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OUR Ref. 2831690CA
LITHIUM COBALT COMPOUND OXIDE AND MANUFACTURING METHODS
THEREOF, AND NON-AQUEOUS ELECTROLYTE SECONDARY CELL
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to lithium cobalt
compound oxides and manufacturing methods thereof, non-
aqueous electrolyte secondary cells, and portable electronic
apparatuses.
2. Description of the Related Art
It has been known that metal ions having an appropriate
size can be introduced onto crystal lattice sites and/or
between crystal lattice planes of a transition metal oxide
having a hexagonal layer crystal structure. In particular,
in lithium-containing interlayer compounds, under a specific
potential-difference condition, lithium ions can be
introduced onto crystal lattice sites and/or between lattice
planes and can then be removed therefrom. In addition,
since a lithium secondary cell using a lithium cobalt
compound oxide, LiCo02, as a positive active material has a
high volume energy density, miniaturization and weight
reduction of portable electronic apparatuses can be realized,
and hence in recent years, the lithium secondary cells have
been increasingly in demand as power sources of portable
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personal computers and mobile phones.
In addition, research has also been carried out in
which inexpensive transition metals, such as nickel or
manganese, are used instead of expensive cobalt (for example,
refer to Japanese Unexamined Patent Application Publication
Nos. 11-71114 and 11-292550). In Japanese Unexamined Patent
Application Publication No. 11-292550, a lithium compound
oxide has been disclosed which is obtained from a starting
material represented by a chemical formula LiCoxNiCl-X~Oz
(0.05Sx<1), and the crystal structure of this lithium
compound oxide (LiCoOz) can be stably maintained even after
the Co component thereof is partly replaced with Ni.
According to the related techniques as described above, it
has been intended to replace expensive cobalt with nickel or
the like.
SUMMARY OF THE INVENTION
However, through intensive research carried out by the
inventors of the present invention, it was found that when
the Co component of the lithium compound oxide (LiCoOz)
functioning as a positive active material is partly replaced
with Ni, the discharge properties of a lithium secondary
cell is adversely deteriorated. Accordingly, an object of
the present invention is to provide_a lithium secondary cell
having improved discharge properties.
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In more particular, the present invention provides the
following.
(1) A lithium cobalt compound oxide is provided which
has a Ni content of 100 ppm or less and which is represented
by the following chemical formula (1).
LiXCol_yMyOZNZ ~ ~ - ( 1 )
In the chemical formula (1), M is at least one element
selected from the group consisting of transition metal
elements except Co and Ni and elements of group II, XIII,
XIV, and XV of the periodic table; N represents a halogen
atom; and 0.101.25, 0_<y_<0.05, and 0<_z<_0.05 are satisfied.
(2) The lithium cobalt compound oxide described in the
above (1) may comprise a sulfate group at a content of 0.01
to 5 percent by weight.
(3) A method for manufacturing a lithium cobalt
compound oxide, is provided which comprises: preparing a
mixture of a lithium compound and cobalt oxyhydroxide having
a Ni content of 100 ppm or less; and heating the mixture to
700 to 1,100°C.
(4) A method for manufacturing a lithium cobalt
compound oxide, is provided which comprises: preparing a
mixture of a lithium compound, cobalt oxyhydroxide having a
Ni content of 100 ppm or less, and at least one element
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selected from the group consisting of a halogen compound, a
sulfate compound, and a compound containing an M element,
the M element being at least one element selected from the
group consisting of transition metal elements except Co and
Ni and elements of group II, XIII, XIV, and XV of the
periodic table; and heating the mixture to 700 to 1,100°C.
(6) A non-aqueous electrolyte secondary cell is
provided which comprises the lithium cobalt compound oxide
according to one of the above (1) to (2) as a positive
active material used for a positive electrode.
(7) A portable electronic apparatus is provided which
comprises the non-aqueous electrolyte secondary cell
according to the above (6).
