U.S. patent application number 13/429580 was filed with the patent office on 2012-09-27 for nonaqueous-electrolyte batteries and nonaqueous electrolytic solutions.
This patent application is currently assigned to MITSUBISHI CHEMICAL CORPORATION. Invention is credited to Hiroyuki TOKUDA.
Application Number | 20120244425 13/429580 |
Document ID | / |
Family ID | 43826131 |
Filed Date | 2012-09-27 |
United States Patent
Application |
20120244425 |
Kind Code |
A1 |
TOKUDA; Hiroyuki |
September 27, 2012 |
NONAQUEOUS-ELECTROLYTE BATTERIES AND NONAQUEOUS ELECTROLYTIC
SOLUTIONS
Abstract
The invention is to provide a nonaqueous-electrolyte battery
which comprises a current collector, a positive electrode
containing a lithium-containing phosphoric acid compound
represented by LixMPO.sub.4 as a positive-electrode active
material, a negative electrode containing a negative-electrode
active material capable of occluding and releasing lithium ions,
and a nonaqueous electrolytic solution containing a chain ether and
a cyclic carbonate having an unsaturated bond.
Inventors: |
TOKUDA; Hiroyuki; (Kanagawa,
JP) |
Assignee: |
MITSUBISHI CHEMICAL
CORPORATION
Tokyo
JP
|
Family ID: |
43826131 |
Appl. No.: |
13/429580 |
Filed: |
March 26, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/JP10/66413 |
Sep 22, 2010 |
|
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13429580 |
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Current U.S.
Class: |
429/199 ;
429/188 |
Current CPC
Class: |
H01M 10/0568 20130101;
H01M 4/587 20130101; H01M 10/0564 20130101; H01M 2004/027 20130101;
H01M 10/0569 20130101; H01M 10/0567 20130101; H01M 4/5825 20130101;
H01M 2300/0037 20130101; H01M 10/0525 20130101; Y02E 60/10
20130101; H01M 2004/028 20130101 |
Class at
Publication: |
429/199 ;
429/188 |
International
Class: |
H01M 10/0564 20100101
H01M010/0564; H01M 10/056 20100101 H01M010/056 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 29, 2009 |
JP |
2009-223809 |
Claims
1. A nonaqueous-electrolyte battery which comprises: a current
collector; a positive electrode containing a lithium-containing
phosphoric acid compound represented by LixMPO.sub.4 (wherein M is
at least one element selected from the group consisting of Group-2
to Group-12 metals of the periodic table, and x satisfies
0<x.ltoreq.1.2) as a positive-electrode active material; a
negative electrode containing a negative-electrode active material
capable of occluding and releasing lithium ions; and a nonaqueous
electrolytic solution, wherein the nonaqueous electrolytic solution
contains (1) a chain ether and (2) a cyclic carbonate having an
unsaturated bond.
2. A nonaqueous-electrolyte battery which comprises: a current
collector; a positive electrode containing a lithium-containing
phosphoric acid compound represented by LixMPO.sub.4 (wherein M is
at least one element selected from the group consisting of Group-2
to Group-12 metals of the periodic table, and x satisfies
0<x.ltoreq.1.2) as a positive-electrode active material; a
negative electrode containing a negative-electrode active material
capable of occluding and releasing lithium ions; and a nonaqueous
electrolytic solution, wherein the nonaqueous electrolytic solution
contains (1) a chain ether and (2) at least one compound selected
from lithium fluorophosphates, lithium sulfonates, imide lithium
salts, sulfonic acid esters, and sulfurous acid esters.
3. The nonaqueous-electrolyte battery according to claim 1 or 2,
wherein the lithium-containing phosphoric acid compound is
represented by LixMPO.sub.4 (wherein M is at least one element
selected from the group consisting of the Group-4 to Group-11
transition metals in the fourth period of the periodic table, and x
satisfies 0<x.ltoreq.1.2).
4. The nonaqueous-electrolyte battery according to claim 1, wherein
the content of the cyclic carbonate having an unsaturated bond is
0.001-5% by mass based on the whole electrolytic solution.
5. The nonaqueous-electrolyte battery according to claim 1 or 2,
wherein the nonaqueous electrolytic solution contains ethylene
carbonate in an amount of 10% by volume or more.
6. The nonaqueous-electrolyte battery according to claim 1 or 2,
wherein the chain ether is represented by R.sup.1OR.sup.2 (wherein
R.sup.1 and R.sup.2 each represent a monovalent organic group which
has 1-8 carbon atoms and may have a fluorine atom, and R.sup.1 and
R.sup.2 may be the same or different).
7. The nonaqueous-electrolyte battery according to claim 1 or 2,
wherein the negative-electrode active material is a carbonaceous
material.
8. The nonaqueous-electrolyte battery according to claim 1 or 2,
wherein the current collector has an electroconductive layer on the
surface thereof, the electroconductive layer being different from
the current collector in compound composition.
9. A nonaqueous electrolytic solution for use in the
nonaqueous-electrolyte battery according to claim 1 or 2.
Description
TECHNICAL FIELD
[0001] The present invention relates to nonaqueous-electrolyte
batteries. More particularly, the invention relates to
nonaqueous-electrolyte batteries which employ nonaqueous
electrolytic solutions that bring about excellent high-output
characteristics and excellent durability when iron lithium
phosphate is used as the positive electrode.
BACKGROUND ART
[0002] Nonaqueous-electrolyte batteries including lithium secondary
batteries are being put to practical use in extensive applications
ranging from power sources for appliances for so-called public use,
such as portable telephones and notebook type personal computers,
to vehicle-mounted power sources for driving motor vehicles or the
like. However, nonaqueous-electrolyte batteries are increasingly
required to have higher performance in recent years, and are
required to attain battery characteristics, such as, for example,
high capacity, high output, high-temperature storability, cycle
characteristics, and high safety, on a high level.
[0003] In nonaqueous-electrolyte batteries, LiCoO.sub.2 is
generally used in the positive electrodes and a carbon material
capable of occluding and releasing lithium is generally used in the
negative electrodes. As the nonaqueous electrolytic solutions, use
is being made of electrolytic solutions prepared by dissolving an
electrolyte salt represented by LiPF.sub.6 in a nonaqueous organic
solvent such as ethylene carbonate or ethyl methyl carbonate.
[0004] Lithium cobalt oxide (LiCoO.sub.2), which is used as a
positive-electrode active material as shown above, has a drawback
that this substance in a charged state has low thermal stability
and reduces battery safety. Extensive substances have hence been
investigated in search of a positive-electrode active material
usable as a substitute for LiCoO.sub.2.
[0005] As one class of substances among these, lithium-containing
metal oxides having an olivine structure have recently received
attention. For example, nonaqueous-electrolyte batteries employing
LiFePO.sub.4 as a positive-electrode active material can be made to
have improved cycle characteristics and improved battery safety by
taking advantage of the high thermal and chemical stability of
LiFePO.sub.4. In the case of applications such as, for example,
hybrid vehicles, such properties are exceedingly useful from the
standpoint of increasing the size of mounted batteries to thereby
improve energy density per unit weight or improve output energy
density or for attaining life prolongation of batteries.
[0006] However, LiFePO.sub.4 is known to be lower in the electronic
conductivity of inner parts of the positive-electrode active
material and in high-rate discharge characteristics as compared
with LiCoO.sub.2, LiNiO.sub.2, LiMnO.sub.2,
Li(Ni.sub.1/3Mn.sub.1/3Co.sub.1/3)O.sub.2, and the like.
[0007] Furthermore, nonaqueous-electrolyte batteries employing
LiFePO.sub.4 as a positive-electrode active material have had the
following problem. When these batteries are repeatedly charged and
discharged in a high-temperature environment of, for example, about
60.degree. C., elements including iron which are contained in the
active material partly dissolve away with repetitions of
charge/discharge, and the dissolved iron adversely affects the
negative-electrode active material constituted of a carbon
material, etc. As a result, the negative electrode itself is
impaired in charge/discharge reversibility and other properties and
is hence reduced in reactivity, and this tends to result in a
decrease in the capacity or output of the nonaqueous-electrolyte
batteries.
[0008] Patent document 1 discloses a nonaqueous-electrolyte battery
which includes a positive-electrode mix layer that includes a
positive-electrode active material including iron lithium
phosphate, a conductive material, and a binder and has a density
regulated to 1.7 g/cc and which further includes a nonaqueous
electrolytic solution containing a solvent including ethylene
carbonate and a chain ether, as a nonaqueous-electrolyte battery
which can have an improved discharge capacity even during high-rate
discharge in which the battery is discharged at a relatively high
current.
[0009] Patent document 2 discloses a nonaqueous-electrolyte battery
which includes a positive-electrode active material including iron
lithium phosphate of an olivine structure as a main component, an
electrolyte including LiPF.sub.6 as a main component, and a
nonaqueous solvent that includes, as a main component, a mixed
solvent composed of ethylene carbonate and diethyl carbonate or a
mixed solvent composed of ethylene carbonate and ethyl methyl
carbonate and that further contains at least vinylene carbonate
and/or vinylethylene carbonate, as a nonaqueous-electrolyte battery
which has a high capacity and high output and can retain the high
capacity equal to the initial value even after repeatedly
charged/discharged in a high-temperature environment of, for
example, 60.degree. C., and which is prevented from decreasing in
ordinary-temperature output and low-temperature output, for
example, output at around -30.degree. C., and shows the high output
equal to the initial value.
PRIOR-ART DOCUMENTS
Patent Documents
[0010] Patent Document 1: JP-A-2006-236809 [0011] Patent Document
2: JP-A-2009-4357
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0012] According to the techniques disclosed in patent document 1,
electronic conductivity within the positive electrode is improved
by improving close contact between the positive-electrode active
material and the conductive material, between the conductive
material and the current collector, and between the
positive-electrode active material and the current collector.
Furthermore, by using a solvent prepared by adding dimethoxyethane,
which has an exceedingly low viscosity, to ethylene carbonate,
which has a high permittivity, not only the solvent can be
sufficiently infiltrated into the positive-electrode mix layer but
also the rate of movement of lithium ions is improved. These
techniques are thought to improve discharge capacity during
high-rate discharge in which the battery is discharged at a
relatively high current.
[0013] However, in the case where a carbon material, which is the
most general material at present, is used in the negative
electrode, the battery according to patent document 1 is still
insufficient in durability such as high-temperature storability and
cycle characteristics because of the poor stability of the
negative-electrode coating film.
[0014] According to the techniques disclosed in patent document 2,
at least part of the vinylene carbonate and/or vinylethylene
carbonate decomposes on the electrodes to form a stable deposit,
e.g., a coating film, on the surface of the active material of the
positive electrode and/or negative electrode. As a result, iron and
other elements are inhibited from dissolving away from the
positive-electrode active material and such dissolved elements are
inhibited from adversely affecting the negative-electrode active
material. It is thought that even when the battery is repeatedly
charged/discharged, the insertion and release of lithium ions
proceed smoothly and capacity deterioration and an increase in
internal resistance can be inhibited to thereby inhibit the output
from decreasing.
[0015] However, the nonaqueous electrolytic solution has a higher
viscosity and lower ionic conductivity as compared with
nonaqueous-electrolyte employing a chain ether. Because of this,
when compared in initial output with the nonaqueous electrolytic
solutions employing a chain ether, this prior-art nonaqueous
electrolytic solution is still insufficient in output in a
low-temperature region, such as output at room temperature or
-30.degree. C.
[0016] An object of the invention is to provide a
nonaqueous-electrolyte battery which has a high initial output at
ordinary temperature and -30.degree. C., attains a high discharge
capacity even during high-rate discharge, and has a high capacity
retention after a durability test such as a high-temperature
storage test or cycle test, and which, even after the durability
test, has the excellent initial output performance and high-rate
discharge capacity. Another object is to provide a nonaqueous
electrolytic solution which renders the nonaqueous-electrolyte
battery possible.
Means for Solving the Problems
[0017] The present inventors diligently made investigations in
order to overcome the problems described above. As a result, the
inventors have found that a nonaqueous-electrolyte battery which
has a high initial output at ordinary temperature and -30.degree.
C., attains a high discharge capacity even during high-rate
discharge, and has a high capacity retention after a durability
test such as a high-temperature storage test or cycle test, and
which, even after the durability test, has the excellent output
performance and high-rate discharge capacity equal to the initial
values can be rendered possible using a lithium-containing metal
oxide having an olivine structure as a positive-electrode active
material, by incorporating a chain ether and either a compound
having the function of forming a negative-electrode coating film or
a compound having the function of protecting the positive
electrode, in a specific proportion, into an electrolytic-solution
composition. The invention has been thus completed.
[0018] Namely, essential points of the invention are as
follows.
[0019] 1. A nonaqueous-electrolyte battery which comprises: a
current collector; a positive electrode containing a
lithium-containing phosphoric acid compound represented by
LixMPO.sub.4 (wherein M is at least one element selected from the
group consisting of Group-2 to Group-12 metals of the periodic
table, and x satisfies 0<x.ltoreq.1.2) as a positive-electrode
active material; a negative electrode containing a
negative-electrode active material capable of occluding and
releasing lithium ions; and a nonaqueous electrolytic solution,
[0020] wherein the nonaqueous electrolytic solution contains
(1) a chain ether and (2) a cyclic carbonate having an unsaturated
bond.
[0021] 2. A nonaqueous-electrolyte battery which comprises: a
current collector; a positive electrode containing a
lithium-containing phosphoric acid compound represented by
LixMPO.sub.4 (wherein M is at least one element selected from the
group consisting of Group-2 to Group-12 metals of the periodic
table, and x satisfies 0<x.ltoreq.1.2) as a positive-electrode
active material; a negative electrode containing a
negative-electrode active material capable of occluding and
releasing lithium ions; and a nonaqueous electrolytic solution,
[0022] wherein the nonaqueous electrolytic solution contains
(1) a chain ether and (2) at least one compound selected from
lithium fluorophosphates, lithium sulfonates, imide lithium salts,
sulfonic acid esters, and sulfurous acid esters.
[0023] 3. The nonaqueous-electrolyte battery according to 1. or 2.
above wherein the lithium-containing phosphoric acid compound is
represented by LixMPO.sub.4 (wherein M is at least one element
selected from the group consisting of the Group-4 to Group-11
transition metals in the fourth period of the periodic table, and x
satisfies 0<x.ltoreq.1.2).
[0024] 4. The nonaqueous-electrolyte battery according to 1. above
wherein the content of the cyclic carbonate having an unsaturated
bond is 0.001-5% by mass based on the whole electrolytic
solution.
[0025] 5. The nonaqueous-electrolyte battery according to 1. or 2.
above wherein the nonaqueous electrolytic solution contains
ethylene carbonate in an amount of 10% by volume or more.
[0026] 6. The nonaqueous-electrolyte battery according to 1. or 2.
above wherein the chain ether is represented by R.sup.1OR.sup.2
(wherein R.sup.1 and R.sup.2 each represent a monovalent organic
group which has 1-8 carbon atoms and may have a fluorine atom, and
R.sup.1 and R.sup.2 may be the same or different).
[0027] 7. The nonaqueous-electrolyte battery according to 1. or 2.
above wherein the negative-electrode active material is a
carbonaceous material.
[0028] 8. The nonaqueous-electrolyte battery according to 1. or 2.
above wherein the current collector has an electroconductive layer
on the surface thereof, the electroconductive layer being different
from the current collector in compound composition.
[0029] 9. A nonaqueous electrolytic solution for use in the
nonaqueous-electrolyte battery according to any one of 1. to 8.
above.
Effects of the Invention
[0030] According to the nonaqueous-electrolyte batteries of the
invention, an improvement in high-rate discharge capacity and an
increase in output are attained in the case where a
lithium-containing metal oxide having an olivine structure is used
as a positive-electrode active material, by incorporating a chain
ether into a nonaqueous electrolytic solution and thereby lowering
the viscosity of the nonaqueous electrolytic solution and improving
the ionic conductivity thereof. Furthermore, by incorporating, in a
specific proportion, a compound having the function of forming a
negative-electrode coating film, the resistance of the coating film
on the surface of the negative electrode is prevented from
increasing excessively, while maintaining thermal and chemical
durability. As a result, not only high high-temperature storability
and cycle characteristics can be imparted, but also an improvement
in high-rate characteristics and an increase in output can be
attained in the battery which has undergone a durability test.
[0031] Moreover, by incorporating, in a specific proportion, a
compound having the function of protecting the positive electrode,
metal dissolution from the positive-electrode active material is
inhibited and the resistance of the coating film on the surface of
the positive electrode is prevented from increasing excessively,
while maintaining thermal and chemical durability. As a result, not
only high high-temperature storability and cycle characteristics
can be imparted, but also an improvement in high-rate
characteristics and an increase in output can be attained in the
battery which has undergone a durability test.
MODES FOR CARRYING OUT THE INVENTION
[0032] Embodiments of the invention will be explained below.
However, the invention should not be construed as being limited to
the following embodiments, and can be modified at will.
[0033] [Nonaqueous Electrolytic Solutions]
[0034] The nonaqueous electrolytic solution for use in the first
aspect of the invention contains
(1) a chain ether and (2) a cyclic carbonate having an unsaturated
bond.
[0035] The nonaqueous electrolytic solution for use in the second
aspect of the invention contains
(1) a chain ether and (2) at least one compound selected from
lithium fluorophosphates, lithium sulfonates, imide lithium salts,
sulfonic acid esters, and sulfurous acid esters.
[0036] <Chain Ether>
[0037] The chain ether preferably is a compound represented by the
general formula R.sup.1OR.sup.2. In the formula, R.sup.1 and
R.sup.2 each represent a monovalent organic group which has 1-8
carbon atoms and may have a fluorine atom, and R.sup.1 and R.sup.2
may be the same or different.
[0038] More preferred are chain ethers having 3-10 carbon
atoms.
