U.S. patent application number 12/303167 was filed with the patent office on 2009-10-08 for nonaqueous electrolytic solutions and nonaqueous-electrolyte batteries.
This patent application is currently assigned to Mitsubishi Chemical Corporation. Invention is credited to Shinichi Kinoshita, Minoru Kotato.
Application Number | 20090253045 12/303167 |
Document ID | / |
Family ID | 38801378 |
Filed Date | 2009-10-08 |
United States Patent
Application |
20090253045 |
Kind Code |
A1 |
Kotato; Minoru ; et
al. |
October 8, 2009 |
NONAQUEOUS ELECTROLYTIC SOLUTIONS AND NONAQUEOUS-ELECTROLYTE
BATTERIES
Abstract
Nonaqueous electrolytes capable of providing a battery having a
high capacity and excellent in storability and cycle
characteristics are provided. Also provided are
nonaqueous-electrolyte batteries produced with the electrolytes.
The nonaqueous electrolytes include ones which (1) contains a
cyclic carbonate having an unsaturated bond and further contains a
fluorinated cyclic carbonate having two or more fluorine atoms, (2)
contains an aromatic compound having 7-18 carbon atoms in total and
further contains a fluorinated cyclic carbonate having two or more
fluorine atoms, (3) contains diethyl carbonate and further contains
a fluorinated cyclic carbonate having two or more fluorine atoms,
or (4) contains at least one compound selected from the group
consisting of cyclic sulfonic acid ester compounds, di-sulfonic
acid ester compounds, nitrile compounds, and compounds represented
by general formula (1) and further contains a fluorinated cyclic
carbonate having two or more fluorine atoms. The electrolytes
further include (5) a nonaqueous electrolyte characterized by being
an electrolytic solution for use in a high-voltage battery having a
final charge voltage of 4.3 V or higher and containing a
fluorinated cyclic carbonate having two or more fluorine atoms.
Inventors: |
Kotato; Minoru; (Ibaraki,
JP) ; Kinoshita; Shinichi; (Ibaraki, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Mitsubishi Chemical
Corporation
Tokyo
JP
|
Family ID: |
38801378 |
Appl. No.: |
12/303167 |
Filed: |
May 31, 2007 |
PCT Filed: |
May 31, 2007 |
PCT NO: |
PCT/JP2007/061114 |
371 Date: |
April 1, 2009 |
Current U.S.
Class: |
429/326 ;
429/188; 429/199; 429/203 |
Current CPC
Class: |
H01M 6/168 20130101;
H01M 10/052 20130101; H01M 2300/0025 20130101; H01M 10/0566
20130101; H01M 10/0525 20130101; Y02E 60/10 20130101; H01M 10/0567
20130101 |
Class at
Publication: |
429/326 ;
429/188; 429/199; 429/203 |
International
Class: |
H01M 6/04 20060101
H01M006/04; H01M 6/16 20060101 H01M006/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 2, 2006 |
JP |
2006-155251 |
Claims
1. A nonaqueous electrolyte comprising an electrolyte and a
nonaqueous solvent for dissolving the electrolyte, wherein the
nonaqueous electrolyte comprises a cyclic carbonate having an
unsaturated bond and a fluorinated cyclic carbonate having two or
more fluorine atoms.
2. A nonaqueous electrolyte comprising an electrolyte and a
nonaqueous solvent for dissolving the electrolyte, wherein the
nonaqueous electrolyte comprises an aromatic compound having 7-18
carbon atoms in total and a fluorinated cyclic carbonate having two
or more fluorine atoms.
3. A nonaqueous electrolyte comprising an electrolyte and a
nonaqueous solvent for dissolving the electrolyte, wherein the
nonaqueous solvent comprises diethyl carbonate and a fluorinated
cyclic carbonate having two or more fluorine atoms.
4. A nonaqueous electrolyte comprising an electrolyte and a
nonaqueous solvent for dissolving the electrolyte, wherein the
nonaqueous electrolyte comprises: at least one compound selected
from the group consisting of cyclic sulfonic acid ester compounds,
di-sulfonic acid ester compounds, nitrile compounds, and compounds
represented by formula (1); and a fluorinated cyclic carbonate
having two or more fluorine atoms: ##STR00004## wherein R.sup.1 to
R.sup.3 each independently represent an alkyl group which has 1-12
carbon atoms and may be substituted with a fluorine atom; and n
represents an integer of 0-6.
5. The nonaqueous electrolyte according to claim 1, wherein the
cyclic carbonate having an unsaturated bond is at least one
compound selected from the group consisting of vinylene carbonate
compounds, vinylethylene carbonate compounds, and
methylene-ethylene carbonate compounds.
6. The nonaqueous electrolyte according to claim 1, wherein the
cyclic carbonate having an unsaturated bond is at least one of
vinylene carbonate and vinylethylene carbonate.
7. The nonaqueous electrolyte according to claim 1, wherein the
proportion of the cyclic carbonate having an unsaturated bond in
the nonaqueous electrolyte is from 0.001% by weight to 8% by
weight.
8. The nonaqueous electrolyte according to claim 2, wherein the
aromatic compound has 10-18 carbon atoms in total.
9. The nonaqueous electrolyte according to claim 2, wherein the
aromatic compound having 7-18 carbon atoms in total is at least one
compound selected from the group consisting of biphenyl,
alkylbiphenyl, terphenyl, partly hydrogenated terphenyl,
cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether,
and dibenzofuran.
10. The nonaqueous electrolyte according to claim 2, wherein the
proportion of the aromatic compound having 7-18 carbon atoms in
total in the nonaqueous electrolyte is from 0.001% by weight to 5%
by weight.
11. The nonaqueous electrolyte according to claim 3, wherein the
proportion of the diethyl carbonate in the whole nonaqueous solvent
is from 10% by volume to 90% by volume.
12. The nonaqueous electrolyte according to claim 4, wherein the
cyclic sulfonic acid ester compounds are at least one compound
selected from the group consisting of 1,3-propanesultone,
1,4-butanesultone, 1,3-propenesultone, and 1,4-butenesultone.
13. The nonaqueous electrolyte according to claim 4, wherein the
di-sulfonic acid ester compounds are at least one compound selected
from the group consisting of ethanediol disulfonate,
1,2-propanediol disulfonate, 1,3-propanediol disulfonate,
1,2-butanediol disulfonate, 1,3-butanediol disulfonate and
1,4-butanediol disulfonate.
14. The nonaqueous electrolyte according to claim 4, wherein the
nitrile compounds are at least one compound selected from the group
consisting of acetonitrile, propinonitrile, butyronitrile,
valeronitrile, crotononitrile, 3-methylcrotononitrile,
malononitrile, succinonitrile, glutaronitrile, adiponitrile, and
fumaronitrile.
15. The nonaqueous electrolyte according to claim 4, wherein
R.sup.1 to R.sup.3 in formula (1) have 2-8 carbon atoms.
16. The nonaqueous electrolyte according to claim 4, wherein n in
formula (1) is 0, 1, or 2.
17. The nonaqueous electrolyte according to claim 4, wherein the
total content of the at least one compound selected from the group
consisting of cyclic sulfonic acid ester compounds, di-sulfonic
acid ester compounds, nitrile compounds, and compounds represented
by formula (1) in the nonaqueous electrolyte is from 0.001% by
weight to 5% by weight.
18. The nonaqueous electrolyte according to claim 1, wherein the
fluorinated cyclic carbonate having two or more fluorine atoms is a
fluorinated ethylene carbonate having two or more fluorine
atoms.
19. The nonaqueous electrolyte according to claim 1, wherein the
fluorinated cyclic carbonate having two or more fluorine atoms is
at least one compound selected from the group consisting of
cis-4,5-difluoro-1,3-dioxolan-2-one,
trans-4,5-difluoro-1,3-dioxolan-2-one, and
4,4-difluoro-1,3-dioxolan-2-one.
20. The nonaqueous electrolyte according to claim 1, wherein the
proportion of the fluorinated cyclic carbonate having two or more
fluorine atoms in the nonaqueous electrolyte is 0.001-10% by
weight.
21. The nonaqueous electrolyte according to claim 1, wherein the
proportion of the fluorinated cyclic carbonate having two or more
fluorine atoms in the nonaqueous electrolyte is 0.01-4% by
weight.
22. A nonaqueous electrolyte comprising an electrolyte and a
nonaqueous solvent for dissolving the electrolyte, wherein the
nonaqueous electrolyte is an electrolyte for use in a high-voltage
battery having a final charge voltage of 4.3 V or higher, and
comprises a fluorinated cyclic carbonate having two or more
fluorine atoms.
23. The nonaqueous electrolyte according to claim 22, which
comprises a cyclic carbonate having an unsaturated bond.
24. The nonaqueous electrolyte according to claim 22, which
comprises an aromatic compound having 7-18 carbon atoms in
total.
25. The nonaqueous electrolyte according to claim 22, which
comprises diethyl carbonate.
26. The nonaqueous electrolyte according to claim 22, which
comprises at least one compound selected from the group consisting
of cyclic sulfonic acid ester compounds, di-sulfonic acid ester
compounds, nitrile compounds, and compounds represented by formula
(1): ##STR00005## wherein R.sup.1 to R.sup.3 each independently
represent an alkyl group which has 1-12 carbon atoms and may be
substituted with a fluorine atom; and n represents an integer of
0-6.
27. A nonaqueous-electrolyte battery comprising: a negative
electrode and a positive electrode which are capable of
occluding/releasing lithium ions; and a nonaqueous electrolyte,
wherein the nonaqueous electrolyte is the nonaqueous electrolyte
according to claim 1.
28. The nonaqueous-electrolyte secondary battery according to claim
27, wherein the negative electrode comprises at least one of
carbonaceous materials and metal compounds capable of occluding and
releasing lithium.
29. The nonaqueous-electrolyte secondary battery according to claim
27, wherein the positive electrode comprises a lithium-transition
metal composite oxide material.
30. The nonaqueous electrolyte according to claim 5, wherein the
cyclic carbonate having an unsaturated bond is at least one of
vinylene carbonate and vinylethylene carbonate.
31. The nonaqueous electrolyte according to claim 8, wherein the
aromatic compound having 7-18 carbon atoms in total is at least one
compound selected from the group consisting of biphenyl,
alkylbiphenyl, terphenyl, partly hydrogenated terphenyl,
cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether,
and dibenzofuran.
32. The nonaqueous electrolyte according to claim 15, wherein n in
formula (1) is 0, 1, or 2.
Description
TECHNICAL FIELD
[0001] The present invention relates to nonaqueous electrolytes and
nonaqueous-electrolyte batteries employing the same.
BACKGROUND ART
[0002] Nonaqueous-electrolyte batteries including lithium secondary
batteries are being put to practical use in extensive applications
ranging from power supplies for so-called domestic use, e.g., ones
for portable telephones and notebook type personal computers, to
on-vehicle power supplies for driving, e.g., ones for motor
vehicles. However, the recent desire for higher performances in
nonaqueous-electrolyte batteries is growing more and more, and
improvements in battery characteristics are required.
[0003] The electrolytes for use in nonaqueous-electrolyte batteries
are usually constituted mainly of an electrolyte and a nonaqueous
solvent. As main components of the nonaqueous solvent, use is being
made of compounds such as cyclic carbonates, e.g., ethylene
carbonate and propylene carbonate; chain carbonates, e.g., dimethyl
carbonate, diethyl carbonate, and ethyl methyl carbonate; and
cyclic carboxylic acid esters, e.g., .gamma.-butyrolactone and
.gamma.-valerolactone.
[0004] Various investigations are being made on nonaqueous solvents
and electrolytes in order to improve battery characteristics of
such nonaqueous-electrolyte batteries, such as load
characteristics, cycle characteristics, and storability.
[0005] Patent document 1 proposes the use of ethyl methyl carbonate
and dimethyl carbonate in order to inhibit the deterioration in
overcharge characteristics and the deterioration through standing
in a high-temperature environment which are attributable to the
reaction of diethyl carbonate with lithium.
[0006] Patent document 2 proposes the use of a mixture of an
asymmetric chain carbonate and a cyclic carbonate having a double
bond as a nonaqueous solvent. There is a statement therein to the
effect that the cyclic carbonate having a double bond reacts
preferentially with the negative electrode to form a coating film
of satisfactory quality on the surface of the negative electrode
and this inhibits the asymmetric chain carbonate from forming a
nonconductor coating film on the surface of the negative electrode,
whereby storability and cycle characteristics are improved.
[0007] Patent document 3 proposes that an additive which
polymerizes at a battery voltage not lower than a maximum operating
voltage of a battery is incorporated into the electrolytic solution
to thereby enable the battery to increase in internal resistance
and thus protect the battery. Patent document 4 proposes that an
additive which polymerizes at a battery voltage not lower than a
maximum operating voltage of a battery to cause gas evolution and a
pressure increase is incorporated into the electrolytic solution to
thereby enable an internal circuit breaker disposed for protection
against overcharge to work without fail. Disclosed as these
additives are aromatic compounds such as biphenyl, thiophene, and
furan.
[0008] Patent document 5 discloses that a lithium secondary battery
having a high capacity and excellent cycle characteristics can be
provided by using an electrolytic solution containing an alkylene
carbonate having a fluorine group, such as
cis-4,5-difluoro-1,3-dioxolan-2-one or
trans-4,5-difluoro-1,3-dioxolan-2-one.
Patent Document 1: JP-A-7-14607
Patent Document 2: JP-A-11-185806
Patent Document 3: JP-A-9-106835
Patent Document 4: JP-A-9-171840
Patent Document 5: JP-A-2004-319317
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009] However, the recent desire for higher performances in
batteries is growing increasingly, and batteries are required to
attain a high capacity, high-temperature storability, and cycle
characteristics on a high level.