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross-sectional view of a non-aqueous
electrolyte secondary cell according to an embodiment of the
present invention;
Fig, 2 is a graph showing the relationship between the
voltage and the discharge capacity of a non-aqueous
electrolyte secondary cell according to the present
invention;
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, a positive active material and a non-
aqueous electrolyte secondary cell, according to the present
invention, will be described in detail.
<Lithium Cobalt Compound Oxide>
A lithium cobalt compound oxide of the present
invention is a lithium oxide represented by the chemical
formula LiXCol_yMy02NZ. In the formula described above, M is
at least one element selected from the group consisting of
transition metal elements except Co and Ni and elements of
group II, XIII, XIV, and XV of the periodic table; N
represents a halogen atom; and 0.10<_xS1.25, OSy<_0.05, and
0_<z_<0.05 are satisfied. It is more preferable when
0.41.0, 0_<y_<0.01, and 0<_z_<0.01 are satisfied. The
lithium cobalt compound oxide described above can be
preferably used as a positive active material for a lithium
ion secondary cell using a non-aqueous electrolyte.
In addition to the elements mentioned above, for
example, the lithium cobalt compound oxide of the present
invention may also contain at least one element selected
from the group consisting of 8, Mg, Si, Cu, Ce, Y, Ti, V, Mn,
Fe, Sn, Zr, Sb, Nb, Ru, Pb, Hf, Ta, La, Pr, and Nd.
In the lithium cobalt compound oxide of the present
invention, the Ni content is 100 ppm or less. The Ni
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content is preferably 70 ppm or less and more preferably 60
ppm or less. In the lithium cobalt compound oxide contained
in a positive electrode of a lithium secondary cell, the
discharge capacity thereof is increased as the Ni content in
the lithium cobalt compound oxide is decreased, and as a
result, it becomes easier to maintain the voltage. Hence,
the volume energy density of a lithium secondary cell is
increased, and as a result, miniaturization and weight
reduction of portable electronic apparatuses can be realized.
In addition, the content of a sulfate group contained
in the lithium cobalt compound oxide of the present
invention is preferably in the range of from 0.01 to 5
percent by weight and more preferably in the range of from
0.05 to 2 percent by weight.
The sulfate group mentioned above may be obtained by
firing a sulfate in reaction performed for the lithium
cobalt compound oxide, the sulfate being provided beforehand
when starting materials are mixed together. As the sulfate,
for example, calcium sulfate or cobalt sulfate may be
mentioned.
For the quantitative determination of sulfate groups,
various methods may be used, and for example, a method may
be performed in which a sample is totally dissolved in
nitric acid/hydrogen peroxide or the like, followed by
quantitative determination of a sulfate group using ion
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chromatography. In addition, ICP spectrometric analysis or
titrimetric analysis may also be used for quantitative
determination. In the ICP spectrometric analysis, a sample
is dissolved in nitric acid and perchloric acid, and the
quantity of sulfur is then determined by ICP spectrometric
analysis, followed by conversion into the quantity of the
sulfate group.
In the titrimetric analysis, after barium chromate and
a diluted hydrochloric acid solution are added to a sample,
neutralization by ammonia is preformed, followed by
filtration, and Cr042- obtained in a filtrate by replacement
of the sulfate group is then titrated by iodometry, thereby
indirectly determining the quantity of the sulfate group (in
accordance with the description in "Jikken Kagaku Koza, vol.
15, Bunseki Kagaku (II)"(Courses in Experimental Chemistry,
vol. 15, Analytical Chemistry (II)), edited by "The Chemical
Society of Japan").
In addition, as the halogen atom contained in the
lithium cobalt compound oxide, for example, fluorine or
bromine may be mentioned, and fluorine is preferably used.
The content of the halogen atom described above is 0.005 to
2.5 percent by weight and preferably 0.05 to 1.5 percent by
weight.
The average particle diameter of the lithium cobalt
compound oxide of the present invention is 10 to 15 ~m and
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preferably 10 to 13 Vim. For the measurement of the average
particle diameter, the value of cumulative 50~ (D50) of the
particle distribution, which is obtained by a laser
scattering particle size distribution analyzer, is used.
