U.S. patent application number 15/364460 was filed with the patent office on 2017-03-23 for nonaqueous electrolyte secondary battery and method for manufacturing nonaqueous electrolyte secondary battery.
This patent application is currently assigned to SANYO Electric Co., Ltd.. The applicant listed for this patent is SANYO Electric Co., Ltd.. Invention is credited to Naoki Imachi, Hiroyuki Matsumoto, Takeshi Ogasawara.
Application Number | 20170084952 15/364460 |
Document ID | / |
Family ID | 43648033 |
Filed Date | 2017-03-23 |
United States Patent
Application |
20170084952 |
Kind Code |
A1 |
Ogasawara; Takeshi ; et
al. |
March 23, 2017 |
NONAQUEOUS ELECTROLYTE SECONDARY BATTERY AND METHOD FOR
MANUFACTURING NONAQUEOUS ELECTROLYTE SECONDARY BATTERY
Abstract
In a nonaqueous electrolyte secondary battery including a
positive electrode, a negative electrode and a nonaqueous
electrolytic solution, the nonaqueous electrolytic solution
contains a nitrile compound having a chain saturated hydrocarbon
group and a nitrile group, the number of carbon atoms in the
nitrile compound is four or more, and the positive electrode
contains a positive-electrode active material on the surface of
which particles of a rare earth element compound are deposited in
dispersed form.
Inventors: |
Ogasawara; Takeshi;
(Daito-shi, JP) ; Matsumoto; Hiroyuki; (Daito-shi,
JP) ; Imachi; Naoki; (Daito-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SANYO Electric Co., Ltd. |
Osaka |
|
JP |
|
|
Assignee: |
SANYO Electric Co., Ltd.
Osaka
JP
|
Family ID: |
43648033 |
Appl. No.: |
15/364460 |
Filed: |
November 30, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12872125 |
Aug 31, 2010 |
|
|
|
15364460 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y10T 29/49115 20150115;
H01M 4/505 20130101; H01M 4/525 20130101; H01M 2004/021 20130101;
H01M 10/0567 20130101; H01M 10/0525 20130101; Y02E 60/10 20130101;
Y02P 70/50 20151101; H01M 4/366 20130101; H01M 10/4235 20130101;
H01M 2220/30 20130101 |
International
Class: |
H01M 10/0567 20060101
H01M010/0567; H01M 4/36 20060101 H01M004/36; H01M 10/42 20060101
H01M010/42; H01M 4/525 20060101 H01M004/525 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 4, 2009 |
JP |
2009-204274 |
Dec 18, 2009 |
JP |
2009-287286 |
Claims
1. A nonaqueous electrolyte secondary battery including a positive
electrode, a negative electrode and a nonaqueous electrolytic
solution, wherein the nonaqueous electrolytic solution contains a
nitrile compound having a chain saturated hydrocarbon group and a
nitrile group, the number of carbon atoms in the nitrile compound
is four or more, and the positive electrode contains a
positive-electrode active material comprising a lithium-containing
transition metal composite oxide on the surface of which particles
of a rare earth compound are uniformly dispersed deposits; wherein
the rare earth compound comprises at least one selected from the
group consisting of erbium, ytterbium, terbium, dysprosium,
holmium, thulium, and lutetium, and wherein the particles of the
rare earth compound have an average particle diameter of 100 nm or
less, wherein the lithium-containing transition metal composite
oxide consists of only one type.
2. The nonaqueous electrolyte secondary battery according to claim
1, wherein the nitrile compound has two or more nitrile groups.
3. The nonaqueous electrolyte secondary battery according to claim
2, wherein the nitrile compound is at least one selected from
glutaronitrile, adiponitrile and pimelonitrile.
4. The nonaqueous electrolyte secondary battery according to claim
1, wherein the rare earth compound is a hydroxide or an
oxyhydroxide.
5. The nonaqueous electrolyte secondary battery according to claim
1, wherein an amount of the rare earth compound deposited on the
positive-electrode active material is within the range from 0.005%
to 0.3% by mass.
6. The nonaqueous electrolyte secondary battery according to claim
1, wherein the rare earth compound is erbium.
7. The nonaqueous electrolyte secondary battery according to claim
1, wherein the lithium-containing transition metal composite oxide
is only one type selected from lithium cobalt oxide,
Ni--Co--Mn-containing lithium composite oxide,
Ni--Mn--Al-containing lithium composite oxide and
Ni--Co--Al-containing lithium composite oxide.
8. The nonaqueous electrolyte secondary battery according to claim
1, wherein the lithium-containing transition metal composite oxide
is only one type selected from lithium cobalt oxide,
Ni--Co--Mn-containing lithium composite oxide and
Ni--Co--Al-containing lithium composite oxide.
9. The nonaqueous electrolyte secondary battery according to claim
8, wherein only the particles of the rare earth compound are
uniformly dispersed deposits on the lithium-containing transition
metal composite oxide.
10. The nonaqueous electrolyte secondary battery according to claim
1, wherein the particles of the rare earth compound have an average
particle diameter of 100 nm or less.
11. The nonaqueous electrolyte secondary battery according to claim
1, wherein the rare earth compound comprises at least one selected
from the group consisting of a hydroxide, an oxyhydroxide and an
oxide.
Description
[0001] This application is a divisional of U.S. application Ser.
No. 12/872,125, filed on Aug. 31, 2010, which claims priority to
Japanese priority application No. 2009-204274 filed on Sep. 4, 2009
and Japanese priority application No. 2009-287286 filed on Dec. 18,
2009, which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] Field of the Invention
[0003] This invention relates to a nonaqueous electrolyte secondary
battery and a method for manufacturing a nonaqueous electrolyte
secondary battery, and particularly relates to a nonaqueous
electrolyte secondary battery improved in characteristics after
continuous charging and a method for manufacturing such a
nonaqueous electrolyte secondary battery.
[0004] Description of Related Arts
[0005] In recent years, size and weight reduction of mobile
information terminals, such as cellular phones, notebook computers
and PDAs, has rapidly progressed. Batteries used as their driving
power sources are being required to achieve a higher capacity. To
meet such a request, nonaqueous electrolyte secondary batteries
using a nonaqueous electrolytic solution to perform charge and
discharge by moving lithium ions between positive and negative
electrodes are widely utilized as new secondary batteries having
high output and high energy density.
[0006] In these nonaqueous electrolyte secondary batteries,
materials commonly used as positive-electrode active materials
include lithium cobalt oxide (LiCoO.sub.2), spinel lithium
manganate (LiMn.sub.2O.sub.4), cobalt-nickel-manganese-containing
lithium composite oxides, and aluminum-nickel-cobalt-containing
lithium composite oxides. On the other hand, materials used as
negative-electrode active materials include carbon materials, such
as graphite, and materials capable of forming an alloy with
lithium, such as Si and Sn.
[0007] However, in more recent years, mobile information terminals
have enhanced their entertainment features including a video
playing feature and a gaming feature and have thereby tended to
further increase the power consumption. Therefore, nonaqueous
electrolyte secondary batteries are being required to achieve a
still higher capacity.
[0008] Possible measures for increasing the capacity of a
nonaqueous electrolyte secondary battery include (1) increasing the
capacity of the active material, (2) increasing the charge voltage,
and (3) increasing the amount of active material packed, i.e.,
increasing the packing density.
[0009] Particularly if the charge voltage is increased, there
arises a problem of ease of decomposition of the electrolytic
solution. More particularly, if the battery is stored or
continuously charged at high temperatures, the electrolytic
solution may decompose to produce gas, thereby causing problems of
swelling of the battery and increased internal pressure of the
battery.
