U.S. patent application number 12/585108 was filed with the patent office on 2009-12-31 for nonaqueous electrolyte secondary battery.
Invention is credited to Hiroyuki Fujimoto, Shin Fujitani, Masanobu Takeuchi, Seiji Yoshimura.
Application Number | 20090325077 12/585108 |
Document ID | / |
Family ID | 38428619 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090325077 |
Kind Code |
A1 |
Takeuchi; Masanobu ; et
al. |
December 31, 2009 |
Nonaqueous electrolyte secondary battery
Abstract
In a nonaqueous electrolyte secondary battery having a positive
electrode containing a positive electrode active material, a
negative electrode containing a negative electrode active material,
and a nonaqueous electrolyte, as the positive electrode active
material or as the negative electrode active material, a mixture
containing molybdenum dioxide and lithium titanate in a weight
ratio (molybdenum dioxide:lithium titanate) of 90:10 to 50:50 is
used.
Inventors: |
Takeuchi; Masanobu;
(Kobe-city, JP) ; Fujimoto; Hiroyuki; (Kobe-city,
JP) ; Yoshimura; Seiji; (Osaka, JP) ;
Fujitani; Shin; (Kobe-city, JP) |
Correspondence
Address: |
KUBOVCIK & KUBOVCIK
SUITE 1105, 1215 SOUTH CLARK STREET
ARLINGTON
VA
22202
US
|
Family ID: |
38428619 |
Appl. No.: |
12/585108 |
Filed: |
September 3, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11656008 |
Jan 22, 2007 |
|
|
|
12585108 |
|
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|
Current U.S.
Class: |
429/338 ;
429/231.1; 429/231.3 |
Current CPC
Class: |
H01M 4/5825 20130101;
H01M 10/0569 20130101; H01M 4/485 20130101; H01M 4/131 20130101;
Y02E 60/10 20130101; H01M 10/0525 20130101; H01M 4/625 20130101;
H01M 4/525 20130101 |
Class at
Publication: |
429/338 ;
429/231.1; 429/231.3 |
International
Class: |
H01M 4/48 20060101
H01M004/48; H01M 6/16 20060101 H01M006/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2006 |
JP |
014988/2006 |
Dec 28, 2006 |
JP |
354764/2006 |
Claims
1. A nonaqueous electrolyte secondary battery comprising: a
positive electrode containing a positive electrode active material;
a negative electrode containing a negative electrode active
material; and a nonaqueous electrolyte, wherein as the positive
electrode active material or as the negative electrode active
material, a mixture containing molybdenum dioxide and lithium
titanate in a weight ratio (molybdenum dioxide lithium titanate) of
90:10 to 50:50 is used.
2. The nonaqueous electrolyte secondary battery according to claim
1, wherein in the positive electrode or negative electrode using
the mixture, as a conductive agent, graphitized vapor-grown carbon
fiber having lattice constant C.sub.0 in the range of 6.7
.ANG..ltoreq.C.sub.0.ltoreq.6.8 .ANG., a ratio L.sub.a/L.sub.c of
crystallite dimensions (L.sub.a and L.sub.c) in a base surface
(surface a) and in a lamination direction (surface c) in the range
of 4.ltoreq.L.sub.a/L.sub.c.ltoreq.6 is used.
3. The nonaqueous electrolyte secondary battery according to claim
2, wherein as the conductive agent, massive artificial graphite
having lattice constant C.sub.0 in the range of 6.7
.ANG..ltoreq.C.sub.0.ltoreq.6.8 .ANG. is used in mixture with the
vapor-grown carbon fiber.
4. The nonaqueous electrolyte secondary battery according to claim
1, wherein the mixture of molybdenum dioxide and lithium titanate
is used as the negative electrode active material, and lithium
cobaltate is used as the positive electrode active material.
5. The nonaqueous electrolyte secondary battery according to claim
4, wherein when weight of lithium cobaltate used as the positive
electrode active material is denoted by W.sub.LCO, weight of
molybdenum dioxide used as the negative electrode active material
by W.sub.MoO2, and weight of lithium titanate used as the negative
electrode active material by W.sub.LTO,
100.ltoreq.(175.times.W.sub.LTO+210.times.W.sub.MoO2)/W.sub.LCO.ltoreq.16-
5 is satisfied.
6. The nonaqueous electrolyte secondary battery according to claim
1, wherein as a solvent of the nonaqueous electrolyte, 5-30% by
volume of ethylene carbonate is contained in the solvent.