In addition, as another characteristic feature of the
lithium cobalt compound oxide of the present invention, the
content of lithium carbonate remaining therein is 0.1
percent by weight or less and preferably 0.5 percent by
weight or less.
Furthermore, the lithium cobalt compound oxide
described above is preferably mixed with at least one metal
oxide selected from the group consisting of magnesium oxide,
titanium oxide, and zirconium oxide. The metal oxides
mentioned above may be used alone or in combination.
In the present invention, the mixture containing the
metal oxide described above is called a "mixed lithium
cobalt compound oxide" in order to discriminate it from the
lithium cobalt compound oxide described above.
The mixing method may be performed by any one of
methods including a dry and a wet method; however, a dry
mixing method is preferable from an industrial point of view.
<Cobalt Oxyhydroxide>
In a manufacturing method of the present invention, a
cobalt oxyhydroxide having a Ni content of 100 ppm or less
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_ g _
is used. A cobalt oxyhydroxide having a Ni content of 70
ppm or less is preferably used, and cobalt oxyhydroxide
having a Ni content of 60 ppm or less is more preferably
used. The reason for this is that the Ni content in the
lithium cobalt compound oxide obtained by the manufacturing
method of the present invention is decreased as the Ni
content in a starting material is decreased. It is believed
that the cobalt oxyhydroxide is primarily composed of Co00H;
however, Co304, CoC03, and the like may also be contained.
A method for manufacturing the cobalt oxyhydroxide used
for the manufacturing method of the present invention is.not
particularly limited.
For example, a material may be used which is formed by
oxidizing a compound containing divalent cobalt, such as
cobalt nitrate, cobalt chloride, or cobalt sulfate, with an
oxidizing agent, followed by neutralization with an alkaline
material.
The oxidizing agent mentioned above is not particularly
limited, and for example, there may be mentioned air, oxygen,
and ozone; permanganic acid (HMn04) and salts thereof
represented by M3Mn04 and the like; chromic acid (Cr03)and
related compounds thereof represented by M3zCr20,, M32Cr04,
M3Cr03X, Cr02X2, and the like; halogens such as F2, C12, Br2,
and I2; peroxides such as HZOZ, Na202, and Ba02; peroxo acids,
compounds represented, for example, by M32Sa0g, M32S05, H2C03,
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and CH3C03H and salts thereof; and oxygen acids, compounds
represented, for example, by M3MC10, M3Br0, M3I0, M3C103,
M3Br03, M3I03, M3C104, M3I04, Na3H2I06, and KI04, and the salts
thereof: In the formula, M3 indicates an alkaline metal
element.
The alkaline metal element mentioned above is not
particularly limited, and for example, lithium, sodium,
potassium, and rubidium may be mentioned.
In addition, X indicates a halogen atom.
The alkaline materials used for neutralization are not
particularly limited, and an aqueous solution containing an
inorganic hydroxide such as lithium hydroxide, sodium
hydroxide, potassium hydroxide, magnesium hydroxide, calcium
hydroxide, barium hydroxide, or ammonium hydroxide may be
preferably used.
The cobalt oxyhydroxide described above can be obtained
by the steps of dissolving a compound containing divalent
cobalt such as cobalt nitrate, cobalt chloride, or cobalt
sulfate in water for forming an aqueous solution, and
subsequently adding the oxidizing agent and the alkaline
material described above for simultaneously performing the
neutralization and oxidation. Alternatively, the cobalt
oxyhydroxide can also be obtained by synthesizing divalent
cobalt hydroxide by adding the alkaline material to an
aqueous solution containing the above divalent cobalt
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compound, followed by oxidation with an oxidizing agent.
Furthermore, the cobalt oxyhydroxide may also be obtained by
neutralization by addition of the alkaline material after
the oxidizing agent is added to an aqueous solution
containing the above divalent cobalt compound.