[0010] Published Japanese Patent Application No. 2007-538365
proposes a lithium secondary battery using a nitrile
group-containing compound for an electrolytic solution, wherein the
positive-electrode active material used is a lithium-containing
composite oxide which contains one or more elements selected from
the group consisting of alkali metals, alkaline earth metals, 13th
group elements, 14th group elements, 15th group elements,
transition metals and rare earth elements and is doped with a
heterogeneous metal selected from Al, Mg, Zr, Fe, Zn, Ga, Sn, Si
and Ge. The document describes that thus the nitrile groups bind to
the surface of the positive electrode at high temperatures to form
a complex, and the complex serves as a protective film for blocking
active sites on the surface of the positive electrode. The document
also describes that the protective film inhibits part of a
transition metal from being eluted from the positive-electrode
active material and deposited on the negative electrode and
inhibits the occurrence of a side reaction and gas generation due
to reactions between the electrolytic solution and the positive
electrode, whereby lithium can be smoothly storaged and released
even at high temperatures to thereby inhibit the deterioration in
lifetime characteristic.
[0011] Published Japanese Patent Application No. 2008-108586
proposes that an oxide of a lithium-containing transition metal
containing at least one metal element selected from Mg, Ti, Zr, Ge,
Nb, Al and Sn is used as a positive-electrode active material, and
a compound having two or more nitrile groups in the molecule is
contained in the electrolytic solution. The document describes that
the nitrile compound has the function of forming a coating on the
surface of the positive-electrode active material, and the
formation of such a surface protective coating prevents direct
contact of the electrolytic solution with the positive-electrode
active material to inhibit gas generation, whereby the crystal
structure of the positive-electrode active material can be
stabilized to increase the storage performance and safety of the
battery.
[0012] Published Japanese Patent Application No. 2002-279991
proposes to use a positive-electrode active material in which two
or more compound layers each made of a hydroxide, an oxyhydroxide,
an oxycarbonate or a hydroxycarbonate of Mg, Al, Co, K, Na, Ca, Si,
Ti, V, Sn, Ge, Ga, B, As or Zr are formed on the core of the
positive-electrode active material. The document describes that
thus the cycle characteristics are improved.
[0013] These conventional techniques, however, do not describe any
approach for reducing the drop in discharge voltage after storage
at high temperatures or after continuous charging at high
temperatures.
[0014] Meanwhile, Published Japanese Patent Application No.
2008-226495 describes a means for impregnating particulate powder
of a lithium-containing composite oxide (positive-electrode active
material) with a solution (impregnating solution) containing a
lanthanum source by spraying. The document also describes that the
amount of impregnating solution is preferably controlled within the
range from 0.1% to 80% by weight relative to the weight of a matrix
used, more preferably within the range from 1% to 75% by weight,
and particularly preferably within the range from 1% to 40% by
weight. In other words, the method described in Published Japanese
Patent Application No. 2008-226495 shows that the amount of
impregnating solution may be within the range from 40% to 80% by
weight. Therefore, according to this method, the impregnating
solution can be sprayed to such an extent that the
positive-electrode active material is soaked therein.
SUMMARY OF THE INVENTION
[0015] An object of the present invention is to provide a
nonaqueous electrolyte secondary battery that can not only inhibit
gas generation during continuous charging but also significantly
reduce the deterioration in discharge characteristic after
continuous charging.
[0016] An aspect of the present invention is directed to a
nonaqueous electrolyte secondary battery including a positive
electrode, a negative electrode and a nonaqueous electrolytic
solution, wherein the nonaqueous electrolytic solution contains a
nitrile compound having a chain saturated hydrocarbon group and a
nitrile group, the number of carbon atoms in the nitrile compound
is four or more, and the positive electrode contains a
positive-electrode active material on the surface of which
particles of a rare earth element compound are deposited in
dispersed form.
[0017] In this aspect of the present invention, a
positive-electrode active material on the surface of which
particles of a rare earth element compound are deposited in
dispersed form is used, and a nonaqueous electrolytic solution
containing the above nitrile compound is used. Therefore, not only
gas generation during continuous charging can be inhibited, but
also the deterioration in discharge characteristic after continuous
charging can be significantly reduced.
[0018] Since in this aspect of the present invention the above
nitrile compound is contained in the nonaqueous electrolytic
solution, a coating of the nitrile compound is formed on the
surface of the positive-electrode active material during initial
charging, and the coating can inhibit the decomposition reaction of
the nonaqueous electrolytic solution.
[0019] Furthermore, in this aspect of the present invention,
particles of a rare earth element compound are deposited in
dispersed form on the surface of the positive-electrode active
material. By using such a positive-electrode active material, the
nonaqueous electrolytic solution can be inhibited from further
decomposing as in gaps in the coating formed by the decomposition
of the nitrile compound.
[0020] Therefore, not only gas generation during continuous
charging can be inhibited, but also the deterioration in discharge
characteristic after continuous charging can be significantly
reduced.
[0021] The number of carbon atoms in the nitrile compound used in
this aspect of the present invention is four or more. If the number
of carbon atoms is less than four, the nitrile compound becomes
very likely to decompose. In this case, even with the use of a
positive-electrode active material on the surface of which
particles of a rare earth element compound are deposited, the
deterioration in discharge characteristic after continuous charging
cannot be sufficiently inhibited.
[0022] The upper limit of the number of carbons in the chain
saturated hydrocarbon group in the nitrile compound used in this
aspect of the present invention is preferably 12 or less. The
reason for this is that if the nitrile compound has an excessively
large number of carbons, it will have a small number of nitrile
groups even if having the same mass as other nitrile compounds.
[0023] In this aspect of the present invention, the average
particle diameter of particles of the rare earth element compound
is preferably 100 nm or less, and more preferably 50 nm or less.
Furthermore, the lower limit of the average particle diameter is
preferably 1 nm or more, and more preferably 10 nm or more.
[0024] One reason for the above is that if the average particle
diameter of particles of the rare earth element compound is 100 nm
or less, the particles can be deposited in uniformly dispersed form
on the surface of the positive-electrode active material. Another
reason for the above is that it has been found from the
below-described results of Examples that if the average particle
diameter of the particles is 50 nm or less, the decomposition
reaction of the electrolytic solution can be effectively
inhibited.
[0025] On the other hand, if the average particle diameter of
particles of the rare earth element compound is less than 1 nm, it
can be assumed that the surface of the positive-electrode active
material is excessively coated with the particles, whereby the
discharge performance is lowered. Furthermore, if the average
particle diameter is not less than 10 nm, this ensures a sufficient
area of the positive-electrode active material surface deposited
with the particles.
[0026] The nitrile compound used in this aspect of the present
invention preferably has two or more nitrile groups. Examples of
the nitrile compound having two nitrile groups include at least one
selected from glutaronitrile, adiponitrile and pimelonitrile.
Examples of the nitrile compound having three nitrile groups
include at least one selected from 1,2,3-propanetricarbonitrile and
1,3,5-pentanetricarbonitrile.
[0027] Nitrile compounds having three or less nitrile groups are
generally commercially available and therefore easily
obtainable.
[0028] In this aspect of the present invention, the amount of the
nitrile compound contained in the nonaqueous electrolytic solution
is not particularly limited, but is preferably within the range
from 0.1% to 10% by mass and more preferably within the range from
0.1% to 5% by mass. If the amount of the nitrile compound in the
nonaqueous electrolytic solution is less than 0.1% by mass, a
coating of the nitrile compound cannot be sufficiently formed
during initial charging and discharging, whereby it may not
sufficiently inhibit the decomposition of the nonaqueous
electrolytic solution. On the other hand, if the amount of the
nitrile compound is above 10% by mass, the viscosity of the
nonaqueous electrolytic solution becomes high, which may present a
problem in that the permeability of the nonaqueous electrolytic
solution is lowered. In addition, if the amount of the nitrile
compound is above 5% by mass, the coating of the nitrile compound
increases in thickness in proportion to the content of the nitrile
compound, which may lower the discharge performance.