7. The nonaqueous electrolyte secondary battery according to claim
2, wherein as a solvent of the nonaqueous electrolyte, 5-30% by
volume of ethylene carbonate is contained in the solvent.
8. The nonaqueous electrolyte secondary battery according to claim
3, wherein as a solvent of the nonaqueous electrolyte, 5-30% by
volume of ethylene carbonate is contained in the solvent.
9. The nonaqueous electrolyte secondary battery according to claim
4, wherein as a solvent of the nonaqueous electrolyte, 5-30% by
volume of ethylene carbonate is contained in the solvent.
10. The nonaqueous electrolyte secondary battery according to claim
2, wherein the mixture of molybdenum dioxide and lithium titanate
is used as the negative electrode active material, and lithium
cobaltate is used as the positive electrode active material.
11. The nonaqueous electrolyte secondary battery according to claim
10, wherein as a solvent of the nonaqueous electrolyte, 5-30% by
volume of ethylene carbonate is contained in the solvent.
12. The nonaqueous electrolyte secondary battery according to claim
3, wherein the mixture of molybdenum dioxide and lithium titanate
is used as the negative electrode active material, and lithium
cobaltate is used as the positive electrode active material.
13. The nonaqueous electrolyte secondary battery according to claim
12, wherein as a solvent of the nonaqueous electrolyte, 5-30% by
volume of ethylene carbonate is contained in the solvent.
14. The nonaqueous electrolyte secondary battery according to claim
10, wherein when weight of lithium cobaltate used as the positive
electrode active material is denoted by W.sub.LCO, weight of
molybdenum dioxide used as the negative electrode active material
by W.sub.MoO2, and weight of lithium titanate used as the negative
electrode active material by W.sub.LTO,
100.ltoreq.(175.times.W.sub.LTO+.sup.210.times.W.sub.MoO2)/W.sub.LCO.ltor-
eq..sup.165 is satisfied.
15. The nonaqueous electrolyte secondary battery according to claim
12, wherein when weight of lithium cobaltate used as the positive
electrode active material is denoted by W.sub.LCO, weight of
molybdenum dioxide used as the negative electrode active material
by W.sub.MoO2, and weight of lithium titanate used as the negative
electrode active material by W.sub.LTO,
100.ltoreq.(175.times.W.sub.LTO+210.times.W.sub.MoO2)/W.sub.LCO.ltoreq.16-
5 is satisfied.
Description
[0001] This application is a division of application Ser. No.
11/656,008, filed Jan. 22, 2007, which claims priority based on
Japanese Patent Application Nos. 2006-014988 and 2006-354764, filed
Jan. 24, 2006, and Dec. 28, 2006, respectively, and which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to nonaqueous electrolyte
secondary batteries, and more specifically to a nonaqueous
electrolyte secondary battery which can be used as a power source
for memory backup of a portable device.
[0004] 2. Description of the Related Art
[0005] In recent years, high electromotive force nonaqueous
electrolyte secondary batteries using nonaqueous electrolyte have
been widely used as secondary batteries of high output and high
energy density. Such nonaqueous electrolyte secondary batteries are
used as a power source for memory backup of portable device, as
well as a main power source of portable device, and in recent
years, increase in energy density is demanded not only in a main
power source of portable device but also in a power source for
memory backup.
[0006] As a secondary battery for memory backup, for example, a
battery in which lithium cobaltate (LiCoO.sub.2) is used as a
positive electrode active material and lithium titanate
(Li.sub.4Ti.sub.5O.sub.12) having spinel structure is used as a
negative electrode active material has already been brought into
practical use. However, the density and weight specific capacity of
lithium titanate used as a negative electrode active material are
3.47 g/mL and 175 mAh/g, respectively, so that there is a problem
that energy density per volume is low. In contrast, molybdenum
dioxide reversely reacts with lithium in a similar potential region
to that of lithium titanate and has density and weight specific
capacity of 6.44 g/mL and 210 mAh/g, respectively, and has higher
volume energy density than lithium titanate. Use of molybdenum
dioxide as an alternative to lithium titanate enables increases in
energy density per volume of battery.
[0007] For example, Japanese Patent Laid-open No. 2000-243445
proposes to use lithium-containing manganese oxide as a positive
electrode active material and molybdenum dioxide for negative
electrode.
