to
thereby obtain a case. This polypropylene film had a melting point
of 165.degree. C. The electrode group described above was packed
into the case so that the current collector terminals protruded
outward from the unsealed part in the cup top. Twenty milliliters
of the nonaqueous electrolyte was introduced into the case and
sufficiently infiltrated into the electrodes. This case was sealed
by heat-sealing the cup top under reduced pressure to produce a
battery. This battery was nearly prismatic. The ratio of the total
electrode area of the positive electrodes to the overall surface
area of the battery case (surface area of the casing material
excluding the heat-sealed part) was 22.6, and (2.times.S.sub.1/T)
was 411.
<<Battery Evaluation>>
(Method of Measuring Battery Capacity)
[2226] The fresh battery which had undergone no charge/discharge
cycling was subjected to 5 cycles of initial charge/discharge at
25.degree. C. in a voltage range of 4.1-3.0 V at a current of 0.2 C
(the value of current at which the rated capacity based on
1-hour-rate discharge capacity is discharged over 1 hour is taken
as 1 C; the same applies hereinafter). The 0.2-C discharge capacity
thereof as measured in the fifth cycle in this operation was taken
as "battery capacity" (Ah). The results of battery evaluation are
shown in Structure [6] Table 3.
(Method of Measuring Direct-Current Resistance Component)
[2227] In a 25.degree. C. environment, the battery was charged at a
constant current of 0.2 C for 150 minutes and a 10-kHz alternating
current was applied thereto to measure the impedance, which was
taken as "direct-current resistance component" (m.OMEGA.). The
results of battery evaluation are shown in Structure [6] Table
3.
(Method of Measuring Initial Output)
[2228] In a 25.degree. C. environment, the battery was charged for
150 minutes at a constant current of 0.2 C and then discharged for
10 seconds at each of 0.1 C, 0.3 C, 1.0 C, 3.0 C, and 10.0 C. The
voltage was measured at 10 seconds after the discharge initiation.
The area of the triangle surrounded by the resultant
current-voltage straight line and the final lower voltage (3 V) was
taken as "initial output" (W). The results of battery evaluation
are shown in Structure [6] Table 3.
(Cycle Test)
[2229] (Method of Measuring Battery Capacity after Durability Test,
Direct-Current Resistance Component after Durability Test, and
Output after Durability Test)
[2230] The battery was subjected to a cycle test in a
high-temperature environment of 60.degree. C., which is regarded as
the upper-limit temperature for the actual use of lithium secondary
batteries. The battery was charged by the constant-current
constant-voltage method at 2 C to a final charge voltage of 4.1 V
and then discharged at a constant current of 2 C to a final
discharge voltage of 3.0 V. This charge/discharge operation as one
cycle was repeated to conduct 500 cycles in total. The battery
which had undergone the cycle test was subjected to 3 cycles of
charge/discharge in a 25.degree. C. environment at a current of 0.2
C. The 0.2-C discharge capacity thereof as measured in the third
cycle was taken as "battery capacity after durability test".
Furthermore, the battery which had undergone the cycle test was
examined for direct-current resistance component, which was taken
as "direct-current resistance component after durability test", and
further examined for output, which was taken as "output after
durability test". The results of battery evaluation are shown in
Structure [6] Table 3.
(Overcharge Test)
[2231] An overcharge test was conducted in a 25.degree. C.
environment. The battery in a discharged state (3 V) was charged at
a constant current of 3 C and the behavior thereof was examined.
The term "valve working" represents the phenomenon in which the gas
release valve works to release nonaqueous-electrolyte ingredients,
while "rupture" represents the phenomenon in which the battery can
breaks violently and the contents are forcedly released. The
results of battery evaluation are shown in Structure [6] Table
3.
(Method of Measuring Volume Change)
[2232] In a 25.degree. C. environment, the volume of the battery in
a discharged state (3 V) was measured. The volume thereof was
determined by placing ethanol in a graduated vessel and immersing
the battery in the ethanol. The proportion of the volume as
measured after the cycle test to the volume as measured before the
cycle test was taken as "volume change". The results of evaluation
are shown in Structure [6] Table 3.
Structure [6]
Example 2
[2233] A battery was produced in the same manner as in Structure
[6] Example 1, except that 0.3% by mass trimethylsilyl
methanesulfonate was incorporated in place of the
hexamethylcyclotrisiloxane into the nonaqueous electrolyte. This
battery was evaluated in the same manners. The results also are
shown in Structure [6] Table 3.
Structure [6]
Example 3
[2234] A battery was produced in the same manner as in Structure
[6] Example 1, except that 0.3% by mass phenyldimethylfluorosilane
was incorporated in place of the hexamethylcyclotrisiloxane into
the nonaqueous electrolyte. This battery was evaluated in the same
manners. The results also are shown in Structure [6] Table 3.
Structure [6]
Example 4
[2235] A battery was produced in the same manner as in Structure
[6] Example 1, except that 0.3% by mass lithium difluorophosphate
was incorporated in place of the hexamethylcyclotrisiloxane into
the nonaqueous electrolyte. This battery was evaluated in the same
manners. The results also are shown in Structure [6] Table 3.
