U.S. patent application number 10/845094 was filed with the patent office on 2004-10-21 for cathode active material for non-aqueous electrolyte secondary cell and process for producing the same.
This patent application is currently assigned to Toda Kogyo Corporation. Invention is credited to Fujino, Shoichi, Hatatani, Mitsuaki, Maeda, Hideaki, Sadamura, Hideaki, Sugiyama, Norimiki, Watanabe, Hiroyasu.
Application Number | 20040208818 10/845094 |
Document ID | / |
Family ID | 18834616 |
Filed Date | 2004-10-21 |
United States Patent
Application |
20040208818 |
Kind Code |
A1 |
Maeda, Hideaki ; et
al. |
October 21, 2004 |
Cathode active material for non-aqueous electrolyte secondary cell
and process for producing the same
Abstract
A cathode active material for a non-aqueous electrolyte
secondary cell having a c-axis length of lattice constant of 14.080
to 14.160 .ANG., an average particle size of 0.1 to 5.0 .mu.m, and
a composition represented by the formula:
LiCo.sub.(1-x-y)Mn.sub.xMg.sub.yO.sub.2 where x is a number of
0.008 to 0.18; and y is a number of 0 to 0.18. This cathode active
material is capable of maintaining an initial discharge capacity
required for secondary cells and shows improved charge/discharge
cycle characteristics under high temperature conditions.
Inventors: |
Maeda, Hideaki; (Ube-shi,
JP) ; Fujino, Shoichi; (Onoda-shi, JP) ;
Hatatani, Mitsuaki; (Ube-shi, JP) ; Watanabe,
Hiroyasu; (Ube-shi, JP) ; Sugiyama, Norimiki;
(Ube-shi, JP) ; Sadamura, Hideaki; (Onoda-shi,
JP) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
Toda Kogyo Corporation
Hiroshima-ken
JP
|
Family ID: |
18834616 |
Appl. No.: |
10/845094 |
Filed: |
May 14, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10845094 |
May 14, 2004 |
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09994903 |
Nov 28, 2001 |
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6756154 |
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Current U.S.
Class: |
423/594.6 ;
423/599; 429/224; 429/231.3; 429/231.6 |
Current CPC
Class: |
C01G 51/50 20130101;
H01M 4/505 20130101; C01G 51/44 20130101; C01P 2002/60 20130101;
C01P 2004/64 20130101; H01M 2004/028 20130101; H01M 2004/021
20130101; H01M 4/131 20130101; C01P 2004/62 20130101; C01P 2006/40
20130101; Y02E 60/10 20130101; H01M 10/0525 20130101; C01G 45/1228
20130101; C01P 2002/52 20130101; H01M 4/525 20130101; B82Y 30/00
20130101; C01P 2006/12 20130101; C01P 2002/54 20130101; C01P
2002/77 20130101; H01M 4/02 20130101; C01P 2006/80 20130101 |
Class at
Publication: |
423/594.6 ;
429/231.3; 429/231.6; 429/224; 423/599 |
International
Class: |
C01G 051/04; H01M
004/48; H01M 004/50; C01G 045/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2000 |
JP |
2000-363511 |
Claims
1.-4. Canceled.
5. A process for producing the cathode active material for a
non-aqueous electrolyte secondary cell, comprising: adding an
aqueous alkali solution to a solution containing a cobalt salt and
a manganese salt with or without a magnesium salt to conduct a
neutralization reaction therebetween; oxidizing a resultant mixed
solution by passing an oxygen-containing gas therethrough to obtain
a cobalt oxide containing manganese or both manganese and
magnesium; mixing the cobalt oxide with a lithium compound; and
heat-treating a resultant mixture of the cobalt oxide and the
lithium compound.
6. A process according to claim 5, wherein the manganese salt is
added in an amount of 0.1 to 20 mol %, calculated as manganese,
based on cobalt; the magnesium salt is added in an amount of not
more than 20 mol %, calculated as magnesium, based on cobalt; the
equivalent ratio of alkali contained in the aqueous alkali solution
added for the neutralization reaction to a neutralized component of
the whole metal salts is 1.0 to 1.2; the oxidation reaction
temperature is not less than 30.degree. C.; the mixing molar ratio
of lithium to a sum of cobalt and manganese is 0.95 to 1.05; and
the heat-treating temperature is 600 to 950.degree. C.
7.-10. Canceled.
11. The process of claim 5 wherein cathode active material has a
c-axis length of lattice constant of 14.080 to 14.160 .ANG., an
average particle size of 0.1 to 5.0 .mu.m, and a composition
represented by the formula: LiCo.sub.(1-x-y)Mn.sub.xMg.sub.yO.sub.2
wherein x is a number of 0.008 to 0.18; and y is a number of 0 to
0.18.
12. The process according to claim 11 wherein the cathode active
material has an a-axis length of lattice constant of 2.81 to 2.83
.ANG..
13. The process according to claim 11 wherein the cathode active
material has a BET specific surface area of 0.1 to 2.5 m.sup.2/g
and a crystallite size of 400 to 1,200 .ANG..