[0008] A secondary battery for memory backup is mounted as a
battery to be incorporated into a device, and used without a
protective circuit from view points of implementation area and
cost. Therefore, it is assumed that over discharge condition may
occur as the condition that electric current is not supplied from
the main power source lasts for a long time, and hence it is
demanded that capacity decrease is small even if over discharge
cycle is conducted.
[0009] As described above, molybdenum dioxide is superior in energy
density per volume to lithium titanate. However, examination made
by the present inventor revealed that in a nonaqueous electrolyte
secondary battery in which lithium cobaltate is used as a positive
electrode active material and molybdenum dioxide is used as a
negative electrode active material, rapid decrease in capacity
occurs with lapse of over discharge cycle, and sufficient cycle
characteristic is not obtained.
[0010] Further, when molybdenum dioxide is uses as a negative
electrode active material, expansion and contraction at the time of
occluding and releasing of lithium are large, so that it is
impossible to obtain sufficient cycle characteristic.
[0011] It is an object of the present invention to provide a
nonaqueous electrolyte secondary battery which is applicable as a
power source for memory backup, and having large battery capacity
and excellent over discharge cycle characteristic.
SUMMARY OF THE INVENTION
[0012] The present invention provides a nonaqueous electrolyte
secondary battery which comprises a positive electrode containing a
positive electrode active material, a negative electrode containing
a negative electrode active material, and a nonaqueous electrolyte,
wherein as the positive electrode active material or the negative
electrode active material, a mixture containing molybdenum dioxide
and lithium titanate in a weight ratio (molybdenum dioxide lithium
titanate) ranging from 90:10 to 50:50 is used.
[0013] In an over discharge condition of a nonaqueous electrolyte
secondary battery having a negative electrode using molybdenum
dioxide, lithium concentration in molybdenum dioxide is extremely
low, and electrode potential is high. Examination made by the
present inventor demonstrated that in such a condition, molybdenum
dioxide is extremely instable in an electrolyte and molybdenum
elutes into the electrolyte (see reference experiments as described
later). It can be supposed that this eluting molybdenum deposits on
surface of negative electrode, and inhibits occluding and releasing
of lithium, to cause decrease in capacity with over discharge
cycle.
[0014] According to the present invention, by mixing lithium
titanate into molybdenum dioxide, elution of molybdenum is
suppressed even in the condition that lithium concentration in
molybdenum dioxide is low (see reference experiments as described
later). Although details about the reason for the above are not
apparent, about 3.1 V (vs. Li/Li.sup.+) of potential is exhibited
by only molybdenum dioxide, while about 2.9 V (vs. Li/Li.sup.+) of
potential which is lower by about 0.2 V is exhibited when lithium
titanate is mixed. Molybdenum dioxide seems to easily elute at more
electropositive potential. By mixing with lithium titanate,
electrode is less likely to have electropositive potential even in
over discharge condition. This may suppress elution of molybdenum.
It is expected that decrease in capacity due to over discharge
cycle is suppressed because elution of molybdenum into electrolyte
is suppressed in this manner.
[0015] In the case where molybdenum dioxide is used for a positive
electrode, as the capacity is increased by elevating charging
voltage, lithium concentration in molybdenum dioxide which is a
positive electrode active material is very low, and in such a
condition, elution of molybdenum into electrolyte occurs as
described above. This exerts adverse affect on cycle
characteristic. According to the present invention, by mixing
lithium titanate into molybdenum dioxide, even when molybdenum
dioxide is used for a positive electrode, it is possible to
suppress elution of molybdenum and improve the cycle
characteristic.
[0016] When molybdenum dioxide is used for a positive electrode,
for example, lithium metal, lithium containing graphite, Li--Al
alloy, Li--Si alloy or the like is used for a negative electrode,
and a battery having operating voltage ranging from about 2.0 to
1.0 V can be realized.
[0017] In a battery used for memory backup, an operating voltage of
the same band region with that of a driving voltage of
semiconductor for which backup is to be conducted is requested.
Molybdenum dioxide and lithium titanate as an active material
exhibit similar band regions of operating voltage, and when it is
used for a negative electrode in combination with lithium cobaltate
or the like, a battery having an operating voltage of about 3.0 to
2.0 V can be realized, whereas when it is used for a positive
electrode in combination with carbon or aluminum, silicon or the
like, a battery having an operating voltage of about 2.0 to 1.0 V
can be realized. Therefore, by using molybdenum dioxide and lithium
titanate as an active material, various requests for voltage band
regions can be satisfied.