[0029] Another aspect of the present invention is directed to a
method for manufacturing a nonaqueous electrolyte secondary
battery, and the method includes the steps of: preparing a
suspension containing a positive-electrode active material
dispersed therein; preparing a solution in which a rare earth salt
is dissolved; adding the solution to the suspension; filtering the
resultant solution after the addition to obtain the
positive-electrode active material on the surface of which a
hydroxide of the rare earth element is deposited; producing a
positive electrode using the positive-electrode active material on
the surface of which the hydroxide of the rare earth element is
deposited; producing an electrode assembly using the positive
electrode, a negative electrode and a separator; and impregnating
the electrode assembly with a nonaqueous electrolytic solution.
[0030] When the suspension containing a positive-electrode active
material dispersed therein is stirred and a rare earth salt is
introduced into the suspension being stirred, the rare earth salt
is made into particles of a hydroxide. Furthermore, since the
suspension still continues to be stirred, the hydroxide particles
collide with the positive-electrode active material and are thereby
deposited in uniformly dispersed form on the surface of the
positive-electrode active material. Thus, particles of the rare
earth element compound can be deposited in uniformly dispersed form
on the surface of the positive-electrode active material.
[0031] Note that in this case, an alkaline aqueous solution, such
as sodium hydroxide or potassium hydroxide, is added to the
suspension to maintain the pH of the suspension at 6 to 13. Thus,
the particles of the rare earth element hydroxide can be
precipitated.
[0032] Still another aspect of the present invention is directed to
a method for manufacturing a nonaqueous electrolyte secondary
battery, and the method includes the steps of: preparing a solution
in which a rare earth salt is dissolved in an aqueous solvent;
depositing a hydroxide of the rare earth element on the surface of
a positive-electrode active material by spraying the solution on
the positive-electrode active material being stirred to moisten the
surface of the positive-electrode active material with the
solution; producing a positive electrode using the
positive-electrode active material on the surface of which the
hydroxide of the rare earth element is deposited; producing an
electrode assembly using the positive electrode, a negative
electrode and a separator; and impregnating the electrode assembly
with a nonaqueous electrolytic solution.
[0033] While the positive-electrode active material is stirred, a
solution containing a rare earth salt dissolved therein is sprayed
on the positive-electrode active material. Thus, particles of the
rare earth element hydroxide can be deposited uniformly on the
surface of the positive-electrode active material. According to
this method, the step of separating the positive-electrode active
material from the solution can be eliminated.
[0034] It is only necessary that the amount of the solution sprayed
be such as to moisten the surface of the positive-electrode active
material. If the amount of the solution is excessive, the step of
separating the positive-electrode active material from the solution
will be needed. The optimal value of the amount of the solution
sprayed depends on the particle diameter and specific surface area
of the positive-electrode active material. However, if the mass of
the solution sprayed is 10% to 20%, both inclusive, of the mass of
the positive-electrode active material, the hydroxide of the rare
earth element can be deposited uniformly on the surface of the
positive-electrode active material.
[0035] Examples of means that can be used to stir the
positive-electrode active material include drum mixers, Loedige
mixers and twin screw kneaders.
[0036] In either manufacturing method described above, a
water-soluble rare earth salt, such as a nitrate salt, a sulfate
salt, a chloride salt or an acetate salt of the rare earth element,
can be used as the rare earth salt.
[0037] In either manufacturing method, after the deposition of the
rare earth element hydroxide, the positive-electrode active
material is dried and subjected to a heat treatment. The
temperature of the heat treatment is generally preferably within
the range from 80.degree. C. to 600.degree. C., and particularly
preferably within the range from 80.degree. C. to 400.degree. C. If
the heat treatment temperature is above 600.degree. C., some of
fine particles of the rare earth compound deposited on the surface
of the positive-electrode active material are diffused into the
active material. This lowers the initial charge/discharge
efficiency. Therefore, in order to obtain a high-capacity active
material having a rare earth compound deposited more selectively on
the surface thereof, the heat treatment temperature is preferably
600.degree. C. or below. The hydroxide deposited on the surface of
the positive-electrode active material is made into the form of a
hydroxide, an oxyhydroxide, an oxide, etc. depending on the
subsequent heat treatment. Therefore, the rare earth element
compound deposited on the surface of the positive-electrode active
material according to the above methods of the present invention
takes the final form of a hydroxide, an oxyhydroxide, an oxide or
others. If in this case the heat treatment is performed at
400.degree. C. or below, the rare earth element compound mainly
takes the form of a hydroxide or an oxyhydroxide. The heat
treatment time is generally preferably for three to seven
hours.
[0038] In the present invention, the amount of rare earth element
compound deposited on the positive-electrode active material is
preferably within the range from 0.005% to 0.5% by mass, and more
preferably within the range from 0.01% to 0.3% by mass. If the
amount of rare earth element compound deposited is small,
improvement in discharge characteristic after continuous charging
may not be sufficiently achieved. If the amount of rare earth
element compound deposited is too large, the surface of the
positive-electrode active material is excessively coated with
particles of the rare earth compound not contributing to charge and
discharge reactions, which may lower the discharge performance.
[0039] Note that the amount of rare earth element compound
deposited is the amount thereof deposited relative to the
positive-electrode active material. For example, if the amount of
rare earth element compound deposited is 0.1% by mass, 0.1 parts by
mass of rare earth element compound is deposited on the
positive-electrode active material relative to 100 parts by mass of
positive-electrode active material in a state where no rare earth
element compound is deposited. Furthermore, the amount of rare
earth element compound deposited is expressed in terms of the
amount of rare earth element.
[0040] The rare earth element of the rare earth element compound in
the present invention is not particularly limited so long as it is
a rare earth element, but examples thereof include erbium,
samarium, neodymium, ytterbium, terbium, dysprosium, holmium,
thulium and lutetium.
[0041] Examples of the positive-electrode active material in the
present invention include lithium-containing transition metal
composite oxides containing a transition metal, such as cobalt,
nickel or manganese. Specific examples of the positive-electrode
active material include lithium cobalt oxide, Ni--Co--Mn-containing
lithium composite oxides, Ni--Mn--Al-containing lithium composite
oxides and Ni--Co--Al-containing lithium composite oxides. These
positive-electrode active materials may be used singly or in
mixture.
[0042] The negative-electrode active material used in the present
invention is not particularly limited so long as it can be used as
a negative-electrode active material for a nonaqueous electrolyte
secondary battery. Examples of the negative-electrode active
material include carbon materials, such as graphite and coke, tin
oxide, metal lithium, metals capable of forming an alloy with
lithium, such as silicon, and alloys of the metals.
[0043] The nonaqueous electrolytic solution used in the present
invention is not particularly limited so long as it can be used for
nonaqueous electrolyte secondary batteries. A general example of
the nonaqueous electrolytic solution is one containing a supporting
salt and a solvent.
[0044] Examples of the supporting salt include LiBF.sub.4,
LiPF.sub.6, LiN(SO.sub.2CF.sub.3).sub.2,
LiN(SO.sub.2C.sub.2F.sub.5) and
LiPF.sub.6-x(C.sub.nF.sub.2n+1).sub.x where 1<x<6 and n=1 or
2. These materials can be used singly or in a mixture of two or
more of them. The concentration of the supporting salt is not
particularly limited but is preferably within the range from 0.8 to
1.5 mol/L.
[0045] Preferred solvents to be used are carbonate solvents, such
as ethylene carbonate, propylene carbonate, .gamma.-butylolactone,
diethylene carbonate, ethyl methyl carbonate and dimethyl
carbonate, and carbonate solvents of such kinds in which one or
some of hydrogen atoms are substituted with fluorine atom or atoms.
The solvent is preferably used in a combination of a cyclic
carbonate and a chain carbonate.
BRIEF DESCRIPTION OF DRAWINGS
[0046] FIG. 1 is a perspective view showing an electrode assembly
produced in Examples.
[0047] FIG. 2 is a plan view showing a lithium ion secondary
battery produced in Examples.