14. The process according to claim 11 wherein the cathode active
material in which y is 0.01 to 0.15.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a cathode active material
for a non-aqueous electrolyte secondary cell and a process for
producing the active material, and more particularly, to a cathode
active material for a non-aqueous electrolyte secondary cell which
is capable of maintaining an initial discharge capacity required
for secondary cells and showing improved charge/discharge cycle
characteristics under high temperature conditions, and a process
for producing such a cathode active material.
[0002] With the recent rapid development of portable and cordless
electronic devices such as audio-visual (AV) devices and personal
computers, it has been increasingly demanded to use as a power
source thereof, secondary cells or batteries having a small size, a
light weight and a high energy density. Under this circumstance,
among the secondary cells, lithium ion secondary cells have been
noticed because of advantages such as high charge/discharge
voltages as well as high charge/discharge capacities.
[0003] Hitherto, as cathode active materials useful for high
energy-type lithium secondary cells exhibiting a 4V-grade voltage,
there are generally known LiMn.sub.2O.sub.4 having a spinel
structure, LiMnO.sub.2 having a corrugated layer structure,
LiCoO.sub.2 having a layered rock salt-type structure,
LiCo.sub.1-xNi.sub.xO.sub.2, LiNiO.sub.2 or the like. Among
secondary cells using these active materials, lithium ion secondary
cells using LiCoO.sub.2 are more excellent because of high
charge/discharge voltages and high charge/discharge capacities.
These lithium ion secondary cells have been, however, required to
be further improved in properties thereof.
[0004] Specifically, when the lithium ion secondary cell using
LiCoO.sub.2 as an active material is repeatedly subjected to
charge/discharge cycles, the discharge capacity of the secondary
cell tends to be deteriorated. This is because LiCoO.sub.2
undergoes Jahn-Teller distortion due to conversion of Co.sup.3+ of
LiCoO.sub.2 into Co.sup.4+ when lithium ions are released
therefrom. As the amount of lithium released increases, the crystal
structure of the active material is transformed from hexagonal
system into monoclinic system, and further from monoclinic system
into hexagonal system. In addition, when such release and insertion
reactions of lithium ions are repeated, the lattice of LiCoO.sub.2
suffers from contraction and expansion, resulting in destruction of
the crystal structure of LiCoO.sub.2. As a result, it is assumed
that the charge/discharge cycle characteristics of the secondary
cell are deteriorated.
[0005] Also, when the secondary cell is repeatedly subjected to
charge/discharge reactions (i.e., release and insertion reactions
of lithium ions), the crystal structure of the active material
becomes unstable, thereby causing release of oxygen from the
crystal lattice, or undesired reaction with an electrolyte
solution. Further, the reaction with an electrolyte solution is
more active under high temperature conditions. Therefore, in order
to ensure safety of the secondary cell, it has been required to
provide active materials exhibiting a stable structure and a high
heat stability even under high temperature conditions.
[0006] Since electronic devices using the secondary cell as a power
source, such as note-type personal computers, are exposed to high
temperature due to heat generation upon use, it has been required
to provide secondary cells showing excellent charge/discharge cycle
characteristics even under high temperature conditions.
[0007] Also, the secondary cell using LiCoO.sub.2 can be operated
with a high voltage. However, since the LiCoO.sub.2 is readily
reacted with an electrolyte solution under a high-voltage
condition, the secondary cell tends to be deteriorated in
charge/discharge cycle characteristics.
[0008] For this reason, it has been required to provide lithium
cobaltate particles which are capable of producing secondary cells
exhibiting excellent charge/discharge cycle characteristics even
under high temperature conditions.
[0009] Hitherto, in order to improve various properties, e.g., in
order to stabilize a crystal structure thereof, there are known a
method of incorporating manganese into lithium cobaltate particles
(Japanese Patent Publication (KOKOKU) No. 7-32017 (1995) and
Japanese Patent Application Laid-Open (KOKAI) No. 4-28162 (1992));
a method of incorporating magnesium into lithium cobaltate
particles (Japanese Patent Application Laid-Open (KOKAI) Nos.
6-168722 (1994), 11-102704 (1999), 2000-11993 and 2000-123834); a
method of mixing manganese or magnesium with lithium cobaltate
particles by a wet method (Japanese Patent Application Laid-Open
(KOKAI) Nos. 10-1316 (1998) and 11-67205 (1999)); a method of
controlling a lattice constant of lithium cobaltate to improve
properties thereof (Japanese Patent Application Laid-Open (KOKAI)
No. 6-181062 (1994)); or the like.
[0010] At present, it has been required to provide cathode active
materials satisfying the above requirements. However, such cathode
active materials have not been obtained until now.
[0011] That is, in Japanese Patent Application Laid-Open (KOKAI)
Nos. 7-32017 (1995), 4-28162 (1992), 6-168722 (1994), 11-102704
(1999), 2000-11993 and 2000-123834, it is described that a cobalt
compound, a lithium compound and manganese or magnesium are
dry-mixed with each other, thereby obtaining lithium cobaltate
particles containing manganese or magnesium. However, since the
obtained lithium cobaltate particles have a non-uniform
distribution of manganese or magnesium, the crystal structure
thereof tends to undergo contraction and expansion upon the release
and insertion reactions of lithium ions, resulting in destruction
of the crystal lattice. Thus, the secondary cells using such
lithium cobaltate particles fail to show excellent charge/discharge
cycle characteristics under high temperature conditions.