[0010] Techniques for packing an active material as much as
possible into a limited battery volume in order to increase
capacity are being investigated. Generally used are a method in
which an active-material layer of an electrode is densified by
pressing and a method in which a battery is designed so as to
minimize the volume occupied by substances other than the active
materials within the battery. However, to density an
active-material layer of an electrode by pressing or to reduce the
amount of an electrolytic solution makes it impossible to evenly
use an active material. Such techniques are hence apt to pose a
problem that reactions proceed unevenly to cause partial lithium
deposition and accelerate active-material deterioration and
sufficient characteristics are not obtained. Nonaqueous-electrolyte
secondary batteries employing the electrolytes described in patent
documents 1 to 5 have been still insufficient from the standpoint
of reconciling cycle characteristics and high-temperature
storability.
[0011] On the other hand, for the purpose of heightening energy
density, an attempt is being made to charge a battery to a high
voltage exceeding 4.2 V to heighten the operating voltage of the
battery. However, there has been a problem that the higher the
charge voltage, the more the deterioration of battery
characteristics becomes remarkable.
Means for Solving the Problems
[0012] The present inventors repeatedly made investigations in
order to accomplish the objects. As a result, it has been found
that the problems described above can be overcome by incorporating
a compound having a specific structure into a specific electrolytic
solution. The invention has been thus completed.
[0013] Namely, essential points of the invention are as
follows.
(1) A nonaqueous electrolyte comprising an electrolyte and a
nonaqueous solvent for dissolving the electrolyte, wherein the
nonaqueous electrolyte comprises a cyclic carbonate having an
unsaturated bond and a fluorinated cyclic carbonate having two or
more fluorine atoms. (2) A nonaqueous electrolyte comprising an
electrolyte and a nonaqueous solvent for dissolving the
electrolyte, wherein the nonaqueous electrolyte comprises an
aromatic compound having 7-18 carbon atoms in total and a
fluorinated cyclic carbonate having two or more fluorine atoms. (3)
A nonaqueous electrolyte comprising an electrolyte and a nonaqueous
solvent for dissolving the electrolyte, wherein the nonaqueous
solvent comprises diethyl carbonate and a fluorinated cyclic
carbonate having two or more fluorine atoms. (4) A nonaqueous
electrolyte comprising an electrolyte and a nonaqueous solvent for
dissolving the electrolyte, wherein the nonaqueous electrolyte
comprises: at least one compound selected from the group consisting
of cyclic sulfonic acid ester compounds, di-sulfonic acid ester
compounds, nitrile compounds, and compounds represented by the
following general formula (1); and a fluorinated cyclic carbonate
having two or more fluorine atoms:
##STR00001##
wherein R.sup.1 to R.sup.3 each independently represent an alkyl
group which has 1-12 carbon atoms and may be substituted with a
fluorine atom; and n represents an integer of 0-6. (5) The
nonaqueous electrolyte according to (1), wherein the cyclic
carbonate having an unsaturated bond is at least one compound
selected from the group consisting of vinylene carbonate compounds,
vinylethylene carbonate compounds, and methylene-ethylene carbonate
compounds. (6) The nonaqueous electrolyte according to (1) or (5),
wherein the cyclic carbonate having an unsaturated bond is at least
one of vinylene carbonate and vinylethylene carbonate. (7) The
nonaqueous electrolyte according to (1), (5), or (6), wherein the
proportion of the cyclic carbonate having an unsaturated bond in
the nonaqueous electrolyte is from 0.001% by weight to 8% by
weight. (8) The nonaqueous electrolyte according to (2), wherein
the aromatic compound has 10-18 carbon atoms in total. (9) The
nonaqueous electrolyte according to (2) or (8), wherein the
aromatic compound having 7-18 carbon atoms in total is at least one
compound selected from the group consisting of biphenyl,
alkylbiphenyl, terphenyl, partly hydrogenated terphenyl,
cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether,
and dibenzofuran. (10) The nonaqueous electrolyte according to (2),
(8), or (9), wherein the proportion of the aromatic compound having
7-18 carbon atoms in total in the nonaqueous electrolyte is from
0.001% by weight to 5% by weight. (11) The nonaqueous electrolyte
according to (3), wherein the proportion of the diethyl carbonate
in the whole nonaqueous solvent is from 10% by volume to 90% by
volume. (12) The nonaqueous electrolyte according to (4), wherein
the cyclic sulfonic acid ester compounds are at least one compound
selected from the group consisting of 1,3-propanesultone,
1,4-butanesultone, 1,3-propenesultone, and 1,4-butenesultone. (13)
The nonaqueous electrolyte according to (4), wherein the
di-sulfonic acid ester compounds are at least one compound selected
from the group consisting of ethanediol disulfonate,
1,2-propanediol disulfonate, 1,3-propanediol disulfonate,
1,2-butanediol disulfonate, 1,3-butanediol disulfonate and
1,4-butanediol disulfonate. (14) The nonaqueous electrolyte
according to (4), wherein the nitrile compounds are at least one
compound selected from the group consisting of acetonitrile,
propinonitrile, butyronitrile, valeronitrile, crotononitrile,
3-methylcrotononitrile, malononitrile, succinonitrile,
glutaronitrile, adiponitrile, and fumaronitrile. (15) The
nonaqueous electrolyte according to (4), wherein R.sup.1 to R.sup.3
in general formula (1) have 2-8 carbon atoms. (16) The nonaqueous
electrolyte according to (4) or (15), wherein n in general formula
(1) is 0, 1, or 2. (17) The nonaqueous electrolyte according to (4)
or any one of (12) to (16), wherein the total content of the at
least one compound selected from the group consisting of cyclic
sulfonic acid ester compounds, di-sulfonic acid ester compounds,
nitrile compounds, and compounds represented by the general formula
(1) in the nonaqueous electrolyte is from 0.001% by weight to 5% by
weight. (18) The nonaqueous electrolyte according to any one of (1)
to (17), wherein the fluorinated cyclic carbonate having two or
more fluorine atoms is a fluorinated ethylene carbonate having two
or more fluorine atoms. (19) The nonaqueous electrolyte according
to any one of (1) to (18), wherein the fluorinated cyclic carbonate
having two or more fluorine atoms is at least one compound selected
from the group consisting of cis-4,5-difluoro-1,3-dioxolan-2-one,
trans-4,5-difluoro-1,3-dioxolan-2-one, and
4,4-difluoro-1,3-dioxolan-2-one. (20) The nonaqueous electrolyte
according to any one of (1) to (19), wherein the proportion of the
fluorinated cyclic carbonate having two or more fluorine atoms in
the nonaqueous electrolyte is 0.001-10% by weight. (21) The
nonaqueous electrolyte according to any one of (1) to (20), wherein
the proportion of the fluorinated cyclic carbonate having two or
more fluorine atoms in the nonaqueous electrolyte is 0.01-4% by
weight. (22) A nonaqueous electrolyte comprising an electrolyte and
a nonaqueous solvent for dissolving the electrolyte, wherein the
nonaqueous electrolyte is an electrolyte for use in a high-voltage
battery having a final charge voltage of 4.3 V or higher, and
comprises a fluorinated cyclic carbonate having two or more
fluorine atoms. (23) The nonaqueous electrolyte according to (22),
which comprises a cyclic carbonate having an unsaturated bond. (24)
The nonaqueous electrolyte according to (22), which comprises an
aromatic compound having 7-18 carbon atoms in total. (25) The
nonaqueous electrolyte according to (22), which comprises diethyl
carbonate. (26) The nonaqueous electrolyte according to (22), which
comprises at least one compound selected from the group consisting
of cyclic sulfonic acid ester compounds, di-sulfonic acid ester
compounds, nitrile compounds, and compounds represented by the
following general formula (1):
##STR00002##
wherein R.sup.1 to R.sup.3 each independently represent an alkyl
group which has 1-12 carbon atoms and may be substituted with a
fluorine atom; and n represents an integer of 0-6. (27) A
nonaqueous-electrolyte battery comprising: a negative electrode and
a positive electrode which are capable of occluding/releasing
lithium ions; and a nonaqueous electrolyte, wherein the nonaqueous
electrolyte is the nonaqueous electrolyte according to any one of
(1) to (26). (28) The nonaqueous-electrolyte secondary battery
according to (27), wherein the negative electrode comprises at
least one of carbonaceous materials and metal compounds capable of
occluding and releasing lithium. (29) The nonaqueous-electrolyte
secondary battery according to (27), wherein the positive electrode
comprises a lithium-transition metal composite oxide material.
ADVANTAGES OF THE INVENTION
[0014] According to the invention, nonaqueous-electrolyte batteries
having a high capacity and excellent in storability and cycle
characteristics can be provided. These nonaqueous-electrolyte
batteries can have a smaller size and higher performances.
BEST MODE FOR CARRYING OUT THE INVENTION
[0015] Modes for carrying out the invention will be explained below
in detail. However, the following explanations on constituent
elements are for embodiments (typical embodiments) of the invention
and should not be construed as limiting the invention.
<Nonaqueous Electrolytic Solutions>
[0016] The nonaqueous electrolytes of the invention include at
least one electrolyte and a nonaqueous solvent for dissolving the
electrolyte, like common nonaqueous electrolytes. Usually, the
electrolyte and the nonaqueous solvent are major components of the
electrolytes.
(Electrolyte)
[0017] One or more lithium salts are usually used as the
electrolyte. As the lithium salts, any desired lithium salts known
to be usable in this application can be used without particular
limitations. Examples thereof include the following.
[0018] Examples of the lithium salts include inorganic lithium
salts such as LiPF.sub.6 and LiBF.sub.4; fluorine-containing
organic lithium salts such as LiCF.sub.3SO.sub.3,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2,
lithium salt of cyclic 1,2-perfluoroethanedisulfonylimide, lithium
salt of cyclic 1,3-perfluoropropanedisulfonylimide,
LiN(CF.sub.3SO.sub.2)(C.sub.4F.sub.9SO.sub.2),
LiC(CF.sub.3SO.sub.2).sub.3, LiPF.sub.4 (CF.sub.3).sub.2,
LiPF.sub.4 (C.sub.2F.sub.5).sub.2, LiPF.sub.4
(CF.sub.3SO.sub.2).sub.2, LiPF.sub.4
(C.sub.2F.sub.5SO.sub.2).sub.2, LiBF.sub.2 (CF.sub.3).sub.2,
LiBF.sub.2 (C.sub.2F.sub.5).sub.2, LiBF.sub.2
(CF.sub.3SO.sub.2).sub.2, and
LiBF.sub.2(C.sub.2F.sub.5SO.sub.2).sub.2; and lithium
bis(oxalate)borate.
[0019] Of these, LiPF.sub.6, LiBF.sub.4, LiCF.sub.3SO.sub.3,
LiN(CF.sub.3SO.sub.2).sub.2, or LiN(C.sub.2F.sub.5SO.sub.2).sub.2
is preferred from the standpoint of battery performance
improvement. Especially preferred is LiPF.sub.6 or LiBF.sub.4.
[0020] These lithium salts may be used alone or in combination of
two or more thereof.
[0021] In the case where two or more lithium salts are used in
combination, a preferred example is a combination of LiPF.sub.6 and
LiBF.sub.4. This combination has the effect of improving cycle
characteristics. In this case, the proportion of LiBF.sub.4 in the
sum of both is preferably 0.01% by weight or higher, especially
preferably 0.1% by weight or higher, and is preferably 20% by
weight or lower, especially preferably 5% by weight or lower. When
the proportion of LiBF.sub.4 is lower than the lower limit, there
are cases where the desired effect is difficult to obtain. When the
proportion thereof exceeds the upper limit, there are cases where
battery characteristics decrease through high-temperature
storage.
[0022] Another example is a combination of an inorganic lithium
salt and a fluorine-containing organic lithium salt. In this case,
the proportion of the inorganic lithium salt in the sum of both
desirably is 70% by weight or higher and 99% by weight or lower.
The fluorine-containing organic lithium salt preferably is any of
LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2,
lithium salt of cyclic 1,2-perfluoroethanedisulfonylimide, and
lithium salt of cyclic 1,3-perfluoropropanedisulfonylimide. This
combination has the effect of inhibiting the deterioration caused
by high-temperature storage.
[0023] When the nonaqueous solvent is one containing
.gamma.-butyrolactone in an amount of 55% by volume or larger, the
lithium salts preferably are either LiBF.sub.4 or a combination of
LiBF.sub.4 and one or more other lithium salts. In this case, it is
preferred that LiBF.sub.4 accounts for at least 40% by mole of all
lithium salts. An especially preferred combination is one in which
the proportion of LiBF.sub.4 in all lithium salts is 40% by mole or
higher and 95% by mole or lower and the remainder is one or more
members selected from the group consisting of LiPF.sub.6,
LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2, and
LiN(C.sub.2F.sub.5SO.sub.2).sub.2.
[0024] The concentration of these electrolytes in the nonaqueous
electrolyte is not particularly limited from the standpoint of
producing the effects of the invention. 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 3 mol/L or lower, preferably 2 mol/L or lower,
more preferably 1.8 mol/L or lower, even more preferably 1.5 mol/L
or lower. When the concentration thereof is too low, there are
cases where the electrolytic solution has insufficient electrical
conductivity. On the other hand, when the concentration thereof is
too high, there are cases where the resultant increase in viscosity
lowers electrical conductivity or reduces battery performances.