[0048] FIG. 3 is a cross-sectional view taking along the line A-A
of FIG. 2.
[0049] FIG. 4 is a graph for illustrating a method for measuring
the maximum discharge voltage drop .DELTA.V1 during measurement of
the remaining capacity after continuous charging and a method for
measuring the maximum discharge voltage drop .DELTA.V2 during
measurement of the recovery discharge capacity.
[0050] FIG. 5 is a scanning electron micrograph (.times.30000)
showing the surface of a positive-electrode active material
produced in Example A1.
[0051] FIG. 6 is a scanning electron micrograph (.times.30000)
showing the surface of a positive-electrode active material
produced in Comparative Example a4.
[0052] FIG. 7 is a scanning electron micrograph (.times.30000)
showing the surface of a positive-electrode active material on
which no rare-earth element compound is deposited.
[0053] FIG. 8 is a scanning electron micrograph (.times.30000)
showing the surface of a positive-electrode active material
produced in Example C1.
[0054] FIG. 9 is a graph showing discharge curves after storage in
Example D1 and Comparative Example d1.
DETAILED DESCRIPTION OF PREFERRED EXAMPLES
[0055] Hereinafter, the present invention will be described with
reference to specific examples. However, the present invention is
not limited by the following examples, and can be embodied in
various other forms appropriately modified without changing the
spirit of the invention.
Experiment 1
Example A1
Synthesis of Positive-Electrode Active Material
[0056] Lithium cobalt oxide was prepared in which 1.5% by mole of
Mg and 1.5% by mole of Al were dissolved in solid solution and
0.05% by mole of Zr was contained. The lithium cobalt oxide was
obtained by using Li.sub.2CO.sub.3, Co.sub.3O.sub.4, MgO,
Al.sub.2O.sub.3 and ZrO.sub.2 to subject them to a heat treatment
at 850.degree. C. for 24 hours in an air atmosphere.
[0057] One thousand grams of the lithium cobalt oxide was added to
3 L of pure water and stirred to prepare a suspension containing
the lithium cobalt oxide dispersed therein. A solution in which
3.18 g of erbium nitrate pentahydrate was dissolved was added to
the suspension. During the addition, the suspension was
continuously stirred. While the solution in which erbium nitrate
pentahydrate was dissolved was added to the suspension, 10% by mass
of sodium hydroxide aqueous solution was also added to maintain the
pH of the solution containing the lithium cobalt oxide at 9.
[0058] Next, the resultant mixture was suction filtered and then
washed with water, and the resultant powder was dried at
120.degree. C. Thus, a product was obtained in which erbium
hydroxide was uniformly deposited on the surface of the lithium
cobalt oxide (hereinafter, the product in which erbium hydroxide
was deposited on the surface of the lithium cobalt oxide is also
referred to as an Er-coated positive-electrode active
material).
[0059] Note that during the addition, the solution in which erbium
nitrate pentahydrate is dissolved is preferably added stepwise to
the suspension rather than all at once. If the full amount of the
solution is added at once, the hydroxide, when changed from the
solution containing erbium nitrate dissolved therein, is deposited
only on part of the positive-electrode active material, whereby
erbium hydroxide cannot be uniformly deposited on the surface of
the lithium cobalt oxide.
[0060] The lithium cobalt oxide on which erbium hydroxide was
deposited was subjected to a heat treatment at 300.degree. C. for
five hours in an air atmosphere. When the resultant
positive-electrode active material was observed with a scanning
electron microscopy (SEM), particles of an erbium compound having
an average particle diameter of 100 nm or less were deposited in
uniformly dispersed form on the surface of the positive-electrode
active material as shown in FIG. 5. The amount of the erbium
compound deposited was 0.12% by mass, in terms of erbium element,
relative to the mass of the lithium cobalt oxide. Note that the
amount of the erbium compound deposited was measured by ICP
(Inductively Coupled Plasma) spectrometry.
[0061] [Production of Positive Electrode]
[0062] The positive-electrode active material obtained in the above
manner, acetylene black as an electronic conductor, and
N-methyl-2-pyrrolidinone in which poly(vinylidene fluoride) as a
binder was dissolved were mixed, thereby preparing a slurry in
which the positive-electrode active material, the electronic
conductor and poly(vinylidene fluoride) were mixed to give a mass
ratio of 95:2.5:2.5.
[0063] The slurry was applied on both surfaces of aluminum foil,
dried and rolled to have a packing density of 3.60 g/cm.sup.3,
thereby producing a positive electrode.
[0064] [Production of Negative Electrode]
[0065] For the production of a negative electrode, a carbon
material (graphite), CMC (carboxymethylcellulose sodium) and SBR
(styrene butadiene rubber) were mixed in an aqueous solution to
give a mass ratio of 98:1:1, thereby preparing a slurry.
[0066] The slurry was applied on both surfaces of copper foil,
dried and rolled, thereby producing a negative electrode. Note that
the packing density of the negative-electrode active material was
1.7 g/cm.sup.3.
[0067] [Preparation of Nonaqueous Electrolytic Solution]
[0068] LiPF.sub.6 was dissolved in a mixed solvent of ethylene
carbonate (EC) and methylethyl carbonate (MEC) having a volume
ratio EC:MEC of 3:7 to give a concentration of 1.0 mole per liter
of the solvent. The resultant mixture was mixed with 1% by mass of
vinylene carbonate and 1% by mass of adiponitrile as a nitrile
compound, thereby preparing a nonaqueous electrolytic solution.
[0069] [Assembly of Battery]
[0070] Lead terminals were attached to the positive and negative
electrodes produced in the above manners, and a separator was
interposed between the positive and negative electrodes. Then,
these components were helically winded up together and pressed down
in a flattened form, thereby producing an electrode assembly.
[0071] FIG. 1 is a perspective view showing the electrode assembly.
As shown in FIG. 1, the positive electrode is provided with a
positive electrode current collector tab 1 attached thereto, the
negative electrode is provided with a negative electrode current
collector tab 2 attached thereto, and these electrodes are
helically winded up together to form an electrode assembly 3.
[0072] The electrode assembly was placed into a battery outer
package made of an aluminum laminate, thereby producing a
battery.
[0073] FIG. 2 is a plan view showing the produced battery. FIG. 3
is a cross-sectional view taking along the line A-A of FIG. 2.
[0074] As shown in FIGS. 2 and 3, the outer package made of an
aluminum laminate includes an electrode assembly section 4 in which
the electrode assembly 3 is placed, and a spare room section 5 for
storaging gas generated from the electrode assembly 3. The
electrode assembly section 4 and the spare room section 5 are
internally communicated with each other.
[0075] With three sides of the outer package heat-sealed at
respective heat-sealing closure ends 6, the electrode assembly 3
was inserted into the electrode assembly section 4 of the outer
package in a CO.sub.2 atmosphere at 25.degree. C. and 1 atmospheric
pressure. Then, the remaining one side of the outer package was
heat-sealed at the remaining heat-sealing closure end 6, whereby
the outer package was fully sealed.
[0076] The size of the electrode assembly section 4 was 4 mm
(thickness).times.34 mm.times.54 mm. By sandwiching the electrode
assembly section 4 between plates, a structural pressure was always
applied to it. The size of the spare room section 5 was 54
mm.times.50 mm. As described above, gas generated in the electrode
assembly section 4 moves to the spare room section 5. Therefore,
even if gas is generated, the electrode assembly 3 can be prevented
from being swelled by the gas and thereby increasing the resistance
and deteriorating the charge and discharge characteristics.
[0077] Note that the design capacity of the produced battery was
set at 750 mAh with reference to an end-of-charge voltage of 4.4
V.
[0078] In the way described above, the battery of Example A1 was
produced.