[0012] In Japanese Patent Application Laid-Open (KOKAI) No. 10-1316
(1998), there is described the process for producing lithium
cobaltate particles by dispersing a cobalt compound and manganese
compound or magnesium compound in an aqueous lithium hydroxide
solution and heat-treating the resultant dispersion. However, this
process requires an additional hydrothermal treatment and,
therefore, is industrially disadvantageous.
[0013] In Japanese Patent Application Laid-Open (KOKAI) No.
11-67205 (1999), there is described the process for producing
lithium cobaltate particles by mixing a solution containing
water-soluble salts of lithium, cobalt and manganese with a citric
acid solution, gelling the resultant mixed solution by removing a
solvent therefrom, and drying and sintering the obtained gel.
However, the obtained lithium cobaltate particles have a large BET
specific surface area and, therefore, undesirably show a high
reactivity with an electrolyte solution when used in secondary
cells.
[0014] Also, in Japanese Patent Application Laid-Open (KOKAI) No.
6-181062 (1994), there is described lithium cobaltate having a
c-axis length of lattice constant of not less than 14.05 .ANG..
However, secondary cells using the above lithium cobaltate cannot
be sufficiently improved in charge/discharge cycle characteristics
under high temperature conditions as compared to those using
lithium cobaltate particles containing manganese or magnesium.
[0015] As a result of the present inventors' earnest studies for
solving the above problems, it has been found that by adding an
aqueous alkali solution to a solution containing a cobalt salt and
a manganese salt with or without a manganese salt to conduct a
neutralization reaction therebetween; oxidizing the resultant
solution to obtain a cobalt oxide containing manganese or both
manganese and magnesium; mixing the thus obtained cobalt oxide with
a lithium compound; and heat-treating the resultant mixture, the
obtained lithium cobaltate particles are useful as a cathode active
material for a non-aqueous secondary cell not only having an
excellent initial discharge capacity, but also exhibiting excellent
charge/discharge cycle characteristics under high temperature
conditions. The present invention has been attained based on the
finding.
SUMMARY OF THE INVENTION
[0016] It is an object of the present invention to provide lithium
cobaltate particles which are useful as a cathode active material
for a non-aqueous secondary cell not only having an excellent
initial discharge capacity but also exhibiting excellent
charge/discharge cycle characteristics under high temperature
conditions.
[0017] It is another object of the present invention to provide a
process for producing the lithium cobaltate particles which are
useful as a cathode active material for a non-aqueous secondary
cell not only having an excellent initial discharge capacity
required for secondary cells, but also exhibiting excellent
charge/discharge cycle characteristics under high temperature
conditions.
[0018] To accomplish the aim, in a first aspect of the present
invention, there is provided a cathode active material for a
non-aqueous electrolyte secondary cell, having a c-axis length of
lattice constant of 14.080 to 14.160 .ANG., an average particle
size of 0.1 to 5.0 .mu.m, and a composition represented by the
formula:
LiCo.sub.(1-x-y)Mn.sub.xMg.sub.yO.sub.2
[0019] wherein x is a number of 0.008 to 0.18; and y is a number of
0 to 0.18.
[0020] In a second aspect of the present invention, there is
provided a process for producing the cathode active material for a
non-aqueous electrolyte secondary cell, comprising:
[0021] adding an aqueous alkali solution to a solution containing a
cobalt salt and a manganese salt with or without a magnesium salt
to conduct a neutralization reaction therebetween;
[0022] oxidizing a resultant mixed solution by passing an
oxygen-containing gas therethrough to obtain a cobalt oxide
containing manganese or both manganese and magnesium;
[0023] mixing the cobalt oxide with a lithium compound; and
[0024] heat-treating a resultant mixture of the cobalt oxide and
the lithium compound.
[0025] In a third aspect of the present invention, there is
provided a cathode active material for a non-aqueous electrolyte
secondary cell, having a c-axis length of lattice constant of
14.080 to 14.160 .ANG., an average particle size of 0.1 to 5.0
.mu.m, and a composition represented by the formula:
LiCo.sub.(1-x-y)Mn.sub.xMg.sub.yO.sub.2
[0026] wherein x is a number of 0.008 to 0.18; and y is a number of
0 to 0.18,
[0027] produced by a process comprising:
[0028] adding an aqueous alkali solution to a solution containing a
cobalt salt and a manganese salt with or without a magnesium salt
to conduct a neutralization reaction therebetween;
[0029] oxidizing a resultant mixed solution by passing an
oxygen-containing gas therethrough to obtain a cobalt oxide
containing manganese or both manganese and magnesium;
[0030] mixing the cobalt oxide with a lithium compound; and
[0031] heat-treating a resultant mixture of the cobalt oxide and
the lithium compound.
[0032] In a fourth aspect of the present invention, there is
provided a cathode active material for a non-aqueous electrolyte
secondary cell, having a c-axis length of lattice constant of
14.080 to 14.160 .ANG., an a-axis length of lattice constant of
2.81 to 2.83 .ANG., a crystallite size of 400 to 1,200 .ANG., an
average particle size of 0.1 to 5.0 .mu.m, and a composition
represented by the formula:
LiCo.sub.(1-x-y)Mn.sub.xMg.sub.yO.sub.2
[0033] wherein x is a number of 0.008 to 0.18; and y is a number of
0 to 0.18.