Structure [6]
Comparative Example 1
[2236] A battery was produced in the same manner as in Structure
[6] Example 1, except that hexamethylcyclotrisiloxane was not
incorporated into the nonaqueous electrolyte. This battery was
evaluated in the same manners. The results also are shown in
Structure [6] Table 3.
TABLE-US-00067 STRUCTURE [6] TABLE 2 Specific compound in
nonaqueous No. electrolyte Example 1 hexamethylcyclotrisiloxane
Example 2 trimethylsilyl methanesulfonate Example 3
phenyldimethylfluorosilane Example 4 lithium difluorophosphate
Comparative Example 1 none
TABLE-US-00068 Structure [6] Table 3 Direct-current Direct-current
resistance component Battery resistance Battery capacity after
durability Output after capacity component Initial after durability
test durability Volume No. Ah m.OMEGA. output W test W m.OMEGA.
test W Over-charge test change Example 1 6.1 4.3 583 5.3 4.2 394
valveworking 1.01 Example 2 6.1 4.2 585 5.3 4.4 398 valveworking
1.01 Example 3 6.1 4.3 579 5.3 4.5 388 valveworking 1.01 Example 4
6.1 4.3 595 5.3 4.5 407 valveworking 1.01 Comparative 6.1 4.5 508
5.0 4.9 318 rupture 1.03 Example 1
[2237] The following was found from the results given in Structure
[6] Table 3. Due to the nonaqueous electrolytes each containing the
specific compound, the batteries of the Examples have been improved
in initial output, capacity retention, and safety and, even after
the cycle test, retain a sufficient battery capacity and a
sufficient output and have a small volume change.
INDUSTRIAL APPLICABILITY
[2238] Applications of the lithium secondary batteries of the
invention are not particularly limited, and the batteries can be
used in various known applications. Examples thereof include
notebook personal computers, pen-input personal computers, mobile
personal computers, electronic-book players, portable telephones,
portable facsimile telegraphs, portable copiers, portable printers,
headphone stereos, video movie cameras, liquid-crystal TVs, handy
cleaners, portable CD players, mini-disk players, transceivers,
electronic pocketbooks, electronic calculators, memory cards,
portable tape recorders, radios, backup power sources, motors,
motor vehicles, motorbikes, bicycles fitted with a motor, bicycles,
illuminators, toys, game machines, clocks and watches, power tools,
stroboscopes, and cameras. The lithium secondary batteries of the
invention have a high capacity, satisfactory life, and high output,
are reduced in gas evolution, and are highly safe even when
overcharged. The batteries can hence be extensively and
advantageously used especially in fields where a high capacity is
required.
[2239] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope
thereof. This application is based on the following Japanese patent
applications, the entire contents thereof being herein incorporated
by reference.
Application No. 2005-331255 (filing date: Nov. 16, 2005)
Application No. 2005-331362 (filing date: Nov. 16, 2005)
Application No. 2005-331391 (filing date: Nov. 16, 2005)
Application No. 2005-331477 (filing date: Nov. 16, 2005)
Application No. 2005-331513 (filing date: Nov. 16, 2005)
Application No. 2005-339794 (filing date: Nov. 25, 2005)
Application No. 2006-019863 (filing date: Jan. 27, 2006)
Application No. 2006-005622 (filing date: Jan. 13, 2005)
Application No. 2005-367747 (filing date: Dec. 21, 2005)
Application No. 2005-377366 (filing date: Dec. 28, 2005)
Application No. 2005-349052 (filing date: Dec. 2, 2005) Application
No. 2005-359061 (filing date: Dec. 13, 2005) Application No.
2006-019879 (filing date: Jan. 27, 2006) Application No.
2006-013664 (filing date: Jan. 23, 2005) Application No.
2005-314043 (filing date: Oct. 28, 2005) Application No.
2005-331585 (filing date: Nov. 16, 2005) Application No.
2005-305368 (filing date: Oct. 20, 2005) Application No.
2005-344732 (filing date: Nov. 29, 2005) Application No.
2005-343629 (filing date: Nov. 29, 2005) Application No.
2005-332173 (filing date: Nov. 16, 2005) Application No.
2005-305300 (filing date: Oct. 20, 2005) Application No.
2005-353005 (filing date: Dec. 7, 2005) Application No. 2005-314260
(filing date: Oct. 28, 2005) Application No. 2005-369824 (filing
date: Dec. 22, 2005) Application No. 2005-370024 (filing date: Dec.
22, 2005)
BRIEF DESCRIPTION OF THE DRAWINGS
[2240] FIG. 1 (a) is a scanning electron microscope (SEM)
photograph of a section of the thin-film negative electrode (1)
used in the Examples 1 to 3 and Comparative Example 1 for negative
electrode [7].
[2241] FIG. 1 (b) is a photograph taken with an electron probe
microanalyzer (EPMA), the photograph showing a mass concentration
distribution of the element Si in the film thickness direction in
the thin-film negative electrode (1) in negative electrode [7].
[2242] FIG. 1 (c) is a photograph taken with an electron probe
microanalyzer (EPMA), the photograph showing a mass concentration
distribution of the element C in the film thickness direction in
the thin-film negative electrode (1) in negative electrode [7].
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