(Nonaqueous Solvent)
[0025] The nonaqueous solvent to be use may also be one suitably
selected from nonaqueous solvents which have been known as solvents
for nonaqueous electrolytes. Examples thereof include cyclic
carbonates having no unsaturated bond, chain carbonates, cyclic
ethers, chain ethers, cyclic carboxylic acid esters, chain
carboxylic acid esters, and phosphorus-containing organic
solvents.
[0026] Examples of the cyclic carbonates having no carbon-carbon
unsaturated bond include alkylene carbonates having an alkylene
group with 2-4 carbon atoms, such as ethylene carbonate, propylene
carbonate, and butylene carbonate. Of these, ethylene carbonate and
propylene carbonate are preferred from the standpoint of improving
battery characteristics. Especially preferred is ethylene
carbonate.
[0027] The chain carbonates preferably are dialkyl carbonates, in
which the constituent alkyl groups each have preferably 1-5 carbon
atoms, especially preferably 1-4 carbon atoms. Examples thereof
include dialkyl carbonates such as symmetric chain alkyl
carbonates, e.g., dimethyl carbonate, diethyl carbonate, and
di-n-propyl carbonate, and asymmetric chain alkyl carbonates, e.g.,
ethyl methyl carbonate, methyl n-propyl carbonate, and ethyl
n-propyl carbonate. Preferred of these from the standpoint of
battery characteristics (in particular, high-load discharge
characteristics) are dimethyl carbonate, diethyl carbonate, and
ethyl methyl carbonate.
[0028] Examples of the cyclic ethers include tetrahydrofuran and
2-methyltetrahdyrofuran.
[0029] Examples of the chain ethers include dimethoxyethane and
dimethoxymethane.
[0030] Examples of the cyclic carboxylic acid esters include
.gamma.-butyrolactone and .gamma.-valerolactone.
[0031] Examples of the chain carboxylic acid esters include methyl
acetate, methyl propionate, ethyl propionate, and methyl
butyrate.
[0032] Examples of the phosphorus-containing organic solvents
include trimethyl phosphate, triethyl phosphate, dimethyl ethyl
phosphate, methyl diethyl phosphate, ethylene-methyl phosphate, and
ethylene-ethyl phosphate.
[0033] These compounds may be used alone or in combination of two
or more thereof. However, it is preferred to use two or more
compounds in combination. For example, it is preferred to use a
combination of a high-permittivity solvent, e.g., an alkylene
carbonate or cyclic carboxylic acid ester, and a low-viscosity
solvent, e.g., a dialkyl carbonate or chain carboxylic acid
ester.
[0034] One preferred example of combinations for use as a
nonaqueous solvent is combinations consisting mainly of at least
one alkylene carbonate and at least one dialkyl carbonate.
Especially preferred is a combination in which the proportion of
the sum of the alkylene carbonate and the dialkyl carbonate in the
whole nonaqueous solvent is 70% by volume or higher, preferably 80%
by volume or higher, more preferably 90% by volume or higher, and
the proportion of the alkylene carbonate to the sum of the alkylene
carbonate and the dialkyl 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, even
more preferably 25% lower. Use of this combination as a nonaqueous
solvent is preferred because the battery produced with this solvent
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).
[0035] Preferred examples of the combination of at least one
alkylene carbonate and at least one dialkyl 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.
[0036] Preferred combination examples further include those
combinations of ethylene carbonate and one or more dialkyl
carbonates to which propylene carbonate has been further added.
[0037] In the case where propylene carbonate is contained, the
ethylene carbonate/propylene carbonate volume ratio is preferably
from 99:1 to 40:60, especially preferably from 95:5 to 50:50. The
proportion of propylene carbonate in the whole nonaqueous solvent
is generally 0.1% by volume or higher, preferably 1% by volume or
higher, more preferably 2% by volume or higher, and the upper limit
thereof is generally 20% by volume or lower, preferably 8% by
volume or lower, more preferably 5% by volume or lower. This
combination containing propylene carbonate in a concentration
within that range is preferred because it has excellent
low-temperature properties while retaining the properties of the
combination of ethylene carbonate and one or more dialkyl
carbonates.
[0038] More preferred of the combinations of ethylene carbonate and
one or more dialkyl carbonates are ones in which the dialkyl
carbonates include one or more asymmetric chain alkyl carbonates.
In particular, a combination composed of ethylene carbonate, one or
more symmetric chain alkyl carbonates, and one or more asymmetric
chain alkyl carbonates is preferred because it brings about a
satisfactory balance between cycle characteristics and high-current
discharge characteristics. Examples of this combination include:
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. Preferred of such combinations are ones in
which the asymmetric chain alkyl carbonates include ethyl methyl
carbonate and each alkyl group of the alkyl carbonates has 1-2
carbon atoms.
[0039] The proportion of dimethyl carbonate in the whole nonaqueous
solvent may be generally 10% by volume or higher, preferably 20% by
volume or higher, more preferably 25% by volume or higher, even
more preferably 30% by volume or higher, and the upper limit
thereof may be generally 90% by volume or lower, preferably 80% by
volume or lower, more preferably 75% by volume or lower, even more
preferably 70% by volume or lower. Use of the nonaqueous solvent
containing dimethyl carbonate in an amount within such range is
preferred because it gives a battery having improved load
characteristics.
[0040] The proportion of ethyl methyl carbonate in the whole
nonaqueous solvent may be generally 10% by volume or higher,
preferably 20% by volume or higher, more preferably 25% by volume
or higher, even more preferably 30% by volume or higher, and the
upper limit thereof may be generally 90% by volume or lower,
preferably 80% by volume or lower, more preferably 75% by volume or
lower, even more preferably 70% by volume or lower. Use of the
nonaqueous solvent containing ethyl methyl carbonate in an amount
within such range is preferred because it gives a battery having a
satisfactory balance between cycle characteristics and
storability.
[0041] Other solvents may be incorporated into the combinations
consisting mainly of at least one alkylene carbonate and at least
one dialkyl carbonate described above. Although the additional
solvents are not particularly limited from the standpoint of
producing the effects of the invention, it is preferred not to
incorporate a cyclic carboxylic acid ester when load
characteristics are important.
[0042] Another preferred example of the nonaqueous solvent is one
at least 60% by volume of which is accounted for by at least one
organic solvent selected from the group consisting of ethylene
carbonate, propylene carbonate, .gamma.-butyrolactone, and
.gamma.-valerolactone or by a mixed solvent composed of two or more
organic solvents selected from that group. The nonaqueous
electrolyte employing this mixed solvent is less apt to suffer
solvent vaporization or liquid leakage even when used at high
temperatures. In particular, when a nonaqueous solvent in which the
proportion of the sum of ethylene carbonate and
.gamma.-butyrolactone is preferably 80% by volume or higher, more
preferably 90% by volume or higher, and the ethylene
carbonate/.gamma.-butyrolactone volume ratio is from 5:95 to 45:55
is used or when a nonaqueous solvent in which the proportion of the
sum of ethylene carbonate and propylene carbonate is preferably 80%
by volume or higher, more preferably 90% by volume or higher, and
the ethylene carbonate/propylene carbonate volume ratio is from
30:70 to 60:40 is used, then a better balance among cycle
characteristics, high-temperature storability, etc. is
obtained.
[0043] In this description, component volumes in the nonaqueous
solvent are values measured at 25.degree. C. However, in the case
of components which are solid at 25.degree. C., such as ethylene
carbonate, values measured at the melting points are used.
(Fluorinated Cyclic Carbonate Having Two or More Fluorine
Atoms)
[0044] The nonaqueous electrolytes according to the invention,
which include the electrolyte and nonaqueous solvent described
above, further contain at least one fluorinated cyclic carbonate
having two or more fluorine atoms.
[0045] The number of the fluorine atoms of the fluorinated cyclic
carbonate having two or more fluorine atoms is not particularly
limited. In the case of fluorinated ethylene carbonates, however,
the lower limit of the number thereof is generally 2 or larger and
the upper limit thereof is generally 4 or smaller, preferably 3 or
smaller.
[0046] In the case of fluorinated propylene carbonates, the lower
limit of the number of the fluorine atoms is generally 2 or larger
and the upper limit thereof is generally 6 or smaller, preferably 5
or smaller. In particular, one having two or more fluorine atoms
each bonded to any of the carbon atoms constituting a ring
structure is preferred from the standpoint of improving cycle
characteristics and storability.
[0047] Examples of the fluorinated cyclic carbonate having two or
more fluorine atoms include fluorinated ethylene carbonates such as
cis-4,5-difluoro-1,3-dioxolan-2-one,
trans-4,5-difluoro-1,3-dioxolan-2-one,
4,4-difluoro-1,3-dioxolan-2-one, trifluoro-1,3-dioxolan-2-one, and
tetrafluoro-1,3-dioxolan-2-one and fluorinated propylene carbonates
such as 4,5-difluoro-4-methyl-1,3-dioxolan-2-one,
4,4-difluoro-5-methyl-1,3-dioxolan-2-one,
4,4,5-trifluoro-5-methyl-1,3-dioxolan-2-one, and
4,5-difluoro-4-trifluoromethyl-1,3-dioxolan-2-one.
[0048] Preferred of these from the standpoint of improving battery
characteristics are the fluorinated ethylene carbonates having two
or more fluorine atoms. Especially preferred of these are
cis-4,5-difluoro-1,3-dioxolan-2-one,
trans-4,5-difluoro-1,3-dioxolan-2-one, and
4,4-difluoro-1,3-dioxolan-2-one.
[0049] Such fluorinated cyclic carbonates having two or more
fluorine atoms may be used alone or in combination of two or more
thereof. The proportion of the fluorinated cyclic carbonates having
two or more fluorine atoms in each nonaqueous electrolyte is not
particularly limited from the standpoint of producing the effects
of the invention. However, the proportion thereof is generally
0.001% by weight or higher, preferably 0.01% by weight or higher,
more preferably 0.1% by weight or higher, especially preferably
0.2% by weight or higher, most preferably 0.25% by weight or
higher. When the concentration thereof is lower than that, there
are cases where the effects of the invention are difficult to
produce. Conversely, too high concentrations thereof may result in
cases where the battery undergoes enhanced swelling during
high-temperature storage. Because of this, the upper limit of the
proportion of the fluorinated cyclic carbonates is generally 10% by
weight or lower, preferably 4% by weight or lower, more preferably
2% by weight or lower, especially preferably 1% by weight or lower,
most preferably 0.5% by weight or lower.
[0050] One aspect of the invention is a nonaqueous electrolyte
which includes at least one electrolyte and a nonaqueous solvent
for dissolving the electrolyte, and is characterized by containing
at least one cyclic carbonate having an unsaturated bond and
further containing at least one fluorinated cyclic carbonate having
two or more fluorine atoms.
[0051] Examples of the cyclic carbonate having an unsaturated bond
include vinylene carbonate compounds such as vinylene carbonate,
methylvinylene carbonate, ethylvinylene carbonate,
4,5-dimethylvinylene carbonate, 4,5-diethylvinylene carbonate, and
fluorovinylene carbonate; vinylethylene carbonate compounds such as
vinylethylene carbonate, 4-methyl-4-vinylethylene carbonate,
4-ethyl-4-vinylethylene carbonate, 4-n-propyl-4-vinylethylene
carbonate, 5-methyl-4-vinylethylene carbonate, 4,4-divinylethylene
carbonate, and 4,5-divinylethylene carbonate; and
methylene-ethylene carbonates such as
4,4-dimethyl-5-methylene-ethylene carbonate and
4,4-diethyl-5-methylene-ethylene carbonate.
[0052] Of these, vinylene carbonate, vinylethylene carbonate,
4-methyl-4-vinylethylene carbonate, or 4,5-divinylethylene
carbonate is preferred from the standpoint of improving cycle
characteristics. More preferred of these is vinylene carbonate or
vinylethylene carbonate. Those compounds may be used alone or in
combination of two or more thereof.
[0053] In the case where two or more compounds are used in
combination, it is preferred to use a combination of vinylene
carbonate and vinylethylene carbonate.
[0054] The proportion of the cyclic carbonate having an unsaturated
bond in this nonaqueous electrolyte is not particularly limited
from the standpoint of producing the effects of the invention.
However, the proportion thereof is generally 0.001% by weight or
higher, preferably 0.1% by weight or higher, especially preferably
0.3% by weight or higher, most preferably 0.5% by weight or higher.
When the content of the cyclic carbonate having an unsaturated bond
in the molecule is too low, there are cases where the effect of
improving battery cycle characteristics cannot be sufficiently
produced. However, in case where the content of the cyclic
carbonate having an unsaturated bond is too high, there is a
tendency that gas evolution during high-temperature storage is
enhanced or low-temperature discharge characteristics decrease.
Because of this, the upper limit of the proportion of this cyclic
carbonate is generally 8% by weight or lower, preferably 4% by
weight or lower, especially preferably 3% by weight or lower.
[0055] The reasons why this nonaqueous electrolyte according to the
invention improves high-temperature storability and cycle
characteristics have not been elucidated. However, the improvement
is presumed to be attained by the following mechanism, although the
invention should not be construed as being limited by the following
mechanism.