[0039] Examples of the chain ethers having 3-10 carbon atoms
include diethyl ether, di(2-fluoroethyl)ether,
di(2,2-difluoroethyl)ether, di(2,2,2-trifluoroethyl)ether, ethyl
2-fluoroethyl ether, ethyl 2,2,2-trifluoroethyl ether, ethyl
1,1,2,2-tetrafluoroethyl ether, 2-fluoroethyl 2,2,2-trifluoroethyl
ether, 2-fluoroethyl 1,1,2,2-tetrafluoroethyl ether,
2,2,2-trifluoroethyl 1,1,2,2-tetrafluoroethyl ether, ethyl n-propyl
ether, ethyl 3-fluoro-n-propyl ether, ethyl
3,3,3-trifluoro-n-propyl ether, ethyl 2,2,3,3-tetrafluoro-n-propyl
ether, ethyl 2,2,3,3,3-pentafluoro-n-propyl ether, 2-fluoroethyl
n-propyl ether, 2-floroethyl 3-fluoro-n-propyl ether, 2-fluoroethyl
3,3,3-trifluoro-n-propyl ether, 2-fluoroethyl
2,2,3,3-tetrafluoro-n-propyl ether, 2-fluoroethyl
2,2,3,3,3-pentafluoro-n-propyl ether, 2,2,2-trifluoroethyl n-propyl
ether, 2,2,2-trifluoroethyl 3-fluoro-n-propyl ether,
2,2,2-trifluoroethyl 3,3,3-trifluoro-n-propyl ether,
2,2,2-trifluoroethyl 2,2,3,3-tetrafluoro-n-propyl ether,
2,2,2-trifluoroethyl 2,2,3,3,3-pentafluoro-n-propyl ether,
1,1,2,2-tetrafluoroethyl n-propyl ether, 1,1,2,2-tetrafluoroethyl
3-fluoro-n-propyl ether, 1,1,2,2-tetrafluoroethyl
3,3,3-trifluoro-n-propyl ether, 1,1,2,2-tetrafluoroethyl
2,2,3,3-tetrafluoro-n-propyl ether, 1,1,2,2-tetrafluoroethyl
2,2,3,3,3-pentafluoro-n-propyl ether, di-n-propyl ether, n-propyl
3-fluoro-n-propyl ether, n-propyl 3,3,3-trifluoro-n-propyl ether,
n-propyl 2,2,3,3-tetrafluoro-n-propyl ether, n-propyl
2,2,3,3,3-pentafluoro-n-propyl ether, di(3-fluoro-n-propyl)ether,
3-fluoro-n-propyl 3,3,3-trifluoro-n-propyl ether, 3-fluoro-n-propyl
2,2,3,3-tetrafluoro-n-propyl ether, 3-fluoro-n-propyl
2,2,3,3,3-pentafluoro-n-propyl ether,
di(3,3,3-trifluoro-n-propyl)ether, 3,3,3-trifluoro-n-propyl
2,2,3,3-tetrafluoro-n-propyl ether, 3,3,3-trifluoro-n-propyl
2,2,3,3,3-pentafluoro-n-propyl ether,
di(2,2,3,3-tetrafluoro-n-propyl)ether, 2,2,3,3-tetrafluoro-n-propyl
2,2,3,3,3-pentafluoro-n-propyl ether,
di(2,2,3,3,3-pentafluoro-n-propyl)ether, di-n-butyl ether,
dimethoxymethane, methoxyethoxymethane,
methoxy(2-fluoroethoxy)methane,
methoxy(2,2,2-trifluoroethoxy)methane,
methoxy(1,1,2,2-tetrafluoroethoxy)methane, diethoxymethane,
ethoxy(2-fluoroethoxy)methane,
ethoxy(2,2,2-trifluoroethoxy)methane,
ethoxy(1,1,2,2-tetrafluoroethoxy)methane,
di(2-fluoroethoxy)methane,
2-fluoroethoxy(2,2,2-trifloroethoxy)methane,
2-fluoroethoxy(1,1,2,2-tetrafluoroethoxy)methane,
di(2,2,2-trifluoroethoxy)methane,
2,2,2-trifluoroethoxy(1,1,2,2-tetrafluoroethoxy)methane,
di(1,1,2,2-tetrafluoroethoxy)methane, dimethoxyethane,
methoxyethoxyethane, methoxy(2-fluoroethoxy)ethane,
methoxy(2,2,2-trifluoroethoxy)ethane,
methoxy(1,1,2,2-tetrafluoroethoxy)ethane, diethoxyethane,
ethoxy(2-fluoroethoxy)ethane, ethoxy(2,2,2-trifluoroethoxy)ethane,
ethoxy(1,1,2,2-tetrafluoroethoxy)ethane, di(2-fluoroethoxy)ethane,
2-fluoroethoxy(2,2,2-trifloroethoxy)ethane,
2-fluoroethoxy(1,1,2,2-tetrafluoroethoxy)ethane,
di(2,2,2-trifluoroethoxy)ethane,
2,2,2-trifluoroethoxy(1,1,2,2-tetrafluoroethoxy)ethane,
di(1,1,2,2-tetrafluoroethoxy)ethane, ethylene glycol di-n-propyl
ether, ethylene glycol di-n-butyl ether, and diethylene glycol
dimethyl ether.
[0040] Preferred of these are dimethoxymethane, diethoxymethane,
ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene
glycol di-n-butyl ether, and diethylene glycol dimethyl ether from
the standpoints of having the high ability to solvate lithium ions
and improving dissolution into ions.
[0041] From the standpoints of having high oxidation resistance,
bringing about a high capacity retention after a durability test
such as a high-temperature storage test or a cycle test, and
enabling the battery to have the excellent output performance and
high-rate discharge capacity equal to the initial values even after
the durability test, the following chain ethers are preferred of
those: 2,2,2-trifluoroethyl 2,2,3,3-tetrafluoro-n-propyl ether,
1,1,2,2-tetrafluoroethyl 3,3,3-trifluoro-n-propyl ether,
1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoro-n-propyl ether,
3,3,3-trifluoro-n-propyl 2,2,3,3-tetrafluoro-n-propyl ether,
3,3,3-trifluoro-n-propyl 2,2,3,3,3-pentafluoro-n-propyl ether, and
di(2,2,3,3-tetrafluoro-n-propyl)ether.
[0042] Especially preferred of those are dimethoxymethane,
diethoxymethane, ethoxymethoxymethane, 2,2,2-trifluoroethyl
2,2,3,3-tetrafluoro-n-propyl ether, 1,1,2,2-tetrafluoroethyl
3,3,3-trifluoro-n-propyl ether, and 1,1,2,2-tetrafluoroethyl
2,2,3,3-tetrafluoro-n-propyl ether because these chain ethers have
low viscosity, impart high ionic conductivity, and bring about
excellent battery durability.
[0043] One chain ether may be used alone, or two or more chain
ethers may be used in any desired combination and proportion. The
amount of the chain ether to be incorporated into each nonaqueous
electrolytic solution of the invention is desirably as follows. The
concentration of the chain ether in the whole nonaqueous solvent is
generally 5% by volume or higher, preferably 8% by volume or
higher, more preferably 10% by volume or higher, and is generally
70% by volume or less, preferably 60% by volume or less, more
preferably 50% by volume or less. In case where the concentration
thereof is too low, there is a tendency that it is difficult to
obtain the effect of improving ionic conductivity that is
attributable to both the improvement in the degree of dissociation
into lithium ions and the decrease in viscosity which are to be
brought about by the chain ether. In case where the concentration
thereof is too high, there are cases where the chain ether is
inserted into the negative carbon electrode together with lithium
ions, resulting in a decrease in capacity.
[0044] The term "whole nonaqueous solvent" in this description
means the whole nonaqueous electrolytic solution excluding the
cyclic carbonate having an unsaturated bond, sulfonic acid esters,
sulfurous acid esters, lithium fluorophosphates, lithium
sulfonates, imide lithium salts, and electrolytes which will be
described later.
[0045] <Cyclic Carbonate Having Unsaturated Bond>
[0046] In the nonaqueous electrolytic solutions of the invention, a
cyclic carbonate having an unsaturated bond (hereinafter often
abbreviated to "unsaturated cyclic carbonate") can be used in order
to form a coating film on the surface of the negative electrode of
the nonaqueous-electrolyte battery to attain battery life
prolongation.
[0047] The unsaturated cyclic carbonate is not particularly limited
so long as the cyclic carbonate has a carbon-carbon double bond,
and any desired unsaturated carbonate can be used. Cyclic
carbonates having an aromatic ring are also included in unsaturated
cyclic carbonates.
[0048] Examples of the unsaturated cyclic carbonate include
vinylene carbonate and derivatives thereof, ethylene carbonates
substituted with one or more aromatic rings or substituents having
a carbon-carbon double bond, phenyl carbonates, vinyl carbonates,
allyl carbonates, and catechol carbonates.
[0049] Examples of the vinylene carbonate and derivatives thereof
include vinylene carbonate, methylvinylene carbonate,
4,5-dimethylvinylene carbonate, phenylvinylene carbonate,
4,5-diphenylvinylene carbonate, vinylvinylene carbonate,
4,5-divinylvinylene carbonate, allylvinylene carbonate, and
4,5-diallylvinylene carbonate.
[0050] Examples of the ethylene carbonates substituted with one or
more aromatic rings or substituents having a carbon-carbon double
bond include vinylethylene carbonate, 4,5-divinylethylene
carbonate, 4-methyl-5-vinylethylene carbonate,
4-allyl-5-vinylethylene carbonate, phenylethylene carbonate,
4,5-diphenylethylene carbonate, 4-phenyl-5-vinylethylene carbonate,
4-allyl-5-phenylethylene carbonate, allylethylene carbonate,
4,5-diallylethylene carbonate, and 4-methyl-5-allylethylene
carbonate.
[0051] Especially preferred unsaturated cyclic carbonates among
these are vinylene carbonate, methylvinylene carbonate,
4,5-dimethylvinylene carbonate, vinylvinylene carbonate,
4,5-divinylvinylene carbonate, allylvinylene carbonate,
4,5-diallylvinylene carbonate, vinylethylene carbonate,
4,5-divinylethylene carbonate, 4-methyl-5-vinylethylene carbonate,
allylethylene carbonate, 4,5-diallylethylene carbonate,
4-methyl-5-allylethylene carbonate, and 4-allyl-5-vinylethylene
carbonate. These carbonates are more suitable because the
carbonates form a stable interface-protective coating film.
[0052] The molecular weight of the unsaturated cyclic carbonate is
not particularly limited, and the carbonate may have any desired
molecular weight unless the effects of the invention are
considerably lessened thereby. The molecular weight thereof is
preferably 50-250. So long as the unsaturated cyclic carbonate has
a molecular weight within that range, it is easy to ensure the
solubility of the unsaturated cyclic carbonate in the nonaqueous
electrolytic solution and the effects of the invention are apt to
be sufficiently produced. The molecular weight of the unsaturated
cyclic carbonate is more preferably 80 or higher, and is more
preferably 150 or lower. Methods for producing the unsaturated
cyclic carbonate are not particularly limited, and the carbonate
can be produced by a known method selected at will.
[0053] One unsaturated cyclic carbonate may be used alone, or two
or more unsaturated cyclic carbonates may be used in any desired
combination and proportion. The amount of the unsaturated cyclic
carbonate to be incorporated is not particularly limited, and the
carbonate may be incorporated in any desired amount unless the
effects of the invention are considerably lessened thereby. The
amount of the unsaturated cyclic carbonate to be incorporated per
100% by mass the nonaqueous solvent, i.e., the amount thereof based
on the whole nonaqueous electrolytic solution, is preferably 0.001%
by mass or more, more preferably 0.01% by mass or more, even more
preferably 0.1% by mass or more, and is preferably 5% by mass or
less, more preferably 4% by mass or less, even more preferably 3%
by mass or less. So long as the amount of the unsaturated cyclic
carbonate is within that range, it is easy to produce the effect of
sufficiently improving the cycle characteristics of the
nonaqueous-electrolyte battery. In addition, it is easy to avoid
the trouble that the battery has reduced high-temperature
storability and evolves a gas in an increased amount, resulting in
a decrease in discharge capacity retention.
[0054] The cyclic carbonate having an unsaturated bond may have a
fluorine atom. The number of fluorine atoms possessed by the
unsaturated cyclic carbonate having a fluorine atom (hereinafter
often abbreviated to "fluorinated unsaturated cyclic carbonate") is
not particularly limited so long as the number thereof is 1 or
more. In particular, the number of the fluorine atoms is generally
6 or less, preferably 4 or less. Most preferred are fluorinated
unsaturated cyclic carbonates having one or two fluorine atoms.
[0055] Examples of the fluorinated unsaturated cyclic carbonate
include fluorinated vinylene carbonate derivatives and fluorinated
ethylene carbonate derivatives substituted with one or more
aromatic rings or substituents having a carbon-carbon double
bond.
[0056] Examples of the fluorinated vinylene carbonate derivatives
include 4-fluorovinylene carbonate, 4-fluoro-5-methylvinylene
carbonate, 4-fluoro-5-phenylvinylene carbonate,
4-allyl-5-fluorovinylene carbonate, and 4-fluoro-5-vinylvinylene
carbonate.
[0057] Examples of the fluorinated ethylene carbonate derivatives
substituted with one or more aromatic rings or substituents having
a carbon-carbon double bond include 4-fluoro-4-vinylethylene
carbonate, 4-fluoro-4-allylethylene carbonate,
4-fluoro-5-vinylethylene carbonate, 4-fluoro-5-allylethylene
carbonate, 4,4-difluoro-4-vinylethylene carbonate,
4,4-difluoro-4-allylethylene carbonate,
4,5-difluoro-4-vinylethylene carbonate,
4,5-difluoro-4-allylethylene carbonate, [0058]
4-fluoro-4,5-divinylethylene carbonate,
4-fluoro-4,5-diallylethylene carbonate,
4,5-difluoro-4,5-divinylethylene carbonate,
4,5-difluoro-4,5-diallylethylene carbonate,
4-fluoro-4-phenylethylene carbonate, 4-fluoro-5-phenylethylene
carbonate, 4,4-difluoro-5-phenylethylene carbonate, and
4,5-difluoro-4-phenylethylene carbonate.
[0059] Especially preferred fluorinated unsaturated cyclic
carbonates among these are 4-fluorovinylene carbonate,
4-fluoro-5-methylvinylene carbonate, 4-fluoro-5-vinylvinylene
carbonate, 4-allyl-5-fluorovinylene carbonate,
4-fluoro-4-vinylethylene carbonate, 4-fluoro-4-allylethylene
carbonate, 4-fluoro-5-vinylethylene carbonate,
4-fluoro-5-allylethylene carbonate, 4,4-difluoro-4-vinylethylene
carbonate, 4,4-difluoro-4-allylethylene carbonate,
4,5-difluoro-4-vinylethylene carbonate,
4,5-difluoro-4-allylethylene carbonate,
4-fluoro-4,5-divinylethylene carbonate,
4-fluoro-4,5-diallylethylene carbonate,
4,5-difluoro-4,5-divinylethylene carbonate, and
4,5-difluoro-4,5-diallylethylene carbonate. These carbonates are
more suitable because the carbonates form a stable
interface-protective coating film.
[0060] The molecular weight of the fluorinated unsaturated cyclic
carbonate is not particularly limited, and the carbonate may have
any desired molecular weight unless the effects of the invention
are considerably lessened thereby. The molecular weight thereof is
preferably 50-250. So long as the fluorinated unsaturated cyclic
carbonate has a molecular weight within that range, it is easy to
ensure the solubility of the fluorinated cyclic carbonate in the
nonaqueous electrolytic solution and the effects of the invention
are apt to be produced. Methods for producing the fluorinated
unsaturated cyclic carbonate are not particularly limited, and the
carbonate can be produced by a known method selected at will. The
molecular weight of the fluorinated unsaturated cyclic carbonate is
more preferably 100 or higher, and is more preferably 200 or
lower.
[0061] One fluorinated unsaturated cyclic carbonate may be used
alone, or two or more fluorinated unsaturated cyclic carbonates may
be used in any desired combination and proportion. The amount of
the fluorinated unsaturated cyclic carbonate to be incorporated is
not particularly limited, and the carbonate may be incorporated in
any desired amount unless the effects of the invention are
considerably lessened thereby. The amount of the fluorinated
unsaturated cyclic carbonate to be incorporated per 100% by mass
the nonaqueous electrolytic solution is usually preferably 0.01% by
mass or more, more preferably 0.1% by mass or more, even more
preferably 0.2% by mass or more, and is preferably 5% by mass or
less, more preferably 4% by mass or less, even more preferably 3%
by mass or less. So long as the amount of the fluorinated
unsaturated cyclic carbonate is within that range, it is easy to
produce the effect of sufficiently improving the cycle
characteristics of the nonaqueous-electrolyte battery. In addition,
it is easy to avoid the trouble that the battery has reduced
high-temperature storability and evolves a gas in an increased
amount, resulting in a decrease in discharge capacity
retention.
[0062] <Sulfonic Acid Esters>
[0063] Sulfonic acid esters can be used in the nonaqueous
electrolytic solutions to be used in the invention, in order to
attain battery life prolongation. Examples of the sulfonic acid
esters include cyclic sulfonic acid esters having 3-6 carbon atoms
and chain sulfonic acid esters having 1-4 carbon atoms.
[0064] Examples of the cyclic sulfonic acid esters having 3-6
carbon atoms include 1,3-propanesultone,
1-methyl-1,3-propanesultone, 2-methyl-1,3-propanesultone,
3-methyl-1,3-propanesultone, 1-ethyl-1,3-propanesultone,
2-ethyl-1,3-propanesultone, 3-ethyl-1,3-propanesultone,
1-fluoro-1,3-propanesultone, 2-fluoro-1,3-propanesultone,
3-fluoro-1,3-propanesultone, 1,4-butanesultone,
1-methyl-1,4-butanesultone, 2-methyl-1,4-butanesultone,
3-methyl-1,4-butanesultone, 4-methyl-1,4-butanesultone,
1-ethyl-1,4-butanesultone, 2-ethyl-1,4-butanesultone,
3-ethyl-1,4-butanesultone, 4-ethyl-1,4-butanesultone,
1-propene-1,3-sultone, 1-fluoro-1-propene-1,3-sultone,
2-fluoro-1-propene-1,3-sultone, 3-fluoro-1-propene-1,3-sultone,
1,4-butanesultone, 1-butene-1,4-sultone, and
3-butene-1,4-sultone.
[0065] More suitable of these are 1,3-propanesultone,
1-methyl-1,3-propanesultone, 2-methyl-1,3-propanesultone,
3-methyl-1,3-propanesultone, 1-ethyl-1,3-propanesultone,
1,4-butanesultone, 1-methyl-1,4-butanesultone,
2-methyl-1,4-butanesultone, 3-methyl-1,4-butanesultone,
4-methyl-1,4-butanesultone, 1-propene-1,3-sultone, and the like
because such cyclic sulfonic acid esters have the high ability to
protect the electrode interface based on interaction with the
electrode surface and improve storability and cycle durability.
[0066] Examples of the chain sulfonic acid esters having 1-4 carbon
atoms include methyl fluorosulfonate, ethyl fluorosulfonate, propyl
fluorosulfonate, methyl methanesulfonate, ethyl methanesulfonate,
propyl methanesulfonate, methyl ethanesulfonate, ethyl
ethanesulfonate, methyl vinylsulfonate, and ethyl
vinylsulfonate.
[0067] More suitable of these are methyl fluorosulfonate, ethyl
fluorosulfonate, methyl methanesulfonate, ethyl methanesulfonate,
methyl ethanesulfonate, ethyl ethanesulfonate, methyl
vinylsulfonate, ethyl vinylsulfonate, and the like because such
chain sulfonic acid esters have the high ability to protect the
electrode interface based on interaction with the electrode surface
and improve storability and cycle durability.
[0068] One sulfonic acid ester may be used alone, or two or more
sulfonic acid esters may be used in any desired combination and
proportion. The amount of the sulfonic acid ester to be
incorporated is not particularly limited, and the sulfonic acid
ester may be incorporated in any desired amount unless the effects
of the invention are considerably lessened thereby. The amount of
the sulfonic acid ester to be incorporated per 100% by mass the
nonaqueous electrolytic solution usually is preferably 0.01% by
mass or more, more preferably 0.1% by mass or more, even more
preferably 0.2% by mass or more, and is preferably 5% by mass or
less, more preferably 4% by mass or less, even more preferably 3%
by mass or less. So long as the amount of the sulfonic acid ester
is within that range, it is easy to produce the effect of
sufficiently improving the cycle characteristics of the
nonaqueous-electrolyte battery. In addition, it is easy to avoid
the trouble that the battery has reduced high-temperature
storability and evolves a gas in an increased amount, resulting in
a decrease in discharge capacity retention.
[0069] <Sulfurous Acid Esters>
[0070] Cyclic sulfurous acid esters can be used in the nonaqueous
electrolytic solutions to be used in the invention, in order to
attain battery life prolongation. Examples of the sulfurous acid
esters include cyclic sulfurous acid esters having 3-6 carbon
atoms.
[0071] Examples of the cyclic sulfurous acid esters having 3-6
carbon atoms include ethylene sulfite, 4-methylethylene sulfite,
4,4-dimethylethylene sulfite, 4,5-dimethylethylene sulfite,
4-ethylethyene sulfite, 4,4-diethylethylene sulfite, and
4,5-diethylethylene sulfite.
[0072] More suitable of these are ethylene sulfite and
4-methylethylene sulfite because these cyclic sulfurous acid esters
have the high ability to protect the electrode interface based on
interaction with the electrode surface and improve storability and
cycle durability.
[0073] One sulfurous acid ester may be used alone, or two or more
sulfurous acid esters may be used in any desired combination and
proportion. The amount of the sulfurous acid ester to be
incorporated is not particularly limited, and the sulfurous acid
ester may be incorporated in any desired amount unless the effects
of the invention are considerably lessened thereby. The amount of
the cyclic sulfurous acid ester to be incorporated per 100% by mass
the nonaqueous electrolytic solution usually is preferably 0.01% by
mass or more, more preferably 0.1% by mass or more, even more
preferably 0.2% by mass or more, and is preferably 5% by mass or
less, more preferably 4% by mass or less, even more preferably 3%
by mass or less. So long as the amount of the cyclic sulfurous acid
ester is within that range, it is easy to produce the effect of
sufficiently improving the cycle characteristics of the
nonaqueous-electrolyte battery. In addition, it is easy to avoid
the trouble that the battery has reduced high-temperature
storability and evolves a gas in an increased amount, resulting in
a decrease in discharge capacity retention.