Example A2
[0079] A positive-electrode active material was produced in the
same manner as in Example A1 except that instead of erbium nitrate
pentahydrate, 3.54 g of samarium nitrate hexahydrate was used. When
the resultant positive-electrode active material was observed with
a SEM, particles of a samarium compound having an average particle
diameter of 100 nm or less were uniformly deposited on the surface
of the positive-electrode active material, like the
positive-electrode active material in Example A1. The amount of the
samarium compound deposited was 0.12% by mass, in terms of samarium
element, relative to the mass of the lithium cobalt oxide.
[0080] Using the above positive-electrode active material, a
battery of Example A2 was produced in the same manner as in Example
A1.
Example A3
[0081] A positive-electrode active material was produced in the
same manner as in Example A1 except that instead of erbium nitrate
pentahydrate, 3.65 g of neodymium nitrate hexahydrate was used.
When the resultant positive-electrode active material was observed
with a SEM, particles of a neodymium compound having an average
particle diameter of 100 nm or less were uniformly deposited on the
surface of the positive-electrode active material, like the
positive-electrode active material in Example A1. The amount of the
neodymium compound deposited was 0.12% by mass, in terms of
neodymium element, relative to the mass of the lithium cobalt
oxide.
[0082] Using the above positive-electrode active material, a
battery of Example A3 was produced in the same manner as in Example
A1.
Comparative Example a1
[0083] A battery of Comparative Example a1 was produced in the same
manner as in Example A1 except that in Example A1 no adiponitrile
was added to the nonaqueous electrolytic solution.
Comparative Example a2
[0084] In producing a positive-electrode active material in Example
Al, erbium oxide was added to a source material for the lithium
cobalt oxide, thereby producing lithium cobalt oxide in which 0.12%
by mass of erbium was dissolved in solid solution. A battery of
Comparative Example a2 was produced in the same manner as in
Example A1 except that the obtained lithium cobalt oxide was used
as a positive-electrode active material.
Comparative Example a3
[0085] A battery of Comparative Example a3 was produced in the same
manner as in Example A1 except that the positive-electrode active
material produced in Comparative Example a2 was used and no
adiponitrile was added to the nonaqueous electrolytic solution.
Comparative Example a4
[0086] An amount of 1.37 g of erbium oxide was added to 1 kg of the
lithium cobalt oxide obtained in the production of the
positive-electrode active material in Example A1, followed by
mixing in a mortar mixer, thereby obtaining lithium cobalt oxide in
which erbium oxide was mixed. When the obtained positive-electrode
active material was observed with a SEM, particles of erbium oxide
had an average particle size of 400 nm or more and were localized
on the surface of the lithium cobalt oxide. A battery of
Comparative Example a4 was produced in the same manner as in
Example A1 except that the above positive-electrode active material
was used.
Comparative Example a5
[0087] A battery of Comparative Example a5 was produced in the same
manner as in Example A1 except that the positive-electrode active
material produced in Comparative Example a4 was used as a
positive-electrode active material and no adiponitrile was added to
the nonaqueous electrolytic solution.
Comparative Example a6
[0088] A positive-electrode active material was produced in the
same manner as in Example A1 except that instead of erbium nitrate
pentahydrate, 3.51 g of zirconium oxynitrate dihydrate was used.
When the resultant positive-electrode active material was observed
with a SEM, particles of a zirconium compound having an average
particle diameter of 100 nm or less were uniformly deposited on the
surface of the positive-electrode active material, like the
positive-electrode active material in Example A1 (hereinafter, the
product in which a zirconium compound was deposited on the surface
of the lithium cobalt oxide is also referred to as a Zr-coated
positive-electrode active material).
[0089] A battery of Comparative Example a6 was produced in the same
manner as in Example A1 except that the above positive-electrode
active material was used.
Comparative Example a7
[0090] A battery of Comparative Example a7 was produced in the same
manner as in Example A1 except that the positive-electrode active
material produced in Comparative Example a6 was used and no
adiponitrile was added to the nonaqueous electrolytic solution.
Comparative Example a8
[0091] A battery of Comparative Example a8 was produced in the same
manner as in Example A1 except that in the preparation of the
electrolytic solution in Example Al, malononitrile was used instead
of adiponitrile.
Comparative Example a9
[0092] A battery of Comparative Example a9 was produced in the same
manner as in Example A1 except that the positive-electrode active
material, produced in Comparative Example a6, on the surface of
which the Zr compound was deposited was used and malononitrile was
used instead of adiponitrile.
Comparative Example a10
[0093] A battery of Comparative Example a10 was produced in the
same manner as in Example A2 except that in Example A2 no
adiponitrile was added to the nonaqueous electrolytic solution.
Comparative Example a11
[0094] A battery of Comparative Example a11 was produced in the
same manner as in Example A3 except that in Example A3 no
adiponitrile was added to the nonaqueous electrolytic solution.
[0095] <Evaluation of Batteries>
[0096] In the measurement of initial charge/discharge capacity and
60.degree. C. continuous charge test described below, each battery
was subjected to a charge-discharge cycle test under the following
charge and discharge conditions.
[0097] Charge: Each battery was charged to 4.4 V at a constant
current of 1 It (750 mA) and then charged to a current of 37.5 mA
at a constant voltage of 4.4 V.
[0098] Discharge: Each battery was discharged to 2.75 V at a
constant current of 1 It (750 mA).
[0099] Pause: The pause interval between charging and discharging
was set at 10 minutes.
[0100] [Measurement of Initial Discharge Capacity]
[0101] After the battery was subjected to a single charge-discharge
cycle test under the above charge and discharge conditions, its
initial discharge capacity (Q.sub.0) was measured.
[0102] [60.degree. C. Continuous Charge Test]
[0103] The battery was subjected to a single charge-discharge cycle
test under the above charge and discharge conditions to confirm
that the same initial discharge capacity (Q.sub.0) as above was
obtained. Then, the battery was allowed to stand for an hour in a
constant temperature bath at 60.degree. C. Thereafter, the battery
was charged to 4.4 V at a constant current of 750 mA while being
still placed in an environment at 60.degree. C., and further
charged at a constant voltage of 4.4 V. The total charging period
of time was 80 hours.
[0104] After the continuous charging, the increase in battery
thickness of the spare room section from that before the continuous
charging was measured. Thereafter, the battery was cooled down to
the room temperature, the first discharge capacity (Q.sub.1) after
the continuous charge test and the second discharge capacity
(Q.sub.2) after the continuous charge test were measured at the
room temperature, and the remaining capacity rate and the recovery
capacity rate were obtained from the following equations.
Remaining capacity rate (%)={(first discharge capacity (Q.sub.1)
after continuous charge test)/(discharge capacity (Q.sub.0) before
continuous charge test)}.times.100
Recovery capacity rate (%)={(second discharge capacity (Q.sub.2)
after continuous charge test)/(discharge capacity (Q.sub.0) before
continuous charge test)}.times.100
[0105] Furthermore, as shown in FIG. 4, a maximum discharge voltage
drop .DELTA.V1 was obtained from the initial discharge curve during
measurement of the remaining capacity and the initial discharge
curve during measurement of the initial discharge capacity, and a
maximum discharge voltage drop .DELTA.V2 was obtained from the
initial discharge curve during measurement of the recovery capacity
and the initial discharge curve during measurement of the initial
discharge capacity. Note that in FIG. 4 the initial discharge curve
during measurement of the initial discharge capacity is indicated
by "Before continuous charging", the initial discharge curve during
measurement of the remaining capacity is indicated by "Remaining",
and the initial discharge curve during measurement of the recovery
capacity is indicated by "Recovery".
[0106] TABLE 1 shows the results of the remaining capacity rates,
recovery capacity rates, .DELTA.V1, .DELTA.V2 and thickness
increases of the battery spare room sections of the above Examples
and Comparative Examples.