[0056] First, the cyclic carbonate having an unsaturated bond,
e.g., vinylene carbonate, is reduced during initial charge to form
a stable coating film containing a polymeric ingredient on the
surface of the negative electrode. As a result, storability and
cycle characteristics can be improved. However, the cyclic
carbonate having an unsaturated bond is apt to react with the
positive-electrode material in a charged state. There has been a
problem that the reaction with the positive-electrode material
proceeds especially in a high-temperature atmosphere to accelerate
deterioration of the positive-electrode active material, resulting
in reduced battery characteristics or enhanced gas evolution. In
contrast, when the nonaqueous electrolyte contains a fluorinated
cyclic carbonate having two or more fluorine atoms, then this
fluorinated cyclic carbonate having two or more fluorine atoms and
the cyclic carbonate having an unsaturated bond form a composite
coating film on the surface of the negative electrode through
reduction reactions. In this stage, part of the product of
reduction of the fluorinated cyclic carbonate having two or more
fluorine atoms moves to the surface of the positive electrode to
form a coating film on the surface of the positive electrode. This
coating film prevents the positive electrode from coming into
contact with the cyclic carbonate having an unsaturated bond. It is
thought that the cyclic carbonate having an unsaturated bond can be
thus inhibited from undergoing a side reaction with the
positive-electrode material.
[0057] Furthermore, the fluorinate cyclic carbonate having two or
more fluorine atoms is more apt to undergo reduction reaction than
fluorinated cyclic carbonates having less than two fluorine atoms.
The former cyclic carbonate hence has the high ability to form a
coating film on the negative electrode and the high ability to
protect the positive electrode, and can inhibit side reactions from
occurring in the battery.
[0058] In case where the cyclic carbonate having an unsaturated
bond is not contained, a coating film consisting mainly of a
product of reductional decomposition of the fluorinated cyclic
carbonate having two or more fluorine atoms is formed on the
surface of the negative electrode. However, compared to this
coating film, the composite coating film formed from both the
cyclic carbonate having an unsaturated bond and the fluorinated
cyclic carbonate having two or more fluorine atoms contains a
larger amount of a polymeric ingredient and has better stability.
The negative electrode is thought to be thus inhibited from
undergoing side reactions with other components of the electrolytic
solution.
[0059] As described above, improvements in high-temperature
storability and cycle characteristics can be attained by the
interaction between the cyclic carbonate having an unsaturated bond
and the fluorinated cyclic carbonate having two or more fluorine
atoms.
[0060] Another aspect of the invention is a nonaqueous electrolyte
which includes at least one electrolyte and a nonaqueous solvent
for dissolving the electrolyte, and is characterized by containing
at least one aromatic compound having 7-18 carbon atoms in total
and further containing at least one fluorinated cyclic carbonate
having two or more fluorine atoms.
[0061] The lower limit of the number of the carbon atoms of the
aromatic compound having 7-18 carbon atoms is generally 7 or
larger, preferably 8 or larger, more preferably 10 or larger. The
lower limit thereof is generally 18 or smaller.
[0062] When the number thereof is larger than the lower limit,
satisfactory overcharge-preventive properties are obtained. When
the number thereof is smaller than the upper limit, this aromatic
compound has satisfactory solubility in the electrolytic
solution.
[0063] Examples of the aromatic compound having 7-18 carbon atoms
in total include aromatic compounds such as biphenyl,
alkylbiphenyls such as 2-methylbiphenyl, terphenyl, partly
hydrogenated terphenyls, cyclopentylbenzene, cyclohexylbenzene,
t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran;
products of the partial fluorination of these aromatic compounds,
such as 2-fluorobiphenyl, 3-fluorobiphenyl, 4-fluorobiphenyl,
o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; and
fluorine-containing anisole compounds such as 2,4-difluoroanisole,
2,5-difluoroanisole, 2,6-difluoroanisole, and
3,5-difluoroanisole.
[0064] Preferred of these are aromatic compounds such as biphenyl,
alkylbiphenyls, terphenyl, partly hydrogenated terphenyls,
cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether,
and dibenzofuran.
[0065] Two or more of these compounds may be used in combination.
In the case of using two or more compounds in combination, the
following combinations are especially preferred from the standpoint
of a balance between overcharge-preventive properties and
high-temperature storability: a combination of cyclohexylbenzene
and either t-butylbenzene or t-amylbenzene; and a combination of
one or more members selected from aromatic compounds containing no
oxygen, such as biphenyl, alkylbiphenyls, terphenyl, partly
hydrogenated terphenyls, cyclohexylbenzene, t-butylbenzene, and
t-amylbenzene, and one or more members selected from
oxygen-containing aromatic compounds such as diphenyl ether and
dibenzofuran.
[0066] The reasons why this nonaqueous electrolyte according to the
invention is excellent in safety in overcharge and improves
high-temperature storability and cycle characteristics have not
been elucidated. However, these effects are presumed to be attained
by the following mechanism, although the invention should not be
construed as being limited by the following mechanism.
[0067] In general, aromatic compounds each having 7-18 carbon atoms
in total have the effect of improving safety in overcharge.
However, these compounds are more apt to react on the positive
electrode and negative electrode than solvent ingredients. Because
of this, the compounds undesirably react at highly active sites on
the electrodes even during high-temperature storage. The reaction
of these compounds considerably increases the internal resistance
of the battery and evolves a gas, and this has been a cause of a
considerable decrease in discharge characteristics after
high-temperature storage. It is thought that when the electrolytic
solution containing a fluorinated cyclic carbonate having two or
more fluorine atoms is used, then a coating film of a reduction
reaction product derived from the fluorinated cyclic carbonate
having two or more fluorine atoms is efficiently formed on the
surface of the negative electrode from initial charge and this
coating film inhibits the negative electrode from reacting with the
aromatic compound having 7-18 carbon atoms in total. Furthermore,
part of the product of reduction of the fluorinated cyclic
carbonate having two or more fluorine atoms moves to the surface of
the positive electrode to form a coating film on the surface of the
positive electrode. This coating film prevents the positive
electrode from coming into contact with the aromatic compound
having 7-18 carbon atoms in total. It is thought that the aromatic
compound having 7-18 carbon atoms in total can be thus inhibited
from undergoing a side reaction with the positive-electrode
material.
[0068] It is thought that by thus inhibiting the aromatic compound
having 7-18 carbon atoms in total from undergoing a side reaction
with the negative electrode and positive electrode, discharge
characteristics are inhibited from considerably decreasing through
high-temperature storage.
[0069] The proportion of the aromatic compound having 7-18 carbon
atoms in total in the nonaqueous electrolyte is not particularly
limited from the standpoint of producing the effects of the
invention. However, the proportion thereof is generally 0.001% by
weight or higher, preferably 0.1% by weight or higher, especially
preferably 0.3% by weight or higher, most preferably 0.5% by weight
or higher. The upper limit thereof is generally 5% by weight or
lower, preferably 3% by weight or lower, especially preferably 2%
by weight or lower. When the concentration of the aromatic compound
is lower than that lower limit, there are cases where the effect of
improving safety in overcharge is difficult to produce. Conversely,
too high concentrations thereof may result in cases where battery
characteristics including high-temperature storability
decrease.
[0070] Still another aspect of the invention is a nonaqueous
electrolyte which includes at least one electrolyte and a
nonaqueous solvent for dissolving the electrolyte, and is
characterized in that the nonaqueous solvent contains diethyl
carbonate and further contains at least one fluorinated cyclic
carbonate having two or more fluorine atoms.
[0071] The proportion of the diethyl carbonate in the whole
nonaqueous solvent is not particularly limited from the standpoint
of producing the effects or the invention. However, the proportion
thereof is generally 10% by volume or higher, preferably 20% by
volume or higher, more preferably 25% by volume or higher, even
more preferably 30% by volume or higher. The upper limit thereof is
generally 90% by volume or lower, preferably 80% by volume or
lower, more preferably 75% by volume or lower, even more preferably
70% by volume or lower. The incorporation of diethyl carbonate in
an amount within that range is preferred because this inhibits
battery swelling during high-temperature storage.
[0072] The reasons why this nonaqueous electrolyte according to the
invention improves high-temperature storability and cycle
characteristics while inhibiting battery swelling during
high-temperature storage have not been elucidated. However, these
effects are presumed to be attained by the following mechanism,
although the invention should not be construed as being limited by
the following mechanism.
[0073] Diethyl carbonate has a higher boiling point than dimethyl
carbonate and ethyl methyl carbonate, and does not generate methane
gas even when decomposed. Diethyl carbonate is hence a preferred
solvent from the standpoint of inhibiting battery swelling during
high-temperature storage. However, diethyl carbonate tends more to
react with lithium than dimethyl carbonate and ethyl methyl
carbonate. Especially in batteries in which lithium deposition is
apt to occur due to an increased density, the reaction with the
deposited lithium has been a case of a decrease in battery
characteristics. It is thought that when the electrolytic solution
containing a fluorinated cyclic carbonate having two or more
fluorine atoms is used, then a coating film of a reduction reaction
product derived from the fluorinated cyclic carbonate having two or
more fluorine atoms is efficiently formed on the surface of the
negative electrode from initial charge and part of the reduction
product of that compound moves to the surface of the positive
electrode to form a coating film on the surface of the positive
electrode. These coating films inhibit the negative electrode and
positive electrode from undergoing side reactions with other
components of the electrolytic solution. It is thought that the
active materials are therefore used evenly and lithium deposition
can be inhibited to thereby produce those effects. Furthermore, it
is thought that even when lithium deposition has occurred, the
fluorinated cyclic carbonate having two or more fluorine atoms
forms a coating film on the surface of the lithium to inhibit the
lithium from reacting with the diethyl carbonate and thereby
produce those effects.
[0074] A further aspect of the invention is a nonaqueous
electrolyte which includes at least one electrolyte and a
nonaqueous solvent for dissolving the electrolyte, and is
characterized by containing at least one compound selected from the
group consisting of cyclic sulfonic acid ester compounds,
di-sulfonic acid ester compounds, nitrile compounds, and compounds
represented by the following general formula (1), and by further
containing at least one fluorinated cyclic carbonate having two or
more fluorine atoms:
##STR00003##
(wherein R.sup.1 to R.sup.3 each independently represent an alkyl
group having 1-12 carbon atoms and optionally substituted with one
or more fluorine atoms; and n represents an integer of 0-6).
[Cyclic Sulfonic Acid Ester Compounds]
[0075] The cyclic sulfonic acid ester compounds are not
particularly limited in their kinds so long as they are compounds
having a cyclic structure part of which is a sulfonic acid ester
structure. Examples of the cyclic sulfonic acid ester compounds
include 1,3-propanesultone, 1,4-butanesultone, 1,3-propenesultone,
1,4-butenesultone, 1-methyl-1,3-propanesultone,
3-methyl-1,3-propanesultone, 1-fluoro-1,3-propanesultone, and
3-fluoro-1,3-propanesultone.
[0076] Of these, 1,3-propanesultone, 1,4-butanesultone,
1,3-propenesultone, and 1,4-butenesultone are preferred from the
standpoint of improving storability. More preferred of these are
1,3-propanesultone and 1,3-propenesultone.