[0074] <Other Ingredients>
[0075] In the invention, it is possible to use cyclic carbonates
including ethylene carbonate, chain carbonates, cyclic and chain
esters other than carbonic acid esters, cyclic ethers, sulfone
compounds, and the like.
[0076] <Cyclic Carbonates>
[0077] Ethylene Carbonate
[0078] It is preferred that the nonaqueous electrolytic solutions
to be used in the invention each should contain ethylene carbonate,
and the content thereof based on the whole nonaqueous solvent is as
follows. The lower limit thereof is preferably 10% by volume or
more, and the upper limit thereof is preferably 70% by volume or
less.
[0079] Cyclic Carbonates Other than Ethylene Carbonate
[0080] Examples of the cyclic carbonates other than ethylene
carbonate include cyclic carbonates having an alkylene group with 3
or 4 carbon atoms.
[0081] Specifically, examples of the cyclic carbonates having an
alkylene group with 3 or 4 carbon atoms include propylene carbonate
and butylene carbonate. Especially preferred of these is propylene
carbonate from the standpoint of improving battery characteristics
on the basis of an improvement in the degree of dissociation into
lithium ions.
[0082] It is desirable in the invention that one or more such
cyclic carbonates other than ethylene carbonate should be
incorporated in a concentration of generally 5% by volume or
higher, preferably 10% by volume or higher, based on the whole
nonaqueous solvent in the nonaqueous electrolytic solution. In case
where the concentration thereof is less than the lower limit, the
incorporation thereof brings about little increase in the
electrical conductivity of the nonaqueous electrolytic solution of
the invention. In particular, there are cases where the
incorporation thereof does not contribute to an improvement in the
high-current discharge characteristics of the
nonaqueous-electrolyte battery of the invention. It is also
desirable that one or more cyclic carbonates other than ethylene
carbonate should be incorporated in a concentration of generally
40% by volume or less, preferably 35% by volume or less. In case
where the concentration thereof exceeds the range, there is a
tendency that the nonaqueous electrolytic solution has an increased
viscosity coefficient and this reduces the electrical conductivity
thereof. In particular, there are cases where the
nonaqueous-electrolyte battery is reduced in high-current discharge
characteristics.
[0083] With respect to the term "whole nonaqueous solvent" used
here also, this term means the whole nonaqueous electrolytic
solution excluding the cyclic carbonate having an unsaturated bond,
sulfonic acid esters, and sulfurous acid esters which were
described above, the lithium fluorophosphates, lithium sulfonates,
and imide lithium salts which will be described later, and the
electrolytes which will be described later, as in the case of
ethylene carbonate.
[0084] Saturated Cyclic Carbonates Having Fluorine Atom(s)
[0085] Saturated cyclic carbonates having a fluorine atom
(hereinafter often abbreviated to "fluorinated saturated cyclic
carbonates") are not particularly limited. Examples thereof include
derivatives of saturated cyclic carbonates having an alkylene group
with 2-6 carbon atoms. Specific examples thereof include ethylene
carbonate derivatives. Examples of the ethylene carbonate
derivatives include products of fluorination of either ethylene
carbonate or ethylene carbonate substituted with one or more alkyl
groups (e.g., alkyl groups having 1-4 carbon atoms). Preferred of
these are such fluorination products having 1-8 fluorine atoms.
[0086] Specific examples thereof include monofluoroethylene
carbonate, 4,4-difluoroethylene carbonate, 4,5-difluoroethylene
carbonate, 4-fluoro-4-methylethylene carbonate,
4,5-difluoro-4-methylethylene carbonate, 4-fluoro-5-methylethylene
carbonate, 4,4-difluoro-5-methylethylene carbonate,
4-(fluoromethyl)ethylene carbonate, 4-(difluoromethyl)ethylene
carbonate, 4-(trifluoromethyl)ethylene carbonate,
4-(fluoromethyl)-4-fluoroethylene carbonate,
4-(fluoromethyl)-5-fluoroethylene carbonate,
4-fluoro-4,5-dimethylethylene carbonate,
4,5-difluoro-4,5-dimethylethylene carbonate, and
4,4-difluoro-5,5-dimethylethylene carbonate.
[0087] More preferred of these is at least one member selected from
the group consisting of monofluoroethylene carbonate,
4,4-difluoroethylene carbonate, 4,5-difluoroethylene carbonate, and
4,5-difluoro-4,5-dimethylethylene carbonate because these
carbonates impart high ionic conductivity and satisfactorily form
an interface-protective coating film.
[0088] One fluorinated saturated cyclic carbonate may be used
alone, or two or more fluorinated saturated cyclic carbonates may
be used in any desired combination and proportion. The amount of
the fluorinated saturated cyclic carbonate to be incorporated is
not particularly limited, and the carbonate may be used in any
desired amount unless the effects of the invention are considerably
lessened thereby. However, the amount thereof per 100% by mass the
nonaqueous solvent is preferably 0.01% by mass or more, more
preferably 0.1% by mass or more, even more preferably 0.2% by mass
or more. With respect to the upper limit thereof, the amount of the
carbonate is less than 50% by mass, preferably 45% by mass or less.
So long as the amount of the fluorinated saturated cyclic carbonate
is within that range, it is easy to produce the effect of
sufficiently improving the cycle characteristics of the
nonaqueous-electrolyte battery. In addition, it is easy to avoid
the trouble that the battery has reduced high-temperature
storability and evolves a gas in an increased amount, resulting in
a decrease in discharge capacity retention.
[0089] <Chain Carbonates>
[0090] Chain carbonates having 3-7 carbon atoms are preferred.
[0091] Examples of the chain carbonates having 3-7 carbon atoms
include dimethyl carbonate, diethyl carbonate, di-n-propyl
carbonate, diisopropyl carbonate, n-propyl isopropyl carbonate,
ethyl methyl carbonate, methyl n-propyl carbonate, n-butyl methyl
carbonate, isobutyl methyl carbonate, t-butyl methyl carbonate,
ethyl n-propyl carbonate, n-butyl ethyl carbonate, isobutyl ethyl
carbonate, and t-butyl ethyl carbonate.
[0092] Preferred of these are dimethyl carbonate, diethyl
carbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propyl
isopropyl carbonate, ethyl methyl carbonate, and methyl n-propyl
carbonate.
[0093] Especially preferred of these are dimethyl carbonate,
diethyl carbonate, and ethyl methyl carbonate.
[0094] Chain carbonates having a fluorine atom (hereinafter, the
carbonates are often abbreviated to "fluorinated chain carbonates")
also are suitable. The number of fluorine atoms possessed by each
fluorinated chain carbonate also is not particularly limited so
long as the number thereof is 1 or more. However, the number
thereof is generally 6 or less, preferably 4 or less. In the case
where a fluorinated chain carbonate has a plurality of fluorine
atoms, these fluorine atoms may be the same or different. Examples
of the fluorinated chain carbonates include ethylene carbonate
derivatives, dimethyl carbonate derivatives, ethyl methyl carbonate
derivatives, and diethyl carbonate derivatives.
[0095] Examples of the dimethyl carbonate derivatives include
fluoromethyl methyl carbonate, difluoromethyl methyl carbonate,
trifluoromethyl methyl carbonate, bis(fluoromethyl) carbonate,
bis(difluoro)methyl carbonate, and bis(trifluoro)methyl
carbonate.
[0096] Examples of the ethyl methyl carbonate derivatives include
2-fluoroethyl methyl carbonate, ethyl fluoromethyl carbonate,
2,2-difluoroethyl methyl carbonate, 2-fluoroethyl fluoromethyl
carbonate, ethyl difluoromethyl carbonate, 2,2,2-trifluoroethyl
methyl carbonate, 2,2-difluoroethyl fluoromethyl carbonate,
2-fluoroethyl difluoromethyl carbonate, and ethyl trifluoromethyl
carbonate.
[0097] Examples of the diethyl carbonate derivatives include ethyl
2-fluoroethyl carbonate, ethyl 2,2-difluoroethyl carbonate,
bis(2-fluoroethyl) carbonate, ethyl 2,2,2-trifluoroethyl carbonate,
2,2-difluoroethyl 2'-fluoroethyl carbonate, bis(2,2-difluoroethyl)
carbonate, 2,2,2-trifloroethyl 2'-fluoroethyl carbonate,
2,2,2-trifluoroethyl 2',2'-difluoroethyl carbonate, and
bis(2,2,2-trifluoroethyl) carbonate.
[0098] With respect to the chain carbonates explained above also,
any one of these chain carbonates may be incorporated alone into
each nonaqueous electrolytic solution of the invention or two or
more thereof may be incorporated in any desired combination and
proportion.
[0099] In each nonaqueous electrolytic solution to be used in the
invention, it is desirable that at least one chain carbonate should
be incorporated in a concentration of preferably 15% by volume or
higher, more preferably 20% by volume or higher, even more
preferably 25% by volume or higher, based on the whole nonaqueous
solvent in the nonaqueous electrolytic solution. It is also
desirable that the chain carbonate be incorporated in a
concentration of 85% by volume or less, more preferably 80% by
volume or less, even more preferably 75% by volume or less.
[0100] So long as the concentration thereof is within that range,
the nonaqueous electrolytic solution suffers neither an increase in
viscosity nor a decrease in ionic conductivity and, hence, has
satisfactory high-current electrical conduction
characteristics.
[0101] In each nonaqueous electrolytic solution of the invention,
ethylene carbonate and a specific chain carbonate may be
incorporated in specific amounts together with a specific chain
ether compound. Thus, the performance of the electrolytic solution
can be greatly improved.
[0102] For example, in the case where dimethoxyethane was selected
as the chain ether, it is preferred to select ethyl methyl
carbonate as the specific chain carbonate. In this case, it is
especially preferred that ethylene carbonate should be incorporated
in an amount of 15% by volume to 40% by volume, dimethoxyethane be
incorporated in an amount of 10% by volume to 40% by volume, and
ethyl methyl carbonate be incorporated in an amount of 30% by
volume to 60% by volume. By selecting ethyl methyl carbonate as the
chain carbonate and by selecting these incorporation amounts, the
compatibility temperature range can be widened and the
lower-temperature-side precipitation temperature for lithium salts
can be lowered.
[0103] In the case where diethoxyethane was selected as the chain
ether in another example, it is preferred to select dimethyl
carbonate as the specific chain carbonate. In this case, it is
especially preferred that ethylene carbonate should be incorporated
in an amount of 15% by volume to 40% by volume, diethoxyethane be
incorporated in an amount of 20% by volume to 60% by volume, and
dimethyl carbonate be incorporated in an amount of 15% by volume to
60% by volume. By selecting dimethyl carbonate as the chain
carbonate and by selecting these incorporation amounts, the
nonaqueous electrolytic solution can be made to have a reduced
viscosity and improved ionic conductivity while lowering the
lower-temperature-side precipitation temperature for lithium salts,
and high output can be obtained even at low temperatures.
[0104] <Cyclic Esters other than Cyclic Carbonic Acid
Esters>
[0105] Examples of the cyclic esters other than cyclic carbonic
acid esters include cyclic esters having 3-12 carbon atoms.
[0106] Specific examples thereof include .gamma.-butyrolactone,
.gamma.-valerolactone, .gamma.-caprolactone, and
.epsilon.-caprolactone. Of these, .gamma.-butyrolactone is
especially preferred from the standpoint of the improvement in
battery characteristics which is attributable to an improvement in
the degree of dissociation into lithium ions.
[0107] It is desirable in the invention that such a cyclic ester
should be incorporated in a concentration of generally 5% by volume
or higher, preferably 10% by volume or higher, based on the whole
nonaqueous solvent in the nonaqueous electrolytic solution. In case
where the concentration thereof is lower than the lower limit, the
effect of increasing the electrical conductivity of the nonaqueous
electrolytic solution of the invention is low and, in particular,
there are cases where the incorporation of the cyclic ester does
not contribute to an improvement in the high-current discharge
characteristics of the nonaqueous-electrolyte battery. It is also
desirable that the cyclic ester should be incorporated in a
concentration of generally 40% by volume or less, preferably 35% by
volume or less. In case where the concentration thereof exceeds the
range, there is a tendency that the nonaqueous electrolytic
solution has an increased viscosity coefficient and this reduces
the electrical conductivity thereof or increases the resistance of
the negative electrode. In particular, there are cases where the
nonaqueous-electrolyte battery has reduced high-current discharge
characteristics.
[0108] <Chain Esters Other than Chain Carbonic Acid
Esters>
[0109] Examples of the chain esters other than chain carbonic acid
esters include chain esters having an alkylene group with 3-7
carbon atoms.
[0110] Specific examples thereof include methyl acetate, ethyl
acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate,
isobutyl acetate, t-butyl acetate, methyl propionate, ethyl
propionate, n-propyl propionate, isopropyl propionate, n-butyl
propionate, isobutyl propionate, t-butyl propionate, methyl
butyrate, ethyl butyrate, n-propyl butyrate, isopropyl butyrate,
methyl isobutyrate, ethyl isobutyrate, n-propyl isobutyrate, and
isopropyl isobutyrate.
[0111] Especially preferred of these are methyl acetate, ethyl
acetate, n-propyl acetate, n-butyl acetate, methyl propionate,
ethyl propionate, n-propyl propionate, isopropyl propionate, methyl
butyrate, ethyl butyrate, and the like from the standpoint of
improving ionic conductivity on the basis of a decrease in
viscosity.
[0112] It is desirable in the invention that such a chain ester
should be incorporated in a concentration of generally 10% by
volume or higher, preferably 15% by volume or higher, based on the
whole nonaqueous solvent in the nonaqueous electrolytic solution.
In case where the concentration thereof is lower than the lower
limit, the effect of increasing the electrical conductivity of the
nonaqueous electrolytic solution of the invention is low and, in
particular, there are cases where the incorporation of the chain
ester does not contribute to an improvement in the high-current
discharge characteristics of the nonaqueous-electrolyte battery. It
is also desirable that the chain ester should be incorporated in a
concentration of generally 60% by volume or less, preferably 50% by
volume or less. In case where the concentration thereof exceeds the
range, there is a tendency that the negative electrode has
increased resistance and the nonaqueous-electrolyte battery suffers
a decrease in high-current discharge characteristics and a decrease
in cycle characteristics.
[0113] <Cyclic Ethers>
[0114] Examples of the cyclic ethers include cyclic ethers having
an alkylene group with 3-6 carbon atoms.
[0115] Specific examples thereof include tetrahydrofuran,
2-methyltetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxane,
2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane, and
compounds formed by fluorinating these.
[0116] Especially preferred of these are 2-methyltetrahydrofuran
and 2-methyl-1,3-dioxane. This is because these cyclic ethers have
low viscosity and the high ability to solvate lithium ions and,
hence, improve dissociation into ions, thereby imparting high ionic
conductivity.
[0117] The amount of the cyclic ether to be incorporated, per 100%
by volume the nonaqueous solvent, usually is preferably 5% by
volume or more, more preferably 10% by volume or more, even more
preferably 15% by volume or more, and is preferably 40% by volume
or less, more preferably 35% by volume or less, even more
preferably 30% by volume or less. So long as the amount thereof is
within that range, it is easy to ensure the effect of improving
ionic conductivity which is attributable to the improvement in the
degree of dissociation into lithium ions and the decrease in
viscosity which are brought about by the cyclic ether. In addition,
in the case where the negative-electrode active material is a
carbonaceous material, it is easy to avoid the trouble that the
cyclic ether is inserted into the active material together with
lithium ions, resulting in a decrease in capacity.
[0118] <Sulfone Compounds>
[0119] Preferred sulfone compounds are cyclic sulfones having 3-6
carbon atoms and chain sulfones having 2-6 carbon atoms. It is
preferred that the number of sulfonyl groups per molecule should be
1 or 2.
[0120] Examples of the cyclic sulfones include monosulfone
compounds such as trimethylene sulfone compounds, tetramethylene
sulfone compounds, and hexamethylene sulfone compounds and
disulfone compounds such as trimethylene disulfone compounds,
tetramethylene disulfone compounds, and hexamethylene disulfone
compounds. From the standpoints of permittivity and viscosity,
tetramethylene sulfone compounds, tetramethylene disulfone
compounds, hexamethylene sulfone compounds, and hexamethylene
disulfone compounds are more preferred of those, and tetramethylene
sulfone compounds (sulfolane compounds) are especially
preferred.
[0121] The sulfolane compounds preferably are sulfolane and/or
sulfolane derivatives (hereinafter, these compounds including
sulfolane are often referred to simply as "sulfolane compounds").
The sulfolane derivatives preferably are sulfolane compounds in
which one or more of the hydrogen atoms bonded to the carbon atoms
constituting the sulfolane ring each have been replaced with a
fluorine atom or an alkyl group.
[0122] Preferred of these are 2-methylsulfolane, 3-methylsulfolane,
2-fluorosulfolane, 3-fluorosulfolane, 2,2-difluorosulfolane,
2,3-difluorosulfolane, 2,4-difluorosulfolane,
2,5-difluorosulfolane, 3,4-difluorosulfolane,
2-fluoro-3-methylsulfolane, 2-fluoro-2-methylsulfolane,
3-fluoro-3-methylsulfolane, 3-fluoro-2-methylsulfolane,
4-fluoro-3-methylsulfolane, 4-fluoro-2-methylsulfolane,
5-fluoro-3-methylsulfolane, 5-fluoro-2-methylsulfolane,
2-fluoromethylsulfolane, 3-fluoromethylsulfolane,
2-difluoromethylsulfolane, 3-difluoromethylsulfolane,
2-trifluoromethylsulfolane, 3-trifluoromethylsulfolane,
2-fluoro-3-(trifluoromethyl)sulfolane,
3-fluoro-3-(trifluoromethyl)sulfolane,
4-fluoro-3-(trifluoromethyl)sulfolane, and
5-fluoro-3-(trifluoromethyl)sulfolane, from the standpoint that
these sulfolane compounds have high ionic conductivity and bring
about high input/output characteristics.
[0123] Examples of the chain sulfones include dimethyl sulfone,
ethyl methyl sulfone, diethyl sulfone, n-propyl methyl sulfone,
n-propyl ethyl sulfone, di-n-propyl sulfone, isopropyl methyl
sulfone, isopropyl ethyl sulfone, diisopropyl sulfone, n-butyl
methyl sulfone, n-butyl ethyl sulfone, t-butyl methyl sulfone,
t-butyl ethyl sulfone, monofluoromethyl methyl sulfone,
difluoromethyl methyl sulfone, trifluoromethyl methyl sulfone,
monofluoroethyl methyl sulfone, difluoroethyl methyl sulfone,
trifluoroethyl methyl sulfone, pentafluoroethyl methyl sulfone,
ethyl monofluoromethyl sulfone, ethyl difluoromethyl sulfone, ethyl
trifluoromethyl sulfone, perfluoroethyl methyl sulfone, ethyl
trifluoroethyl sulfone, ethyl pentafluoroethyl sulfone,
di(trifluoroethyl) sulfone, perfluorodiethyl sulfone, fluoromethyl
n-propyl sulfone, difluoromethyl n-propyl sulfone, trifluoromethyl
n-propyl sulfone, fluoromethyl isopropyl sulfone, difluoromethyl
isopropyl sulfone, trifluoromethyl isopropyl sulfone,
trifluoroethyl n-propyl sulfone, trifluoroethyl isopropyl sulfone,
pentafluoroethyl n-propyl sulfone, pentafluoroethyl isopropyl
sulfone, trifluoroethyl n-butyl sulfone, trifluoroethyl t-butyl
sulfone, pentafluoroethyl n-butyl sulfone, and pentafluoroethyl
t-butyl sulfone.