[0018] Current largest market of secondary battery for backup use
is for secondary batteries which are chargeable and dischargeable
in regions from 3.0 to 2.0 V. As a positive electrode active
material exhibiting charge-discharge potential satisfying the above
requirement, lithium cobaltate is most preferably used. In the case
of lithium nickelate, charge-discharge potential decreases and also
discharge voltage of battery decreases, so that sufficient capacity
is not obtained in discharge of up to 2.0 V. In the case of lithium
manganate, a problem may occur in storage characteristic.
[0019] Since lithium titanate and molybdenum dioxide have similar
operating potentials at the time of occluding/releasing of lithium,
and molybdenum dioxide may have higher electrode density compared
to lithium titanate, it is possible to increase the energy density
while keeping the voltage compatibility with a conventional battery
based on lithium titanate, by applying mixture of molybdenum
dioxide and lithium titanate as an active material according to the
present invention.
[0020] As described above, an electrode using only molybdenum
dioxide faces a problem of poor cycle characteristic due to volume
change or the like of molybdenum dioxide caused by charge and
discharge. That is, electrolyte that is no longer retained in
electrode due to expansion of molybdenum dioxide at the time of
charging migrates to redundant space in the battery system.
However, at the time of discharging, contraction of molybdenum
dioxide causes reabsorption of liquid, so that liquid will not
migrate smoothly from the redundant space. For this reason, in the
electrode using molybdenum dioxide exhibiting large volume change
as an active material, liquid retaining volume is reduced and
charge-discharge capacity is reduced as a result of repeated
charges and discharges. Such phenomenon is considered as a
secondary factor of deterioration in cycle characteristic, in
addition to elution of molybdenum in the aforementioned over
discharge cycle.
[0021] Usually, electrolyte in an electrode is retained in carbon
added as a conductive agent or in gap of active material or in gap
between particles. Since these volumes decrease with expansion of
molybdenum dioxide, the liquid retaining ability will decrease. By
adding inorganic porous particles such as alumina or titania in an
electrode, volumes of these inorganic oxide particles will not
change by charging or discharging, so that surface or fine pores of
particles function as liquid retaining space in the electrode, and
decrease in cycle capacity retention rate can be prevented.
However, since most of these inorganic porous particles lack
ability of occluding/releasing lithium in the molybdenum dioxide
charge-discharge band region, energy density decreases as a result
of addition of these inorganic porous particles to the
electrode.
[0022] In the present invention, lithium titanate used in mixture
with molybdenum dioxide exhibits an operating voltage which is
similar to that of molybdenum dioxide, causes little voltage change
due to charging and discharging, and has an ability of
occluding/releasing lithium, so that it may be used as an active
material. Therefore, by using lithium titanate as inorganic porous
particles in combination with molybdenum dioxide according to the
present invention, it is possible to secure retaining ability of
electrolyte while suppressing decrease in energy density, and hence
it is possible to increase the cycle characteristic.
[0023] In the present invention, molybdenum dioxide and lithium
titanate are mixed in a weight ratio (molybdenum dioxide:lithium
titanate) of 90:10 to 50:50. By setting the mixing amount of
lithium titanate at 10 wt. % or more, it is possible to
sufficiently suppress elution of molybdenum at decreased lithium
concentration. Even when the mixing amount of lithium titanate
exceeds 50 wt. %, ability of suppressing elution of molybdenum is
little observed, and decrease in energy density due to increase in
mixing amount of lithium titanate occurs. Therefore, the mixing
amount of lithium titanate is preferably 50 wt. % or less. The
weight ratio between molybdenum dioxide and lithium titanate is
more preferably in the range of 90:10 to 70:30, and still
preferably in the range of 80:20 to 70:30.
[0024] Preferably, molybdenum dioxide is based on stoichiometric
composition of MoO.sub.2. When molybdenum dioxide having higher
oxidation number such as MoO.sub.2.25 enters, initial efficiency
may decrease and cycle characteristic may deteriorate. Also lithium
titanate is preferably based on stoichiometric composition of
Li.sub.4Ti.sub.5O.sub.12.