<Lithium Compound>
The manufacturing method of the present invention uses
a lithium compound. The lithium compound is not
particularly limited; however, for example, an inorganic
lithium salt, such as lithium hydroxide, lithium carbonate,
or lithium nitrate may be preferably used. As the lithium
compound, lithium carbonate is preferable since being easily
available and inexpensive. A lithium compound having a
higher purity is preferably used.
<Manufacturing Method>
According to the manufacturing method of the present
invention, for example, a mixture is first obtained by
mixing the cobalt oxyhydroxide with a lithium compound,
preferably with lithium carbonate. A wet or a dry mixing
method may be optionally performed; however, a dry mixing
method is preferable since being easily performed. In the
dry mixing method, a blender is preferably used for
uniformly mixing starting materials. The mixing ratio of
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the lithium compound to the cobalt compound, which compounds
are starting materials in the mixing step, on an atomic
basis (Li/Co) is set to 0.99 to 1.06 and is preferably set
to 0.99 to 1.02.
Next, the mixture thus prepared is fired. The firing
temperature is preferably 700 to 1,110°C and more preferably
850 to 1,050°C. The firing time is 1 to 24 hours and
preferably 2 to 10 hours. When the firing temperature is
decreased below 700°C, the lithium cobalt compound oxide
cannot be sufficiently synthesized, and as a result, the
cobalt oxyhydroxide and the lithium compound used as the
starting materials unfavorably remain. On the other hand,
when the firing temperature is increased above 1,100°C, the
decomposition of the desired lithium cobalt compound oxide
will start, and when this lithium cobalt compound oxide is
used as a positive active material, the degradation in cell
properties may occur, that is, in particular, the decrease
in capacity at a voltage at the stage of the end of
discharge and the degradation in cyclic properties
unfavorably occur.
The firing may be performed either in the air or in an
oxygen atmosphere and is not particularly limited. After
the firing, cooling is optionally performed, followed by
pulverization whenever necessary, thereby forming the
lithium cobalt compound oxide. The pulverization performed
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whenever necessary is optionally performed, for example,
when particles of the lithium cobalt compound oxide obtained
by firing are loosely bonded to each other in the form of a
block.
The content of the nickel with respect to the lithium
cobalt compound oxide and cobalt oxyhydroxide is in the
range of from 0 to 100 ppm on a weight basis and
particularly preferably in the range of from 0 to 50 ppm.
The reason for this is that since the nickel atoms are
replaced with the cobalt atoms so as to be located at the
sites at which the cobalt atoms are present, the number of
the cobalt atoms responsible for the charge and discharge
capacity is decreased. In addition, it may also be
considered that since the nickel atoms are replaced with the
cobalt atoms having a valence different therefrom, the
number of trivalent cobalt atoms necessary for charge and
discharge for charge compensation is decreased. The minimum
content of Ni is not particularly limited.
<Measurement Method of Ni Content>
After the lithium cobalt compound oxide (0.5 g) was
fully dissolved while being boiled in perchloric acid,
distilled water was added thereto so as to obtain a total
volume of 100 ml, and the Ni amount in this solution was
measured using an ICP spectroscopic analyzer (manufactured
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by Rigaku Corporation).
<Formation Method of Cell>
As the positive active material for a lithium secondary
cell, the lithium cobalt compound oxide described above is
used. The positive active material is one of stating
materials for a positive electrode compound of a lithium
secondary cell, the positive electrode compound, which will
be described later, being a mixture formed of the positive
active material, a conductive agent, a binder, filler
whenever necessary, and the like. Since the positive active
material of the lithium secondary cell, according to the
present invention, is formed of the lithium cobalt compound
oxide described above, kneading with the other starting
materials can be easily performed when the positive
electrode compound is prepared, and in addition, coating of
a positive electrode collector with the positive electrode
compound thus obtained can also be easily performed.