[0124] Preferred of these are dimethyl sulfone, ethyl methyl
sulfone, diethyl sulfone, n-propyl methyl sulfone, isopropyl methyl
sulfone, n-butyl methyl sulfone, t-butyl methyl sulfone,
monofluoromethyl methyl sulfone, difluoromethyl methyl sulfone,
trifluoromethyl methyl sulfone, monofluoroethyl methyl sulfone,
difluoroethyl methyl sulfone, trifluoroethyl methyl sulfone,
pentafluoroethyl methyl sulfone, ethyl monofluoromethyl sulfone,
ethyl difluoromethyl sulfone, ethyl trifluoromethyl sulfone, ethyl
trifluoroethyl sulfone, ethyl pentafluoroethyl sulfone,
trifluoromethyl n-propyl sulfone, trifluoromethyl isopropyl
sulfone, trifluoroethyl n-butyl sulfone, trifluoroethyl t-butyl
sulfone, trifluoromethyl n-butyl sulfone, trifluoromethyl t-butyl
sulfone, and the like, from the standpoint that these sulfone
compounds have high ionic conductivity and bring about high
input/output characteristics.
[0125] The amount of the sulfone compound to be incorporated, per
100% by mass the nonaqueous solvent, usually is preferably 0.3% by
mass or more, more preferably 0.5% by mass or more, even more
preferably 1% by mass or more, and is preferably 40% by mass or
less, more preferably 35% by mass or less, even more preferably 30%
by mass or less. So long as the amount thereof is within that
range, it is easy to obtain the effect of improving durability such
as cycle characteristics and storability. In addition, the
viscosity of the nonaqueous electrolytic solution can be regulated
so as to be within a proper range, and a decrease in electrical
conductivity can be avoided. Furthermore, it is easy to avoid the
trouble that the nonaqueous-electrolyte battery decreases in
charge/discharge capacity retention when charged and discharged at
a high current density.
[0126] <Electrolytes>
[0127] Examples of electrolytes which can be contained in the
nonaqueous electrolytic solutions to be used in the invention
include lithium fluorophosphates, lithium sulfonates, and imide
lithium salts. Preferred of these lithium salts are compounds
having the high ability to be adsorbed onto or interact with the
surface of the positive-electrode active material. In the case
where a compound having the high ability to be adsorbed onto or
interact with the electrode surface is used, the resistance of the
coating film on the electrode surface can be prevented from
increasing excessively, while maintaining thermal and chemical
durability. As a result, not only high high-temperature storability
and cycle characteristics can be imparted, but also an improvement
in high-rate characteristics and an increase in output can be
attained in the battery which has undergone a durability test.
Besides those lithium salts, any desired lithium salts can be
used.
<Lithium Fluorophosphates>
[0128] Examples of the lithium fluorophosphates include lithium
fluorophosphate and lithium difluorophosphate. Such lithium salts
may be used in combination. In particular, lithium
difluorophosphate is preferred because this lithium salt has the
high ability to be adsorbed onto or interact with the surface of
the electrode active material.
<Lithium Sulfonates>
[0129] Examples of the lithium sulfonates include lithium
methanesulfonate, lithium monofluoromethanesulfonate, lithium
difluoromethanesulfonate, and lithium trifluoromethanesulfonate.
Such lithium salts may be used in combination. In particular,
lithium trifluoromethanesulfonate is preferred because this lithium
salt has the high ability to be adsorbed onto or interact with the
surface of the electrode active material.
[0130] <Imide Lithium Salts>
[0131] Examples of the imide lithium salts include
LiN(FCO.sub.2).sub.2, LiN(FCO)(FSO.sub.2), LiN(FSO.sub.2).sub.2,
LiN(FSO.sub.2)(CF.sub.3SO.sub.2), LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, the lithium salt of cyclic
1,2-perfluoroethanedisulfonylimide, the lithium salt of cyclic
1,3-perfluoropropanedisulfonylimide, and
LiN(CF.sub.3SO.sub.2)(C.sub.4F.sub.9SO.sub.2). Such salts may be
used in combination. In particular, LiN(FCO)(FSO.sub.2),
LiN(FSO.sub.2).sub.2, LiN(FSO.sub.2)(CF.sub.3SO.sub.2),
LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2, and
the lithium salt of cyclic 1,2-perfluoroethanedisulfonylimide are
preferred because these lithium salts have the high ability to be
adsorbed onto or interact with the surface of the electrode active
material.
[0132] <Other Lithium Salts>
[0133] Examples of lithium salts other than the lithium
fluorophosphates, lithium sulfonates, and imide lithium salts
include:
[0134] carboxylic acid lithium salts such as lithium formate,
lithium acetate, lithium monofluoroacetate, lithium
difluoroacetate, and lithium trifluoroacetate;
[0135] lithium methide compounds such as LiC(FSO.sub.2).sub.3,
LiC(CF.sub.3SO.sub.2).sub.3, and
LiC(C.sub.2F.sub.5SO.sub.2).sub.3;
[0136] lithium oxalatoborate salts such as lithium
difluorooxalatoborate and lithium bis(oxalato)borate;
[0137] lithium oxalatophosphate salts such as lithium
tetrafluorooxalatophosphate, lithium difluorooxalatophosphate, and
lithium tris(oxalato)phosphate; and other fluorine-containing
organolithium salts such as LiBF.sub.4, 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).sup.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.
[0138] Preferred of these, for use as main electrolytes for the
nonaqueous electrolytic solutions, are LiPF.sub.6, LiBF.sub.4,
lithium trifluoromethanesulfonate, LiN(FSO.sub.2).sub.2,
LiN(FSO.sub.2)(CF.sub.3SO.sub.2), LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, the lithium salt of cyclic
1,2-perfluoroethanedisulfonylimide, and the lithium salt of cyclic
1,3-perfluoropropanedisulfonylimide from the standpoint of
improving battery performance.
[0139] The concentration of each of these main electrolytes in the
nonaqueous electrolytic solutions is not particularly limited.
However, the concentration thereof is generally 0.5 mol/L or
higher, preferably 0.6 mol/L or higher, more preferably 0.7 mol/L
or higher, and is generally 3 mol/L or less, preferably 2 mol/L or
less, more preferably 1.8 mol/L or less, especially preferably 1.5
mol/L or less. In the case where the concentration of the
electrolyte added is within that range, the effect of improving
battery characteristics is sufficiently produced and it is easy to
avoid the trouble that the resistance of charge movement increases
to reduce charge/discharge performance.
[0140] One of those main electrolytes for the nonaqueous
electrolytic solutions may be used alone, or two or more thereof
may be used in combination. In the case where two or more main
electrolytes are used in combination, a preferred example is a
combination of LiPF.sub.6 with LiBF.sub.4, LiN(FSO.sub.2).sub.2,
LiN(FSO.sub.2)(CF.sub.3SO.sub.2), LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, the lithium salt of cyclic
1,2-perfluoroethanedisulfonylimide, the lithium salt of cyclic
1,3-perfluoropropanedisulfonylimide, or the like. This combination
has the effect of improving output characteristics, high-rate
charge/discharge characteristics, high-temperature storability,
cycle characteristics, etc.
[0141] Also in the case where two or more main electrolytes are
contained in a nonaqueous electrolytic solution, the concentration
of the electrolytes is not particularly limited. However, the total
concentration of the main electrolytes is generally 0.5 mol/L or
higher, preferably 0.6 mol/L or higher, more preferably 0.7 mol/L
or higher, and is generally 3 mol/L or less, preferably 2 mol/L or
less, more preferably 1.8 mol/L or less, especially preferably 1.5
mol/L or less. In the case where the concentration of the
electrolytes added is within that range, the effect of improving
battery characteristics is sufficiently produced and it is easy to
avoid the trouble that the resistance of charge amount increases to
reduce charge/discharge performance.
[0142] It is also preferred that an electrolyte other than the main
electrolytes for the nonaqueous electrolytic solutions should be
added besides one or more of the main electrolytes. Preferred
examples of electrolytes to be added when LiPF.sub.6 is used as a
main electrolyte include LiBF.sub.4, lithium monofluorophosphate,
lithium difluorophosphate, lithium formate, lithium acetate,
lithium monofluoroacetate, lithium difluoroacetate, lithium
trifluoroacetate, lithium methanesulfonate, lithium
monofluoromethanesulfonate, lithium difluoromethanesulfonate,
lithium trifluoromethanesulfonate, LiN(FSO.sub.2).sub.2,
LiN(FSO.sub.2)(CF.sub.3SO.sub.2), lithium difluorooxalatoborate,
lithium tetrafluorooxalatophosphate, and lithium
difluorooxalatophosphate. Addition of these electrolytes has the
effect of improving output characteristics and high-rate
characteristics.
[0143] Furthermore, it is preferred to add lithium fluorophosphate,
lithium difluorophosphate, lithium trifluoromethanesulfonate,
LiN(FSO.sub.2).sub.2, LiN(FSO.sub.2)(CF.sub.3SO.sub.2),
LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2, the
lithium salt of cyclic 1,2-perfluoroethanedisulfonylimide, the
lithium salt of cyclic 1,3-perfluoropropanedisulfonylimide, lithium
difluorooxalatoborate, lithium tetrafluorooxalatophosphate, lithium
difluorooxalatophosphate, or the like besides the main electrolyte,
because the addition thereof has the effect of improving
high-temperature storability and cycle characteristics.
[0144] The concentration of the electrolyte added to a nonaqueous
electrolytic solution besides the main electrolyte is also not
particularly limited. However, the concentration thereof is
preferably 0.01% by mass or higher, more preferably 0.03% by mass
or higher, even more preferably 0.05% by mass or higher, and is
preferably 8% by mass or less, more preferably 6% by mass or less,
even more preferably 5% by mass or less. When the concentration of
the electrolyte added is within that range, the effect of improving
battery characteristics is sufficiently produced and it is easy to
avoid the trouble that the resistance of charge movement increases
to reduce charge/discharge performance.
[0145] Incidentally, LiBF.sub.4, LiN(FSO.sub.2).sub.2,
LiN(FSO.sub.2)(CF.sub.3SO.sub.2), LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, the lithium salt of cyclic
1,2-perfluoroethanedisulfonylimide, the lithium salt of cyclic
1,3-perfluoropropanedisulfonylimide, and the like have the effect
of improving the performance of the battery regardless of whether
these salts are added as main electrolytes or added as electrolytes
besides a main electrolyte.
[0146] <Overcharge Inhibitor>
[0147] An overcharge inhibitor can be used in the nonaqueous
electrolytic solutions of the invention in order to effectively
inhibit the nonaqueous-electrolyte batteries from bursting or
firing when brought into an overcharged state or the like.
[0148] Examples of the overcharge inhibitor include: aromatic
compounds such as biphenyl, alkylbiphenyls, terphenyl, partly
hydrogenated terphenyls, cyclohexylbenzene, t-butylbenzene,
t-amylbenzene, diphenyl ether, and dibenzofuran; products of
partial fluorination of these aromatic compounds, such as
2-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. Preferred of these
are aromatic compounds such as biphenyl, alkylbiphenyls, terphenyl,
partly hydrogenated terphenyls, cyclohexylbenzene, t-butylbenzene,
t-amylbenzene, diphenyl ether, and dibenzofuran. One of these may
be used alone, or two or more thereof may be used in combination.
In the case where two or more compounds are used 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 with t-butylbenzene or t-amylbenzene; and a
combination of at least one member selected from aromatic compounds
containing no oxygen, such as biphenyl, alkylbiphenyls, terphenyl,
partly hydrogenated terphenyls, cyclohexylbenzene, t-butylbenzene,
and t-amylbenzene, with at least one member selected from
oxygen-containing aromatic compounds such as diphenyl ether and
dibenzofuran.
[0149] The amount of the overcharge inhibitor to be incorporated is
not particularly limited, and the overcharge inhibitor may be
incorporated in any desired amount unless the effects of the
invention are considerably lessened thereby. The amount of the
overcharge inhibitor is preferably 0.01-5% by mass per 100% by mass
the nonaqueous solvent. So long as the amount thereof is within
that range, it is easy to sufficiently produce the effect of the
overcharge inhibitor and it is easy to avoid the trouble that
battery characteristics including high-temperature storability
decrease. The amount of the overcharge inhibitor is more preferably
0.01% by mass or more, even more preferably 0.1% by mass or more,
especially preferably 0.2% by mass or more, and is more preferably
3% by mass or less, even more preferably 2% by mass or less.
[0150] <Other Aids>
[0151] Other known aids can be used in the nonaqueous electrolytic
solutions of the invention. Examples of the other aids include:
carbonate compounds such as erythritane carbonate,
spiro-bis-dimethylene carbonate, and methoxyethyl methyl carbonate;
carboxylic acid anhydrides such as succinic anhydride, glutaric
anhydride, maleic anhydride, citraconic anhydride, glutaconic
anhydride, itaconic anhydride, diglycolic anhydride,
cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic
dianhydride, and phenylsuccinic anhydride; Spiro compounds such as
2,4,8,10-tetraoxaspiro[5.5]undecane and
3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane; sulfur-containing
compounds such as busulfan, sulfolene, diphenyl sulfone,
N,N-dimethylmethanesulfonamide, and N,N-diethylmethanesulfonamide;
nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone,
1-methyl-2-piperidone, 3-methyl-2-oxazolidinone,
1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide;
hydrocarbon compounds such as heptane, octane, nonane, decane, and
cycloheptane; and fluorine-containing aromatic compounds such as
fluorobenzene, difluorobenzene, hexafluorobenzene, and
benzotrifluoride. One of these aids may be used alone, or two or
more thereof may be used in combination. By adding these aids,
capacity retentivity after high-temperature storage and cycle
characteristics can be improved.
[0152] The amount of the other aids to be incorporated is not
particularly limited, and the other aids may be incorporated in any
desired amount unless the effects of the invention are considerably
lessened thereby. The amount of the other aids is preferably
0.01-5% by mass per 100% by mass the nonaqueous solvent. So long as
the amount thereof is within that range, it is easy to sufficiently
produce the effects of the other aids and it is easy to avoid the
trouble that battery characteristics including high-load discharge
characteristics decrease. The amount of the other aids to be
incorporated is more preferably 0.1% by mass or more, even more
preferably 0.2% by mass or more, and is more preferably 3% by mass
or less, even more preferably 1% by mass or less.
[0153] The nonaqueous electrolytic solutions described above
include the nonaqueous electrolytic solutions present in inner
parts of the nonaqueous-electrolyte batteries according to the
invention. Specifically, the invention includes: the nonaqueous
electrolytic solution present in a nonaqueous-electrolyte battery
obtained by separately synthesizing constituent elements for a
nonaqueous electrolytic solution, such as a lithium salt, a
solvent, and aids, preparing the nonaqueous electrolytic solution
from the substantially separate constituent elements, and
introducing the nonaqueous electrolytic solution into a battery
separately assembled by the method which will be described later.
The invention further includes: the case in which constituent
elements for a nonaqueous electrolytic solution of the invention
are separately introduced into a battery and mixed together within
the battery to thereby obtain the same composition as the
nonaqueous electrolytic solution of the invention; and the case in
which a compound serving as a component of a nonaqueous
electrolytic solution of the invention is generated within the
nonaqueous-electrolyte battery to obtain the same composition as
the nonaqueous electrolytic solution of the invention.
[0154] [Battery Configuration]
[0155] The nonaqueous-electrolyte batteries of the invention may
have the same battery configuration as conventionally known
nonaqueous-electrolyte batteries. Usually, the batteries of the
invention have a configuration obtained by superposing a positive
electrode and a negative electrode through a porous film
(separator) impregnated with a nonaqueous electrolytic solution of
the invention and disposing the stack in a case (outer case).
Consequently, the shapes of the nonaqueous-electrolyte batteries of
the invention are not particularly limited, and may be any of
cylindrical, prismatic, laminate type, coin type, large-size, and
other shapes.
[0156] [Negative Electrode]
[0157] The negative-electrode active material to be used in the
negative electrode is described below. The negative-electrode
active material is not particularly limited so long as the active
material is capable of electrochemically occluding and releasing
lithium ions. Examples thereof include a carbonaceous material, an
alloy material, and a lithium-containing composite metal oxide
material.
[0158] [Negative-Electrode Active Material]
[0159] Examples of the negative-electrode active material include a
carbonaceous material, an alloy material, and a lithium-containing
composite metal oxide material.
[0160] The carbonaceous material to be used as a negative-electrode
active material preferably is a material selected from:
(1) natural graphites; (2) carbonaceous materials obtained by
subjecting artificial carbonaceous substances and artificial
graphitic substances to a heat treatment at a temperature in the
range of 400-3,200.degree. C. one or more times; (3) carbonaceous
materials giving a negative-electrode active-material layer which
is composed of at least two carbonaceous substances differing in
crystallinity and/or has an interface where at least two
carbonaceous substances differing in crystallinity are in contact
with each other; and (4) carbonaceous materials giving a
negative-electrode active-material layer which is composed of at
least two carbonaceous substances differing in orientation and/or
has an interface where at least two carbonaceous substances
differing in orientation are in contact with each other.
[0161] This is because this carbonaceous material brings about a
satisfactory balance between initial irreversible capacity and
high-current-density charge/discharge characteristics. One of the
carbonaceous materials (1) to (4) may be used alone, or two or more
thereof may be used in any desired combination and proportion.
[0162] Examples of the artificial carbonaceous substances and
artificial graphitic substances in (2) above include natural
graphites, coal coke, petroleum coke, coal pitch, petroleum pitch,
carbonaceous substances obtained by oxidizing these pitches, needle
coke, pitch coke, carbon materials obtained by partly graphitizing
these cokes, products of the pyrolysis of organic substances, such
as furnace black, acetylene black, and pitch-derived carbon fibers,
organic substances capable of carbonization and products of the
carbonization thereof, or solutions obtained by dissolving any of
such organic substances capable of carbonization in a low-molecular
organic solvent, e.g., benzene, toluene, xylene, quinoline, or
n-hexane, and products of the carbonization of these solutions.
[0163] The alloy material to be used as a negative-electrode active
material is not particularly limited so long as the material is
capable of occluding and releasing lithium. Use may be made of
elemental lithium, an elemental metal or alloy which forms a
lithium alloy, or any of compounds thereof, such as oxides,
carbides, nitrides, silicides, sulfides, and phosphides. The
elemental metal or alloy which forms a lithium alloy preferably is
a material including any of the metals and semimetals in Group 13
and Group 14 (that is, carbon is excluded). More preferred are
elemental aluminum, silicon, and tin (hereinafter, these metals are
often referred to as "specific metallic elements") and alloys or
compounds containing one or more atoms of any of these metals. One
of such materials may be used alone, or two or more thereof may be
used in any desired combination and proportion.
[0164] Examples of the negative-electrode active material including
atoms of at least one member selected from the specific metallic
elements include: the elemental metal which is any one of the
specific metallic elements; alloys constituted of two or more
specific metallic elements; alloys constituted of one or more
specific metallic elements and one or more other metallic elements;
compounds containing one or more specific metallic elements; and
composite compounds, e.g., oxides, carbides, nitrides, silicides,
sulfides, or phosphides, of these compounds. By using any of these
elemental metals, alloys, and metal compounds as a
negative-electrode active material, a battery having a higher
capacity can be obtained.
[0165] Examples of the negative-electrode active material further
include compounds formed by the complicated bonding of any of those
composite compounds to one or more elemental metals or alloys or to
several elements, e.g., nonmetallic elements. Specifically, in the
case of silicon and tin, for example, use can be made of an alloy
of those elements with a metal which does not function as a
negative electrode. In the case of tin, for example, use may be
made of a complicated compound constituted of a combination of five
to six elements including tin, a metal which functions as a
negative electrode and is not silicon, a metal which does not
function as a negative electrode, and a nonmetallic element.
[0166] Preferred of those negative-electrode active materials are
the elemental metal which is any one of the specific metallic
elements, alloys of two or more of the specific metallic elements,
and oxides, carbides, nitrides, and other compounds of the specific
metallic elements. This is because these negative-electrode active
materials give a battery having a high capacity per unit mass.
Especially preferred are the elemental metal(s), alloys, oxides,
carbides, nitrides, and the like of silicon and/or tin from the
standpoints of capacity per unit mass and environmental burden.