[0034] In a fifth aspect of the present invention, there is
provided a cathode active material for a non-aqueous electrolyte
secondary cell, having a c-axis length of lattice constant of
14.080 to 14.160 .ANG., an a-axis length of lattice constant of
2.81 to 2.83 .ANG., a crystallite size of 400 to 1,200 .ANG., an
average particle size of 0.1 to 5.0 .mu.m, and a composition
represented by the formula:
LiCo.sub.(1-x-y)Mn.sub.xMg.sub.yO.sub.2
[0035] wherein x is a number of 0.008 to 0.18; and y is a number of
0.01 to 0.15.
[0036] In a sixth aspect of the present invention, there is
provided a non-aqueous electrolyte secondary cell comprising a
lithium ion conductive electrolyte and a pair of electrodes
separated by means of a separator, wherein at least one of said
electrodes comprises a cathode active material having a c-axis
length of lattice constant of 14.080 to 14.160 .ANG., an average
particle size of 0.1 to 5.0 .mu.m, and a composition represented by
the formula:
LiCo.sub.(1-x-y)Mn.sub.xMg.sub.yO.sub.2
[0037] wherein x is a number of 0.008 to 0.18; and y is a number of
0 to 0.18.
DETAILED DESCRIPTION OF THE INVENTION
[0038] The present invention will be described in detail below.
[0039] First, the lithium cobaltate particles as a cathode active
material according to the present invention are described.
[0040] The cathode active material of the present invention is made
of lithium cobaltate particles containing manganese or both
manganese and magnesium. The cathode active material has a
composition represented by the following formula:
LiCo.sub.(1-x-y)Mn.sub.xMg.sub.yO.sub.2.
[0041] The manganese content x is usually 0.008 to 0.18, preferably
0.01 to 0.15. When the manganese content x is less than 0.008, the
effect of improving charge/discharge cycle characteristics of
secondary cells under high temperature conditions may be
insufficient. When the manganese content x is more than 0.18, the
initial discharge capacity thereof may be remarkably deteriorated.
When magnesium is further incorporated into the cathode active
material, it may become possible to further enhance the
charge/discharge cycle characteristics under high temperature
conditions. The magnesium content y is usually not more than 0.18,
preferably 0.01 to 0.15, more preferably 0.001 to 0.1, still more
preferably 0.01 to 0.07. When the magnesium content y is more than
0.18, the effect of increasing the capacity may be
insufficient.
[0042] The cathode active material of the present invention has a
c-axis length of lattice constant of usually 14.080 to 14.160
.ANG., preferably 14.080 to 14.153 .ANG., more preferably 14.085 to
14.153 .ANG.. When the c-axis length of lattice constant is less
than 14.080 .ANG., the crystal lattice of the cathode active
material undergoes remarkable contraction and expansion upon the
release and insertion reactions of lithium ions, resulting in
deterioration in charge/discharge cycle characteristics of
secondary cells under high temperature conditions. Although the
cathode active material having a c-axis length of lattice constant
of more than 14.160 .ANG. is obtainable by increasing the amount of
manganese replaced, such an active material is deteriorated in
initial discharge capacity. In addition, the cathode active
material of the present invention has an a-axis length of lattice
constant of preferably 2.81 to 2.83 .ANG., more preferably 2.815 to
2.825 .ANG..
[0043] The cathode active material of the present invention has an
average particle size of usually 0.1 to 5.0 .mu.m, preferably 0.5
to 5.0 .mu.m, more preferably 1.5 to 5.0 .mu.m, still more
preferably 2.0 to 5.0 .mu.m. When the average particle size is less
than 0.1 .mu.m, the obtained active material may suffer from
disadvantages such as low packing density and increased reactivity
with an electrolyte solution. Also, it may be industrially
difficult to produce such a cathode active material having an
average particle size of more than 5.0 .mu.m.
[0044] The cathode active material of the present invention has a
BET specific surface area of preferably 0.1 to 2.5 m.sup.2/g, more
preferably 0.1 to 2.0 m.sup.2/g, still more preferably 0.1 to 1.7
m.sup.2/g. It may be industrially difficult to produce such a
cathode active material having a BET specific surface area of less
than 0.1 m.sup.2/g. When the BET specific surface area is more than
2.5 m.sup.2/g, the obtained active material may tend to suffer from
disadvantages such as low packing density and increased reactivity
with an electrolyte solution.
[0045] The cathode active material of the present invention has a
crystallite size of preferably 400 to 1,200 .ANG., more preferably
450 to 1,000 .ANG., still more preferably 500 to 850 .ANG..
[0046] The secondary cell produced using the cathode active
material of the present invention exhibits an initial discharge
capacity of preferably 135 to 160 mAh/g, more preferably 140 to 160
mAh/g, still more preferably 150 to 160 mAh/g, and a capacity
retention percentage after 50 cycles at 60%, of preferably 90 to
99%, more preferably 92 to 99%, still more preferably 95 to
99%.
[0047] Next, the process for producing the cathode active material
according to the present invention will be described below.
[0048] The cathode active material of the present invention can be
produced by mixing a cobalt oxide containing manganese or both
manganese and magnesium with a lithium compound, and heat-treating
the resultant mixture.