The lithium secondary cell of the present invention
uses the lithium cobalt compound oxide as a positive active
material and comprises a positive electrode, a negative
electrode, separators, and a non-aqueous electrolyte
containing a lithium salt. The positive electrode is formed,
for example, by applying a positive electrode compound onto
a positive electrode collector, followed by drying, and the
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positive electrode compound is composed of a positive active
material, a conductive agent, a binder, and filler whenever
necessary, and the like.
A material for the positive electrode collector is not
particularly limited as long as being inactive in an
assembled cell in view of chemical reaction, and for example,
there may be mentioned stainless steel, nickel, aluminum,
titanium, baked carbon, and aluminum or stainless steel
surface-treated with carbon, nickel, titanium, or silver.
As the conductive agent, for example, there may be
mentioned conductive materials, such as graphite including
natural graphite and manmade graphite, carbon black,
acetylene black, carbon fiber, carbon nanotube, and metal
such as powdered nickel. As the natural graphite, for
example, scaly graphite, flake graphite, and earthy graphite
may be mentioned. Those mentioned above may be used alone
or in combination. The content of the conductive agent in
the positive electrode compound is 1 to 50 percent by weight
and preferably 2 to 30 percent by weight.
As the binder, for example, there may be mentioned
polysaccharides, thermoplastic resins, and polymers having
elasticity, such as poly(vinylidene fluoride), polyvinyl
chloride), carboxylmethylcellulose, hydroxylpropylcellulose,
recycled cellulose, diacetylcellulose, polyvinyl
pyrrolidone), ethylene-propylene-diene-terpolymer (EPDM),
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sulfonated EPDM, styrene-butadiene rubber, fluorinated
rubber, and polyethylene oxide. Those mentioned above may
be used alone or in combination. The content of the binder
in the positive electrode compound is 2 to 30 percent by
weight and preferably 5 to 15 percent by weight.
The filler of the positive electrode compound has a
function of suppressing the volume expansion or the like of
the positive electrode and is used whenever necessary. As
the filler, any fiber materials may be used as long as being
in active in an assembled cell in view of chemical reaction,
and for example, fibers made of olefinic polymers such as
polypropylene and polyethylene, glass fibers, and carbon
fibers may be used. The content of the filler is not
particularly limited and is preferably 0 to 30 percent by
weight of the positive electrode compound.
The negative electrode is formed by applying a negative
electrode material onto a negative electrode collector,
followed by drying. As the negative electrode collector,
any material may be used as long as being in active in an
assembled cell in view of chemical reaction, and for example,
there may be mentioned stainless steel, nickel, copper,
titanium, aluminum, baked carbon, copper or stainless steel
surface-treated with carbon, nickel, titanium, or silver,
and aluminum-cadmium alloy.
The negative electrode material is not particularly
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limited, and for example, there may be mentioned
carbonaceous materials, metal composite oxides, metal
lithium, and lithium alloys. As the carbonaceous material,
for example, hard-graphitized carbon materials and graphite-
base carbon materials may be mentioned. As the metal
composite oxide, for example, there may be mentioned a
compound represented y SnpNh-pM2qOr (where, Ml is at least one
element selected from the group consisting of Mn, Fe, Pb,
and Ge; Mz is at least one element selected from the group
consisting of A1, B, P, Si, elements of group I, II, and III
of the periodic table, and halogen atoms; and 0<p51, 15q<_3,
and 1_<rS8 are satisfied).
As the separator, an insulating thin film having a high
ion transmittance and a predetermined mechanical strength is
used. Sheets and nonwoven cloths may be used which are made
of glass fibers or an olefinic polymer, such as polyethylene
or polypropylene, having organic-solvent resistance and
hydrophobic properties. The pore diameter of the separator
is not particularly limited as long as effectively used for
a general cell application and is, for example, 0.01 to 10
Vim. The thickness of the separator may be in the range used
for a general cell application and is, for example, 5 to 300
~.m. In addition, in the case in which a solid electrolyte
such as a polymer is used as described later, the solid
electrolyte may also be used as the separator. In addition,
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in order to improve the discharge properties and the charge
and discharge properties, a compound such as pyridine,
triethyl phosphite, or triethanolamine may be added to the
electrolyte.