[0167] The lithium-containing composite metal oxide material to be
used as a negative-electrode active material is not particularly
limited so long as the material is capable of occluding and
releasing lithium. However, from the standpoint of
high-current-density charge/discharge characteristics, materials
containing both titanium and lithium are preferred, and
lithium-containing composite metal oxide materials containing
titanium are more preferred. Even more preferred are composite
oxides of lithium and titanium (hereinafter abbreviated to
"lithium-titanium composite oxides"). Namely, use of a
lithium-titanium composite oxide having a spinel structure is
especially preferred because incorporation of this composite oxide
into a negative-electrode active material for
nonaqueous-electrolyte batteries is effective in considerably
reducing output resistance.
[0168] Also preferred are lithium-titanium composite oxides in
which the lithium or titanium has been replaced by one or more
other metallic elements, e.g., at least one element selected from
the group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn,
and Nb.
[0169] Such metal oxide preferably is a lithium-titanium composite
oxide represented by general formula (1) wherein
0.7.ltoreq.x.ltoreq.1.5, 1.5.ltoreq.y.ltoreq.2.3, and
0.ltoreq.z.ltoreq.1.6, because the structure thereof is stable
during lithium ion doping/undoping.
Li.sub.xTi.sub.yM.sub.zO.sub.4 (1)
[In general formula (1), M represents at least one element selected
from the group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu,
Zn, and Nb.]
[0170] Of the compositions represented by general formula (I),
structures represented by general formula (1) wherein
1.2.ltoreq.x.ltoreq.1.4,1.5.ltoreq.y.ltoreq.1.7, and z=0 (a)
0.9.ltoreq.x.ltoreq.1.1,1.9.ltoreq.y.ltoreq.2.1, and z=0 or (b)
0.7.ltoreq.x.ltoreq.0.9,2.1.ltoreq.y.ltoreq.2.3, and z=0 (c)
are especially preferred because these structures bring about a
satisfactory balance among battery performances.
[0171] Especially preferred representative compositions of those
compounds are: Li.sub.4/3Ti.sub.5/3O.sub.4 for (a),
Li.sub.1Ti.sub.2O.sub.4 for (b), and Li.sub.4/5Ti.sub.11/5O.sub.4
for (c). Preferred examples of the structure wherein z.noteq.0
include Li.sub.4/3Ti.sub.4/3Al.sub.1/3O.sub.4.
[0172] <Properties of Carbonaceous Material>
[0173] In the case where a carbonaceous material is used as a
negative-electrode active material, it is desirable that the
carbonaceous material should have the following properties.
[0174] (X-Ray Parameter)
[0175] The carbonaceous material preferably has a value of d
(interplanar spacing) for the lattice planes (002), as determined
by X-ray diffractometry in accordance with the method of the Japan
Society for Promotion of Scientific Research, of 0.335 nm or
larger. The value of d thereof is generally 0.360 nm or less,
preferably 0.350 nm or less, more preferably 0.345 nm or less. The
crystallite size (Lc) of the carbonaceous material, as determined
by X-ray diffractometry in accordance with the method of the Japan
Society for Promotion of Scientific Research, is preferably 1.0 nm
or larger, more preferably 1.5 nm or larger.
[0176] (Volume-Average Particle Diameter)
[0177] The volume-average particle diameter of the carbonaceous
material, in terms of volume-average particle diameter (median
diameter) as determined by the laser diffraction/scattering method,
is generally 1 .mu.m or more, preferably 3 .mu.m or more, more
preferably 5 .mu.m or more, especially preferably 7 .mu.m or more,
and is generally 100 .mu.m or less, preferably 50 .mu.m or less,
more preferably 40 .mu.m or less, even more preferably 30 .mu.m or
less, especially preferably 25 .mu.m or less.
[0178] When the volume-average particle diameter thereof is less
than the lower limit of that range, there are cases where
irreversible capacity increases, leading to a loss in initial
battery capacity. When the volume-average particle diameter thereof
exceeds the upper limit of that range, there are cases where such a
carbonaceous material is undesirable from the standpoint of battery
production because an uneven coating surface is apt to result when
an electrode is produced through coating fluid application.
[0179] Volume-average particle diameter is determined by dispersing
the carbon powder in a 0.2% by mass aqueous solution (about 10 mL)
of poly(oxyethylene (degree of polymerization, 20)) sorbitan
monolaurate as a surfactant and examining the dispersion with a
laser diffraction/scattering type particle size distribution
analyzer (LA-700, manufactured by HORIBA, Ltd.). The median
diameter determined through this measurement is defined as the
volume-average particle diameter of the carbonaceous material in
the invention.
[0180] (Raman R Value, Raman Half-Value Width)
[0181] The Raman R value of the carbonaceous material as determined
by the argon ion laser Raman spectroscopy is generally 0.01 or
higher, preferably 0.03 or higher, more preferably 0.1 or higher,
and is generally 1.5 or lower, preferably 1.2 or lower, more
preferably 1 or lower, especially preferably 0.5 or lower.
[0182] When the Raman R value thereof is within that range, it is
easy to avoid the trouble that the surface of the particles has too
high crystallinity and the number of intercalation sites into which
lithium comes with charge/discharge decreases, resulting in a
decrease in suitability for charge. In addition, it is possible to
prevent the trouble that when a coating fluid containing a
carbonaceous material is applied to a current collector and the
resultant coating is pressed to heighten the density of the
negative electrode, then the crystals are apt to orient in
directions parallel to the electrode plate and this leads to a
decrease in load characteristics. Furthermore, it is easy to avoid
the trouble that the surface of the particles has reduced
crystallinity and enhanced reactivity with the nonaqueous
electrolytic solution and this leads to a decrease in efficiency
and enhanced gas evolution.
[0183] The Raman half-value width around 1,580 cm.sup.-1 of the
carbonaceous material is not particularly limited. However, the
half-value width thereof is generally 10 cm.sup.-1 or more,
preferably 15 cm.sup.-1 or more, and is generally 100 cm.sup.-1 or
less, preferably 80 cm.sup.-1 or less, more preferably 60 cm.sup.-1
or less, especially preferably 40 cm.sup.-1 or less.
[0184] When the Raman half-value width thereof is within that
range, it is easy to avoid the trouble that the surface of the
particles has too high crystallinity and the number of
intercalation sites into which lithium comes with charge/discharge
decreases, resulting in a decrease in suitability for charge. In
addition, it is possible to prevent the trouble that when a coating
fluid containing a carbonaceous material is applied to a current
collector and the resultant coating is pressed to heighten the
density of the negative electrode, then the crystals are apt to
orient in directions parallel to the electrode plate and this leads
to a decrease in load characteristics. Furthermore, it is easy to
avoid the trouble that the surface of the particles has reduced
crystallinity and enhanced reactivity with the nonaqueous
electrolytic solution and this leads to a decrease in efficiency
and enhanced gas evolution.
[0185] The examination for a Raman spectrum is made with a Raman
spectrometer (Raman spectrometer manufactured by Japan
Spectroscopic Co., Ltd.). In the examination, a sample is charged
into a measuring cell by causing the sample to fall naturally into
the cell and the surface of the sample in the cell is irradiated
with argon ion laser light while rotating the cell in a plane
perpendicular to the laser light. The Raman spectrum obtained is
examined for the intensity I.sub.A of a peak PA around 1,580
cm.sup.-1 and the intensity I.sub.g of a peak PB around 1,360
cm.sup.-1. The ratio between these intensities
R(R.dbd.I.sub.B/I.sub.A) is calculated. The Raman R value
calculated through this examination is defined as the Raman R value
of the carbonaceous material in the invention. Furthermore, the
half-value width of the peak P.sub.A around 1,580 cm.sup.-1 in the
Raman spectrum obtained is measured, and this value is defined as
the Raman half-value width of the carbonaceous material in the
invention.
[0186] Conditions for the Raman spectroscopy are as follows.
[0187] Wavelength of argon ion laser: 514.5 nm
[0188] Laser power on sample: 15-25 mW
[0189] Resolution: 10-20 cm.sup.-1
[0190] Examination range: 1,100 cm.sup.-1 to 1,730 cm.sup.-1
[0191] Analysis for Raman R value and Raman half-value width:
background processing
[0192] Smoothing: simple average; convolution, 5 points
(4) BET Specific Surface Area
[0193] The BET specific surface area of the carbonaceous material,
in terms of the value of specific surface area as determined by the
BET method, is generally 0.1 m.sup.2g.sup.-1 or larger, preferably
0.7 m.sup.2g.sup.-1 or larger, more preferably 1.0 m.sup.2g.sup.-1
or larger, especially preferably 1.5 m.sup.2g.sup.-1 or larger, and
is generally 100 m.sup.2g.sup.-1 or smaller, preferably 25
m.sup.2g.sup.-1 or smaller, more preferably 15 m.sup.2g.sup.-1 or
smaller, especially preferably 10 m.sup.2g.sup.-1 or smaller.
[0194] When the BET specific surface area thereof is within that
range, this carbonaceous material, when used as a
negative-electrode material, readily accepts lithium during charge
and inhibits lithium deposition from occurring on the electrode
surface. Furthermore, when this carbonaceous material is used as a
negative-electrode material, the reactivity thereof with the
nonaqueous electrolytic solution is not so high and, hence, gas
evolution is slight. A preferred battery therefore is easy to
obtain.
[0195] The determination of specific surface area by the BET method
is made with a surface area meter (a fully automatic surface area
measuring apparatus manufactured by Ohkura Riken Co., Ltd.) by
preliminarily drying a sample at 350.degree. C. for 15 minutes in a
nitrogen stream and then measuring the specific surface area
thereof by the gas-flowing nitrogen adsorption BET one-point method
using a nitrogen/helium mixture gas precisely regulated so as to
have a nitrogen pressure of 0.3 relative to atmospheric pressure.
The specific surface area determined through this measurement is
defined as the BET specific surface area of the carbonaceous
material in the invention.
[0196] (Roundness)
[0197] When the carbonaceous material is examined for roundness as
an index to the degree of sphericity thereof, the roundness thereof
is preferably within the range shown below. Roundness is defined by
"Roundness=(length of periphery of equivalent circle having the
same area as projected particle shape)/(actual length of periphery
of projected particle shape)". When a particle has a roundness of
1, this particle theoretically is a true sphere.
[0198] The closer to 1 the roundness of carbonaceous-material
particles having a particle diameter in the range of 3-40 .mu.m,
the more the particles are desirable. The roundness of the
particles is desirably 0.1 or higher, preferably 0.5 or higher,
more preferably 0.8 or higher, even more preferably 0.85 or higher,
especially preferably 0.9 or higher. The higher the roundness, the
more the high-current-density charge/discharge characteristics are
improved. Consequently, when carbonaceous-material particles have a
roundness within that range, the negative-electrode active material
has improved suitability for loading and retains low interparticle
resistance. Consequently, short-time high-current-density
charge/discharge characteristics are less apt to decrease.
[0199] Roundness is determined with a flow type particle image
analyzer (FPIA, manufactured by Sysmex Industrial Corp.). About 0.2
g of a sample is dispersed in a 0.2% by mass aqueous solution
(about 50 mL) of poly(oxyethylene(20)) sorbitan monolaurate as a
surfactant, and an ultrasonic wave of 28 kHz is propagated to the
dispersion for 1 minute at an output of 60 W. Thereafter, particles
having a particle diameter in the range of 3-40 .mu.m are examined
with the analyzer having a detection range set at 0.6-400 .mu.m.
The roundness determined through this measurement is defined as the
roundness of the carbonaceous material in the invention.
[0200] Methods for improving roundness are not particularly
limited. However, a carbonaceous material in which the particles
have been rounded by a rounding treatment is preferred because this
material gives an electrode in which the interstices among
particles are uniform in shape. Examples of the rounding treatment
include: a method in which shear force or compressive force is
applied to thereby mechanically make the shape of the particles
close to sphere; and a method of mechanical/physical treatment in
which fine particles are aggregated into particles by means of the
bonding force of a binder or of the fine particles themselves.
[0201] (Tap Density)
[0202] The tap density of the carbonaceous material is generally
0.1 gcm.sup.-3 or higher, preferably 0.5 gcm.sup.-3 or higher, more
preferably 0.7 gcm.sup.-3 or higher, especially preferably 1
gcm.sup.-3 or higher, and is preferably 2.2 gcm.sup.-3 or less,
more preferably 2.1 gcm.sup.-3 or less, especially preferably 2.0
gcm.sup.-3 or less. In the case where the tap density thereof is
within that range, this carbonaceous material, when used in a
negative electrode, can attain an increase in loading density to
render a high-capacity battery easy to obtain. Furthermore, the
amount of interparticle interstices in the electrode is not
excessively small and, hence, electrical conductivity among the
particles is ensured. Thus, it is easy to obtain preferred battery
characteristics.
[0203] Tap density is determined by dropping a sample through a
sieve having an opening size of 300 .mu.m into a 20-cm.sup.3
tapping cell to fill the cell with the sample up to the brim,
subsequently conducting a tapping operation 1,000 times over a
stroke length of 10 mm using a powder densimeter (e.g., Tap Denser,
manufactured by Seishin Enterprise Co., Ltd.), and calculating the
tap density from the resultant volume of the sample and the weight
thereof. The tap density calculated through this measurement is
defined as the tap density of the carbonaceous material in the
invention.
[0204] (Orientation Ratio)
[0205] The orientation ratio of the carbonaceous material is
generally 0.005 or greater, preferably 0.01 or greater, more
preferably 0.015 or greater, and is generally 0.67 or less. When
the orientation ratio thereof is within that range, it is easy to
avoid the trouble that high-density charge/discharge
characteristics decrease. The upper limit of that range is a
theoretical upper limit of the orientation ratio of carbonaceous
materials.
[0206] Orientation ratio is determined by X-ray diffractometry
after a sample is molded by compaction. A molded object obtained by
packing 0.47 g of a sample into a molding machine having a diameter
of 17 mm and compacting the sample at 58.8 MNm.sup.-2 is set with
clay on a sample holder for examination so as to be flush with the
holder. This sample is examined for X-ray diffraction. From the
intensities of the resultant (110) diffraction peak and (004)
diffraction peak for the carbon, the ratio represented by (110)
diffraction peak intensity/(004) diffraction peak intensity is
calculated. The orientation ratio calculated through this
measurement is defined as the orientation ratio of the carbonaceous
material in the invention.
[0207] Conditions for the X-ray diffractometry are as follows.
Incidentally, "20" represents diffraction angle.
[0208] Target: Cu (K.alpha. line) graphite monochromator
[0209] Slits: [0210] Divergence slit=0.5 degrees [0211] Receiving
slit=0.15 mm [0212] Scattering slit=0.5 degrees
[0213] Examination range and step angle/measuring time:
TABLE-US-00001 (110) plane: 75.degree. .ltoreq. 2.theta. .ltoreq.
80.degree. 1.degree./60 sec (004) plane: 52.degree. .ltoreq.
2.theta. .ltoreq. 57.degree. 1.degree./60 sec
[0214] (Aspect Ratio (Powder))
[0215] The aspect ratio of the carbonaceous material is generally 1
or greater, and is generally 10 or less, preferably 8 or less, more
preferably 5 or less. When the aspect ratio thereof is within that
range, it is easy to avoid the trouble that the carbonaceous
material causes streak lines in electrode plate formation and an
even coating surface cannot be obtained, resulting in a decrease in
high-current-density charge/discharge characteristics.
Incidentally, the lower limit of that range is a theoretical lower
limit of the aspect ratio of carbonaceous materials.
[0216] In determining aspect ratio, particles of the carbonaceous
material are examined with a scanning electron microscope with
enlargement. Fifty are arbitrarily selected from graphite particles
fixed to an edge face of a metal having a thickness of 50 .mu.m or
smaller, and each particle is examined in a three-dimensional
manner while rotating and inclining the stage to which the sample
is fixed. In this examination, the length of the longest axis A of
each carbonaceous-material particle and the length of the shortest
axis B perpendicular to that axis are measured, and the average of
the A/B values is determined. The aspect ratio (A/B) determined
through this measurement is defined as the aspect ratio of the
carbonaceous material in the invention.
<Configuration of Negative Electrode and Method of Production
thereof.
[0217] Any known method can be used for electrode production unless
this considerably lessens the effects of the invention. For
example, a binder and a solvent are added to a negative-electrode
active material optionally together with a thickener, conductive
material, filler, etc. to obtain a slurry and this slurry is
applied to a current collector and dried. Thereafter, the coated
current collector is pressed. Thus, an electrode can be formed.
[0218] In the case where an alloy material is employed, use may be
made of a method in which a thin-film layer containing the
negative-electrode active material described above
(negative-electrode active-material layer) is formed by a technique
such as vapor deposition, sputtering, or plating.
[0219] (Current Collector)
[0220] As the current collector for holding the negative-electrode
active material, a known current collector can be used at will.
Examples of the current collector for the negative electrode
include metallic materials such as copper, nickel, stainless steel,
and nickel-plated steel. Copper is especially preferred from the
standpoints of processability and cost.
[0221] In the case where the current collector is a metallic
material, examples of the shape of the current collector include
metal foils, metal cylinders, metal coils, metal plates, thin metal
films, expanded metals, punching metals, and metal foam. Preferred
of these are thin metal films. More preferred are copper foils.
Even more preferred are a rolled copper foil, which is produced by
the rolling process, and an electrolytic copper foil, which is
produced by the electrolytic process. Either of these can be used
as a current collector.
[0222] The thickness of the current collector is generally 1 .mu.m
or more, preferably 5 .mu.m or more, and is generally 100 .mu.M or
less, preferably 50 .mu.m or less. When the thickness of the
negative-electrode current collector is within that range, the
current collector does not considerably reduce the capacity of the
whole battery and is easy to handle.
[0223] (Thickness Ratio Between Current Collector and
Negative-Electrode Active-Material Layer)
[0224] The thickness ratio between the current collector and the
negative-electrode active-material layer is not particularly
limited. However, the value of "(thickness of the
negative-electrode active-material layer on one surface just before
impregnation with nonaqueous electrolytic solution)/(thickness of
the current collector)" is preferably 150 or less, more preferably
20 or less, especially preferably 10 or less, and is preferably 0.1
or greater, more preferably 0.4 or greater, especially preferably 1
or greater. When the thickness ratio between the current collector
and the negative-electrode active-material layer is within that
range, this current collector is less apt to be heated up by
Joule's heat during high-current-density charge/discharge.
Furthermore, it is easy to avoid the trouble that the proportion by
volume of the current collector to the negative-electrode active
material increases to reduce the capacity of the battery.
[0225] (Binder)
[0226] The binder for binding the negative-electrode active
material is not particularly limited so long as it is stable to the
nonaqueous electrolytic solution and to the solvent to be used for
electrode production.
[0227] Examples thereof include resinous polymers such as
polyethylene, polypropylene, poly(ethylene terephthalate),
poly(methyl methacrylate), aromatic polyamides, cellulose, and
nitrocellulose; rubbery polymers such as SBR (styrene/butadiene
rubbers), isoprene rubbers, butadiene rubbers, fluororubbers, NBR
(acrylonitrile/butadiene rubbers), and ethylene/propylene rubbers;
styrene/butadiene/styrene block copolymers or products of
hydrogenation thereof; thermoplastic elastomeric polymers such as
EPDM (ethylene/propylene/diene terpolymers),
styrene/ethylene/butadiene/styrene copolymers, and
styrene/isoprene/styrene block copolymers or products of
hydrogenation thereof; flexible resinous polymers such as
syndiotactic 1,2-polybutadiene, poly(vinyl acetate), ethylene/vinyl
acetate copolymers, and propylene/.alpha.-olefin copolymers;
fluorochemical polymers such as poly(vinylidene fluoride),
polytetrafluoroethylene, fluorinated poly(vinylidene fluoride), and
polytetrafluoroethylene/ethylene copolymers; and polymer
compositions having the property of conducting alkali metal ions
(especially lithium ions). One of these binders may be used alone,
or two or more thereof may be used in any desired combination and
proportion.