[Di-Sulfonic Acid Ester Compounds]
[0077] The di-sulfonic acid ester compounds are not particularly
limited in their kinds so long as they are compounds having two
sulfonic acid ester structures in the molecule. Examples of the
di-sulfonic acid ester compounds include
ethanediol disulfonates such as ethanediol dimethanesulfonate,
ethanediol diethanesulfonate, ethanediol dipropanesulfonate,
ethanediol dibutanesulfonate, ethanediol
bis(trifluoromethanesulfonate), ethanediol
bis(pentafluoroethanesulfonate), ethanediol
bis(heptafluoropropanesulfonate), ethanediol
bis(perfluorobutanesulfonate), ethanediol
di(fluoromethanesulfonate), ethanediol
bis(difluoromethanesulfonate), ethanediol
di(2-fluoroethanesulfonate), ethanediol
bis(1,1-difluoroethanesulfonate), ethanediol
bis(1,2-difluoroethanesulfonate), ethanediol
bis(2,2-difluoroethanesulfonate), ethanediol
bis(1,1,2-trifluoroethanesulfonate), ethanediol
bis(1,2,2-trifluoroethanesulfonate), ethanediol
bis(2,2,2-trifluoroethanesulfonate), ethanediol
bis(1,1,2,2-tetrafluoroethanesulfonate), and ethanediol
bis(1,2,2,2-tetrafluoroethanesulfonate);
[0078] 1,2-propanediol disulfonates such as 1,2-propanediol
dimethanesulfonate, 1,2-propanediol diethanesulfonate,
1,2-propanediol dipropanesulfonate, 1,2-propanediol
dibutanesulfonate, 1,2-propanediol bis(trifluoromethanesulfonate),
1,2-propanediol bis(pentafluoroethanesulfonate), 1,2-propanediol
bis(heptafluoropropanesulfonate), 1,2-propanediol
bis(perfluorobutanesulfonate), 1,2-propanediol
di(fluoromethanesulfonate), 1,2-propanediol
bis(difluoromethanesulfonate), 1,2-propanediol
di(2-fluoroethanesulfonate), 1,2-propanediol
bis(1,1-difluoroethanesulfonate), 1,2-propanediol
bis(1,2-difluoroethanesulfonate), 1,2-propanediol
bis(2,2-difluoroethanesulfonate), 1,2-propanediol
bis(1,1,2-trifluoroethanesulfonate), 1,2-propanediol
bis(1,2,2-trifluoroethanesulfonate), 1,2-propanediol
bis(2,2,2-trifluoroethanesulfonate), 1,2-propanediol
bis(1,1,2,2-tetrafluoroethanesulfonate), and 1,2-propanediol
bis(1,2,2,2-tetrafluoroethanesulfonate);
[0079] 1,3-propanediol disulfonates such as 1,3-propanediol
dimethanesulfonate, 1,3-propanediol diethanesulfonate,
1,3-propanediol dipropanesulfonate, 1,3-propanediol
dibutanesulfonate, 1,3-propanediol bis(trifluoromethanesulfonate),
1,3-propanediol bis(pentafluoroethanesulfonate), 1,3-propanediol
bis(heptafluoropropanesulfonate), 1,3-propanediol
bis(perfluorobutanesulfonate), 1,3-propanediol
di(fluoromethanesulfonate), 1,3-propanediol
bis(difluoromethanesulfonate), 1,3-propanediol
di(2-fluoroethanesulfonate), 1,3-propanediol
bis(1,1-difluoroethanesulfonate), 1,3-propanediol
bis(1,2-difluoroethanesulfonate), 1,3-propanediol
bis(2,2-difluoroethanesulfonate), 1,3-propanediol
bis(1,1,2-trifluoroethanesulfonate), 1,3-propanediol
bis(1,2,2-trifluoroethanesulfonate), 1,3-propanediol
bis(2,2,2-trifluoroethanesulfonate), 1,3-propanediol
bis(1,1,2,2-tetrafluoroethanesulfonate), and 1,3-propanediol
bis(1,2,2,2-tetrafluoroethanesulfonate); 1,2-butanediol
disulfonates such as 1,2-butanediol dimethanesulfonate,
1,2-butanediol diethanesulfonate, 1,2-butanediol
bis(trifluoromethanesulfonate), 1,2-butanediol
bis(pentafluoroethanesulfonate), 1,2-butanediol
bis(heptafluoropropanesulfonate), 1,2-butanediol
bis(perfluorobutanesulfonate), 1,2-butanediol
di(fluoromethanesulfonate), 1,2-butanediol
bis(difluoromethanesulfonate), 1,2-butanediol
di(2-fluoroethanesulfonate), 1,2-butanediol
bis(2,2-difluoroethanesulfonate), and 1,2-butanediol
bis(2,2,2-trifluoroethanesulfonate);
[0080] 1,3-butanediol disulfonates such as 1,3-butanediol
dimethanesulfonate, 1,3-butanediol diethanesulfonate,
1,3-butanediol bis(trifluoromethanesulfonate), 1,3-butanediol
bis(pentafluoroethanesulfonate), 1,3-butanediol
bis(heptafluoropropanesulfonate), 1,3-butanediol
bis(perfluorobutanesulfonate), 1,3-butanediol
di(fluoromethanesulfonate), 1,3-butanediol
bis(difluoromethanesulfonate), 1,3-butanediol
di(2-fluoroethanesulfonate), 1,3-butanediol
bis(2,2-difluoroethanesulfonate), and 1,3-butanediol
bis(2,2,2-trifluoroethanesulfonate); and
[0081] 1,4-butanediol disulfonates such as 1,4-butanediol
dimethanesulfonate, 1,4-butanediol diethanesulfonate,
1,4-butanediol dipropanesulfonate, 1,4-butanediol
dibutanesulfonate, 1,4-butanediol bis(trifluoromethanesulfonate),
1,4-butanediol bis(pentafluoroethanesulfonate), 1,4-butanediol
bis(heptafluoropropanesulfonate), 1,4-butanediol
bis(perfluorobutanesulfonate), 1,4-butanediol
di(fluoromethanesulfonate), 1,4-butanediol
bis(difluoromethanesulfonate), 1,4-butanediol
di(2-fluoroethanesulfonate), 1,4-butanediol
bis(1,1-difluoroethanesulfonate), 1,4-butanediol
bis(1,2-difluoroethanesulfonate), 1,4-butanediol
bis(2,2-difluoroethanesulfonate), 1,4-butanediol
bis(1,1,2-trifluoroethanesulfonate), 1,4-butanediol
bis(1,2,2-trifluoroethanesulfonate), 1,4-butanediol
bis(2,2,2-trifluoroethanesulfonate), 1,4-butanediol
bis(1,1,2,2-tetrafluoroethanesulfonate), and 1,4-butanediol
bis(1,2,2,2-tetrafluoroethanesulfonate).
[0082] Preferred of these from the standpoint of improving
storability are:
ethanediol disulfonates such as ethanediol dimethanesulfonate,
ethanediol diethanesulfonate, ethanediol
bis(trifluoromethanesulfonate), ethanediol
bis(pentafluoroethanesulfonate), ethanediol
di(fluoromethanesulfonate), ethanediol
bis(difluoromethanesulfonate), ethanediol
di(2-fluoroethanesulfonate), ethanediol
bis(2,2-difluoroethanesulfonate), and ethanediol
bis(2,2,2-trifluoroethanesulfonate); 1,2-propanediol disulfonates
such as 1,2-propanediol dimethanesulfonate, 1,2-propanediol
diethanesulfonate, 1,2-propanediol bis(trifluoromethanesulfonate),
1,2-propanediol bis(pentafluoroethanesulfonate), 1,2-propanediol
di(fluoromethanesulfonate), 1,2-propanediol
bis(difluoromethanesulfonate), 1,2-propanediol
di(2-fluoroethanesulfonate), 1,2-propanediol
bis(2,2-difluoroethanesulfonate), and 1,2-propanediol
bis(2,2,2-trifluoroethanesulfonate);
[0083] 1,3-propanediol disulfonates such as 1,3-propanediol
dimethanesulfonate, 1,3-propanediol diethanesulfonate,
1,3-propanediol bis(trifluoromethanesulfonate), 1,3-propanediol
bis(pentafluoroethanesulfonate), 1,3-propanediol
di(fluoromethanesulfonate), 1,3-propanediol
bis(difluoromethanesulfonate), 1,3-propanediol
di(2-fluoroethanesulfonate), 1,3-propanediol
bis(2,2-difluoroethanesulfonate), and 1,3-propanediol
bis(2,2,2-trifluoroethanesulfonate);
[0084] 1,2-butanediol disulfonates such as 1,2-butanediol
dimethanesulfonate, 1,2-butanediol diethanesulfonate,
1,2-butanediol bis(trifluoromethanesulfonate), 1,2-butanediol
bis(pentafluoroethanesulfonate), 1,2-butanediol
di(fluoromethanesulfonate), 1,2-butanediol
bis(difluoromethanesulfonate), 1,2-butanediol
di(2-fluoroethanesulfonate), 1,2-butanediol
bis(2,2-difluoroethanesulfonate), and 1,2-butanediol
bis(2,2,2-trifluoroethanesulfonate);
[0085] 1,3-butanediol disulfonates such as 1,3-butanediol
dimethanesulfonate, 1,3-butanediol diethanesulfonate,
1,3-butanediol bis(trifluoromethanesulfonate), 1,3-butanediol
bis(pentafluoroethanesulfonate), 1,3-butanediol
di(fluoromethanesulfonate), 1,3-butanediol
bis(difluoromethanesulfonate), 1,3-butanediol
di(2-fluoroethanesulfonate), 1,3-butanediol
bis(2,2-difluoroethanesulfonate), and 1,3-butanediol
bis(2,2,2-trifluoroethanesulfonate); and
[0086] 1,4-butanediol disulfonates such as 1,4-butanediol
dimethanesulfonate, 1,4-butanediol diethanesulfonate,
1,4-butanediol bis(trifluoromethanesulfonate), 1,4-butanediol
bis(pentafluoroethanesulfonate), 1,4-butanediol
di(fluoromethanesulfonate), 1,4-butanediol
bis(difluoromethanesulfonate), 1,4-butanediol
di(2-fluoroethanesulfonate), 1,4-butanediol
bis(2,2-difluoroethanesulfonate), and 1,4-butanediol
bis(2,2,2-trifluoroethanesulfonate).
[0087] Especially preferred of these are
ethanediol disulfonates such as ethanediol
bis(trifluoromethanesulfonate), ethanediol
bis(pentafluoroethanesulfonate), ethanediol
di(fluoromethanesulfonate), ethanediol di(2-fluoroethanesulfonate),
and ethanediol bis(2,2,2-trifluoroethanesulfonate); 1,2-propanediol
disulfonates such as 1,2-propanediol
bis(trifluoromethanesulfonate), 1,2-propanediol
bis(pentafluoroethanesulfonate), 1,2-propanediol
di(fluoromethanesulfonate), 1,2-propanediol
di(2-fluoroethanesulfonate), and 1,2-propanediol
bis(2,2,2-trifluoroethanesulfonate);
[0088] 1,3-propanediol disulfonates such as 1,3-propanediol
bis(trifluoromethanesulfonate), 1,3-propanediol
bis(pentafluoroethanesulfonate), 1,3-propanediol
di(fluoromethanesulfonate), 1,3-propanediol
di(2-fluoroethanesulfonate), and 1,3-propanediol
bis(2,2,2-trifluoroethanesulfonate); 1,2-butanediol disulfonates
such as 1,2-butanediol bis(trifluoromethanesulfonate),
1,2-butanediol bis(pentafluoroethanesulfonate), 1,2-butanediol
di(fluoromethanesulfonate), 1,2-butanediol
di(2-fluoroethanesulfonate), and 1,2-butanediol
bis(2,2,2-trifluoroethanesulfonate);
[0089] 1,3-butanediol disulfonates such as 1,3-butanediol
bis(trifluoromethanesulfonate), 1,3-butanediol
bis(pentafluoroethanesulfonate), 1,3-butanediol
di(fluoromethanesulfonate), 1,3-butanediol
di(2-fluoroethanesulfonate), and 1,3-butanediol
bis(2,2,2-trifluoroethanesulfonate); and 1,4-butanediol
disulfonates such as 1,4-butanediol bis(trifluoromethanesulfonate),
1,4-butanediol bis(pentafluoroethanesulfonate), 1,4-butanediol
di(fluoromethanesulfonate), 1,4-butanediol
di(2-fluoroethanesulfonate), and 1,4-butanediol
bis(2,2,2-trifluoroethanesulfonate).
[Nitrile Compounds]
[0090] The nitrile compounds are not particularly limited in their
kinds so long as they are compounds having a nitrile group in the
molecule. They may be compounds having two or more nitrile groups
in the molecule.
[0091] Examples of the nitrile compounds include
mononitrile compounds such as acetonitrile, propinonitrile,
butyronitrile, isobutyronitrile, valeronitrile, isovaleronitrile,
2-methylbutyronitrile, trimethylacetonitrile, hexanenitrile,
cyclopentanecarbonitrile, cyclohexanecarbonitrile, acrylonitrile,
methacrylonitrile, crotononitrile, 3-methylcrotononitrile,
2-methyl-2-butenenitrile, 2-pentenenitrile,
2-methyl-2-pentenenitrile, 3-methyl-2-penetenenitrile,
2-hexenenitrile, fluoroacetonitrile, difluoroacetonitrile,
trifluoroacetonitrile, 2-fluoropropionitrile,
3-fluoropropionitrile, 2,2-difluoropropionitrile,
2,3-difluoropropionitrile, 3,3-difluoropropionitrile,
2,2,3-trifluoropropionitrile, 3,3,3-trifluoropropionitrile, and
pentafluoropropionitrile; dinitrile compounds such as
malononitrile, succinonitrile, 2-methylsuccinonitrile,
tetramethylsuccinonitrile, glutaronitrile, 2-methylglutaronitrile,
adiponitrile, fumaronitrile, and 2-methyleneglutaronitrile; and
tetranitrile compounds such as tetracyanoethylene.
[0092] Preferred of these from the standpoint of improving
storability are
acetonitrile, propinonitrile, butyronitrile, valeronitrile,
crotononitrile, 3-methylcrotononitrile, malononitrile,
succinonitrile, glutaronitrile, adiponitrile, and
fumaronitrile.
[0093] More preferred are dinitrile compounds such as
malononitrile, succinonitrile, glutaronitrile, adiponitrile, and
fumaronitrile.
[Compounds Represented by General Formula (1)]
[0094] Examples of the alkyl groups having 1-12 carbon atoms in
general formula (1) include linear, branched, or cyclic alkyl
groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl,
isobutyl, sec-butyl, tert-butyl, pentyl, cyclopentyl, and
cyclohexyl. The lower limit of the number of the carbon atoms of
each of R.sup.1 to R.sup.3 is generally 1 or larger, and is
preferably 2 or larger from the standpoint of inhibiting gas
evolution. The upper limit thereof is generally 12 or smaller, and
is preferably 8 or smaller, more preferably 4 or smaller, from the
standpoints of solubility in the electrolytic solution and battery
characteristics.
[0095] The alkyl groups may have been substituted with one or more
fluorine atoms. Examples of the fluorine-atom-substituted groups
include partly fluorinated alkyl and perfluoroalkyl groups derived
from those alkyl groups, such as trifluoromethyl, trifluoroethyl,
and pentafluoroethyl.
[0096] Symbol n in the general formula represents an integer of
0-6.