TABLE-US-00001 TABLE 1 Positive- Remaining Recovery .DELTA.V1
(Initial .DELTA.V2 (Initial Electrode Nitrile Compound Capacity
Capacity Thickness Voltage Drop during Voltage Drop during Active
Amount Rate Rate Increase Remaining Discharge) Recovery Discharge)
Material Type Added (%) (%) (%) (mm) (mV) (mV) Example A1 Er-coated
Adiponitrile 1 89.4 84.9 0.10 83 0 NCC.sub.4H.sub.8CN Example A2
Sm-coated Adiponitrile 1 88 84.5 0.13 90 0 NCC.sub.4H.sub.8CN
Example A3 Nd-coated Adiponitrile 1 88.2 84.6 0.12 90 0
NCC.sub.4H.sub.8CN Comparative Er-coated -- 0 85.4 83.6 0.95 247 60
Example a1 Comparative Er-dissolved in Adiponitrile 1 82.4 76.8
0.56 432 252 Example a2 solid solution NCC.sub.4H.sub.8CN
Comparative Er-dissolved in -- 0 69.6 65.9 3.64 369 204 Example a3
solid solution Comparative Er-added Adiponitrile 1 76.4 66.5 1.47
675 562 Example a4 NCC.sub.4H.sub.8CN Comparative Er-added -- 0
55.5 55.7 4.49 552 427 Example a5 Comparative Zr-coated
Adiponitrile 1 86.4 81.3 0.36 351 90 Example a6 NCC.sub.4H.sub.8CN
Comparative Zr-coated -- 0 78.5 78.6 2.12 230 106 Example a7
Comparative Er-coated Malononitrile 1 70.2 65.2 0.48 249 120
Example a8 NCCH.sub.2CN Comparative Zr-coated Malononitrile 1 46.2
42 3.27 562 460 Example a9 NCCH.sub.2CN Comparative Sm-coated -- 0
82.2 83.1 0.96 251 70 Example a10 Comparative Nd-coated -- 0 82.1
82.9 0.99 260 70 Example a11
[0107] As shown in TABLE 1, the batteries of Examples A1 to A3
according to the present invention exhibit higher remaining
capacity rates and higher recovery capacity rates than those of
Comparative Examples a1 to a11. Furthermore, the batteries of
Examples A1 to A3 according to the present invention significantly
reduce the voltage drop .DELTA.V1 in the initial stage of remaining
discharge and the voltage drop .DELTA.V2 in the initial stage of
recovery discharge as compared to the batteries of Comparative
Examples a1 to a11.
[0108] Since the batteries of Comparative Examples a1, a10 and a11
contain no nitrile compound added thereto, the voltage drop
.DELTA.V1 in the initial stage of remaining discharge is large. It
can be assumed that the reason for this is that since no nitrile
compound is added to the batteries, the decomposition of the
electrolytic solution could not be sufficiently inhibited and a
film was formed during high-temperature continuous charging.
[0109] In Comparative Examples a2 and a3, erbium is not deposited
on the surface of the positive-electrode active material but
dissolved in solid solution in the interior thereof. In Comparative
Example a3 to which no nitrile compound is added, the decomposition
reaction of the electrolytic solution is significant, so that a
thick film of a decomposed material of the electrolytic solution is
formed. This increases .DELTA.V1 and lowers the remaining capacity
itself. On the other hand, in Comparative Example a2 to which a
nitrile compound is added, the decomposition reaction of the
electrolytic solution is inhibited by a coating of the nitrile
compound formed during initial charging and discharging, whereby
gas generation and decrease in remaining capacity are inhibited.
However, .DELTA.V1 is very large.
[0110] In Comparative Examples a4 and a5, an erbium compound is not
deposited in uniformly dispersed form on the surface of the
positive-electrode active material unlike Example A1, but localized
on part of the surface of the positive-electrode active material.
In Comparative Example a5 to which no nitrile compound is added,
the decomposition reaction of the electrolytic solution is
significant, so that a thick film of a decomposed material of the
electrolytic solution is formed. This increases .DELTA.V1 and
lowers the remaining capacity itself. On the other hand, in
Comparative Example a4 to which a nitrile compound is added, the
decomposition reaction of the electrolytic solution is inhibited by
a coating of the nitrile compound formed during initial charging
and discharging, whereby gas generation and decrease in remaining
capacity are inhibited. However, .DELTA.V1 is very large.
[0111] In Comparative Examples a6 and a7, a Zr compound is
deposited in dispersed form on the surface of the
positive-electrode active material. However, the Zr compound is
less effective in inhibiting the decomposition reaction of the
electrolytic solution, so that gas is generated and a film of a
decomposed material of the electrolytic solution is formed.
Therefore, .DELTA.V1 is large. In Comparative Example a6 to which a
nitrile compound is added, a coating of the nitrile compound is
formed during initial charging and discharging. However, even if
the Zr compound is deposited on the surface of the
positive-electrode active material, there is no effect of
inhibiting further decomposition of the electrolytic solution as in
gaps in the coating formed by the decomposition of the nitrile
compound. Therefore, during the subsequent charging and
discharging, a new film is formed from a decomposed material of the
electrolytic solution. For this reason, during measurement of the
remaining capacity, the resistance increases and .DELTA.V1 becomes
larger than that in Comparative Example a7 to which no nitrile
compound is added.
[0112] In Comparative Examples a8 and a9, malononitrile, which is a
nitrile compound whose number of carbon atoms is three, is used as
a nitrile compound. In this case, even if erbium is deposited in
dispersed form on the surface of the positive-electrode active
material, the remaining capacity significantly decreases and
.DELTA.V1 also significantly decreases.
[0113] [Observation with Scanning Electron Microscope (SEM)]
[0114] FIG. 5 is a scanning electron micrograph showing the surface
of the positive-electrode active material produced in Example A1.
FIG. 6 is a scanning electron micrograph showing the surface of the
positive-electrode active material produced in Comparative Example
a4. FIG. 7 is a scanning electron micrograph showing the surface of
the lithium cobalt oxide before erbium hydroxide is deposited
thereon.
[0115] FIG. 5 shows that in the positive-electrode active material
of Example A1, particles having an average particle diameter of 100
nm or less are deposited in uniformly dispersed form on the
surface.
[0116] FIG. 6 shows that in the positive-electrode active material
of Comparative Example a4 to which erbium oxide is added and mixed,
no particles are deposited on the surface. A particle located in
the center of FIG. 6 is an erbium oxide particle having a particle
diameter of 300 to 400 nm.
[0117] As shown in FIG. 7, such particles as shown in FIG. 5 are
not observed on the surface of the positive-electrode active
material on which no rare earth element compound has been
deposited.
Experiment 2
[0118] In this experiment, examination was made by changing the
type of nitrile compound.
Example B1
[0119] A battery of Example B1 was produced in the same manner as
in Example A1 except that instead of adiponitrile, 1% by mass of
succinonitrile was added to the nonaqueous electrolytic
solution.
Example B2
[0120] A battery of Example B2 was produced in the same manner as
in Example A1 except that instead of adiponitrile, 1% by mass of
glutaronitrile was added to the nonaqueous electrolytic
solution.
Example B3
[0121] A battery of Example B3 was produced in the same manner as
in Example A1 except that instead of adiponitrile, 1% by mass of
pimelonitrile was added to the nonaqueous electrolytic
solution.
Example B4
[0122] A battery of Example B4 was produced in the same manner as
in Example A1 except that instead of adiponitrile, 1% by mass of
1,2,3-propanetricarbonitrile was added to the nonaqueous
electrolytic solution.
Example B5
[0123] A battery of Example B5 was produced in the same manner as
in Example A1 except that instead of adiponitrile, 1% by mass of
1,3,5-pentanetricarbonitrile was added to the nonaqueous
electrolytic solution.
Example B6
[0124] A battery of Example B6 was produced in the same manner as
in Example A1 except that instead of adiponitrile, 1% by mass of
butyronitrile was added to the nonaqueous electrolytic
solution.
Example B7
[0125] A battery of Example B7 was produced in the same manner as
in Example A1 except that instead of adiponitrile, 1% by mass of
valeronitrile was added to the nonaqueous electrolytic
solution.