[0049] The cobalt oxide containing manganese or both manganese and
magnesium may be produced by mixing an aqueous cobalt salt solution
with an aqueous manganese salt solution or an aqueous solution
containing a manganese salt and a magnesium salt, adding alkali to
the resultant mixed solution to conduct a neutralization reaction
therebetween, and then subjecting the obtained solution to
oxidation reaction.
[0050] Examples of the alkali may include aqueous solutions of
sodium hydroxide, potassium hydroxide, sodium carbonate, ammonia
and the like. Among these aqueous solutions, an aqueous sodium
hydroxide solution, an aqueous sodium carbonate solution and a
mixed solution thereof are preferred.
[0051] The amount of the manganese salt added is usually 0.1 to 20
mol %, preferably 2 to 18 mol % (calculated as manganese) based on
cobalt. The amount of the magnesium salt added is usually not more
than 20 mol %, preferably 0.1 to 20 mol %, more preferably 1 to 18
mmol % (calculated as magnesium) based on cobalt.
[0052] The alkali used for the neutralization reaction may be used
in such an amount that the equivalent ratio of the alkali to the
neutralized component of whole metal salt contained in hydroxides
of cobalt and manganese or hydroxides of cobalt, manganese and
magnesium, is preferably 1.0 to 1.2.
[0053] The oxidation reaction may be conducted by passing an
oxygen-containing gas, e.g., air through the solution. The
oxidation reaction temperature is preferably not less than
30.degree. C., more preferably 30 to 95.degree. C., and the
oxidation reaction time is preferably 5 to 20 hours.
[0054] The cobalt oxide containing manganese or both manganese and
magnesium has an average particle size of usually 0.01 to 2.0
.mu.m, preferably 0.05 to 1.0 .mu.m; and a BET specific surface
area of usually 0.5 to 50 m.sup.2/g, preferably 10 to 40
m.sup.2/g.
[0055] In the cobalt oxide containing manganese or both manganese
and magnesium, manganese atoms or both manganese and magnesium
atoms are uniformly distributed in the cobalt oxide when observed
at atomic level. Therefore, when such a cobalt oxide is mixed with
the lithium compound and the resultant mixture is heat-treated, the
cobalt sites thereof may be uniformly replaced with the manganese
atoms or manganese and magnesium atoms.
[0056] As described above, the cobalt oxide containing manganese or
both manganese and magnesium is mixed with the lithium compound,
and the resultant mixture is then heat-treated.
[0057] The mixing of the cobalt oxide containing manganese or both
manganese and magnesium, with the lithium compound may be conducted
by either dry or wet method as long as a uniform mixture can be
obtained.
[0058] The mixing molar ratio of lithium to cobalt and manganese is
0.95 to 1.05.
[0059] The heat-treating temperature is preferably 600 to
950.degree. C., at which LiCoO.sub.2 in the form of a
high-temperature regular phase can be produced. When the
heat-treating temperature is less than 600.degree. C., LiCoO.sub.2
produced may be made of a low-temperature phase having a
pseudo-spinel structure. When the heat-treating temperature is more
than 950.degree. C., LiCoO.sub.2 produced may be made of a
high-temperature irregular phase in which lithium and cobalt are
dispersed at random positions.
[0060] In the case where a cathode is produced using the cathode
active material of the present invention, the cathode active
material is mixed with a conductive agent and a binder by an
ordinary method. As the preferred conductive agent, there may be
exemplified acetylene black, carbon black, graphite or the like. As
the preferred binder, there may be exemplified
polytetrafluoroethylene, polyvinylidene fluoride or the like.
[0061] A secondary cell (lithium battery) according to the present
invention comprises a pair of electrodes disposed by means of a
separator in the presence of a lithium ion conductive
electrolyte.
[0062] A cathode and an anode are disposed in a case so as to be
opposed to each other with a separator composed of a porous
thermoplastic resin film. A lithium ion conductive electrolyte is
present in the case.
[0063] In the secondary cell of the present invention, it is only
necessary that the above-described specific cathode active material
is used for at least one electrode, preferably a cathode active
material, and the other active materials may be the known
substances which are conventionally used for a lithium battery.
[0064] The secondary cell containing the cathode active material of
the present invention, is produced by using the above cathode in
combination with an anode and an electrolyte solution.
[0065] As an active material for the anode, there may be used
metallic lithium, lithium/aluminum alloy, lithium/tin alloy,
graphite or the like.
[0066] In addition, as a solvent for the electrolyte solution,
there may be used a mixed solvent of ethylene carbonate and diethyl
carbonate, an organic solvent containing at least one solvent
selected from the group consisting of carbonates such as propylene
carbonate and dimethyl carbonate, and ethers such as
dimethoxyethane, and the like.
[0067] Further, as the electrolyte solution, there may be used a
solution prepared by dissolving lithium phosphate hexafluoride in
the above solvent as well as those solutions prepared by dissolving
at least one lithium salt selected from the group consisting of
lithium perchlorate, lithium borate tetrafluoride and the like, in
the solvent.
[0068] The point of the present invention is that the secondary
cell produced using the cathode active material composed of lithium
cobaltate particles containing manganese or both manganese and
magnesium not only can retain an adequate initial discharge
capacity required for secondary cells, but also can exhibit
excellent charge/discharge cycle characteristics even under high
temperature conditions.