The non-aqueous electrolyte containing a lithium salt
is a mixture of a non-aqueous electrolyte and a lithium salt.
As the non-aqueous electrolyte, a non-aqueous electrolyte or
an organic solid electrolyte is used. As the non-aqueous
electrolyte, for example, there may be mentioned aprotic
organic solvents such as N-methyl-2-pyrrolidinone, propylene
carbonate, ethylene carbonate, butylene carbonate, dimethyl
carbonate, diethyl carbonate, 'y-butyrolactone, 1,2-
dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran,
dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethyl
formamide, dioxolane, acetonitrile, nitromethane, methyl
formate, methyl acetate, a phosphoric acid triester,
trimethoxymethane, a dioxolane derivative, sulfolane, 3-
methyl-2-oxazolidinone, a propylene carbonate derivative, a
tetrahydrofuran derivative, diethyl ether, and 1,3-
propanesultone. Those mentioned above may be used alone or
in combination.
As the organic solid electrolyte, for example, a
polyethylene derivative, a polymer including the same, a
propylene oxide derivative, a polymer including the same,
and a phosphate polymer may be mentioned. As the lithium
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salt, a material dissolved in the non-aqueous electrolyte
described above is used, and for example, LiC104, LiBF4,
LiPF6, LiCF3S03, LiCF3C02, LiAsF6, LiSbF6, LiBlaCllo, LiA1C14,
chloroboran lithium, a lithium lower aliphatic carboxylate,
and lithium tetraphenylborate may be used alone or in
combination.
The shape of the lithium secondary cell Qf the present
invention may be a button, sheet, cylinder, rectangle, or
the like. The application of the secondary cell of the
present invention is not particularly limited and may be
applied to electronic apparatuses, such as notebook personal
computers, laptop personal computers, pocket type word
processors, mobile phones, cordless phone handsets, portable
CD players, and radios, and consumer electronic apparatuses
for automobiles, electric vehicles, and game machines. In
addition, the lithium secondary cell is categorized in a
non-aqueous electrolyte secondary cell.
<Portable Electronic Apparatuses>
The present invention provides a portable electronic
apparatuses incorporating the non-aqueous electrolyte
secondary cell described above. As the portable electronic
apparatuses, for example, notebook personal computers,
pocket type word processors, mobile phones, cordless phone
handsets, portable CD players, radios, and game machines may
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be mentioned.
EXAMPLES
Hereinafter, the lithium cobalt compound oxide and the
non-aqueous electrolyte secondary cell, according to the
present invention, will be further described in detail.
(Lithium Cobalt Compound Oxide)
Example 1
Lithium carbonate (average particle diameter of 11.0
Vim) and cobalt oxyhydroxide having a Ni content of 25 ppm
and an average particle diameter of 12.0 ~m were prepared so
that the ratio Li/Co on an atomic basis was 1.01 and were
then sufficiently mixed in a mortar, thereby forming a
uniform mixture. Subsequently, the mixture thus formed was
placed in an alumina crucible and was then heated to 950°C
for 10 hours in an air atmosphere using an electric furnace,
thereby forming a lithium cobalt compound oxide (LiCo02)
having a Ni content of 28 ppm. The average particle
diameter was 10.2 Vim.
Example 2
Lithium carbonate (average particle diameter of 11.0
~.m) and cobalt oxyhydroxide having a Ni content of 49 ppm
and an average particle diameter of 12.0 ~m were prepared so
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that the ratio Li/Co on an atomic basis was 1.03 and were
then sufficiently mixed in a mortar, thereby forming a
uniform mixture. Subsequently, the mixture thus formed was
placed in an alumina crucible and was then heated to 800°C
for 10 hours in an air atmosphere using an electric furnace,
thereby forming a lithium cobalt compound oxide (LiCo02)
having a Ni content of 45 ppm. The average particle
diameter was 12.0 ~,m.