Example B8
[0126] A battery of Example B8 was produced in the same manner as
in Example A1 except that instead of adiponitrile, 1% by mass of
n-heptanenitrile was added to the nonaqueous electrolytic
solution.
Comparative Example b1
[0127] A battery of Comparative Example b1 was produced in the same
manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of succinonitrile was added to the
nonaqueous electrolytic solution.
Comparative Example b2
[0128] A battery of Comparative Example b2 was produced in the same
manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of glutaronitrile was added to the
nonaqueous electrolytic solution.
Comparative Example b3
[0129] A battery of Comparative Example b3 was produced in the same
manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of pimelonitrile was added to the
nonaqueous electrolytic solution.
Comparative Example b4
[0130] A battery of Comparative Example b4 was produced in the same
manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of 1,2,3-propanetricarbonitrile was added
to the nonaqueous electrolytic solution.
Comparative Example b5
[0131] A battery of Comparative Example b5 was produced in the same
manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of 1,3,5-pentanetricarbonitrile was added
to the nonaqueous electrolytic solution.
Comparative Example b6
[0132] A battery of Comparative Example b6 was produced in the same
manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of butyronitrile was added to the
nonaqueous electrolytic solution.
Comparative Example b7
[0133] A battery of Comparative Example b7 was produced in the same
manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of valeronitrile was added to the
nonaqueous electrolytic solution.
Comparative Example b8
[0134] A battery of Comparative Example b8 was produced in the same
manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of n-heptanenitrile was added to the
nonaqueous electrolytic solution.
Comparative Example b9
[0135] A battery of Comparative Example b9 was produced in the same
manner as in Example A1 except that instead of adiponitrile, 1% by
mass of acetonitrile was added to the nonaqueous electrolytic
solution.
Comparative Example b10
[0136] A battery of Comparative Example b10 was produced in the
same manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of acetonitrile was added to the
nonaqueous electrolytic solution.
Comparative Example b11
[0137] A battery of Comparative Example b11 was produced in the
same manner as in Example A1 except that instead of adiponitrile,
1% by mass of propionitrile was added to the nonaqueous
electrolytic solution.
Comparative Example b12
[0138] A battery of Comparative Example b12 was produced in the
same manner as in Comparative Example a6 except that instead of
adiponitrile, 1% by mass of propionitrile was added to the
nonaqueous electrolytic solution.
[0139] [Evaluation of Batteries]
[0140] In the same manner as in Experiment 1, the obtained
batteries of the above Examples and Comparative Examples were
evaluated. The evaluation results are shown in TABLES 2 and 3.
TABLE-US-00002 TABLE 2 Positive- Remaining Recovery .DELTA.V1
(Initial .DELTA.V2 (Initial Electrode Nitrile Compound Capacity
Capacity Thickness Voltage Drop during Voltage Drop during Active
Amount Rate Rate Increase Remaining Discharge) Recovery Discharge)
Material Type Added (%) (%) (%) (mm) (mV) (mV) Example A1 Er-coated
Adiponitrile 1 89.4 84.9 0.1 83 0 NCC.sub.4H.sub.8CN Example B1
Er-coated Succinonitrile 1 90.2 86 0.1 170 0 NCC.sub.2H.sub.4CN
Comparative Zr-coated Succinonitrile 1 85.7 81.1 0.72 350 138
Example b1 NCC.sub.2H.sub.4CN Example B2 Er-coated Glutaronitrile 1
93.2 88.5 0.1 95 0 NCC.sub.3H.sub.6CN Comparative Zr-coated
Glutaronitrile 1 85.6 81.9 0.08 293 97 Example b2
NCC.sub.3H.sub.6CN Example B3 Er-coated Pimelonitrile 1 93.1 87.2
0.27 95 0 NCC.sub.5H.sub.10CN Comparative Zr-coated Pimelonitrile 1
88.5 86.8 0.12 232 38 Example b3 NCC.sub.5H.sub.10CN Example B4
Er-coated 1,2,3-Propanetri- 1 91.9 86.8 0.01 83 0 carbonitrile
NCCH.sub.2CHCNCH.sub.2CN Comparative Zr-coated 1,2,3-Propanetri- 1
88.4 83.7 0.06 230 65 Example b4 carbonitrile
NCCH.sub.2CHCNCH.sub.2CN Example B5 Er-coated 1,3,5-Pentanetri- 1
92.4 86.7 0.12 84 0 carbonitrile
NCC.sub.2H.sub.4CHCNC.sub.2H.sub.4CN Comparative Zr-coated
1,3,5-Pentanetri- 1 88.9 86.4 0.18 220 30 Example b5 carbonitrile
NCC.sub.2H.sub.4CHCNC.sub.2H.sub.4CN
TABLE-US-00003 TABLE 3 Positive- Remaining Recovery .DELTA.V1
(Initial .DELTA.V2 (Initial Electrode Nitrile Compound Capacity
Capacity Thickness Voltage Drop during Voltage Drop during Active
Amount Rate Rate Increase Remaining Discharge) Recovery Discharge)
Material Type Added (%) (%) (%) (mm) (mV) (mV) Example B6 Er-coated
Butyronitrile 1 87.9 82.3 0.3 193 30 C.sub.3H.sub.7CN Comparative
Zr-coated Butyronitrile 1 77.8 73.7 1.91 334 27 Example b6
C.sub.3H.sub.7CN Example B7 Er-coated Valeronitrile 1 89 84.7 0.44
193 24 C.sub.4H.sub.9CN Comparative Zr-coated Valeronitrile 1 80.3
75.3 0.45 330 29 Example b7 C.sub.4H.sub.9CN Example B8 Er-coated
n-Heptanenitrile 1 89.6 84.3 0.26 174 25 C.sub.6H.sub.13CN
Comparative Zr-coated n-Heptanenitrile 1 83.4 77.3 0.26 330 29
Example b8 C.sub.6H.sub.13CN Comparative Er-coated Acetonitrile 1
53.7 51.3 4.65 746 53 Example b9 CH.sub.3CN Comparative Zr-coated
Acetonitrile 1 51 43.1 5.72 823 50 Example b10 CH.sub.3CN
Comparative Er-coated Propionitrile 1 76.2 72.7 2.25 422 42 Example
b11 C.sub.2H.sub.5CN Comparative Zr-coated Propionitrile 1 61.6
56.2 2.08 586 50 Example b12 C.sub.2H.sub.5CN
[0141] It can be seen from TABLES 2 and 3 that also if the nitrile
compound added to the electrolytic solution is succinonitrile,
glutaronitrile, pimelonitrile, 1,2,3-propanetricarbonitrile,
1,3,5-pentanetricarbonitrile, butyronitrile, valeronitrile or
n-heptanenitrile, the batteries using an Er-coated
positive-electrode active material (Examples B1 to B8) increase the
remaining capacity and recovery capacity after high-temperature
continuous charging and significantly reduce the discharge voltage
drop .DELTA.V1 during measurement of the remaining capacity and the
discharge voltage drop .DELTA.V2 during measurement of the recovery
capacity, as compared to the batteries using a Zr-coated
positive-electrode active material (Comparative Examples b1 to
b8).
[0142] A comparison of Examples A1 and B1 to B5 with Examples B6 to
B8 indicates that the addition of a nitrile compound having two or
more nitrile groups to the electrolytic solution more significantly
reduces the discharge voltage drop .DELTA.V1 during measurement of
the remaining capacity and the discharge voltage drop .DELTA.V2
during measurement of the recovery capacity.
[0143] A comparison of Examples A1 and B1 to B3 with Examples B4
and B5 indicates that the addition of a nitrile compound having
three or more nitrile groups to the electrolytic solution is more
effective in reducing the discharge voltage drop .DELTA.V1 during
measurement of the remaining capacity.