[0069] The reason why the secondary cell produced using the cathode
active material of the present invention can retain an adequate
initial discharge capacity, is considered as follow. That is, the
cathode active material contains manganese or both manganese and
magnesium in such amounts as not to deteriorate the inherent
initial discharge capacity of LiCoO.sub.2.
[0070] The reason why the cathode active material of the present
invention exhibits a large c-axis length of lattice constant, is
considered by the present inventors as follows. That is, since
manganese or both manganese and magnesium are incorporated into the
cobalt oxide by a wet oxidation reaction, manganese atoms or both
manganese and magnesium atoms are uniformly distributed in cobalt
when observed at atomic level. Therefore, it is suggested that in
the cathode active material obtained from such a cobalt oxide, the
cobalt sites in the active material can be uniformly replaced with
manganese and magnesium atoms.
[0071] Further, since the cathode active material has a large
c-axis length of lattice constant, the release and insertion
reactions of lithium ions are readily performed, so that the
contraction and expansion of the crystal structure in the c-axis
direction thereof is inhibited upon the reactions, thereby
preventing destruction of the crystal lattice. As a result, it is
suggested that the secondary cell produced using such a cathode
active material is excellent in charge/discharge cycle
characteristics under high temperature conditions.
[0072] On the other hand, when the lithium compound, the cobalt
compound, and manganese compound or magnesium compound are
dry-mixed with each other and then calcined, manganese atoms or
magnesium atoms cannot be uniformly distributed in the active
material, thereby failing to obtain the effect of the present
invention.
[0073] By using the cathode active material of the present
invention, it becomes possible to obtain a non-aqueous electrolyte
secondary cell capable of maintaining an initial discharge capacity
required as the secondary cell, and exhibiting improved
charge/discharge cycle characteristics under high temperature
conditions.
EXAMPLES
[0074] The present invention is described in more detail by
Examples and Comparative Examples, but the Examples are only
illustrative and, therefore, not intended to limit the scope of the
present invention thereto.
[0075] Various properties were measured by the following
methods.
[0076] (1) The cathode active material was measured and identified
using a Powder X-ray Diffraction Analyzer manufactured by Rigaku
Denki Kogyo Co., Ltd. (Cu-K.alpha.; 40 kV; 40 mA). Also, the
lattice constant of the active material was calculated from
respective diffraction peaks of the powder X-ray diffraction curve
obtained above.
[0077] (2) The crystallite size of the cathode active material was
calculated from the respective diffraction peaks of the powder
X-ray diffraction curve obtained above.
[0078] (3) The elemental analysis was conducted using an
inductively coupled high-frequency plasma atomic emission
spectroscope "SPS-4000 Model" manufactured by Seiko Denshi Kogyo
Co., Ltd.
[0079] (4) The cell characteristics of the cathode active material
were evaluated by a coin-shaped cell constituted from a cathode, an
anode and an electrolyte solution prepared by the following
methods.
[0080] <Preparation of Cathode>
[0081] The cathode active material, acetylene black as a conductive
agent, and polyvinylidene fluoride as a binder were accurately
weighed at a weight ratio of 85:10:5, and intimately mixed with
each other in a mortar. The resultant mixture was dispersed in
N-methyl-2-pyrrolidone to prepare a cathode slurry. Then, the
obtained slurry was applied onto an aluminum foil as a current
collector to form a coating film having a thickness of 150 .mu.m
vacuum-dried at 150.degree. C., and then punched into a disc shape
having a diameter of 16 mm, thereby producing a cathode.
[0082] <Preparation of Anode>
[0083] A metallic lithium foil was punched into a disc shape having
a diameter of 16 mm, thereby producing an anode.
[0084] <Preparation of Electrolyte Solution>
[0085] Lithium phosphate hexafluoride (LiPF.sub.6) as an
electrolyte was added in an amount of 1 mol/liter into a mixed
solution containing ethylene carbonate and diethyl carbonate at a
volume ratio of 50:50, thereby preparing an electrolyte
solution.
[0086] <Assembling of Coin-Shaped Cell>
[0087] In a globe box maintained under an argon atmosphere, the
above positive and anodes were fitted via a polypropylene separator
in a casing made of SUS316 stainless steel. Further, the
electrolyte solution was filled in the casing, thereby producing a
CR2032-type coin-shaped cell.
[0088] <Evaluation of Cell>
[0089] The above-produced coin-shaped cell was subjected to a
charge/discharge cycle test for secondary cells. The charge and
discharge cycles were repeated at a temperature of 60.degree. C.
and a cathode current density of 0.2 mA/cm.sup.2 while varying the
cut-off voltage from 3.0 to 4.25 V.
Example 1
[0090] <Production of Cathode Active Material>
[0091] 109.5 ml of manganese sulfate (5 mol % based on cobalt) was
added to 5,800 ml of an aqueous solution containing cobalt in an
amount of 0.5 mol/liter. In addition, an aqueous sodium hydroxide
solution was added in an amount of 1.05 equivalents based on one
equivalent of a neutralized component of cobalt and manganese, to
the resultant mixed solution, thereby conducting a neutralization
reaction therebetween. Then, the obtained solution was subjected to
oxidation reaction at 90.degree. C. for 8 hours while passing air
therethrough, thereby obtaining 240.8 g of a manganese-containing
cobalt oxide. It was conformed that the thus obtained
manganese-containing cobalt oxide was CO.sub.3O.sub.4, and had a
manganese content of 5 mol %, an average particle size of 0.05
.mu.m and a BET specific surface area of 23 m.sup.2/g.