[0228] The proportion of the binder to the negative-electrode
active material is preferably 0.1% by mass or higher, more
preferably 0.5% by mass or higher, especially preferably 0.6% by
mass or higher, and is preferably 20% by mass or lower, more
preferably 15% by mass or lower, even more preferably 10% by mass
or lower, especially preferably 8% by mass or lower. When the
proportion of the binder to the negative-electrode active material
is within that range, it is easy to avoid the trouble that the
proportion of the binder which does not contribute to battery
capacity increases and this leads to a decrease in battery
capacity. Furthermore, the negative electrode is inhibited from
having a reduced strength.
[0229] Especially when the binder includes a rubbery polymer
represented by SBR as the main component, the proportion of this
binder to the negative-electrode active material is generally 0.1%
by mass or higher, preferably 0.5% by mass or higher, more
preferably 0.6% by mass or higher, and is generally 5% by mass or
lower, preferably 3% by mass or lower, more preferably 2% by mass
or lower. In the case where the binder includes a fluorochemical
polymer represented by poly(vinylidene fluoride) as the main
component, the proportion of this binder to the negative-electrode
active material is generally 1% by mass or higher, preferably 2% by
mass or higher, more preferably 3% by mass or higher, and is
generally 15% by mass or lower, preferably 10% by mass or lower,
more preferably 8% by mass or lower.
[0230] (Solvent for Slurry Formation)
[0231] The kind of the solvent to be used for forming a slurry is
not particularly limited so long as the negative-electrode active
material and binder and the thickener and conductive material which
are optionally used according to need can be dissolved or dispersed
therein. Either an aqueous solvent or an organic solvent may be
used.
[0232] Examples of the aqueous solvent include water and alcohols.
Examples of the organic solvent include N-methylpyrrolidone (NMP),
dimethylformamide, dimethylacetamide, methyl ethyl ketone,
cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine,
N,N-dimethylaminopropylamine, tetrahydrofuran (THF), toluene,
acetone, diethyl ether, dimethylacetamide, hexamethylphosphoramide,
dimethyl sulfoxide, benzene, xylene, quinoline, pyridine,
methylnaphthalene, and hexane.
[0233] Especially when an aqueous solvent is used, it is preferred
to add a dispersant or the like in combination with a thickener and
prepare a slurry using a latex of, for example, SBR. One of those
solvents may be used alone, or two or more thereof may be used in
any desired combination and proportion.
[0234] (Thickener)
[0235] A thickener is used generally for the purpose of regulating
the slurry viscosity. The thickener is not particularly limited.
Examples thereof include carboxymethyl cellulose, methyl cellulose,
hydroxymethyl cellulose, ethyl cellulose, poly(vinyl alcohol),
oxidized starch, phosphorylated starch, casein, and salts of these.
One of these thickeners may be used alone, or two or more thereof
may be used in any desired combination and proportion.
[0236] In the case where such a thickener is further added, the
proportion of the thickener to the negative-electrode active
material is generally 0.1% by mass or higher, preferably 0.5% by
mass or higher, more preferably 0.6% by mass or higher, and is
generally 5% by mass or lower, preferably 3% by mass or lower, more
preferably 2% by mass or lower.
[0237] When the proportion of the thickener to the
negative-electrode active material is lower than the lower limit of
that range, there are cases where applicability decreases
considerably. When the proportion thereof is within that range, the
proportion of the negative-electrode active material in the
negative-electrode active-material layer is appropriate, and it is
easy to avoid the problem that battery capacity decreases and the
trouble that resistance among the particles of the
negative-electrode active material increases.
[0238] (Electrode Density)
[0239] When the negative-electrode active material is used to form
an electrode, the electrode structure is not particularly limited.
However, the density of the negative-electrode active material
present on the current collector is preferably 1 gcm.sup.-3 or
higher, more preferably 1.2 gcm.sup.-3 or higher, especially
preferably 1.3 gcm.sup.-3 or higher, and is preferably 2 gcm.sup.-3
or less, more preferably 1.9 gcm.sup.-3 or less, even more
preferably 1.8 gcm.sup.-3 or less, especially preferably 1.7
gcm.sup.-3 or less. When the density of the negative-electrode
active material present on the current collector is within that
range, it is easy to avoid the trouble that the negative-electrode
active-material particles are broken and this increases the initial
irreversible capacity and reduces the infiltration of a nonaqueous
electrolytic solution into around the current
collector/negative-electrode active material interface, resulting
in a deterioration in high-current-density charge/discharge
characteristics. Furthermore, it is also possible to avoid the
trouble that electrical conductivity among the negative-electrode
active-material particles decreases and this increases battery
resistance, resulting in a decrease in capacity per unit
volume.
(Thickness of Negative-Electrode Plate)
[0240] The thickness of the negative-electrode plate is designed so
as to be suited for the positive-electrode plate to be used, and is
not particularly limited. However, it is desirable that the
thickness of the mix layer, i.e., the thickness of the
negative-electrode plate excluding the metal foil serving as a
core, should be generally 15 .mu.m or more, preferably 20 .mu.m or
more, more preferably 30 .mu.m or more, and be generally 150 .mu.m
or less, preferably 120 .mu.m or less, more preferably 100 .mu.m or
less.
[0241] [Positive Electrode]
[0242] The positive electrode to be used in each of the lithium
secondary batteries of the invention is explained below.
[0243] [Positive-Electrode Active Material]
[0244] The positive-electrode active material to be used in the
positive electrode is described below.
[0245] (Composition)
[0246] The nonaqueous-electrolyte batteries of the invention are
equipped with a positive-electrode active material including, as a
basic composition, a lithium-containing phosphoric acid compound
represented by LixMPO.sub.4 (wherein M is at least one element
selected from the group consisting of Group-2 to Group-12 metals of
the periodic table, and x satisfies 0<x.ltoreq.1.2).
[0247] The lithium-containing phosphoric acid compound preferably
is a compound represented by LixMPO.sub.4 (wherein M is at least
one element selected from the group consisting of the Group-4 to
Group-11 transition metals in the fourth period of the periodic
table, and x satisfies 0<x.ltoreq.1.2).
[0248] It is preferred that M in the formula LixMPO.sub.4 should be
at least one element selected from the group consisting of Mg, Zn,
Ca, Cd, Sr, Ba, Co, Ni, Fe, Mn, and Cu. It is more preferred that M
should be at least one element selected from the group consisting
of Co, Ni, Fe, and Mn. Of these phosphoric acid compounds, iron
lithium phosphate of an olivine structure having the basic
composition LiFePO.sub.4 is especially suitable because this
compound is less apt to suffer metal dissolution when in a
high-temperature charged state and is inexpensive.
[0249] The expression "including LixMPO.sub.4 as a basic
composition" used above means that not only compounds having a
composition represented by the empirical formula but also compounds
in which the Fe or other sites in the crystal structure have been
partly replaced by another element are included. Furthermore, that
expression means that not only compounds having the stoichiometric
composition but also compounds having non-stoichiometric
compositions which include, for example, sites where part of the
elements is deficient are included. It is preferred that the
element which replaces should be an element such as Al, Ti, V, Cr,
Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, or Si. In the case where
replacement by such an element is conducted, the degree of
replacement is preferably 0.1-5 mol %, more preferably 0.2-2.5 mol
%.
[0250] Although the positive-electrode active material includes the
LixMPO.sub.4 as a main component, it is possible to use this
compound in combination with a lithium-transition metal composite
oxide such as, for example, a lithium-manganese composite oxide,
lithium-cobalt composite oxide, lithium-nickel composite oxide,
lithium-nickel-cobalt composite oxide, lithium-nickel-manganese
composite oxide, or lithium-nickel-manganese-cobalt composite
oxide. It is preferred that the positive-electrode active material
includes the LixMPO.sub.4 in an amount of 20 wt % or more. In this
case, the charge/discharge cycle characteristics of the
nonaqueous-electrolyte battery can be further improved. It is more
preferred that the content of the LixMPO.sub.4 should be 40 wt % or
higher.
[0251] It is also possible to use two or more compounds represented
by LixMPO.sub.4 in combination. Preferred examples of such
combinations include: LixFePO.sub.4 and LixMnPO.sub.4;
LixFePO.sub.4 and LixCoPO.sub.4; and LixFePO.sub.4 and
LixNiPO.sub.4. Use of such a combination can improve battery
operating voltage while maintaining safety. An especially preferred
combination among these is a combination of LixFePO.sub.4 and
LixMnPO.sub.4, because this combination brings about excellent
durability, such as high-temperature storability and cycle
characteristics, besides the improvements in safety and battery
operating voltage.
[0252] (Surface Coating)
[0253] As a positive-electrode active material, use may be made of
a material composed of LixMPO.sub.4 and, adherent to the surface
thereof, a substance having a composition different therefrom.
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, carbonates such as lithium carbonate, calcium carbonate,
and magnesium carbonate, and carbon.
[0254] Those surface-adherent substances each can be adhered to the
surface of the positive-electrode active material, for example, by:
a method in which the substance is dissolved or suspended in a
solvent and this solution or suspension is infiltrated into the
positive-electrode active material and then dried; a method in
which a precursor for the surface-adherent substance is dissolved
or suspended in a solvent and this solution or suspension is
infiltrated into the positive-electrode active material and then
heated or otherwise treated to react the precursor; or a method in
which the substance is added to a precursor for the
positive-electrode active material and heat-treated together with
the precursor. In the case where carbon is to be adhered, use may
be made of a method in which a carbonaceous substance is
mechanically adhered later in the form of, for example, activated
carbon or the like.
[0255] With respect to the amount of the surface-adherent substance
to be used, the lower limit of the amount thereof, in terms of mass
ppm of the positive-electrode active material, is preferably 0.1
ppm or more, more preferably 1 ppm or more, even more preferably 10
ppm or more. The upper limit thereof is preferably 20% or less,
more preferably 10% or less, even more preferably 5% or less, in
terms of mass % based on the positive-electrode active material.
The surface-adherent substance can inhibit the electrolytic
solution from undergoing an oxidation reaction on the surface of
the positive-electrode active material, and an improvement in
battery life can hence be attained. This effect is enhanced when
the substance has been adhered in an appropriate amount.
[0256] In the invention, a material composed of a
positive-electrode active material made of LixMPO.sub.4 and,
adherent to the surface thereof, a substance having a composition
different from the composition of the active material is also
referred to as "positive-electrode active material".
[0257] (Shape)
[0258] The shape of the particles of the positive-electrode active
material in the invention may be any of massive, polyhedral,
spherical, ellipsoidal, platy, acicular, columnar, and other shapes
such as those in common use. Preferred of these is one in which the
primary particles have aggregated to form secondary particles and
these secondary particles have a spherical or ellipsoidal shape. In
electrochemical elements, the active material in each electrode
usually expands/contracts with the charge/discharge of the element
and, hence, a deterioration, such as active-material breakage or
conduction path breakage, that is caused by the resultant stress is
apt to occur. Consequently, a positive-electrode active material in
which the primary particles have aggregated to form secondary
particles is preferable to an active material composed of primary
particles only since the particles in the former active material
relieve the stress caused by expansion/contraction to prevent the
deterioration. Furthermore, particles of a spherical or ellipsoidal
shape are preferable to particles showing axial orientation, e.g.,
platy particles, because the former particles are less apt to
orient during electrode forming and hence this electrode is reduced
in expansion/contraction during charge/discharge, and because these
particles are apt to be evenly mixed with a conductive material in
electrode production.
[0259] (Tap Density)
[0260] The tap density of the positive-electrode active material is
preferably 0.1 g/cm.sup.3 or higher, more preferably 0.2 g/cm.sup.3
or higher, even more preferably 0.3 g/cm.sup.3 or higher, most
preferably 0.4 g/cm.sup.3 or higher. In case where the tap density
of the positive-electrode active material is lower than the lower
limit of that range, not only it is necessary to use a larger
amount of a dispersion medium and larger amounts of a conductive
material and a binder in forming a positive-electrode
active-material layer, but also there are cases where the loading
of the positive-electrode active material in the positive-electrode
active-material layer is limited, resulting in a limited battery
capacity. By using a composite-oxide power having a high tap
density, a positive-electrode active-material layer having a high
density can be formed. The higher the tap density, the more the
positive-electrode active material is generally preferred. There is
no particular upper limit on the tap density. However, when the tap
density thereof is too high, there are cases where the diffusion of
lithium ions in the positive-electrode active-material layer
through the electrolytic solution as a medium becomes a
rate-determining stage and this is apt to reduce load
characteristics. Consequently, the upper limit thereof is
preferably 2.0 g/cm.sup.3 or lower, more preferably 1.8 g/cm.sup.3
or lower.
[0261] In the invention, the tap density of a positive-electrode
active-material powder is determined by placing 5-10 g of the
powder in a 10-mL measuring cylinder made of glass, conducting a
tapping operation 200 times over a stroke of about 20 mm, and
determining the density of the thus-densified powder (tap density)
in terms of g/cc.
[0262] (Median Diameter d.sub.50)
[0263] The median diameter d.sub.50 (secondary-particle diameter in
the case where the primary particles have aggregated to form
secondary particles) of the particles of the positive-electrode
active material is preferably 0.1 .mu.m or more, more preferably
0.2 .mu.m or more, even more preferably 0.3 .mu.m or more, most
preferably 2 .mu.m or more. The upper limit thereof is preferably
20 .mu.m or less, more preferably 18 .mu.m or less, even more
preferably 16 .mu.m or less, most preferably 15 .mu.m or less. When
the median diameter d.sub.50 thereof is less than the lower limit,
there are cases where a product having a high tap density cannot be
obtained. In case where the median diameter thereof exceeds the
upper limit, lithium diffusion within individual particles requires
a longer period and this results in a decrease in battery
performance. In addition, there are cases where when such
positive-electrode active-material particles are used in producing
a positive electrode for batteries, i.e., when the active material
and other ingredients including a conductive material and a binder
are slurried with a solvent and this slurry is applied in a
thin-film form, then the active material poses a problem, for
example, that streak lines generate. It is possible to further
improve loading during positive-electrode production by mixing two
or more positive-electrode active materials differing in median
diameter d.sub.50.
[0264] Median diameter d.sub.50 in the invention is determined with
a known laser diffraction/scattering type particle size
distribution analyzer. In the case where LA-920, manufactured by
HORIBA Ltd., is used as a particle size distribution analyzer, a
0.1% by mass aqueous solution of sodium hexametaphosphate is used
as a dispersion medium for measurement to conduct a 5-minute
ultrasonic dispersing treatment, before the particles are examined
at a measuring refractive index set at 1.24.
[0265] (Average Primary-Particle Diameter)
[0266] In the case where the primary particles have aggregated to
form secondary particles, the average primary-particle diameter of
this positive-electrode active material is preferably 0.02 .mu.m or
more, more preferably 0.03 .mu.m or more, even more preferably 0.05
.mu.m or more. The upper limit thereof is preferably 2 .mu.m or
less, more preferably 1.6 .mu.m or less, even more preferably 1.3
.mu.m or less, most preferably 1 .mu.m or less. When the average
primary-particle diameter thereof is within that range, spherical
secondary particles are apt to be formed. It is hence easy to avoid
the trouble that the shape of secondary particles adversely affects
powder loading or results in a considerably reduced specific
surface area, resulting in a higher possibility that battery
performance, such as output characteristics, might decrease.
Furthermore, since crystal growth is insufficient, this
positive-electrode active material is less apt to pose problems
such as, for example, poor charge/discharge reversibility.
[0267] Average primary-particle diameter is determined through an
examination with a scanning electron microscope (SEM).
Specifically, arbitrarily selected 50 primary-particle images in a
photograph having a magnification of 10,000 diameters each are
examined for the length of the longest segment of a horizontal line
which extends across the primary-particle image from one side to
the other side of the boundary. These measured lengths are averaged
to determine the average value.
[0268] (BET Specific Surface Area)
[0269] The BET specific surface area of the positive-electrode
active material to be used in the secondary batteries of the
invention is preferably 0.4 m.sup.2/g or larger, more preferably
0.5 m.sup.2/g or larger, even more preferably 0.6 m.sup.2/g or
larger. The upper limit thereof may be 50 m.sup.2/g or smaller,
preferably 40 m.sup.2/g or smaller, even more preferably 30
m.sup.2/g or smaller. When the BET specific surface area thereof is
within that range, battery performance can be inhibited from
decreasing and satisfactory applicability is obtained when a
positive-electrode active-material layer is formed.
[0270] BET specific surface area is measured with a surface area
meter (a fully automatic surface area measuring apparatus
manufactured by Ohkura Riken Co., Ltd.) in the following manner. A
sample is preliminarily dried at 150.degree. C. for 30 minutes in a
nitrogen stream, and the specific surface area thereof is
thereafter determined by the gas-flowing nitrogen adsorption BET
one-point method using a nitrogen/helium mixture gas precisely
regulated so as to have a nitrogen pressure of 0.3 relative to
atmosphere pressure. The value determined through this measurement
is defined as the BET specific surface area of the
positive-electrode active material.
[0271] (Processes for Production)
[0272] For producing the positive-electrode active material,
techniques which are in general use as processes for producing
inorganic compounds may be used. Especially for producing a
spherical or ellipsoidal active material, various techniques may be
used. Examples thereof include a method which includes dissolving
or pulverizing/dispersing a phosphorus source, e.g., phosphoric
acid, and a source of the M as a component of LixMPO.sub.4 in a
solvent, e.g., water, regulating the pH of the solution or
dispersion with stirring to produce a spherical precursor,
recovering and optionally drying the precursor, subsequently adding
thereto a lithium source, e.g., LiOH, Li.sub.2CO.sub.3, or
LiNO.sub.3, and burning the mixture at a high temperature to obtain
the active material.
[0273] For producing the positive electrode to be used in the
invention, one positive-electrode active material represented by
LixMPO.sub.4 and/or one positive-electrode active material
LixMPO.sub.4 coated with the surface-adherent substance may be used
alone, or may be used together with one or more such materials
differing in composition in any desired combination or proportion.
Here, the proportion of the positive-electrode active material
LixMPO.sub.4 and/or the positive-electrode active material
LixMPO.sub.4 coated with the surface-adherent substance is
preferably 30% by mass or higher, more preferably 50% by mass or
higher, based on all positive-electrode active materials. When the
proportion of the positive-electrode active material LixMPO.sub.4
and/or the positive-electrode active material LixMPO.sub.4 coated
with the surface-adherent substance is within that range, a
preferred battery capacity can be provided.
[0274] Incidentally, "the positive-electrode active material
LixMPO.sub.4 and/or the positive-electrode active material
LixMPO.sub.4 coated with the surface-adherent substance" and
"positive-electrode active materials other than the
positive-electrode active material LixMPO.sub.4 and/or the
positive-electrode active material LixMPO.sub.4 coated with the
surface-adherent substance" are inclusively referred to as
"positive-electrode active material".
[Configuration of Positive Electrode]
[0275] The configuration of the positive electrode to be used in
the invention is described below.
[0276] (Electrode Structure and Production Process)
[0277] The positive electrode to be used in the lithium secondary
batteries of the invention is produced by forming a
positive-electrode active-material layer including a
positive-electrode active material and a binder on a current
collector. Namely, the positive electrode for the lithium secondary
batteries of the invention is produced by forming a
positive-electrode active-material layer including the
positive-electrode active material and a binder on a current
collector. The production of the positive electrode using a
positive-electrode active material can be conducted in an ordinary
manner. Namely, a positive-electrode active material and a binder
are mixed together by a dry process optionally together with a
conductive material, thickener, etc. and this mixture is formed
into a sheet and press-bonded to a positive-electrode current
collector. Alternatively, those materials are dissolved or
dispersed in a liquid medium to obtain a slurry and this slurry is
applied to a positive-electrode current collector and dried. Thus,
a positive-electrode active-material layer is formed on the current
collector, and the positive electrode can be thereby obtained.
[0278] In the positive-electrode active-material layer, the content
of the positive-electrode active material for use in the positive
electrodes of the lithium secondary batteries of the invention is
preferably 80% by mass or higher, more preferably 82% by mass or
higher, especially preferably 84% by mass or higher. The upper
limit thereof is preferably 97% by mass or lower, more preferably
95% by mass or lower. When the content of the positive-electrode
active material in the positive-electrode active-material layer is
within that range, an excellent balance between electrical capacity
and the strength of the positive electrode is obtained.