[0025] According to the present invention, in the electrode using
mixture of molybdenum dioxide and lithium titanate as an active
material, it is preferred to use as a conductive agent, graphitized
vapor-grown carbon fiber having lattice constant C.sub.0 in the
range of 6.7 .ANG..ltoreq.C.sub.0.ltoreq.6.8 .ANG., a ratio
L.sub.a/L.sub.c of crystallite dimensions (L.sub.a and L.sub.c) in
a basal surface (surface a) and in a lamination direction (surface
c) in the range of 4.ltoreq.L.sub.a/L.sub.c.ltoreq.6. By using such
graphitized vapor-grown carbon fiber as a conductive agent, it is
possible to suppress decomposition of electrolyte on the conductive
agent, and to realize a nonaqueous electrolyte secondary battery
having excellent cycle characteristic and storage
characteristic.
[0026] In theory, lower limit value of C.sub.0 of graphite material
is 6.7 .ANG.. The value of C.sub.0 is preferably 6.8 .ANG. or less
because larger interlayer distance may possibly accelerate a
decomposition reaction of the electrolyte. Since it is expected
that most of side reaction such as decomposition of electrolyte in
graphite material occurs in surface c, and side reaction occurring
in surface a is insignificant, it is preferred to make exposure of
surface c small. Therefore, the value of L.sub.a/L.sub.c is
preferably 4 or more. However, when L.sub.a is too large, aspect
ratio of fiber shape is large, and formability of electrode and
handling ability of combined material reduce, so that the value of
L.sub.a/L.sub.c is preferably 6 or less.
[0027] In the present invention, it is preferred to use as a
conductive agent, massive artificial graphite having lattice
constant C.sub.0 in the range of 6.7
.ANG..ltoreq.C.sub.0.ltoreq.6.8 .ANG. combined and mixed with the
above vapor-grown carbon fiber. By combinational use of such
massive artificial graphite as a conductive agent, it is possible
to realize an electrode having high strength and excellent
productivity and high availability of active material. Mixing ratio
of the vapor-grown carbon fiber and massive artificial graphite is
preferably in the range of 50:50 to 100:0 by weight ratio
(vapor-grown carbon fiber:massive artificial graphite). Larger
proportion of massive artificial graphite may possibly deteriorate
the cycle characteristic.
[0028] In the present invention, when mixture of molybdenum dioxide
and lithium titanate is used as a positive electrode active
material, for example, a carbon material such as graphite, metal
which is able to form alloy with lithium such as aluminum, silicon
or the like may be used as a negative electrode active material. By
using these materials as a positive electrode active material, it
is possible to realize a nonaqueous electrolyte secondary battery
having operating voltage of about 2.0 to 1.0 V.
[0029] In the present invention, when mixture of molybdenum dioxide
and lithium titanate is used as a negative electrode active
material, lithium containing transition metal complex oxide such as
lithium cobaltate which is conventionally used as a positive
electrode active material of nonaqueous electrolyte secondary
battery may be used as a positive electrode active material.
[0030] When lithium cobaltate is used as a positive electrode
active material and the above mixture is used as a negative
electrode active material, use depth of lithium cobaltate is
preferably in the range of 4.0 to 4.3 V (vs. Li/Li.sup.+) in order
to secure sufficient cycle characteristic. In the region of less
than 4.0 V (vs. Li/Li.sup.+), sufficient specific capacity is not
obtained, and in the region of more than 4.3 V (vs. Li/Li.sup.+),
structure of active material is instable, and sufficient cycle
characteristic may not be obtained. At charge-discharge depth of
4.0 V (vs. Li/Li.sup.+), specific capacity of lithium cobaltate is
about 100 mAh/g, and at charge-discharge depth of 4.3 V (vs.
Li/Li.sup.+), specific capacity of lithium cobaltate is about 165
mAh/g. Further, specific capacities of lithium titanate and
molybdenum dioxide are about 175 mAh/g and about 210 mAh/g,
respectively. From these facts, denoting weight of lithium
cobaltate which is a positive electrode active material by
W.sub.LCO, weight of molybdenum dioxide used as a negative
electrode active material by W.sub.MoO2, and weight of lithium
titanate used as a negative electrode active material by W.sub.LTO,
they are preferably used in the ranges that satisfy
100.ltoreq.(175.times.W.sub.LTO+210.times.W.sub.MoO2)/W.sub.LCO.ltoreq.16-
5. By satisfying this condition, more preferred cycle
characteristic is obtained.