[0144] A comparison of Example B1 with Examples A1 and B2 to B5
indicates that in nitrile compounds having two or more nitrile
groups, those having five or more carbon atoms, i.e., those having
three or more carbons in each chain saturated hydrocarbon group,
are particularly highly effective in reducing .DELTA.V1.
[0145] Furthermore, as seen from Comparative Examples b9 to b12, if
a nitrile compound is added to the electrolytic solution but the
number of carbon atoms in the nitrile compound is three or less,
even batteries using an Er-coated positive-electrode active
material do not provide much improvement in all of the remaining
capacity rate, the recovery capacity rate, the discharge voltage
drop .DELTA.V1 during measurement of the remaining capacity and the
discharge voltage drop .DELTA.V2 during measurement of the recovery
capacity, as compared to the batteries using a Zr-coated
positive-electrode active material.
Experiment 3
[0146] In this experiment, examination was made of methods for
depositing a rare earth compound.
Example C1
[0147] While 1000 g of the same lithium cobalt oxide powder as used
in Example A1 was stirred with a mixer "T. K. HIVIS MIX"
manufactured by PRIMIX Corporation, a solution of 2.98 g of erbium
acetate tetrahydrate in 150 mL of pure water was sprayed on the
lithium cobalt oxide powder. Thereby, the surface of the lithium
cobalt oxide powder was moistened with the erbium salt solution.
Note that the powder and liquid were not separated but the entire
lithium cobalt oxide powder was moistened.
[0148] The powder was dried at 120.degree. C. Thus, a product was
obtained in which erbium hydroxide was uniformly deposited on the
surface of the lithium cobalt oxide. The lithium cobalt oxide on
which erbium hydroxide was deposited was subjected to a heat
treatment at 300.degree. C. for five hours in an air atmosphere.
Thus, a positive-electrode active material on the surface of which
an erbium oxyhydroxide compound was deposited was obtained. The
amount of the erbium compound deposited was 0.12% by mass, in terms
of erbium element, relative to the mass of the lithium cobalt
oxide. Note that the amount of the erbium compound deposited was
measured by ICP spectrometry.
[0149] When the resultant positive-electrode active material was
observed with a scanning electron microscopy (SEM), particles of an
erbium compound having an average particle diameter of 100 nm or
less were deposited in uniformly dispersed form on the surface of
the positive-electrode active material as shown in FIG. 8.
[0150] Using the obtained positive-electrode active material, a
battery of Example C1 was produced in the same manner as in Example
A1. Note that the electrolytic solution used was an electrolytic
solution to which 1% by mass of adiponitrile was added like Example
A1.
Comparative Example c1
[0151] A battery of Comparative Example c1 was obtained in the same
manner as in Example C1 except that in Example C1, an electrolytic
solution to which no adiponitrile was added was used.
[0152] Using the batteries of Example C1 and Comparative Example
c1, the same test as for the battery of Example A1 was performed.
The evaluation results are shown in TABLE 4.
TABLE-US-00004 TABLE 4 Positive- Remaining Recovery .DELTA.V1
(Initial .DELTA.V2 (Initial Electrode Nitrile Compound Capacity
Capacity Thickness Voltage Drop during Voltage Drop during Active
Amount Rate Rate Increase Remaining Discharge) Recovery Discharge)
Material Type Added (%) (%) (%) (mm) (mV) (mV) Example A1 Er-coated
Adiponitrile 1 89.4 84.9 0.1 83 0 NCC.sub.4H.sub.8CN Example C1
Er-coated Adiponitrile 1 89.2 84.5 0.22 90 0 NCC.sub.4H.sub.8CN
Comparative Er-coated -- 0 84.6 83.3 1.25 262 78 Example c1
[0153] As shown in TABLE 4, the battery of Example C1 increases the
remaining capacity and recovery capacity after high-temperature
continuous charging and significantly reduces the battery thickness
increase, the discharge voltage drop .DELTA.V1 during measurement
of the remaining capacity and the discharge voltage drop .DELTA.V2
during measurement of the recovery capacity, as compared to the
battery of Comparative Example c1.
[0154] Now comparing the SEM photograph of FIG. 8 with the SEM
photograph of FIG. 5 corresponding to the surface of the
positive-electrode active material of Example A1, there exist a
large number of 10 to 30 nm diameter particles in the latter, while
there exist a large number of 50 nm or more diameter particles in
the former. It can be assumed that in the method of Example C1, the
rare earth salt solution and the positive-electrode active material
directly react at the surface of the positive-electrode active
material, whereby particles of the erbium compound deposited on the
surface of the positive-electrode active material of Example C1
have larger diameters than those in Example A1. Referring to TABLE
4, the battery of Example C1 is slightly poorer in remaining
capacity and recovery capacity after high-temperature continuous
charging, battery thickness increase and discharge voltage drop
.DELTA.V1 during measurement of the remaining capacity than the
battery of Example A1.
[0155] It can be assumed that this is due to a difference in
particle size between the erbium compounds.
Experiment 4
[0156] In this experiment, evaluation is made of the battery
characteristics of cylindrical batteries.
Example D1
[0157] A positive electrode for an 18650 size cylindrical battery
was produced by the same manufacturing method as in Example A1.
Note that the packing density of the electrode was 3.9 g/cm.sup.3.
A negative electrode was also produced by the same manufacturing
method as in Example A1. Note that the packing density of the
negative electrode was 1.68 g/cm.sup.3.
[0158] A separator was interposed between the obtained positive and
negative electrodes, and these components were helically winded up
together to produce an electrode assembly. The electrode assembly
was inserted into a cylindrical battery casing, and an electrolytic
solution was poured into the battery casing. The electrolytic
solution was prepared as follows: First, EC and MEC were mixed to
have a volume ratio of 3:7. Then added to the mixture was
LiPF.sub.6 to give a concentration of 1 mole per liter of the
mixture. Added to the resultant mixture were 2% by mass of vinylene
carbonate and 1% by mass of adiponitrile as a nitrile compound. The
capacity of the battery was 2500 mAh, and the capacity ratio
between the positive and negative electrodes was 1.05.
Comparative Example d1
[0159] A cylindrical battery was produced in the same manner as in
Example D1 except that the positive-electrode active material of
Comparative Example a6 was used.
[0160] [Evaluation of Battery Characteristics]
[0161] The batteries of Example D1 and Comparative Example d1 were
evaluated for battery characteristics in the following manner.
[0162] Each battery was charged to 4.35 V at a constant current of
1250 mA at the room temperature and then charged to a current value
of 50 mA at a constant voltage of 4.35 V. Next, the battery was
discharged to 2.75 V at 2500 mA at the room temperature, thereby
completing one charge and discharge cycle. This charge and
discharge cycle was carried out three times.
[0163] After the measurement of the third discharge capacity, each
battery was charged again to 4.35 V at a constant current of 1250
mA at the room temperature and then charged to a current value of
50 mA at a constant voltage of 4.35 V. Next, the battery was stored
at 60.degree. C. for 20 days. After cooled down to the room
temperature, the battery was discharged to a cutoff voltage of 2.75
V at 2500 mA. The discharge curves in this case are shown in FIG.
9.
[0164] As shown in FIG. 9, Comparative Example d1 exhibiting a
large .DELTA.V1 in the test of Experiment 1 sharply drops the
voltage to around 2.9 V in the initial stage of discharge. On the
other hand, Example D1 exhibiting a small .DELTA.V1 in the test of
Experiment 1 drops the voltage only to about 3.5 V even in the
initial stage of discharge.
[0165] Note that the cutoff voltage of the battery is not limited
to 2.75 V and the battery may be designed to have a cutoff voltage
of 3V, for example. If the cutoff voltage is set at 3 V, the
battery of Comparative Example d1 has a remaining capacity of only
less than 10% of the discharge capacity before the battery storage.
Therefore, it will be appreciated that it is important to minimize
.DELTA.V1 and .DELTA.V2 in Experiments 1, 2 and 3.
* * * * *