Example 3
Lithium carbonate (average particle diameter of 11.0
Vim) and cobalt oxyhydroxide having a Ni content of 60 ppm
and an average particle diameter of 12.0 ~m were prepared so
that the ratio Li/Co on an atomic basis was 1.00 and were
then sufficiently mixed in a mortar, thereby forming a
uniform mixture. Subsequently, the mixture thus formed was
placed in an alumina crucible and was then heated to 1,050°C
for 10 hours in an air atmosphere using an electric furnace,
thereby forming a lithium cobalt compound oxide (LiCo02)
having a Ni content of 58 ppm. The average particle
diameter was 10.5 ~cn.
Example 4
Lithium carbonate (average particle diameter of 11.0
~,m) and cobalt oxyhydroxide having a Ni content of 98 ppm
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and an average particle diameter of 12.0 ~,tn were prepared so
that the ratio Li/Co on an atomic basis was 0.99 and were
then sufficiently mixed in a mortar, thereby forming a
uniform mixture. Subsequently, the mixture thus formed was
placed in an alumina crucible and was then heated to 1,050°C
for 10 hours in an air atmosphere using an electric furnace,
thereby forming a lithium cobalt compound oxide (LiCoOz)
having a Ni content of 96 ppm. The average particle
diameter was 10.5 ~,m.
Example 5
After lithium carbonate (average particle diameter of
11.0 Vim) and cobalt oxyhydroxide having a Ni content of 49
ppm and an average particle diameter of 12.0 ~m were mixed
together so that the ratio Li/Co on an atomic basis was 1.04,
calcium sulfate was added to this mixture so that the
content of S04 was 2,000 ppm with respect to LiCoOz and was
then sufficiently mixed in a mortar, thereby forming a
uniform mixture. Subsequently, the mixture thus formed was
placed in an alumina crucible and was then heated to 1,060°C
for 5 hours in an air atmosphere using an electric furnace,
thereby forming a lithium cobalt compound oxide (LiCoOz)
having a Ni content of 40 ppm. The average particle
diameter was 12.5 Nm.
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Example 6
After lithium carbonate (average particle diameter of
11.0 wm) and cobalt oxyhydroxide having a Ni content of 49
ppm and an average particle diameter of 12.0 ~m were mixed
together so that the ratio Li/Co on an atomic basis was 1.04,
MgF2 was added to this mixture so that the content of F was
3,000 ppm with respect to LiCo02 and was then sufficiently
mixed in a mortar, thereby forming a uniform mixture.
Subsequently, the mixture thus formed was placed in an
alumina crucible and was then heated to 1,060°C for 5 hours
in an air atmosphere using an electric furnace, thereby
forming a lithium cobalt compound oxide (LiCo02) having a Ni
content of 40 ppm. The average particle diameter was 12.2
Vim.
Example 7
After lithium carbonate (average particle diameter of
11.0 Vim) and cobalt oxyhydroxide having a Ni content of 49
ppm and an average particle diameter of 12.0 dun were mixed
together so that the ratio Li/Co on an atomic basis was 1,04,
calcium sulfate and MgF2 were added to this mixture so that
the contents S04 and F were 1,500 ppm and 2,000 ppm,
respectively, with respect to LiCo02 and were then
sufficiently mixed in a mortar, thereby forming a uniform
mixture. Subsequently, the mixture thus formed was placed
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in an alumina crucible and was then heated to 1,060°C for 5
hours in an air atmosphere using an electric furnace,
thereby forming a lithium cobalt compound oxide (LiCoOz)
having a Ni content of 40 ppm. The average particle
diameter was 12.9 ~cn.