[0279] It is preferred that the positive-electrode active-material
layer obtained by coating fluid application and drying should be
pressed and densified with a handpress, roller press, or the like
in order to heighten the loading density of the positive-electrode
active material. The lower limit of the loading density of the
positive-electrode active-material layer is preferably 1.3
g/cm.sup.3 or higher, more preferably 1.4 g/cm.sup.3 or higher,
even more preferably 1.5 g/cm.sup.3 or higher. The upper limit
thereof is preferably 3.0 g/cm.sup.3 or less, more preferably 2.5
g/cm.sup.3 or less, even more preferably 2.3 g/cm.sup.3 or
less.
[0280] When the density of the positive-electrode active-material
layer is within that range, it is easy to avoid the trouble that an
electrolytic solution shows insufficient infiltration into around
the current collector/active material interface and
charge/discharge characteristics especially at a high current
density decrease, making it impossible to obtain high output, and
the trouble that electrical conductivity among the active-material
particles decreases to increase battery resistance, making it
impossible to obtain high output.
[0281] (Conductive Material)
[0282] As the conductive material, a known conductive material can
be used at will. Examples thereof include metallic materials such
as copper and nickel; graphites such as natural graphites and
artificial graphites; carbon blacks such as acetylene black; and
carbon materials such as amorphous carbon, e.g., needle coke. One
of these materials may be used alone, or two or more thereof may be
used in any desired combination and proportion. The conductive
material may be used so that the material is incorporated in the
positive-electrode active-material layer in an amount of generally
0.01% by mass or more, preferably 0.1% by mass or more, more
preferably 1% by mass or more, the upper limit thereof being
generally 50% by mass or less, preferably 30% by mass or less, more
preferably 15% by mass or less. When the content thereof is within
that range, electrical conductivity can be sufficiently ensured and
a preferred battery capacity can be provided.
[0283] (Binder)
[0284] The binder to be used for producing the positive-electrode
active-material layer is not particularly limited. In the case
where the layer is to be formed through coating fluid application,
any binder may be used so long as it is a material which is soluble
or dispersible in the liquid medium for use in electrode
production. Examples thereof include resinous polymers such as
polyethylene, polypropylene, poly(ethylene terephthalate),
poly(methyl methacrylate), polyimides, aromatic polyamides,
cellulose, and nitrocellulose; rubbery polymers such as SBR
(styrene/butadiene rubbers), NBR (acrylonitrile/butadiene rubbers),
fluororubbers, isoprene rubbers, butadiene rubbers, and
ethylene/propylene rubbers; thermoplastic elastomeric polymers such
as styrene/butadiene/styrene block copolymers or products of
hydrogenation thereof, EPDM (ethylene/propylene/diene terpolymers),
styrene/ethylene/butadiene/ethylene copolymers, and
styrene/isoprene/styrene block copolymers or products of
hydrogenation thereof; flexible resinous polymers such as
syndiotactic 1,2-polybutadiene, poly(vinyl acetate), ethylene/vinyl
acetate copolymers, and propylene/.alpha.-olefin copolymers;
fluorochemical polymers such as poly(vinylidene fluoride) (PVdF),
polytetrafluoroethylene, fluorinated poly(vinylidene fluoride), and
polytetrafluoroethylene/ethylene copolymers; and polymer
compositions having the property of conducting alkali metal ions
(especially lithium ions). One of these substances may be used
alone, or two or more thereof may be used in any desired
combination and proportion.
[0285] The proportion of the binder in the positive-electrode
active-material layer is generally 0.1% by mass or higher,
preferably 1% by mass or higher, more preferably 3% by mass or
higher. The upper limit thereof is generally 80% by mass or lower,
preferably 60% by mass or lower, more preferably 40% by mass or
lower, most preferably 10% by mass or lower. When the proportion of
the binder is within that range, the positive-electrode active
material can be sufficiently held and, hence, a suitable mechanical
strength of the positive electrode can be provided, making it
possible to provide battery performance including excellent cycle
characteristics, without causing a decrease in battery capacity or
electrical conductivity.
[0286] (Liquid Medium)
[0287] The kind of the liquid medium to be used for forming a
slurry is not particularly limited so long as the liquid medium is
a solvent in which the positive-electrode active material,
conductive material, and binder and a thickener, which is used
according to need, can be dissolved or dispersed. Either an aqueous
solvent or an organic solvent may be used. Examples of the aqueous
medium include water and mixed solvents composed of an alcohol and
water. Examples of the organic medium include aliphatic
hydrocarbons such as hexane; aromatic hydrocarbons such as benzene,
toluene, xylene, and methylnaphthalene; heterocyclic compounds such
as quinoline and pyridine; ketones such as acetone, methyl ethyl
ketone, and cyclohexanone; esters such as methyl acetate and methyl
acrylate; amines such as diethylenetriamine and
N,N-dimethylaminopropylamine; ethers such as diethyl ether,
propylene oxide, and tetrahydrofuran (THF); amides such as
N-methylpyrrolidone (NMP), dimethylformamide, and
dimethylacetamide; and aprotic polar solvents such as
hexamethylphosphoramide and dimethyl sulfoxide.
[0288] Especially when an aqueous medium is used, it is preferred
to use a thickener and a latex of, for example, a styrene/butadiene
rubber (SBR) to prepare a slurry. A thickener is used generally for
the purpose of regulating the viscosity of the slurry. The
thickener is not particularly limited, and examples thereof include
carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose,
ethyl cellulose, poly(vinyl alcohol), oxidized starch,
phosphorylated starch, casein, and salts of these. One of these
thickeners may be used alone, or two or more thereof may be used in
any desired combination and proportion. In the case where such a
thickener is further added, the proportion of the thickener to the
active material may be 0.1% by mass or higher, preferably 0.5% by
mass or higher, more preferably 0.6% by mass or higher, and the
upper limit thereof may be 5% by mass or lower, preferably 3% by
mass or lower, more preferably 2% by mass or lower. When the
proportion thereof is within that range, satisfactory applicability
is obtained and the proportion of the active material in the
positive-electrode active-material layer is not excessively low. It
is therefore easy to avoid the problem that battery capacity
decreases and the trouble that resistance among particles of the
positive-electrode active material increases.
[0289] (Current Collector)
[0290] The material of the positive-electrode current collector is
not particularly limited, and a known one can be used at will.
Examples thereof include metallic materials such as aluminum,
stainless steel, nickel-plated materials, titanium, and tantalum;
and carbon materials such as carbon cloths and carbon papers. Of
these, metallic materials are preferred. Especially preferred is
aluminum.
[0291] In the case of a metallic material, examples of the shape of
the current collector include metal foils, metal cylinders, metal
coils, metal plates, thin metal films, expanded metals, punching
metals, and metal foam. In the case of a carbon material, examples
of the collector shape include carbon plates, thin carbon films,
and carbon cylinders. Of these, a thin metal film is preferred. The
thin film may be in a suitable mesh form. Although the thin film
may have any desired thickness, the thickness thereof is generally
1 .mu.m or more, preferably 3 .mu.m or more, more preferably 5
.mu.m or more. The upper limit thereof is generally 1 mm or less,
preferably 100 .mu.m or less, more preferably 50 .mu.m or less.
When the thin film has a thickness within that range, this thin
film can have the strength required of a current collector and is
easy to handle.
[0292] Furthermore, use of a material composed of a current
collector and, formed on the surface thereof, an electroconductive
layer differing in compound composition from the current collector
is also preferred from the standpoint of lowering the resistance of
electronic contact between the current collector and the
positive-electrode active-material layer. Examples of the
electroconductive layer differing in compound composition from the
current collector include electroconductive layers formed from
carbonaceous materials, electroconductive polymers, and noble
metals such as gold, platinum, and silver.
[0293] The thickness ratio between the current collector and the
positive-electrode active-material layer is not particularly
limited. However, the value of (thickness of the positive-electrode
active-material layer on one surface just before impregnation with
electrolytic solution)/(thickness of the current collector) is
preferably 20 or less, more preferably 15 or less, most preferably
10 or less, and the lower limit thereof is preferably 0.5 or
greater, more preferably 0.8 or greater, most preferably 1 or
greater. When the thickness ratio is within that range, it is easy
to avoid the trouble that the current collector is heated up by
Joule's heat during high-current-density charge/discharge or that
the proportion by volume of the current collector to the
positive-electrode active material increases to reduce the capacity
of the battery.
[0294] (Electrode Area)
[0295] In the case where a nonaqueous electrolytic solution of the
invention is used, it is preferred to regulate the
positive-electrode active-material layer so as to have a larger
area than the area of the outer surface of the battery case, from
the standpoints of high output and enhanced high-temperature
stability. Specifically, the total area of the positive electrode
is preferably at least 15 times, more preferably at least 40 times,
the surface area of the case of the secondary battery. In the case
of a bottomed prismatic shape, the term "area of the outer surface
of the case" means the total area calculated from the length,
width, and thickness dimensions of the case part packed with the
power generation elements excluding the projecting parts of the
terminals. In the case of a bottomed cylindrical shape, that term
means a geometrical surface area obtained by approximating to a
cylinder the case part packed with the power generation elements
excluding the projecting parts of the terminals. The term "total
area of the positive electrode" means the geometrical surface area
of the positive-electrode mix layer which faces the mix layer
containing a negative-electrode active material. In the case of a
structure obtained by forming a positive-electrode mix layer on
each of both surfaces of a current collector foil, that term means
the sum of the areas separately calculated for the respective
surfaces.
[0296] (Thickness of Positive-Electrode Plate)
[0297] The thickness of the positive-electrode plate is not
particularly limited. However, from the standpoints of high
capacity and high output, the thickness of the mix layer, i.e., the
thickness of the positive-electrode plate excluding the metal foil
serving as a core, for one surface of the current collector is as
follows. The lower limit thereof is preferably 10 .mu.m or more,
more preferably 20 .mu.m or more, and the upper limit thereof is
preferably 500 .mu.m or less, more preferably 400 .mu.m or
less.
[0298] [Separator]
[0299] A separator is generally interposed between the positive
electrode and the negative electrode in order to prevent
short-circuiting. In this case, a nonaqueous electrolytic solution
of this invention is usually infiltrated into the separator.
[0300] The material and shape of the separator are not particularly
limited, and known separators can be employed at will unless the
effects of the invention are considerably lessened thereby. In
particular, use may be made of separators constituted of materials
stable to the nonaqueous electrolytic solutions of the invention,
such as resins, glass fibers, and inorganic materials. It is
preferred to use a separator which is in the form of a porous
sheet, nonwoven fabric, or the like and has excellent liquid
retentivity.
[0301] As the material of the resinous or glass-fiber separators,
use can be made of, for example, polyolefins such as polyethylene
and polypropylene, polytetrafluoroethylene, polyethersulfones,
glass filters, and the like. Preferred of these are glass filters
and polyolefins. More preferred are polyolefins. One of these
materials may be used alone, or two or more thereof may be used in
any desired combination and proportion.
[0302] The separator may have any desired thickness. However, the
thickness thereof is generally 1 .mu.m or more, preferably 5 .mu.M
or more, more preferably 10 .mu.m or more, and is generally 50
.mu.m or less, preferably 40 .mu.m or less, more preferably 30
.mu.m or less. When the thickness of the separator is within that
range, neither insulating properties nor mechanical strength
decreases, and the battery is less apt to suffer a decrease in
battery performance, e.g., rate characteristics, or a decrease in
the energy density of the nonaqueous-electrolyte battery as a
whole.
[0303] In the case where a porous material such as, e.g., a porous
sheet or nonwoven fabric is used as the separator, this separator
may have any desired porosity. However, the porosity thereof is
generally 20% or higher, preferably 35% or higher, more preferably
45% or higher, and is generally 90% or lower, preferably 85% or
lower, more preferably 75% or lower. When the porosity thereof is
within that range, this separator does not have excessively high
film resistance, and preferred rate characteristics can be
provided. Furthermore, this separator does not have reduced
mechanical strength, and preferred insulating properties also can
be provided.
[0304] The separator may have any desired average pore diameter.
However, the average pore diameter thereof is generally 0.5 .mu.m
or less, preferably 0.2 .mu.m or less, and is generally 0.05 .mu.m
or more. When the average pore diameter thereof is within that
range, short-circuiting is less apt to occur, and this separator
does not have excessively high film resistance and can provide
preferred rate characteristics.
[0305] On the other hand, examples of the inorganic materials which
may be used include oxides such as alumina and silicon dioxide,
nitrides such as aluminum nitride and silicon nitride, and sulfates
such as barium sulfate and calcium sulfate. Such materials of a
particulate shape or fibrous shape may be used.
[0306] With respect to form, a separator of a thin film form may be
used, such as nonwoven fabric, woven fabric, or microporous film.
Suitable separators of a thin film form have a pore diameter of
0.01-1 .mu.m and a thickness of 5-50 .mu.m. Besides such a
separator in an independent thin film form, use can be made of a
separator obtained by forming a composite porous layer containing
particles of the inorganic material using a resinous binder on a
surface layer of the positive electrode and/or negative electrode.
Examples thereof include to form a porous layer from alumina
particles having a 90% particle diameter smaller than 1 .mu.m on
both surfaces of the positive electrode using a fluororesin as a
binder.
[0307] [Battery Design]
[0308] <Electrode Group>
[0309] The electrode group may be either of: an electrode group
having a multilayer structure in which the positive-electrode plate
and negative-electrode plate have been superposed through the
separator; and an electrode group having a wound structure in which
the positive-electrode plate and negative-electrode plate have been
spirally wound through the separator. The proportion of the volume
of the electrode group to the internal volume of the battery
(hereinafter referred to as electrode group proportion) is
generally 40% or higher, preferably 50% or higher, and is generally
90% or lower, preferably 80% or lower.
[0310] When the electrode group proportion is within that range,
not only a preferred battery capacity can be provided, but also a
moderate space volume can be ensured. Consequently, this battery
does not undergo an excessive increase in internal pressure, even
when the battery is heated up to cause members to expand or a
liquid component of the electrolyte to have a heightened vapor
pressure. It is therefore easy to avoid the trouble that the
battery is reduced in various characteristics including
charge/discharge cycling performance and high-temperature
storability, and the trouble that gas release valve, which releases
the internal pressure, works.
[0311] <Structure for Current Collection>
[0312] The structure for current collection is not particularly
limited. However, for more effectively attaining the improvement in
high-current density charge/discharge characteristics which is
brought about by the nonaqueous electrolytic solutions of this
invention, it is preferred to employ a structure reduced in the
resistance of wiring parts and joint parts. In the case where
internal resistance has been reduced in this manner, use of the
nonaqueous electrolytic solutions of the invention produces the
effects thereof especially satisfactorily.
[0313] In the case of an electrode group assembled into the
multilayer structure described above, a structure obtained by
bundling the metallic core parts of respective electrode layers and
welding the bundled parts to a terminal is suitable. When each
electrode has a large area, this results in increased internal
resistance. In this case, it is preferred to dispose a plurality of
terminals in each electrode to reduce the resistance. In the case
of an electrode group having the wound structure described above, a
plurality of lead structures may be disposed on each of the
positive electrode and negative electrode and bundled into a
terminal. Thus, internal resistance can be reduced.
[0314] <Protective Element>
[0315] Examples of the protective element include a PTC (positive
temperature coefficient), which increases in resistance upon
abnormal heating-up or when an excessive current flows, a
temperature fuse, a thermister, and a valve (current breaker valve)
which, upon abnormal heating-up, breaks the current flowing through
the circuit, on the basis of an abrupt increase in the internal
pressure or internal temperature of the battery. It is preferred to
select such a protective element which does not work under ordinary
high-current use conditions. It is more preferred to employ a
design which prevents abnormal heating-up and thermal run-away even
without a protective element.
[0316] <Case>
[0317] The nonaqueous-electrolyte batteries of the invention each
are usually fabricated by housing the nonaqueous electrolytic
solution, negative electrode, positive electrode, separator, etc.
in a case. This case is not particularly limited, and a known case
can be employed at will unless this considerably lessens the
effects of the invention. Specifically, although the case may be
made of any desired material, use is generally made of a metal such
as nickel-plated iron, stainless steel, aluminum or an alloy
thereof, nickel, titanium, or a magnesium alloy or a laminated film
composed of a resin and an aluminum foil. From the standpoint of
weight reduction, a metal which is aluminum or an aluminum alloy or
a laminated film is suitable.
[0318] Examples of the case obtained using any of those metals
include: a case formed by fusion-bonding a metal to itself by laser
welding, resistance welding, or ultrasonic welding to constitute a
sealed structure; or a case formed by caulking any of those metals
through a resinous gasket. Examples of the case obtained using the
laminated film include a case having a sealed structure formed by
thermally fusion-bonding the resin layer to itself. A resin
different from the resin used in the laminated film may be
interposed between the resin layers in order to enhance sealing
properties. Especially when a sealed structure is to be formed by
thermally fusion-bonding resin layers through current-collector
terminals, then metal/resin bonding is involved and, hence, a resin
having polar groups or a modified resin into which polar groups
have been incorporated is suitable for use as the resin to be
interposed.
[0319] The case may have any desired shape. For example, the case
may be any of the cylindrical type, prismatic type, laminate type,
coin type, large type, and the like.
EXAMPLES
[0320] 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.
Example 1
Production of Negative Electrode
[0321] To 98 parts by weight of artificial-graphite powder KS-44
(trade name; manufactured by Timcal) were added 100 parts by weight
of an aqueous dispersion of sodium carboxymethyl cellulose
(concentration of sodium carboxymethyl cellulose, 1% by mass) as a
thickener and 2 parts by weight of an aqueous dispersion of a
styrene/butadiene rubber (concentration of styrene/butadiene
rubber, 50% by mass) as a binder. The ingredients were mixed
together by means of a disperser to obtain a slurry. The slurry
obtained was applied to one surface of a copper foil having a
thickness of 10 .mu.m and dried. This coated foil was rolled with a
pressing machine to a thickness of 75 .mu.m, and a piece of a shape
having an active-material layer size with a width of 30 mm and a
length of 40 mm and having an uncoated area with a width of 5 mm
and a length of 9 mm was cut out of the rolled sheet. Thus, a
negative electrode was obtained.
[Production of Positive Electrode]
[0322] Ninety percents by mass iron lithium phosphate
(LiFePO.sub.4; manufactured by STL Energy Technology Co., Ltd.) as
a positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to obtain a slurry. The slurry obtained was applied to one
surface of an aluminum foil having a thickness of 15 .mu.m and
coated beforehand with a carbonaceous material, and dried. This
coated foil was rolled with a pressing machine to a thickness of 80
.mu.m, and a piece of a shape having an active-material layer size
with a width of 30 mm and a length of 40 mm and having a uncoated
area with a width of 5 mm and a length of 9 mm was cut out of the
rolled sheet. Thus, a positive electrode was obtained.
[Production of Electrolytic Solution]
[0323] In a dry argon atmosphere, 98.5% by mass mixture of ethylene
carbonate (EC), dimethoxyethane (DME), and ethyl methyl carbonate
(EMC) (volume ratio, 2:3:5) was mixed with 0.5% by mass lithium
difluorophosphate (LiPO.sub.2F.sub.2), 0.5% by mass vinylene
carbonate (VC), and 0.5% by mass monofluoroethylene carbonate
(MFEC). Subsequently, sufficiently dried LiPF.sub.6 was dissolved
therein so as to result in a proportion thereof of 1.1 mol/L. Thus,
an electrolytic solution was obtained.
[Production of Lithium Secondary Battery]
[0324] The positive electrode and negative electrode described
above and a separator made of polyethylene were superposed in the
order of negative electrode/separator/positive electrode to produce
a battery element. This battery element was inserted into a bag
constituted of a laminated film obtained by coating both surfaces
of aluminum (thickness, 40 .mu.m) with a resin layer, with the
terminals of the positive and negative electrodes projecting
outward. Thereafter, the electrolytic solution was introduced into
the bag, and this bag was vacuum-sealed to produce a sheet battery.
This battery was evaluated. The components of the electrolytic
solution are shown in Table 1.