[0097] Examples of the compounds represented by general formula (1)
include
compounds in which n=0, such as trimethyl phosphonoformate, methyl
diethyl phosphonoformate, methyl dipropyl phosphonoformate, methyl
diputyl phosphonoformate, triethyl phosphonoformate, ethyl dimethyl
phosphonoformate, ethyl dipropyl phosphonoformate, ethyl diputyl
phosphonoformate, tripropyl phosphonoformate, propyl dimethyl
phosphonoformate, propyl diethyl phosphonoformate, propyl diputyl
phosphonoformate, tributyl phosphonoformate, butyl dimethyl
phosphonoformate, butyl diethyl phosphonoformate, butyl dipropyl
phosphonoformate, methyl bis(2,2,2-trifluoroethyl)
phosphonoformate, ethyl bis(2,2,2-trifluoroethyl) phosphonoformate,
propyl bis(2,2,2-trifluoroethyl) phosphonoformate, and butyl
bis(2,2,2-trifluoroethyl) phosphonoformate;
[0098] compounds in which n=1, such as trimethyl phosphonoacetate,
methyl diethyl phosphonoacetate, methyl dipropyl phosphonoacetate,
methyl diputyl phosphonoacetate, triethyl phosphonoacetate, ethyl
dimethyl phosphonoacetate, ethyl dipropyl phosphonoacetate, ethyl
diputyl phosphonoacetate, tripropyl phosphonoacetate, propyl
dimethyl phosphonoacetate, propyl diethyl phosphonoacetate, propyl
diputyl phosphonoacetate, tributyl phosphonoacetate, butyl dimethyl
phosphonoacetate, butyl diethyl phosphonoacetate, butyl dipropyl
phosphonoacetate, methyl bis(2,2,2-trifluoroethyl)
phosphonoacetate, ethyl bis(2,2,2-trifluoroethyl) phosphonoacetate,
propyl bis(2,2,2-trifluoroethyl) phosphonoacetate, and butyl
bis(2,2,2-trifluoroethyl) phosphonoacetate;
[0099] compounds in which n=2, such as trimethyl
3-phosphonopropionate, methyl diethyl 3-phosphonopropionate, methyl
dipropyl 3-phosphonopropionate, methyl diputyl
3-phosphonopropionate, triethyl 3-phosphonopropionate, ethyl
dimethyl 3-phosphonopropionate, ethyl dipropyl
3-phosphonopropionate, ethyl diputyl 3-phosphonopropionate,
tripropyl 3-phosphonopropionate, propyl dimethyl
3-phosphonopropionate, propyl diethyl 3-phosphonopropionate, propyl
diputyl 3-phosphonopropionate, tributyl 3-phosphonopropionate,
butyl dimethyl 3-phosphonopropionate, butyl diethyl
3-phosphonopropionate, butyl dipropyl 3-phosphonopropionate, methyl
bis(2,2,2-trifluoroethyl) 3-phosphonopropionate, ethyl
bis(2,2,2-trifluoroethyl) 3-phosphonopropionate, propyl
bis(2,2,2-trifluoroethyl) 3-phosphonopropionate, and butyl
bis(2,2,2-trifluoroethyl) 3-phosphonopropionate; and
[0100] compounds in which n=3, such as trimethyl
4-phosphonobutyrate, methyl diethyl 4-phosphonobutyrate, methyl
dipropyl 4-phosphonobutyrate, methyl diputyl 4-phosphonobutyrate,
triethyl 4-phosphonobutyrate, ethyl dimethyl 4-phosphonobutyrate,
ethyl dipropyl 4-phosphonobutyrate, ethyl diputyl
4-phosphonobutyrate, tripropyl 4-phosphonobutyrate, propyl dimethyl
4-phosphonobutyrate, propyl diethyl 4-phosphonobutyrate, propyl
diputyl 4-phosphonobutyrate, tributyl 4-phosphonobutyrate, butyl
dimethyl 4-phosphonobutyrate, butyl diethyl 4-phosphonobutyrate,
and butyl dipropyl 4-phosphonobutyrate.
[0101] Of these, the compounds in which n is 0, 1, or 2 are
preferred from the standpoint of improving battery characteristics
after high-temperature storage. Especially preferred are the
compounds in which n is 1 or 2.
[0102] The at least one compound selected from the group consisting
of those cyclic sulfonic acid ester compounds, di-sulfonic acid
ester compounds, nitrile compounds, and compounds represented by
general formula (1) may be one compound alone or may be any desired
two or more compounds used in combination in any desired
proportion.
[0103] The content of these compounds in the nonaqueous electrolyte
is not particularly limited from the standpoint of producing the
effects of the invention. However, the total content thereof is
generally 0.001% by weight or higher, preferably 0.01% by weight or
higher, more preferably 0.1% by weight or higher, based on the
whole nonaqueous electrolyte. The upper limit of the total content
thereof is generally 5% by weight or lower, preferably 4% by weight
or lower, more preferably 3% by weight or lower. When the
concentration of these compounds is too low, there are cases where
the improving effect is difficult to obtain. On the other hand, too
high concentrations thereof may lead to a decrease in
charge/discharge efficiency.
[0104] The reasons why this nonaqueous electrolyte according to the
invention improves high-temperature storability have not been
elucidated. However, this effect is presumed to be attained by the
following mechanism, although the invention should not be construed
as being limited by the following mechanism.
[0105] Cyclic sulfonic acid ester compounds, di-sulfonic acid ester
compounds, nitrile compounds, and compounds represented by general
formula (1) are adsorbed onto or form a protective coating film on
the surface of the positive electrode and can thus inhibit the
positive electrode from deteriorating during high-temperature
storage. However, these compounds tend to suffer reductional
decomposition on the negative-electrode side. Namely, use of these
compounds tends to enhance side reactions on the negative-electrode
side and increase the resistance of the negative electrode,
resulting in reduced battery characteristics. It is thought that
when the electrolytic solution contains a fluorinated cyclic
carbonate having two or more fluorine atoms, the fluorinated cyclic
carbonate having two or more fluorine atoms can form a coating film
on the surface of the negative electrode before the reaction of
those compounds to thereby inhibit those compounds from undergoing
excessive reactions.
[0106] Still a further aspect of the invention is a nonaqueous
electrolyte which includes at least one electrolyte and a
nonaqueous solvent for dissolving the electrolyte, and is
characterized by being an electrolytic solution for use in a
high-voltage battery to be used at a final charge voltage of 4.3 V
or higher, and by containing at least one fluorinated cyclic
carbonate having two or more fluorine atoms.
(High-Voltage Battery to be Used at Final Charge Voltage of 4.3 V
or Higher)
[0107] This nonaqueous electrolyte according to the invention is
characterized by being for use in a high-voltage battery to be used
at a final charge voltage of 4.3 V or higher.
[0108] The lower limit of voltage for the high-voltage battery to
be used at a final charge voltage of 4.3 V or higher is generally
4.3 V or higher, preferably 4.35 V or higher. Although the upper
limit thereof is not particularly limited, it is 6 V or lower,
preferably 5 V or lower, especially preferably 4.8 V or lower.
Voltages higher than that lower limit are preferred because the
effect of improving energy density and cycle characteristics are
satisfactory.
[0109] A high-voltage battery can be constituted by suitably
selecting the kinds of active materials and a balance between
positive and negative electrodes.
[0110] Details of the constitution thereof will be described
later.
[0111] The battery according to the invention has undergone a
voltage of 4.3 V or higher at least once, because the final charge
voltage thereof is 4.3 V or higher. There has been a problem that
batteries which have undergone a voltage of 4.3 V or higher at
least once deteriorate considerably in battery characteristics
probably due to side reactions between the positive electrode and
the electrolytic solution.
[0112] In contrast, in the battery according to the invention, the
positive electrode and negative electrode and the electrolytic
solution are less apt to decompose. This battery can hence be
repeatedly charged/discharged while retaining high battery
characteristics.
[0113] The nonaqueous electrolytes according to the invention may
be used in combination thereof.
For example, use may be made of a nonaqueous electrolyte which
includes an electrolyte and a nonaqueous solvent for dissolving the
electrolyte and in which the nonaqueous electrolyte contains a
cyclic carbonate having an unsaturated bond and/or an aromatic
compound having 7-18 carbon atoms in total and/or the nonaqueous
solvent contains diethyl carbonate and/or the nonaqueous
electrolyte contains at least one compound selected from the group
consisting of cyclic sulfonic acid ester compounds, di-sulfonic
acid ester compounds, nitrile compounds, and compounds represented
by general formula (1), and which further contains a fluorinated
cyclic carbonate having two or more fluorine atoms. Furthermore,
such nonaqueous electrolytes may be ones for use in a high-voltage
battery to be used at a final charge voltage of 4.3 V or
higher.
(Other Compounds)
[0114] The nonaqueous electrolytes according to the invention each
may contain various other compounds as auxiliaries unless this
lessens the effects of the invention.
[0115] Examples of such optional auxiliaries include
[0116] carbonate compounds such as fluoroethylene carbonate,
erythritan carbonate, spiro-bis-dimethylene carbonate, and
methoxyethyl methyl carbonate;
[0117] carboxylic acid anhydrides such as succinic anhydride,
glutaric anhydride, maleic anhydride, itaconic anhydride,
citraconic anhydride, glutaconic anhydride, diglycolic anhydride,
cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic
dianhydride, and phenylsuccinic anhydride;
[0118] spiro compounds such as 2,4,8,10-tetraoxaspiro[5.5]undecane
and 1,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane;
[0119] sulfur-containing compounds such as ethylene sulfite,
propylene sulfite, methyl methanesulfonate, ethyl methanesulfonate,
methyl methoxymethanesulfonate, methyl 2-methoxyethanesulfonate,
sulfolane, sulfolene, dimethyl sulfone, diphenyl sulfone,
N,N-dimethylmethanesulfonamide, and
N,N-diethylmethanesulfonamide;
[0120] nitrogen-containing compounds such as
1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone,
3-methyl-2-oxazolidinone, 3-dimethyl-2-imidazolidinone, and
N-methylsuccinimide;
[0121] hydrocarbon compounds such as heptane, octane, nonane,
decane, cycloheptane, methylcyclohexane, ethylcyclohexane,
propylcyclohexane, n-butylcyclohexane, t-butylcyclohexane, and
dicyclohexyl; and
[0122] fluorinated benzenes such as fluorobenzene, difluorobenzene,
and hexafluorobenzene.
[0123] These compounds may be used in combination of two or more
thereof.
[0124] The proportion of these auxiliaries in the nonaqueous
electrolyte is not particularly limited from the standpoint of
producing the effects of the invention. However, the proportion
thereof is generally 0.01% by weight or higher, preferably 0.1% by
weight or higher, especially preferably 0.2% by weight or higher.
The upper limit thereof is generally 5% by weight or lower,
preferably 3% by weight or lower, especially preferably 1% by
weight or lower. By adding those auxiliaries, capacity retentivity
and cycle characteristics after high-temperature storage can be
improved. When the concentration thereof is lower than that lower
limit, there are cases where the auxiliaries produce almost no
effect. Conversely, too high concentrations thereof may result in
cases where battery characteristics such as high-load discharge
characteristics decrease.
(Preparation of Electrolytic Solutions)
[0125] The nonaqueous electrolytes according to the invention each
can be prepared by dissolving an electrolyte and at least one
compound selected from the group consisting of fluorinated cyclic
carbonates having two or more fluorine atoms optionally together
with other compound(s) in a nonaqueous solvent. In preparing the
nonaqueous electrolyte, it is preferred that each raw material
should be dehydrated beforehand in order to reduce the water
content of the electrolytic solution to be obtained. It is
desirable that each raw material should be dehydrated to generally
50 ppm or lower, preferably 30 ppm or lower, especially preferably
10 ppm or lower. It is also possible to conduct dehydration,
deacidification treatment, etc. after the electrolytic solution has
been prepared.
[0126] The nonaqueous electrolytes of the invention are suitable
for use as electrolytes for secondary batteries among
nonaqueous-electrolyte batteries, i.e., nonaqueous-electrolyte
secondary batteries such as, e.g., lithium secondary batteries. The
nonaqueous-electrolyte secondary battery employing any of the
electrolytes of the invention is explained below.
<Nonaqueous-Electrolyte Secondary Battery>
[0127] The nonaqueous-electrolyte secondary battery of the
invention is a nonaqueous-electrolyte battery including a negative
electrode and a positive electrode which are capable of
occluding/releasing lithium ions and further including a nonaqueous
electrolyte, and is characterized in that the nonaqueous
electrolyte is any of the nonaqueous electrolytes described
above.
(Battery Constitution)
[0128] The nonaqueous-electrolyte secondary battery according to
the invention is a nonaqueous-electrolyte battery including a
negative electrode and a positive electrode which are capable of
occluding/releasing lithium ions and further including a nonaqueous
electrolyte like known nonaqueous-electrolyte secondary batteries,
provided that it is produced using any of the electrolytes of the
invention described above. Usually, it is obtained by packing a
positive electrode and a negative electrode into a case while
keeping the electrodes separate from each other with a porous film
impregnated with the nonaqueous electrolyte according to the
invention. Consequently, the shape of the secondary battery
according to the invention is not particularly limited, and may be
any of cylindrical, prismatic, laminate, coin, and large-size types
and the like.
(Negative Electrode)
[0129] As a negative-electrode active material, use can be made of
carbonaceous materials or metal compounds which are capable of
occluding/releasing lithium and of lithium metal, lithium alloys,
and the like. These negative-electrode active materials may be used
alone or as a mixture of two or more thereof. Preferred of these
are carbonaceous materials and metal compounds capable of occluding
and releasing lithium.
[0130] Especially preferred of the carbonaceous materials are
graphite and a material obtained by coating the surface of graphite
with carbon which is more amorphous than graphite.
[0131] The graphite preferably is one in which the lattice plane
(002) has a value of d (interplanar spacing), as determined through
X-ray diffraction by the method of the Japan Society for Promotion
of Scientific Research, of 0.335-0.338 nm, especially 0.335-0.337
nm. The crystallite size (Lc) thereof as determined through X-ray
diffraction by the method of the Japan Society for Promotion of
Scientific Research is generally 30 nm or larger, preferably 50 nm
or larger, especially preferably 100 nm or larger. The ash content
thereof is generally 1% by weight or lower, preferably 0.5% by
weight or lower, especially preferably 0.1% by weight or lower.