[0092] The manganese-containing cobalt oxide was intimately mixed
with a lithium compound such that the molar ratio of Li to Co plus
manganese was 1.03. The resultant mixed particles were calcined at
900.degree. C. for 10 hours under an oxidative atmosphere, thereby
obtaining manganese-containing lithium cobaltate particles.
[0093] It was confirmed that the thus obtained manganese-containing
lithium cobaltate particles had an average particle size of 1.0
.mu.m, a BET specific surface area of 0.6 m.sup.2/g, an a-axis
length of lattice constant of 2.820 .ANG., a c-axis length of
lattice constant of 14.100 .ANG. and a crystallite size of 642
.ANG.. In addition, when the composition of the
manganese-containing lithium cobaltate particles was represented by
the formula: LiCO.sub.1-xMn.sub.xO.sub.2, it was confirmed that the
manganese content x was 0.05.
[0094] The thus obtained manganese-containing lithium cobaltate
particles were used as a cathode active material to prepare a
coin-shaped cell. As a result, it was confirmed that the thus
prepared cell exhibited an initial discharge capacity of 150 mAh/g,
and a capacity retention percentage of 97% after 50 cycles at
60.degree. C.
Examples 2 to 8 and Comparative Examples 1 to 6
[0095] The same procedure as defined in Example 1 was conducted
except that the contents of manganese and magnesium were changed
variously, thereby obtaining a cathode active material. Then, a
coin-shaped cell was produced using the cathode active material by
the same method as defined in Example 1.
[0096] Production conditions and various properties of the obtained
cathode active material and cell characteristics of the obtained
coin-shaped cell are shown in Tables 1 to 3.
[0097] Meanwhile, in Comparative Examples 5 and 6, the respective
raw materials were mixed together by a dry-mixing method, and then
the resultant mixture was calcined, thereby obtaining a cathode
active material.
[0098] As apparent from Table 1, it was confirmed that the
coin-shaped cells produced using the cathode active materials of
the present invention exhibited an initial discharge capacity of
not less than 140 mAh/g, and a capacity retention percentage after
50 cycles at 60.degree. C. as high as not less than 95%.
[0099] On the contrary, as apparent from the results of Comparative
Examples, when the manganese content x is less than 0.008, the
effect of adding manganese to the cathode active material was
insufficient. When the manganese content x is more than 0.18, the
initial discharge capacity of the obtained cell became too
deteriorated.
[0100] Also, in the production of the lithium cobaltate particles,
in the case where, upon the production of the cobalt oxide
particles from the aqueous solutions, cobalt oxide particles
containing only magnesium was used, and in the case where the
respective raw components were dry-mixed together, it was confirmed
that the effect of improving the charge/discharge cycle
characteristics under high temperature conditions was not
attained.
1 TABLE 1 Production of Mn-containing cobalt oxide Amount of Ratio
of Examples and aqueous Co Co Amount of Mn added Comparative
solution concentration Mn added (Mn/Co) Examples (ml) (mol/liter)
(ml) (mol %) Example 1 5,800 0.5 109.5 5 Example 2 5,800 0.5 21.9 1
Example 3 5,800 0.5 328.5 15 Example 4 5,800 0.5 21.9 1 Example 5
5,800 0.5 21.9 1 Example 6 5,800 0.5 54.8 2.5 Example 7 5,800 0.5
54.8 2.5 Example 8 5,800 0.5 109.5 5 Example 9 5,800 0.5 109.5 5
Comparative 5,800 0.5 0 0 Example 1 Comparative 5,800 0.5 11.0 0.5
Example 2 Comparative 5,800 0.5 438.0 20 Example 3 Comparative
5,800 0.5 0 0 Example 4 Comparative -- -- -- -- Example 5
Comparative -- -- -- -- Example 6 Production of Mn-containing
cobalt oxide Amount of Ratio of Mg Kind of aqueous Examples and
Amount of added aqueous alkali Comparative Mg added (Mg/Co) alkali
solution Examples (g) (mol %) solution (liter) Example 1 0 0 NaOH
0.349 Example 2 0 0 NaOH 0.336 Example 3 0 0 NaOH 0.382 Example 4
7.39 1 NaOH 0.339 Example 5 7.39 1 NaOH 0.339 Example 6 0 0 NaOH
0.341 Example 7 18.48 2.5 NaOH 0.349 Example 8 0 0 NaOH 0.349