[0031] In the present invention, as a nonaqueous electrolyte
solvent, a solvent that contains 5-30% by volume of ethylene
carbonate in the solvent is preferred. With ethylene carbonate of
less than 5% by volume, sufficient lithium ion conductivity may not
be obtained in nonaqueous electrolyte. With ethylene carbonate of
more than 30% by volume, a film is excessively formed by decomposed
matters of ethylene carbonate, with respect to negative electrode
active material, so that cycle characteristic may be deteriorated.
As other solvent in nonaqueous electrolyte, cyclic carbonate
solvents such as propylene carbonate, butylene carbonate and the
like, and chain-like carbonate solvents such as diethyl carbonate,
ethylmethyl carbonate, dimethyl carbonate and the like can be used,
and preferably, mixture of cyclic carbonate solvent and chain-like
carbonate solvent is desirably used.
[0032] As a solute of nonaqueous electrolyte in the present
invention, lithium hexafluorophosphate (LiPF.sub.6), lithium
borofluoride (LiBF.sub.4), LiTFSI (LiN(CF.sub.3SO.sub.2).sub.2),
LiBETI (LiN(C.sub.2F.sub.5SO.sub.2).sub.2) and the like can be
used.
[0033] According to the present invention, it is possible to
provide a nonaqueous electrolyte secondary battery having large
battery capacity and excellent over discharge cycle
characteristic.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a schematic section view showing a lithium
secondary battery fabricated in Example according to the present
invention; and
[0035] FIG. 2 is a graph showing relationship between mixing ratio
of active material and capacity decrease rate by cycles.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Example 1
Fabrication of Positive Electrode
[0036] LiCoO.sub.2, acetylene black, artificial graphite, and
polyvinylidene fluoride (PVdF) were mixed in weight ratio of
88.8:5:5:1.2 in N-methylpyrrolidone (NMP) solvent, and ground after
drying to give a positive electrode combined material.
[0037] 25.5 mg of the obtained positive electrode combined material
was weighed, and input into a molding jig having a diameter of 4.16
mm, and molded under pressure of 600 kgf to prepare a disc-shaped
positive electrode.
[Fabrication of Negative Electrode]
[0038] After mixing as an active material, MoO.sub.2 and
Li.sub.4Ti.sub.5O.sub.12 in a weight ratio of 90:10, the active
material, graphitized vapor-grown carbon fiber (C.sub.0=6.80 .ANG.,
L.sub.a=900 .ANG., L.sub.c=200 .ANG.), massive artificial graphite
(C.sub.0=6.72 .ANG., L.sub.a=300 .ANG., L.sub.c=300 .ANG.), and
polyvinylidene fluoride (PVdF) serving as a binder were mixed in a
weight ratio of 90:4:3:3, and ground after drying to give a
negative electrode combined material.
[0039] 15.8 mg of the obtained negative electrode combined material
was weighed, and input into a molding jig having a diameter of 4.16
mm, and molded under pressure of 600 kgf to prepare a disc-shaped
negative electrode.
[Preparation of Electrolyte]
[0040] Lithium hexafluorophosphate (LiPF.sub.6) which is a solute
was dissolved in a mixed solvent of 3:7 (by volume) of ethylene
carbonate and diethyl carbonate, so that a concentration was 1
mol/L, to prepare a nonaqueous electrolyte.
[Assembling of Battery]
[0041] Using the above positive electrode, negative electrode and
nonaqueous electrolyte, a flat lithium secondary battery (battery
dimension: diameter 6 mm, thickness 1.4 mm) was prepared. FIG. 1 is
a schematic section view showing a prepared lithium secondary
battery. As shown in FIG. 1, a positive electrode 1 and a negative
electrode 2 are arranged to oppose each other via a separator 3,
and housed in a battery case made up of a positive electrode can 4
and a negative electrode can 5. The positive electrode 1 is
connected to the positive electrode can 4 and the negative
electrode 2 is connected to the negative electrode can 5,
respectively via a conductive paste 7 made of carbon. Outer
periphery of the negative electrode can 5 is fitted inside the
positive electrode can 4 via a gasket 6 of polypropylene. As the
separator 3, nonwoven fabric made of polypropylene is used, and the
positive electrode 1, negative electrode 2 and separator 3 are
immersed with the above nonaqueous electrolyte.