[0132] The material obtained by coating the surface of graphite
with amorphous carbon preferably is one which is constituted of
graphite in which the lattice plane (002) has a value of d as
determined through X-ray diffraction of 0.335-0.338 nm as a core
material and a carbonaceous material adherent to the surface of the
core material and higher than the core material in the value of d
of the lattice plane (002) as determined through X-ray diffraction,
and in which the proportion of the core material to the
carbonaceous material higher than the core material in the value of
d of the lattice plane (002) as determined through X-ray
diffraction is from 99/1 to 80/20 by weight. When this material is
used, a negative electrode having a high capacity and less apt to
react with the electrolytic solution can be produced.
[0133] The particle diameter of each of the carbonaceous materials,
in terms of median diameter 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,
most preferably 7 .mu.m or larger, and is generally 100 .mu.m or
smaller, preferably 50 .mu.m or smaller, more preferably 40 .mu.m
or smaller, most preferably 30 .mu.m or smaller.
[0134] The specific surface area of each of the carbonaceous
materials as determined by the BET method is generally 0.3
m.sup.2/g or larger, preferably 0.5 m.sup.2/g or larger, more
preferably 0.7 m.sup.2/g or larger, most preferably 0.8 m.sup.2/g
or larger, and is generally 25.0 m.sup.2/g or smaller, preferably
20.0 m.sup.2/g or smaller, more preferably 15.0 m.sup.2/g or
smaller, most preferably 10.0 m.sup.2/g or smaller.
[0135] The carbonaceous materials preferably are ones which, when
analyzed by Raman spectroscopy using argon ion laser light, have a
value of R expressed by I.sub.B to I.sub.A ratio (i.e.,
I.sub.B/I.sub.A) in the range of 0.01-0.7, provided that I.sub.A is
the peak intensity of a peak P.sub.A appearing in the range of
1,570-1,620 cm.sup.-1 and I.sub.B is the peak intensity of a peak
P.sub.B appearing in the range of 1,300-1,400 cm.sup.-1. Also
preferred are ones in which the peak appearing in the range of
1,570-1,620 cm.sup.-1 has a half-value width of generally 26
cm.sup.-1 or smaller, especially 25 cm.sup.-1 or smaller.
[0136] Examples of the metal compounds capable of occluding and
releasing lithium include compounds containing a metal such as Ag,
Zn, Al, Ga, In, Si, Ge, Sn, Pb, P, Sb, Bi, Cu, Ni, Sr, or Ba. These
metals may be used as elemental metals, oxides, alloys with
lithium, etc. In the invention, ones containing an element selected
from Si, Sn, Ge, and Al are preferred. More preferred are oxides or
lithium alloys of a metal selected from Si, Sn, and Al. These
materials may be in the form of a powder or thin film or may be
crystalline or amorphous.
[0137] Metal compounds capable of occluding/releasing lithium or
oxides or lithium alloys of the metals generally have a larger
capacity per unit weight than carbonaceous materials represented by
graphite. These metallic materials are hence suitable for use in
lithium secondary batteries, which are required to have a higher
energy density.
[0138] The metal compounds capable of occluding/releasing lithium
or oxides or lithium alloys of the metals are not particularly
limited in their average particle diameters from the standpoint of
producing the effects of the invention. However, the average
particle diameter of each of these metallic materials is generally
50 .mu.m or smaller, preferably 20 .mu.m or smaller, especially
preferably 10 .mu.m or smaller, and is generally 0.1 .mu.m or
larger, preferably 1 .mu.m or larger, especially preferably 2 .mu.m
or larger. When the average particle diameter exceeds that upper
limit, there is a possibility that the electrode might undergo
enhanced expansion, resulting in reduced cycle characteristics.
When the average particle diameter thereof is smaller than that
lower limit, there is a possibility that current collection might
be difficult, resulting in an insufficient capacity.
(Positive Electrode)
[0139] Examples of positive-electrode active materials include
materials capable of occluding/releasing lithium, such as
lithium-transition metal composite oxide materials, e.g.,
lithium-cobalt oxides, lithium-nickel oxides, and lithium-manganese
oxides. These compounds may be Li.sub.xCoO.sub.2,
Li.sub.xNiO.sub.2, Li.sub.xMnO.sub.2,
Li.sub.xCo.sub.1-yM.sub.yO.sub.2, Li.sub.xNi.sub.1-yM.sub.yO.sub.2,
Li.sub.xMn.sub.1-yM.sub.yO.sub.2, etc., wherein M generally is at
least one member selected from Fe, Co, Ni, Mn, Mg, Cu, Zn, Al, Sn,
B, Ga, Cr, V, Sr, and Ti, 0.4.ltoreq.x.ltoreq.1.2, and
0.ltoreq.y.ltoreq.0.6. Examples of those compounds further include
Li.sub.xMn.sub.aNi.sub.bCo.sub.cO.sub.2 (wherein
0.4.ltoreq.x.ltoreq.1.2 and a+b+c=1)
[0140] In particular, composite oxides in which cobalt, nickel, or
manganese has been partly replaced by one or more other metals and
which are represented by Li.sub.xCo.sub.1-yM.sub.yO.sub.2,
Li.sub.xNi.sub.1-yM.sub.yO.sub.2, Li.sub.xMn.sub.1-yM.sub.yO.sub.z,
or the like and ones represented by
Li.sub.xMn.sub.aNi.sub.bCo.sub.cO.sub.2 (wherein
0.4.ltoreq.x.ltoreq.1.2, a+b+c=1, and |a-b|<0.1) are preferred
because these composite oxides can have a stabilized structure.
[0141] Such positive-electrode active materials may be used alone
or in combination of two or more thereof.
[0142] It is also possible to use a material constituted of any of
those positive-electrode active materials and a substance adherent
to the surface of the positive-electrode active material as a base
and differing in composition from the material constituting the
active material. Examples of the 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.
[0143] The amount of the surface-adherent substance is not
particularly limited from the standpoint of producing the effects
of the invention. However, 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% by mass or smaller, more preferably 10% by mass or
smaller, even more preferably 5% by mass or smaller, based on the
positive-electrode active material. The surface-adherent substance
can inhibit the nonaqueous electrolyte from undergoing an oxidation
reaction on the surface of the positive-electrode active material,
whereby an improvement in battery life can be attained. However, in
case where the amount of the substance adhered is too small, the
effect of the adherent substance is not sufficiently produced. When
the amount thereof is too larger, there are cases where the
adherent substance inhibits the occlusion/release of lithium ions,
resulting in increased resistance.
(Electrodes)
[0144] As a binder for binding an active material, use can be made
of any desired material stable to the solvent and electrolytic
solution to be used for electrode production. Examples thereof
include fluororesins such as poly(vinylidene fluoride) and
polytetrafluoroethylene, polyolefins such as polyethylene and
polypropylene, polymers having unsaturated bonds and copolymers
thereof, such as styrene/butadiene rubbers, isoprene rubbers, and
butadiene rubbers, and acrylic acid polymers and copolymers
thereof, such as ethylene/acrylic acid copolymers and
ethylene/methacrylic acid copolymers.
[0145] A thickener, conductive material, filler, and the like may
be incorporated into each electrode in order to enhance mechanical
strength and electrical conductivity.
[0146] Examples of the thickener include carboxymethyl cellulose,
methyl cellulose, hydroxymethyl cellulose, ethyl cellulose,
poly(vinyl alcohol), oxidized starch, phosphorylated starch, and
casein.
[0147] Examples of the conductive material include metallic
materials such as copper and nickel and carbon materials such as
graphite and carbon black.
[0148] The electrodes may be produced in an ordinary manner. For
example, each electrode can be produced by adding a binder,
thickener, conductive material, solvent, etc to a
positive-electrode or negative-electrode active material to obtain
a slurry, applying this slurry to a current collector, drying the
slurry applied, and then pressing the coated current collector.
[0149] Furthermore, a mixture obtained by adding a binder,
conductive material, etc. to an active material may be subjected as
it is to roll forming to obtain a sheet electrode or to compression
molding to obtain a pellet electrode. Alternatively, a thin film of
an electrode material may be formed on a current collector by a
technique such as, e.g., vapor deposition, sputtering, or
plating.
[0150] In the case where graphite is used as a negative-electrode
active material, the negative-electrode active-material layer after
drying and pressing has a density of generally 1.45 g/cm.sup.3 or
higher, preferably 1.55 g/cm.sup.3 or higher, more preferably 1.60
g/cm.sup.3 or higher, especially preferably 1.65 g/cm.sup.3 or
higher.
[0151] The positive-electrode active-material layer after drying
and pressing has a density of generally 2.0 g/cm.sup.3 or higher,
preferably 2.5 g/cm.sup.3 or higher, more preferably 3.0 g/cm.sup.3
or higher.
[0152] As current collectors, various materials can be used.
However, metals or alloys are generally used. Examples of the
current collector for the negative electrode include copper,
nickel, and stainless steel. Preferred is copper. Examples of the
current collector for the positive electrode include metals such as
aluminum, titanium, and tantalum and alloys thereof. Preferred is
aluminum or an alloy thereof.
(Separator and Case)
[0153] A porous film (separator) is disposed between the positive
electrode and negative electrode in order to prevent
short-circuiting. In this case, the electrolytic solution is
incorporated by infiltrating it into the porous film. The material
and shape of the porous film are not particularly limited so long
as the porous film is stable to the electrolytic solution and has
excellent liquid retentivity. It is preferred to use a porous
sheet, nonwoven fabric, or the like produced from a polyolefin such
as polyethylene or polypropylene.
[0154] The battery case to be used in the battery according to the
invention also is made of any desired material. Use may be made of
nickel-plated iron, stainless steel, aluminum or an alloy thereof,
nickel, titanium, a laminated film, or the like.
[0155] The operating voltage of the nonaqueous-electrolyte
secondary battery of the invention described above is generally in
the range of from 2 V to 6 V.
EXAMPLES
[0156] 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.
[0157] Methods used for evaluating the batteries obtained in the
following Examples and Comparative Examples are shown below.
[Capacity Evaluation]
[0158] At 25.degree. C., a nonaqueous-electrolyte secondary battery
kept in the state of being sandwiched between glass plates for the
purpose of enhancing contact between the electrodes was charged to
4.2 V at a constant current corresponding to 0.2 C and then
discharged to 3 V at a constant current of 0.2 C. This operation as
one cycle was conducted three times to stabilize the battery. In a
fourth cycle, the battery was charged to 4.2 V at a constant
current of 0.5 C, subsequently charged at a constant voltage of 4.2
V until the current value reached 0.05 C, and then discharged to 3
V at a constant current of 0.2 C to determine an initial discharge
capacity.
[0159] In this connection, 1 C means the value of current at which
the reference capacity of a battery is discharged over 1 hour; 0.2
C means 1/5 the current value.
[Evaluation of Cycle Characteristics]
[0160] A battery which had undergone the capacity evaluation test
was subjected at 45.degree. C. to a cycle test in which the battery
was charged to 4.2 V at a constant current of 0.5 C, subsequently
charged at a constant voltage of 4.2 V until the current value
reached 0.05 C, and then discharged to 3 V at a constant current of
1 C. The discharge capacity determined in the first cycle was taken
as 100, and the discharge capacity (%) after 300 cycles was
determined.
[Evaluation of High-Voltage Cycle Characteristics]
[0161] A battery which had undergone the capacity evaluation test
was subjected at 45.degree. C. to a cycle test in which the battery
was charged to 4.35 V at a constant current of 0.5 C, subsequently
charged at a constant voltage of 4.35 V until the current value
reached 0.05 C, and then discharged to 3 V at a constant current of
1 C. The discharge capacity determined in the first cycle was taken
as 100, and the discharge capacity (%) after 50 cycles was
determined.
[Evaluation of Discharge Storage Characteristics]
[0162] A battery which had undergone the capacity evaluation test
was stored at 60.degree. C. and examined for voltage change. The
time period required for the battery to decrease in voltage from 3
V to 2.5 V was measured as discharge storage period. The longer the
discharge storage period, the more the battery is inhibited from
deteriorating during storage (deteriorating due to side reactions
within the battery, mainly side reactions on the negative-electrode
side) and the more the battery is stable.
[Evaluation of Continuous-Charge Characteristics]
[0163] A battery which had undergone the capacity evaluation test
was immersed in an ethanol bath to measure the volume of the
battery. Thereafter, at 60.degree. C., this battery was charged at
a constant current of 0.5 C. At the time when the voltage reached
4.25 V, the charging was changed to constant-voltage charging and
the battery was continuously charged for 1 week.
[0164] The battery was cooled and then immersed in an ethanol bath
to measure the volume of the battery. The amount of a gas evolved
was determined from a volume change through the continuous
charge.
[0165] After the determination of the evolved-gas amount, the
battery was discharged at 25.degree. C. to 3 V at a constant
current of 0.2 C to determine the residual capacity remaining after
the continuous charge test. The proportion of the discharge
capacity after the continuous charge test to the initial discharge
capacity was determined, and this proportion was taken as residual
capacity after continuous charge (%).
Example 1
Production of Negative Electrode
[0166] Ninety-four parts by weight of a natural-graphite powder in
which the lattice plane (002) had a value of d as determined by
X-ray diffraction of 0.336 nm, which had a crystallite size (Lc) of
652 nm, an ash content of 0.07% by weight, a median diameter as
determined by the laser diffraction/scattering method of 12 .mu.m,
a specific surface area as determined by the BET method of 7.5
m.sup.2/g, and a value of R (=I.sub.B/I.sub.A) as determined by
Raman spectroscopy using argon ion laser light of 0.12, and in
which the peak appearing in the range of 1,570-1,620 cm.sup.-1 had
a half-value width of 19.9 cm.sup.-1 was mixed with a 6 parts by
weight of poly(vinylidene fluoride). N-Methyl-2-pyrrolidone was
added to the mixture to slurry it. This slurry was evenly applied
to one side of a copper foil having a thickness of 12 .mu.m and
dried. The resultant coated film was pressed so as to result in a
negative-electrode active-material layer having a density of 1.65
g/cm.sup.3. Thus, a negative electrode was obtained.