Example 9 36.96 5 NaOH 0.365 Comparative 0 0 NaOH 0.333 Example 1
Comparative 0 0 NaOH 0.334 Example 2 Comparative 0 0 NaOH 0.398
Example 3 Comparative 36.96 5 NaOH 0.349 Example 4 Comparative --
-- -- -- Example 5 Comparative -- -- -- -- Example 6 Production of
Mn-containing cobalt oxide Amount of alkali Oxidation Examples and
added reaction Oxidation Comparative (equivalent temperature
reaction time Examples ratio) (.degree. C.) (hour) Example 1 1.02
90 8 Example 2 1.02 90 8 Example 3 1.02 90 8 Example 4 1.02 90 8
Example 5 1.02 90 8 Example 6 1.02 90 8 Example 7 1.02 90 8 Example
8 1.02 90 8 Example 9 1.02 90 8 Comparative 1.02 90 8 Example 1
Comparative 1.02 90 8 Example 2 Comparative 1.02 90 8 Example 3
Comparative 1.02 90 8 Example 4 Comparative -- -- -- Example 5
Comparative -- -- -- Example 6
[0101]
2 TABLE 2 Properties of cobalt oxide particles BET Average specific
Examples and Mn content Mg content particle surface Comparative
(molar (molar size area Examples ratio) ratio) (.mu.m) (m.sup.2/g)
Example 1 5 0 0.05 23 Example 2 1 0 0.05 23 Example 3 15 0 0.05 23
Example 4 1 1 0.05 23 Example 5 1 1 0.05 23 Example 6 2.5 0 0.05 23
Example 7 2.5 2.5 0.05 23 Example 8 5 0 0.05 23 Example 9 5 5 0.05
23 Comparative 0 0 0.05 23 Example 1 Comparative 0.5 0 0.05 23
Example 2 Comparative 20 0 0.05 23 Example 3 Comparative 0 5 0.05
23 Example 4 Comparative -- -- 2.5 1.3 Example 5 Comparative -- --
2.5 1.3 Example 6 Production method of lithium cobaltate particles
Examples Molar ratio and Kind of of lithium Calcination Calcination
Comparative lithium [Li/ temperature time Examples compound (Mn +
Mg + Co)] (.degree. C.) (hour) Example 1 Li.sub.2CO.sub.3 1.03 900
10 Example 2 Li.sub.2CO.sub.3 1.03 900 10 Example 3
Li.sub.2CO.sub.3 1.03 900 10 Example 4 Li.sub.2CO.sub.3 1.03 900 10
Example 5 Li.sub.2CO.sub.3 1.03 950 10 Example 6 Li.sub.2CO.sub.3
1.035 950 10 Example 7 Li.sub.2CO.sub.3 1.03 900 10 Example 8
Li.sub.2CO.sub.3 1.03 930 10 Example 9 Li.sub.2CO.sub.3 1.03 900 10
Comparative Li.sub.2CO.sub.3 1.03 900 10 Example 1 Comparative
Li.sub.2CO.sub.3 1.03 900 10 Example 2 Comparative Li.sub.2CO.sub.3
1.03 900 10 Example 3 Comparative Li.sub.2CO.sub.3 1.03 900 10
Example 4 Comparative Li.sub.2CO.sub.3 1.02 900 10 Example 5
Comparative Li.sub.2CO.sub.3 1.03 900 10 Example 6
[0102]
3TABLE 3 Examples and Properties of cathode active material
Comparative Mn content Mg content Lattice constant (.ANG.) Examples
(x) (y) a-axis c-axis Example 1 0.05 0 2.820 14.100 Example 2 0.01
0 2.817 14.085 Example 3 0.15 0 2.817 14.150 Example 4 0.01 0.01
2.817 14.089 Example 5 0.01 0.01 2.817 14.089 Example 6 0.025 0
2.817 14.091 Example 7 0.025 0.025 2.819 14.095 Example 8 0.05 0
2.819 14.101 Example 9 0.05 0.05 2.819 14.105 Comparative 0 0 2.817
14.052 Example 1 Comparative 0.005 0 2.817 14.064 Example 2
Comparative 0.2 0 2.817 14.167 Example 3 Comparative 0 0.05 2.819
14.072 Example 4 Comparative 0.025 0 2.817 14.069 Example 5
Comparative 0.05 0 2.817 14.075 Example 6 Examples Properties of
cathode active material and BET specific Crystallite Comparative
Particle size surface area size Examples (.mu.m) (m.sup.2/g)
(.ANG.) Example 1 1.0 0.6 642 Example 2 1.0 0.6 612 Example 3 0.5
0.9 683 Example 4 0.5 0.6 638 Example 5 4.0 0.5 792 Example 6 5.0
0.4 813 Example 7 0.5 0.9 598 Example 8 3.0 0.5 728 Example 9 0.5
0.9 604 Comparative 3.0 0.5 842 Example 1 Comparative 1.0 0.9 651
Example 2 Comparative 0.5 0.9 632 Example 3 Comparative 3.0 0.5 785
Example 4 Comparative 5.0 0.4 788 Example 5 Comparative 7.0 0.3 999
Example 6 Examples Properties of secondary cell and Initial
discharge Capacity retention Comparative capacity percentage after
50 Examples (mAh/g) cycles (%/50cy.) Example 1 150 97 Example 2
149.8 95 Example 3 140.2 97 Example 4 147.8 95 Example 5 148 96
Example 6 147.2 97 Example 7 145.1 96 Example 8 150 97 Example 9
155.1 98 Comparative 149.5 85 Example 1 Comparative 149.2 87
Example 2 Comparative 131.1 88 Example 3 Comparative 147.8 89
Example 4 Comparative 146.7 83 Example 5 Comparative 148.4 84
Example 6
* * * * *