Example 2
[0042] A lithium secondary battery was prepared in a similar manner
to Example 1 using a positive electrode and nonaqueous electrolyte
similar to those of Example 1 except that MoO.sub.2 and
Li.sub.4Ti.sub.5O.sub.12 serving as a negative electrode active
material were mixed in a weight ratio of 75:25. The amount of
positive electrode combined material was 24.5 mg and the amount of
negative electrode combined material was 15.5 mg.
Example 3
[0043] A lithium secondary battery was prepared in a similar manner
to Example 1 using a positive electrode and nonaqueous electrolyte
similar to those of Example 1 except that MoO.sub.2 and
Li.sub.4Ti.sub.5O.sub.12 serving as a negative electrode active
material were mixed in a weight ratio of 50:50. The amount of
positive electrode combined material was 23.4 mg and the amount of
negative electrode combined material was 15.4 mg.
Comparative Example 1
[0044] A lithium secondary battery was prepared in a similar manner
to Example 1 using a positive electrode and nonaqueous electrolyte
similar to those of Example 1 except that only MoO.sub.2 was used
as a negative electrode active material. The amount of positive
electrode combined material was 26.4 mg and the amount of negative
electrode combined material was 16.1 mg.
Comparative Example 2
[0045] A lithium secondary battery was prepared in a similar manner
to Example 1 using a positive electrode and nonaqueous electrolyte
similar to those of Example 1 except that only
Li.sub.4Ti.sub.5O.sub.12 was used as a negative electrode active
material. The amount of positive electrode combined material was
20.3 mg and the amount of negative electrode combined material was
14.4 mg.
[0046] Constructions of batteries according to these Examples and
Comparative Examples are shown in Table 1.
TABLE-US-00001 TABLE 1 Mixing Ratio of Positive Negative Negative
Electrode Positive Positive Electrode Negative Negative Electrode
Active Material Electrode Electrode Filling Electrode Electrode
Filling (175 .times. MoO.sub.2 Li.sub.4Ti.sub.5O.sub.12 Weight
Thickness Density Weight Thickness Density W.sub.LTO + 210 .times.
wt. % wt. % mg mm g/ml mg mm g/ml W.sub.MoO2)/W.sub.LCO Comp. 100 0
26.4 0.59 3.3 16.1 0.311 3.83 129.8 Ex. 1 Ex. 1 90 10 25.5 0.568
3.3 15.8 0.333 3.49 129.7 Ex. 2 75 25 24.5 0.547 3.3 15.5 0.353
3.24 129.0 Ex. 3 50 50 23.4 0.521 3.3 15.4 0.379 3.00 128.4 Comp. 0
100 20.3 0.516 3.3 14.4 0.384 2.92 125.8 Ex. 2
[Evaluation of Charge-Discharge Characteristic]
[0047] Initial charge-discharge characteristic and charge-discharge
cycle characteristic were evaluated for each of the above Examples
and Comparative Examples. Measurement conditions are as
follows.
[0048] <Measurement Condition of Initial Charge-Discharge
Characteristic>
[0049] Charge: constant current-constant voltage charge 100
.mu.A-3.2 V 5 .mu.A cut
[0050] Discharge: Step variable constant current discharge 100
.mu.A, 50 .mu.A, 30 .mu.A, 10 .mu.A, 5 .mu.A-2.0 V cut
[0051] Pause: 10 seconds
[0052] Initial charge capacity, initial discharge capacity and
initial efficiency of each battery measured in the above
measurement conditions are shown in Table 2.
[0053] <Measurement Condition of Normal Cycle
Characteristic>
[0054] Charge: constant current charge 100 .mu.A 3.2 V cut
[0055] Discharge: constant current discharge 100 .mu.A 2.0 V
cut
[0056] Pause: 10 seconds
[0057] <Measurement Condition of Over Discharge Cycle
Characteristic>
[0058] Charge: constant current charge 100 .mu.A 3.2 V cut
[0059] Discharge: constant current discharge 100 .mu.A 0.01 V
cut
[0060] Pause: 10 seconds
[0061] Respective discharge capacities after 50 cycles measured in
the above conditions are shown in Table 2 as discharge capacity
after normal cycle and discharge capacity after over discharge
cycle.
[0062] 1-10 cyc. capacity decrease rate and 11-50 cyc. capacity
decrease rate measured in accordance with the above measurement
condition of over discharge cycle characteristic are shown in FIG.
2. These capacity decrease rates are calculated according to the
following formula.