Comparative Example 1
Lithium carbonate (average particle diameter of 11.0
wm) and cobalt oxyhydroxide having a Ni content of 205 ppm
and an average particle diameter of 12.0 ~m were prepared so
that the ratio Li/Co on an atomic basis was 1.01 and were
then sufficiently mixed in a mortar, thereby forming a
uniform mixture. Subsequently, the mixture thus formed was
placed in an alumina crucible and was then heated to 1,060°C
for 10 hours in an air atmosphere using an electric furnace,
thereby forming a lithium cobalt compound oxide (LiCo02)
having a Ni content of 201 ppm. The average particle
diameter was 10.7 Eun.
Comparative Example 2
Lithium carbonate (average particle diameter of 11.0
~,m) and cobalt oxyhydroxide having a Ni content of 502 ppm
and an average particle diameter of 12.0 ~tm were prepared so
that the ratio Li/Co on an atomic basis was 1.04 and were
then sufficiently mixed in a mortar, thereby forming a
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uniform mixture. Subsequently, the mixture thus formed was
placed in an alumina crucible and was then heated to 820°C
for 10 hours in an air atmosphere using an electric furnace,
thereby forming a lithium cobalt compound oxide (LiCo02)
having a Ni content of 490 ppm. The average particle
diameter was 11.2 Vim.
CA 02462062 2004-03-26
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<Cell Performance Test>
(I) Formation of Lithium Secondary Cell
A positive electrode material was formed by mixing 91
percent by weight of each of the lithium cobalt compound
oxides formed in accordance with Examples 1 to 7 and
Comparative Examples 1 and 2, 6 percent by weight of
powdered graphite, and 3 percent by weight of poly(vinyliden
fluoride), and the positive electrode material thus obtained
was dispersed in N-methyl-2-pyrrolidinone, thereby forming a
paste compound. After this paste compound was applied onto
an aluminum foil and then dried, a disc 15 mm in diameter
was punched out therefrom, and hence a positive electrode
plate was obtained.
As shown in Fig. 1, a lithium secondary cell, that is,
a non-aqueous electrolyte secondary cell, was formed by
using a separator 1, a negative electrode 2, a positive
electrode 3, collectors 4, mounting tags 5, exterior
terminals 6, an electrolyte 7, and the like. Among the
members mentioned above, a lithium metal foil was used as
the negative electrode, and the electrolyte was formed by
dissolving one mole of LiPF3 in one liter of a mixed
solution of ethylene carbonate and diethyl carbonate at a
ratio of 1 to 1.
(II) Evaluation of Cell Performance
CA 02462062 2004-03-26
w
,,~..
- 27 -
The lithium secondary cell thus formed was operated at
room temperature, and the initial discharge capacity was
measured fox evaluation of the cell performance.
(III) Evaluation Method
The discharge capacity was measured by the steps of
charging a cell in a CCCV (constant-current, constant-
voltage, 1.OC) mode up to 4.3 V with respect to the positive
electrode and then discharging it down to 2.7 V. Lithium
secondary cells formed from the lithium cobalt compound
oxides as a positive active material, which were obtained in
Examples 1 to 7 and Comparative Examples 1 and 2, were
evaluated as described above, and the relationship between
the voltage and the discharge capacity is shown in Fig. 2.
Experimental Result
A constant-current charge test was performed at a
potential 2.7 to 4.3 V (vs. LijLi+) using an electrode
coated with the active material obtained in each of Examples
1 to 7 and Comparative Examples 1 and 2. The discharge
curves are shown in Fig. 2. The test was performed at a
charge and discharge current of 0.2C. The active material
obtained in each of the Examples 1 to 7 had an initial
discharge capacity of 158 mAH/g or more, and hence it was
found that compared to the active materials obtained in
CA 02462062 2004-03-26
- 28 -
Comparative Examples 1 and 2, a large discharge capacity
could be obtained. The reason for this is believed that
since Ni contained in the lithium cobalt compound oxide is
replaced with Co to form a solid solution, the Co amount
responsible for the capacity is decreased, so that the
decrease in capacity occurs. From the results described
above, it was confirmed that the lithium cobalt compound
oxides according to Examples 1 to 7 are superior as a
material for a non-aqueous electrolyte secondary cell.