[Production of Positive Electrode]
[0167] Ninety parts by weight of LiCoO.sub.2 was mixed with 4 parts
by weight of carbon black and 6 parts by weight of poly(vinylidene
fluoride) (trade name "KF-1000", manufactured by Kureha Chemical
Industry Co., Ltd.). N-Methyl-2-pyrrolidone was added to the
mixture to slurry it. This slurry was evenly applied to each side
of an aluminum foil having a thickness of 15 .mu.m and dried. The
resultant coated foil was pressed so as to result in a
positive-electrode active-material layer having a density of 3.0
g/cm.sup.3. Thus, a positive electrode was obtained.
[Production of Electrolytic Solution]
[0168] In a dry argon atmosphere, 97 parts by weight of an ethylene
carbonate/ethyl methyl carbonate mixture (volume ratio, 3:7) was
mixed with 2 parts by weight of vinylene carbonate and 1 part by
weight of cis-4,5-difluoro-1,3-dioxolan-2-one. Subsequently,
sufficiently dried LiPF.sub.6 was dissolved in the resultant
mixture in such an amount as to result in a proportion of 1.0
mol/L. Thus, an electrolytic solution was obtained.
[Production of Lithium Secondary Battery]
[0169] The positive electrode and negative electrode described
above and a separator made of polyethylene were superposed in the
order of negative electrode/separation/positive
electrode/separator/negative electrode to produce a battery
element. This battery element was inserted into a bag made of a
laminated film obtained by coating each side of aluminum
(thickness, 40 .mu.m) with a resin layer, with the terminals of the
positive and negative electrodes protruding from the bag.
Thereafter, the electrolytic solution was injected into the bag,
which was sealed under vacuum to produce a sheet-form battery. This
battery was evaluated for cycle characteristics and discharge
storage characteristics. The results of the evaluation are shown in
Table 1.
Example 2
[0170] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 97.5 parts by weight of an
ethylene carbonate/ethyl methyl carbonate mixture (volume ratio,
3:7) with 2 parts by weight of vinylene carbonate and 0.5 parts by
weight of cis-4,5-difluoro-1,3-dioxolan-2-one and then dissolving
sufficiently dried LiPF.sub.6 therein in such an amount as to
result in a proportion of 1.0 mol/L. This battery was evaluated for
cycle characteristics and discharge storage characteristics. The
results of the evaluation are shown in Table 1.
Example 3
[0171] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 97.5 parts by weight of an
ethylene carbonate/ethyl methyl carbonate mixture (volume ratio,
3:7) with 1.5 parts by weight of vinylene carbonate, 0.5 parts by
weight of vinylethylene carbonate, and 0.5 parts by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one and then dissolving
sufficiently dried LiPF.sub.6 therein in such an amount as to
result in a proportion of 1.0 mol/L. This battery was evaluated for
cycle characteristics. The results of the evaluation are shown in
Table 1.
Example 4
[0172] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that
trans-4,5-difluoro-1,3-dioxolan-2-one was used in place of the
cis-4,5-difluoro-1,3-dioxolan-2-one in preparing the electrolytic
solution of Example 1. This battery was evaluated for cycle
characteristics. The results of the evaluation are shown in Table
1.
Example 5
[0173] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 97 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate
mixture (volume ratio, 2:4:4) with 2 parts by weight of vinylene
carbonate, and 1 part by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one and then dissolving
sufficiently dried LiPF.sub.6 therein in such an amount as to
result in a proportion of 1.0 mol/L. This battery was evaluated for
cycle characteristics. The results of the evaluation are shown in
Table 1.
Comparative Example 1
[0174] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 98 parts by weight of an
ethylene carbonate/ethyl methyl carbonate mixture (volume ratio,
3:7) with 2 parts by weight of vinylene carbonate and then
dissolving sufficiently dried LiPF.sub.6 therein in such an amount
as to result in a proportion of 1.0 mol/L. This battery was
evaluated for cycle characteristics and discharge storage
characteristics. The results of the evaluation are shown in Table
1.
Comparative Example 2
[0175] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 98 parts by weight of an
ethylene carbonate/ethyl methyl carbonate mixture (volume ratio,
3:7) with 2 parts by weight of cis-4,5-difluoro-1,3-dioxolan-2-one
and then dissolving sufficiently dried LiPF.sub.6 therein in such
an amount as to result in a proportion of 1.0 mol/L. This battery
was evaluated for cycle characteristics and discharge storage
characteristics. The results of the evaluation are shown in Table
1.
Comparative Example 3
[0176] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by dissolving sufficiently dried
LiPF.sub.6 in an ethylene carbonate/ethyl methyl carbonate mixture
(volume ratio, 3:7) in such an amount as to result in a proportion
of 1.0 mol/L. This battery was evaluated for cycle characteristics
and discharge storage characteristics. The results of the
evaluation are shown in Table 1.
Comparative Example 4
[0177] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 97 parts by weight of an
ethylene carbonate/ethyl methyl carbonate mixture (volume ratio,
3:7) with 2 parts by weight of vinylene carbonate and 1 part by
weight of 4-fluoro-1,3-dioxolan-2-one and then dissolving
sufficiently dried LiPF.sub.6 therein in such an amount as to
result in a proportion of 1.0 mol/L. This battery was evaluated for
cycle characteristics. The results of the evaluation are shown in
Table 1.
TABLE-US-00001 TABLE 1 Cycle Characteristics and Discharge Storage
Characteristics Evaluation of discharge Cycle characteristics
storage characteristics (%) (hr) Example 1 86 294 Example 2 85 345
Example 3 87 -- Example 4 84 -- Example 5 88 -- Comparative Example
1 54 379 Comparative Example 2 77 90 Comparative Example 3 52 155
Comparative Example 4 69 --
[0178] As apparent from Table 1, it can be seen that the batteries
according to the invention are excellent in cycle characteristics
and storability.
Example 6
[0179] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 96.5 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate
mixture (volume ratio, 2:4:4) with 2 parts by weight of vinylene
carbonate, 0.5 parts by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one, and 1 part by weight of
cyclohexylbenzene and then dissolving sufficiently dried LiPF.sub.6
therein in such an amount as to result in a proportion of 1.0
mol/L. This battery was evaluated for continuous-charge
characteristics. The results of the evaluation are shown in Table
2.
Example 7
[0180] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that 2,4-dilfluoroaniole was
used in place of the cyclohexylbenzene in preparing the
electrolytic solution of Example 6. This battery was evaluated for
continuous-charge characteristics. The results of the evaluation
are shown in Table 2.
Comparative Example 5
[0181] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 97 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate
mixture (volume ratio, 2:4:4) with 2 parts by weight of vinylene
carbonate and 1 part by weight of cyclohexylbenzene and then
dissolving sufficiently dried LiFF.sub.6 therein in such an amount
as to result in a proportion of 1.0 mol/L. This battery was
evaluated for continuous-charge characteristics. The results of the
evaluation are shown in Table 1.
Comparative Example 6
[0182] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that 2,4-dilfluoroaniole was
used in place of the cyclohexylbenzene in preparing the
electrolytic solution of Comparative Example 5. This battery was
evaluated for continuous-charge characteristics. The results of the
evaluation are shown in Table 2.
TABLE-US-00002 TABLE 2 Continuous-Charge Characteristics Amount of
gas Residual generated through capacity after continuous charge
continuous charge (mL) (%) Example 6 0.87 89 Example 7 0.74 91
Comparative Example 5 1.76 74 Comparative Example 6 1.34 82
[0183] As apparent from Table 2, the batteries according to the
invention can be inhibited from increasing in gas evolution and
decreasing considerably in discharge characteristics through
high-temperature storage (through the continuous charge test),
although the electrolytes each contain an aromatic compound having
7-18 carbon atoms in total.
Example 8
[0184] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 99.5 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/diethyl carbonate mixture
(volume ratio, 3:1:6) with 0.5 parts by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one and then dissolving
sufficiently dried LiPF.sub.6 therein in such an amount as to
result in a proportion of 1.0 mol/L. This battery was evaluated for
high-voltage cycle characteristics and continuous-charge
characteristics. The results of the evaluation are shown in Table
3.
Comparative Example 7
[0185] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by dissolving sufficiently dried
LiPF.sub.6 in ethylene carbonate and ethyl methyl carbonate (volume
ratio, 3:7) in such an amount as to result in a proportion of 1.0
mol/L. This battery was evaluated for high-voltage cycle
characteristics and continuous-charge characteristics. The results
of the evaluation are shown in Table 3.
Comparative Example 8
[0186] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by dissolving sufficiently dried
LiPF.sub.6 in an ethylene carbonate/ethyl methyl carbonate/diethyl
carbonate mixture (volume ratio, 3:1:6) in such an amount as to
result in a proportion of 1.0 mol/L. This battery was evaluated for
high-voltage cycle characteristics and continuous-charge
characteristics. The results of the evaluation are shown in Table
3.
Example 9
[0187] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 99 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/diethyl carbonate mixture
(volume ratio, 3:1:6) with 0.5 parts by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one and 0.5 parts by weight of
1,3-propanesultone and then dissolving sufficiently dried
LiPF.sub.6 therein in such an amount as to result in a proportion
of 1.0 mol/L. This battery was evaluated for high-voltage cycle
characteristics and continuous-charge characteristics. The results
of the evaluation are shown in Table 3.
Example 10
[0188] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 99 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/diethyl carbonate mixture
(volume ratio, 3:1:6) with 0.5 parts by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one and 0.5 parts by weight of
1,4-butanediol bis(2,2,2-trifluoroethanesulfonate) and then
dissolving sufficiently dried LiPF.sub.6 therein in such an amount
as to result in a proportion of 1.0 mol/L. This battery was
evaluated for high-voltage cycle characteristics and
continuous-charge characteristics. The results of the evaluation
are shown in Table 3.
Example 11
[0189] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 99 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/diethyl carbonate mixture
(volume ratio, 3:1:6) with 0.5 parts by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one and 0.5 parts by weight of
succinonitrile and then dissolving sufficiently dried LiPF.sub.6
therein in such an amount as to result in a proportion of 1.0
mol/L. This battery was evaluated for high-voltage cycle
characteristics and continuous-charge characteristics. The results
of the evaluation are shown in Table 3.
Example 12
[0190] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 99 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/diethyl carbonate mixture
(volume ratio, 3:1:6) with 0.5 parts by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one and 0.5 parts by weight of
triethyl phosphonoacetate and then dissolving sufficiently dried
LiPF.sub.6 therein in such an amount as to result in a proportion
of 1.0 mol/L. This battery was evaluated for high-voltage cycle
characteristics and continuous-charge characteristics. The results
of the evaluation are shown in Table 3.
Example 13
[0191] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 98.5 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/diethyl carbonate mixture
(volume ratio, 3:1:6) with 0.5 parts by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one, 0.5 parts by weight of
vinylene carbonate, and 0.5 parts by weight of triethyl
phosphonoacetate and then dissolving sufficiently dried LiPF.sub.6
therein in such an amount as to result in a proportion of 1.0
mol/L. This battery was evaluated for high-voltage cycle
characteristics and continuous-charge characteristics. The results
of the evaluation are shown in Table 3.
Example 14
[0192] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 99 parts by weight of an
ethylene carbonate/ethyl methyl carbonate mixture (volume ratio,
3:7) with 0.5 parts by weight of
cis-4,5-difluoro-1,3-dioxolan-2-one and 0.5 parts by weight of
triethyl phosphonoacetate and then dissolving sufficiently dried
LiPF.sub.6 therein in such an amount as to result in a proportion
of 1.0 mol/L. This battery was evaluated for high-voltage cycle
characteristics and continuous-charge characteristics. The results
of the evaluation are shown in Table 3.
Comparative Example 9
[0193] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that use was made of an
electrolytic solution prepared by mixing 99.5 parts by weight of an
ethylene carbonate/ethyl methyl carbonate/diethyl carbonate mixture
(volume ratio, 3:1:6) with 0.5 parts by weight of succinonitrile
and then dissolving sufficiently dried LiPF.sub.6 therein in such
an amount as to result in a proportion of 1.0 mol/L. This battery
was evaluated for high-voltage cycle characteristics and
continuous-charge characteristics. The results of the evaluation
are shown in Table 3.
TABLE-US-00003 TABLE 3 High-Voltage Cycle Characteristics and
Continuous-Charge Characteristics Amount of gas Residual
High-voltage cycle generated through capacity after characteristics
continuous charge continuous charge (%) (mL) (%) Example 8 88 0.38
92 Comparative 70 0.39 67 Example 7 Comparative 64 0.29 86 Example
8 Example 9 88 0.33 94 Example 10 89 0.33 94 Example 11 88 0.30 93
Example 12 90 0.28 96 Example 13 90 0.31 98 Example 14 91 0.34 97
Comparative 61 0.28 79 Example 9
[0194] As apparent from Table 3, the batteries according to the
invention have excellent cycle characteristics, are inhibited from
evolving a gas through high-temperature storage (through the
continuous charge test), and can have improved discharge
characteristics.
[0195] 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.
[0196] This application is based on a Japanese patent application
filed on Jun. 2, 2006 (Application No. 2006-155251), the contents
thereof being herein incorporated by reference.
INDUSTRIAL APPLICABILITY
[0197] Nonaqueous electrolytes capable of providing a battery
having a high capacity and excellent in storability and cycle
characteristics can be provided, and nonaqueous-electrolyte
batteries produced with these electrolytes can be provided.
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