[0063] (1-10 cyc. capacity decrease rate)=100-(10 cyc. discharge
capacity)/(1 cyc. discharge capacity).times.100(%)
[0064] (11-50 cyc. capacity decrease rate)=100-(50 cyc. discharge
capacity)/(11 cyc. discharge capacity).times.100(%)
TABLE-US-00002 TABLE 2 Mixing Ratio of Negative Electrode Initial
Initial Discharge Capacity Discharge Capacity Active Material
Charge Discharge Initial After After Over MoO.sub.2
Li.sub.4Ti.sub.5O.sub.12 Capacity Capacity Efficiency Normal Cycle
Discharge Cycle wt. % wt. % mAh mAh % mAh mAh Comp. 100 0 3.19 2.81
87.9 1.71 0.28 Ex. 1 Ex. 1 90 10 3.12 2.74 88.0 1.75 0.96 Ex. 2 75
25 2.99 2.64 88.4 1.72 1.63 Ex. 3 50 50 2.71 2.42 89.2 1.75 1.86
Comp. 0 100 2.38 2.19 92.0 1.66 1.38 Ex. 2
[0065] As is apparent from the result shown in Table 2, in Examples
1 to 3 using as a negative electrode active material, a mixture
containing 10 to 50% by weight of lithium titanate, relative to the
total amount of molybdenum dioxide and lithium titanate according
to the present invention, high initial charge capacity and initial
discharge capacity, excellent initial efficiency, and high
discharge capacity after 50 cycles in over discharge cycle are
observed, demonstrating excellent over discharge cycle
characteristic. As to normal cycle characteristic, such a
significant difference as is the case of over discharge cycle was
not observed between Examples 1 to 3 according to the present
invention and Comparative Examples 1 and 2. This reveals that the
effect of the present invention is particularly outstanding in over
discharge cycle characteristic.
[0066] As is apparent from FIG. 2, decrease in capacity is notably
suppressed in the cycles following cycle 11.
[0067] <Reference Experiments>
[0068] (Reference Experiment A)
[0069] A slurry prepared by mixing molybdenum dioxide, graphitized
vapor-grown carbon fiber and PVdF in a weight ratio of 90:5:5 in
NMP solvent was applied and dried on aluminum foil and then
compressed to form an applied polar plate. This was then cut into a
rectangular shape of 2.5.times.5.0 cm. The amount of molybdenum
dioxide in the polar plate was 147.3 mg. This polar plate was
immersed with a nonaqueous electrolyte (1M (mol/litter) LiPF.sub.6
EC/DEC=3/7), and then stored for 5 days at 60.degree. C. while the
nonaqueous atmosphere was kept, and Mo element eluted into the
electrolyte was quantified by using ICP. The proportion of quantity
of eluted Mo element, relative to quantity of Mo element contained
in the polar plate before storage was 350.7 ppm.
[0070] (Reference Experiment B)
[0071] After electrically inserting lithium into a polar plate
which is identical to that used in Reference Experiment A until
Li.sub.0.25MoO.sub.2 (1.6 V(vs.Li/Li.sup.+)) was achieved, storage
was conducted in the same manner as in Reference Experiment A, and
Mo element eluted into the electrolyte was quantified by using ICP.
The proportion of quantity of eluted Mo element, relative to
quantity of Mo element contained in the polar plate before storage
was 93.6 ppm.
(Reference Experiment C)
[0072] A slurry prepared by mixing mixture of 75:25 (by weight) of
molybdenum dioxide and lithium titanate, vapor-grown carbon fiber
and PVdF in a weight ratio of 90:5:5 in NMP solvent was applied and
dried on aluminum foil and then compressed to form an applied polar
plate. This was then cut into a rectangular shape of 2.5.times.5.0
cm. The amount of molybdenum dioxide in the polar plate was 110.7
mg. Storage in electrolyte was conducted in the same manner as in
Reference Experiment A, and Mo element eluted into the electrolyte
was quantified by using ICP. The proportion of quantity of eluted
Mo element, relative to quantity of Mo element contained in the
polar plate before storage was 80.2 ppm.
[0073] From comparison between Reference Experiment A and Reference
Experiment B, it was found that molybdenum dioxide is easy to elute
especially when the lithium concentration in polar plate is
low.
[0074] From comparison between Reference Experiment A and Reference
Experiment C, it was found that by mixing lithium titanate into
molybdenum dioxide, elution of molybdenum is suppressed even when
lithium concentration in molybdenum dioxide is low.
* * * * *