[Evaluation of Initial Discharge Capacity]
[0325] The lithium secondary battery was evaluated in the state of
being sandwiched between glass plates in order to enhance contact
between the electrodes. At 25.degree. C., this battery was charged
to 4.0 V at a constant current corresponding to 0.2 C and then
discharged to 2.5 V at a constant current of 0.1 C. Two cycles of
this charge/discharge were conducted to stabilize the battery. In
each of the third to sixth cycles, the battery was charged to 4.0 V
at a constant current of 0.2 C, subsequently charged at a constant
voltage of 4.0 V until the current value became 0.05 C, and then
discharged to 2.5 V at a constant current of 0.2 C. Thereafter, in
the seventh cycle, the battery was charged to 4.0 V at a constant
current of 0.2 C, subsequently charged at a constant voltage of 4.0
V until the current value became 0.05 C, and then discharged to 2.5
V at a constant current of 0.2 C to determine initial discharge
capacity. Here, "1 C" means a current value at which the reference
capacity of the battery is discharged over 1 hour; "5 C" means the
current value which is 5 times the current of 1 C, "0.1 C" means
the current value which is 1/10 the current of 1 C, and "0.2 C"
means the current value which is 1/5 the current of 1 C.
[Evaluation of 25.degree. C. Output]
[0326] The battery which had undergone the evaluation of initial
discharge capacity was charged at 25.degree. C. and a constant
current of 0.2 C to a half of the initial discharge capacity. This
battery was discharged at 25.degree. C. for 10 seconds at each of 1
C, 2 C, 4 C, 7 C, 10 C, and 15 C, and the voltage was measured at
the time when the 10 seconds had passed. The area of the triangle
surrounded by the current-voltage line and the lower-limit voltage
(2.5 V) was regarded as output (W). The relative value (%) of the
output was calculated, with the output value at 25.degree. C. of
Comparative Example 1 being taken as 100.
[Evaluation of -30.degree. C. Output]
[0327] The battery which had undergone the evaluation of initial
discharge capacity was charged at 25.degree. C. and a constant
current of 0.2 C to a half of the initial discharge capacity. This
battery was discharged at -30.degree. C. for 10 seconds at each of
0.2 C, 0.4 C, 0.8 C, 1 C, and 2 C, and the voltage was measured at
the time when the 10 seconds had passed. The area of the triangle
surrounded by the current-voltage line and the lower-limit voltage
(2.5 V) was regarded as output (W). The relative value (%) of the
output was calculated, with the output value at -30.degree. C. of
Comparative Example 1 being taken as 100.
[Evaluation of High-Temperature Cycle Characteristics]
[0328] At 60.degree. C., the battery which had undergone the test
for evaluating initial discharge capacity was charged to 3.6 V at a
constant current of 2 C and then discharged to 2.5 V at a constant
current of 2 C. This operation was taken as one cycle, and 500
cycles were conducted. The discharge capacity (%) in the 500th
cycle was determined as a value relative to the discharge capacity
measured in the first cycle, which was taken as 100. This discharge
capacity (%) was taken as discharge capacity retention.
[Evaluation of High-Rate Discharge Characteristics]
[0329] The battery which had undergone the high-temperature cycle
test was subjected to the following test at 25.degree. C. The
battery was charged to 3.6 V at a constant current of 0.2 C and
then charged at a constant voltage of 3.6 V until the current value
became 0.05 C. This battery was discharged to 2.5 V at each of
constant currents of 2 C and 5 C. The discharge capacities (%) at 2
C and 5 C after the high-temperature cycle test were determined as
values relative to the discharge capacity determined in the initial
discharge capacity test, which was taken as 100.
[0330] The results of the evaluation are shown in Tables 2 and
3.
Example 2
[0331] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.5% by mass mixture of ethylene carbonate (EC),
dimethoxyethane (DME), and ethyl methyl carbonate (EMC) (volume
ratio, 2:3:5) with 0.5% by mass vinylene carbonate (VC) and then
dissolving sufficiently dried LiPF.sub.6 in the resultant mixture
so as to result in a proportion thereof of 1.1 mol/L. The
components of the electrolytic solution and the results of the
evaluation are shown in Table 1 to Table 3.
Example 3
[0332] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.5% by mass mixture of ethylene carbonate (EC),
dimethoxyethane (DME), and ethyl methyl carbonate (EMC) (volume
ratio, 2:3:5) with 0.5% by mass lithium difluorophosphate
(LiPO.sub.2F.sub.2) and then dissolving sufficiently dried
LiPF.sub.6 in the resultant mixture so as to result in a proportion
thereof of 1.1 mol/L. The components of the electrolytic solution
and the results of the evaluation are shown in Table 1 to Table
3.
Example 4
[0333] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 98.5% by mass mixture of ethylene carbonate (EC),
dimethoxyethane (DME), and ethyl methyl carbonate (EMC) (volume
ratio, 3:2:5) with 0.5% by mass lithium difluorophosphate
(LiPO.sub.2F.sub.2), 0.5% by mass vinylene carbonate (VC), and 0.5%
by mass monofluoroethylene carbonate (MFEC) and then dissolving
sufficiently dried LiPF.sub.6 in the resultant mixture so as to
result in a proportion thereof of 1.1 mol/L. The components of the
electrolytic solution and the results of the evaluation are shown
in Table 1 to Table 3.
Example 5
[0334] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.5% by mass mixture of ethylene carbonate (EC),
dimethoxyethane (DME), and ethyl methyl carbonate (EMC) (volume
ratio, 3:2:5) with 0.5% by mass vinylene carbonate (VC) and then
dissolving sufficiently dried LiPF.sub.6 in the resultant mixture
so as to result in a proportion thereof of 1.1 mol/L. The
components of the electrolytic solution and the results of the
evaluation are shown in Table 1 to Table 3.
Example 6
[0335] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.0% by mass mixture of ethylene carbonate (EC),
diethoxyethane (DEE), and ethyl methyl carbonate (EMC) (volume
ratio, 3:1:6) with 0.5% by mass vinylene carbonate (VC) and 0.5% by
mass lithium difluorophosphate (LiPO.sub.2F.sub.2) and then
dissolving sufficiently dried LiPF.sub.6 in the resultant mixture
so as to result in a proportion thereof of 1.1 mol/L. The
components of the electrolytic solution and the results of the
evaluation are shown in Table 1 to Table 3.
Example 7
[0336] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.0% by mass mixture of ethylene carbonate (EC),
ethoxy(2,2,2-trifluoroethoxy)ethane (ETFEE), and ethyl methyl
carbonate (EMC) (volume ratio, 3:1:6) with 0.5% by mass vinylene
carbonate (VC) and 0.5% by mass lithium difluorophosphate
(LiPO.sub.2F.sub.2) and then dissolving sufficiently dried
LiPF.sub.6 in the resultant mixture so as to result in a proportion
thereof of 1.1 mol/L. The components of the electrolytic solution
and the results of the evaluation are shown in Table 1 to Table
3.
Example 8
[0337] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.0% by mass mixture of ethylene carbonate (EC),
1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoro-n-propyl ether
(TFETFPE), and ethyl methyl carbonate (EMC) (volume ratio, 3:1:6)
with 0.5% by mass vinylene carbonate (VC) and 0.5% by mass lithium
difluorophosphate (LiPO.sub.2F.sub.2) and then dissolving
sufficiently dried LiPF.sub.6 in the resultant mixture so as to
result in a proportion thereof of 1.1 mol/L. The components of the
electrolytic solution and the results of the evaluation are shown
in Table 1 to Table 3.
Example 9
[0338] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 98.5% by mass mixture of ethylene carbonate (EC),
dimethoxyethane (DME), and ethyl methyl carbonate (EMC) (volume
ratio, 3:2:5) with 0.5% by mass vinylene carbonate (VC) and 1% by
mass lithium trifluoromethanesulfonate (CF.sub.3SO.sub.3Li) and
then dissolving sufficiently dried LiPF.sub.6 in the resultant
mixture so as to result in a proportion thereof of 1.1 mol/L. The
components of the electrolytic solution and the results of the
evaluation are shown in Table 1 to Table 3.
Example 10
[0339] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 98.5% by mass mixture of ethylene carbonate (EC),
dimethoxyethane (DME), and ethyl methyl carbonate (EMC) (volume
ratio, 3:2:5) with 0.5% by mass vinylene carbonate (VC) and 1% by
mass lithium bis(fluorosulfonyl)imide (LiFSI) and then dissolving
sufficiently dried LiPF.sub.6 in the resultant mixture so as to
result in a proportion thereof of 1.1 mol/L. The components of the
electrolytic solution and the results of the evaluation are shown
in Table 1 to Table 3.
Example 11
[0340] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.0% by mass mixture of ethylene carbonate (EC),
dimethoxyethane (DME), and ethyl methyl carbonate (EMC) (volume
ratio, 3:2:5) with 0.3% by mass vinylene carbonate (VC), 0.2% by
mass 1,3-propanesultone (PS), and 0.5% by mass lithium
difluorophosphate (LiPO.sub.2F.sub.2) and then dissolving
sufficiently dried LiPF.sub.6 in the resultant mixture so as to
result in a proportion thereof of 1.1 mol/L. The components of the
electrolytic solution and the results of the evaluation are shown
in Table 1 to Table 3.
Example 12
[0341] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.0% by mass mixture of ethylene carbonate (EC),
dimethoxyethane (DME), and ethyl methyl carbonate (EMC) (volume
ratio, 3:2:5) with 0.3% by mass vinylene carbonate (VC), 0.2% by
mass ethylene sulfite (ES), and 0.5% by mass lithium
difluorophosphate (LiPO.sub.2F.sub.2) and then dissolving
sufficiently dried LiPF.sub.6 in the resultant mixture so as to
result in a proportion thereof of 1.1 mol/L. The components of the
electrolytic solution and the results of the evaluation are shown
in Table 1 to Table 3.
Comparative Example 1
[0342] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.5% by mass mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4) with 0.5% by mass vinylene carbonate (VC) and then
dissolving sufficiently dried LiPF.sub.6 in the resultant mixture
so as to result in a proportion thereof of 1 mol/L. The components
of the electrolytic solution and the results of the evaluation are
shown in Table 1 to Table 3.
Comparative Example 2
[0343] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99% by mass mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4) with 1% by mass vinylene carbonate (VC) and then dissolving
sufficiently dried LiPF.sub.6 in the resultant mixture so as to
result in a proportion thereof of 1 mol/L. The components of the
electrolytic solution and the results of the evaluation are shown
in Table 1 to Table 3.
Comparative Example 3
[0344] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.5% by mass mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4) with 0.5% by mass vinylethylene carbonate (VEC) and then
dissolving sufficiently dried LiPF.sub.6 in the resultant mixture
so as to result in a proportion thereof of 1 mol/L. The components
of the electrolytic solution and the results of the evaluation are
shown in Table 1 to Table 3.
Comparative Example 4
[0345] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing 99.5% by mass mixture of ethylene carbonate (EC), dimethyl
carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio,
3:3:4) with 0.5% by mass 1,3-propanesultone (PS) and then
dissolving sufficiently dried LiPF.sub.6 in the resultant mixture
so as to result in a proportion thereof of 1 mol/L. The components
of the electrolytic solution and the results of the evaluation are
shown in Table 1 to Table 3.
Comparative Example 5
[0346] A sheet-form lithium secondary battery was produced and
evaluated in the same manners as in Example 1, except that an
electrolytic solution was obtained in a dry argon atmosphere by
mixing a mixture of ethylene carbonate (EC), dimethoxyethane (DME),
and ethyl methyl carbonate (EMC) (volume ratio, 3:2:5) and then
dissolving sufficiently dried LiPF.sub.6 in the resultant mixture
so as to result in a proportion thereof of 1.1 mol/L. The
components of the electrolytic solution and the results of the
evaluation are shown in Table 1 to Table 3.
Comparative Example 6
Production of Positive Electrode
[0347] Ninety percents by mass
Li(Ni.sub.1/3Mn.sub.1/3Co.sub.1/3)O.sub.2 (NMC) as a
positive-electrode active material was mixed with 5% by mass
acetylene black as a conductive material and 5% by mass
poly(vinylidene fluoride) (PVdF) as a binder in N-methylpyrrolidone
solvent to obtain a slurry. The slurry obtained was applied to one
surface of an aluminum foil having a thickness of 15 .mu.m and
coated beforehand with a conduction aid, and dried. This coated
foil was rolled with a pressing machine to a thickness of 80 .mu.m,
and a piece of a shape having an active-material layer size with a
width of 30 mm and a length of 40 mm and having a uncoated area
with a width of 5 mm and a length of 9 mm was cut out of the rolled
sheet. Thus, a positive electrode was obtained.
[Production of Electrolytic Solution]
[0348] A sheet-form lithium secondary battery was produced in the
same manner as in Example 1, except that an electrolytic solution
was obtained in a dry argon atmosphere by mixing 99.0% by mass
mixture of ethylene carbonate (EC), dimethoxyethane (DME), and
ethyl methyl carbonate (EMC) (volume ratio, 2:3:5) with 1.0% by
mass LiN(FSO.sub.2).sub.2 (LiFSI) and then dissolving sufficiently
dried LiPF.sub.6 in the resultant mixture so as to result in a
proportion thereof of 1.1 mol/L. The components of the electrolytic
solution are shown in Table 1.
[Evaluation of Initial Discharge Capacity]
[0349] The lithium secondary battery produced above was evaluated
in the state of being sandwiched between glass plates in order to
enhance contact between the electrodes. At 25.degree. C., this
battery was charged to 4.1 V at a constant current corresponding to
0.2 C and then discharged to 3 V at a constant current of 0.2 C.
Two cycles of this charge/discharge were conducted to stabilize the
battery. In the third cycle, the battery was charged to 4.2 V at a
constant current of 0.2 C, subsequently charged at a constant
voltage of 4.2 V until the current value became 0.05 C, and then
discharged to 3 V at a constant current of 0.2 C. Thereafter, in
the fourth cycle, the battery was charged to 4.2 V at a constant
current of 0.2 C, subsequently charged at a constant voltage of 4.2
V until the current value became 0.05 C, and then discharged to 3 V
at a constant current of 0.2 C to determine initial discharge
capacity. Here, "1 C" means a current value at which the reference
capacity of the battery is discharged over 1 hour; "5 C" means the
current value which is 5 times the current of 1 C, "0.1 C" means
the current value which is 1/10 the current of 1 C, and "0.2 C"
means the current value which is 1/5 the current of 1 C.
[Evaluation of High-Temperature Cycle Characteristics]
[0350] At 60.degree. C., the battery which had undergone the test
for evaluating initial discharge capacity was charged to 4.2 V at a
constant current of 2 C and then discharged to 3 V at a constant
current of 2 C. This operation was taken as one cycle, and 500
cycles were conducted. The discharge capacity (%) in the 500th
cycle was determined as a value relative to the discharge capacity
measured in the first cycle, which was taken as 100. This discharge
capacity (%) was taken as discharge capacity retention.
[Evaluation of High-Rate Discharge Characteristics]
[0351] The battery which had undergone the high-temperature cycle
test was subjected to the following test at 25.degree. C. The
battery was charged to 4.2 V at a constant current of 0.2 C and
then charged at a constant voltage of 4.2 V until the current value
became 0.05 C. This battery was discharged to 3 V at each of
constant currents of 2 C and 5 C. The discharge capacities (%) at 2
C and 5 C after the high-temperature cycle test were determined as
values relative to the discharge capacity determined in the initial
discharge capacity test, which was taken as 100.
[0352] The results of the evaluation are shown in Tables 2 and
3.
TABLE-US-00002 TABLE 1 Solvent (ratio) Additive (mass %) Example 1
EC:DME:EMC LiPO.sub.2F.sub.2 (0.5) (20:30:50) VC (0.5) MFEC (0.5)
Example 2 EC:DME:EMC VC (0.5) (20:30:50) Example 3 EC:DME:EMC
LiPO.sub.2F.sub.2 (0.5) (20:30:50) Example 4 EC:DME:EMC
LiPO.sub.2F.sub.2 (0.5) (30:20:50) VC (0.5) MFEC (0.5) Example 5
EC:DME:EMC VC (0.5) (30:20:50) Example 6 EC:DEE:EMC VC (0.5)
(30:10:60) LiPO.sub.2F.sub.2 (0.5) Example 7 EC:ETFEE:EMC VC (0.5)
(30:10:60) LiPO.sub.2F.sub.2 (0.5) Example 8 EC:TFETFPE:EMC VC
(0.5) (30:10:60) LiPO.sub.2F.sub.2 (0.5) Example 9 EC:DME:EMC VC
(0.5) (30:20:50) CF.sub.3SO.sub.3Li (1) Example 10 EC:DME:EMC VC
(0.5) (30:20:50) LiFSI (1) Example 11 EC:DME:EMC VC (0.3)
(30:20:50) PS (0.2) LiPO.sub.2F.sub.2 (0.5) Example 12 EC:DME:EMC
VC (0.3) (30:20:50) ES (0.2) LiPO.sub.2F.sub.2 (0.5) Comparative
EC:DMC:EMC VC (0.5) Example 1 (30:30:40) Comparative EC:DMC:EMC VC
(1.0) Example 2 (30:30:40) Comparative EC:DMC:EMC VEC (0.5) Example
3 (30:30:40) Comparative EC:DMC:EMC PS (0.5) Example 4 (20:30:50)
Comparative EC:DME:EMC none Example 5 (20:30:50) Comparative
EC:DME:EMC LiFSI (1) Example 6 (20:30:50)
TABLE-US-00003 TABLE 2 Output relative to Comparative Example 1 (%)
25.degree. C. -30.degree. C. Example 1 115.5 149.3 Example 2 112.2
114.0 Example 3 116.6 176.6 Example 4 105.7 118.8 Example 5 108.0
105.9 Example 6 111.3 133.3 Example 7 110.4 128.5 Example 8 107.8
122.0 Example 9 112.0 136.5 Example 10 112.6 134.9 Example 11 111.0
130.1 Example 12 113.0 131.7 Comparative 100 100 Example 1
Comparative 98.5 93.1 Example 2 Comparative 91.5 94.7 Example 3
Comparative 90.8 99.5 Example 4
TABLE-US-00004 TABLE 3 Discharge capacity 2-C discharge 5-C
discharge retention after capacity after capacity after 500 cycles
(%) 500 cycles (%) 500 cycles (%) Example 1 66.4 59.9 58.7 Example
2 65.1 59.3 57.6 Example 3 51.4 39.1 37.9 Example 4 77.8 72.4 69.5
Example 6 71.3 64.8 61.5 Example 7 65.4 60.9 57.4 Example 8 71.7
62.1 58.6 Example 9 67.8 66.5 63.9 Example 10 68.1 65.6 62.9
Example 11 64.8 58.7 55.9 Example 12 67.8 60.0 56.9 Comparative
23.4 26.1 23.5 Example 4 Comparative 40.7 33.9 29.7 Example 5
Comparative 45.0 35.7 30.5 Example 6
[0353] As apparent from Table 2, the nonaqueous-electrolyte
batteries of the invention are superior in initial output at
25.degree. C. and -30.degree. C. As apparent from Table 3, the
batteries of the invention are superior in high-temperature cycle
characteristics and in high-current-density discharge
characteristics determined after the high-temperature cycle test.
It was hence found that the batteries of the invention have high
durability. In contrast, the batteries employing nonaqueous
electrolytic solutions which are not the nonaqueous electrolytic
solutions according to the invention are lower in initial output at
25.degree. C. and -30.degree. C. than the batteries employing the
nonaqueous electrolytic solutions according to the invention, and
are inferior in high-temperature cycle characteristics and in
high-current-density discharge characteristics determined after the
high-temperature cycle test.
[0354] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope
thereof. This application is based on a Japanese patent application
filed on Sep. 29, 2009 (Application No. 2009-223809), the contents
thereof being incorporated herein by reference.
INDUSTRIAL APPLICABILITY
[0355] The nonaqueous-electrolyte batteries of the invention have a
high initial output at ordinary temperature and -30.degree. C.,
attain a high discharge capacity even during high-rate discharge,
and have a high capacity retention after a durability test such as
a high-temperature storage test or cycle test. In addition, even
after the durability test, the batteries of the invention have the
excellent output performance and high-rate discharge capacity equal
to the initial values.
* * * * *