Cathode active material for lithium ion secondary battery, and method for its production
To provide a cathode active material for a lithium ion secondary battery, which has high packing properties and high volume capacity density, and a method for its production. A cathode active material for a lithium ion secondary battery is used, which comprises a mixture B of a collection of many large particles having different particle sizes and a collection of many small particles having different particle sizes, wherein the function F(x) of a particle size x of a particle contained in the mixture B and its frequency F has a relation of the formula 1, the function E(x) of a particle size x of a particle contained in a mixture B′ after the mixture B is pressed and its frequency E has a relation of the formula 2, the rate of change of the median size μ′ g relative to μ g is at most 10%, and the rate of change of the median size μ′ h relative to μ h is at least 20%, F ( x ) = A g × 1 2 π σ g x exp { - ( log x - μ g ) 2 2 σ g 2 } + A h × 1 2 π σ h x exp { - ( log x - μ h ) 2 2 σ h 2 } Formula 1 E ( x ) = A g ′ × 1 2 π σ g ′ x exp { - ( log x - μ g ′ ) 2 2 σ g ′2 } + A h ′ × 1 2 π σ h ′ x exp { - ( log x - μ h ′ ) 2 2 σ h ′2 } Formula 2
1. A cathode active material for a lithium ion secondary battery, comprising:
a mixture B of a collection of many large particles having different particle sizes and a collection of many small particles having different particle sizes,
wherein
a particle size distribution function F(x) of a particle size x of a particle contained in the mixture B and having a particle size frequency F has a relation of the formula 1 (provided that in the formula 1, in the collection of the large particles, a median size μ g is 10 μm≤μ g ≤30 μm and standard deviation σ g is 1.16≤σ g ≤1.65, in the collection of the small particles, a median size μ h is 0.1 μm≤μ h ≤10 μm and standard deviation σ h is 1.16≤σ h ≤1.65, A g +A h =1, 0<A g <1, 0<A h <1, and 1≤A g /A h ≤9),
a particle size distribution function E(x) of a particle size x of a particle contained in a mixture B′ after the mixture B is pressed by 1.92 t/cm 2 and having a particle size frequency E has a relation of the formula 2 (provided that in the formula 2, in the collection of the large particles, a median size p′ is 10 μm≤p′<30 μm and a standard deviation σ′ g is 1.16≤σ′ g ≤1.65, in the collection of the small particles, a median size μ′ h is 0.1 μm≤μ′ h ≤10 μm and a standard deviation σ′ h is 1.16≤σ′ h ≤2.1, A′ g +A′ h =1, 0<A′ g <1, 0<A′ h <1, and 1≤A′ g /A′ h ≤9), and
a rate of change of the median size μ′ g relative to μ g is at most 10%, and a rate of change of the median size μ′ h relative to μ h is at least 20%,
F
(
x
)
=
A
g
×
1
2
π
σ
g
x
exp
{
-
(
log
x
-
μ
g
)
2
2
σ
g
2
}
+
A
h
×
1
2
π
σ
h
x
exp
{
-
(
log
x
-
μ
h
)
2
2
σ
h
2
}
Formula
1
E
(
x
)
=
A
g
′
×
1
2
π
σ
g
′
x
exp
{
-
(
log
x
-
μ
g
′
)
2
2
σ
g
′2
}
+
A
h
′
×
1
2
π
σ
h
′
x
exp
{
-
(
log
x
-
μ
h
′
)
2
2
σ
h
′2
}
Formula
2
wherein A g and A′ g are mixing ratios of the large particles before and after pressing at 1.92 t/cm 2 , respectively, and A h and A′ h are mixing ratios of the small particles before and after pressing at 1.92 t/cm 2 , respectively; and
wherein the cathode active material is a lithium composite oxide having a composition represented by Li p Ni x Co y Mn z M q O r F a ,
where
M is at least one element selected from the group consisting of Al, Group 2 elements and transition metal elements other than Ni, Co and Mn, and
p, x, v, z, a, r and a satisfy 0.9<p<1.5, 0<x<0.8, 0.1<y<0.5, 0<z<0.5, 0<q<0.1, 1.9<r<2.1 and 0<a<0.1, respectively.
2. The cathode active material for a lithium ion secondary battery according to claim 1 , wherein a porosity when the cathode active material is pressed by 1.92 t/cm 2 is from 15 to 25%.
3. The cathode active material for a lithium ion secondary battery according to claim 1 , wherein the cathode active material has a specific surface area of from 0.2 to 1.5 m 2 /g.
4. The cathode active material for a lithium ion secondary battery according to claim 1 , wherein the cathode active material has an alkali content of from 0.1 to 1.5 mol %.
5. The cathode active material for a lithium ion secondary battery according to claim 1 , wherein when the cathode active material is pressed by 1.92 t/cm 2 , the volume cumulative fraction of particles having particle sizes of at most 0.75 μm is at most 1.5%.
6. The cathode active material for a lithium ion secondary battery according to claim 1 , wherein the rate of change of the median size μ′ g relative to μ g is from 0.5 to 5%, and 10 the rate of change of the median size μ′ h relative to μ h is from 20 to 80%.
7. The cathode active material of claim 1 , wherein μ g is from 14.3 to 15.4 m, μ h is from 4.5 to 4.8 m, μ′ g is from 13.7 to 15.2 m and μ′ h is from 1.8 to 2.7 m.
8. The cathode active material of claim 1 , wherein the rate of change in μ g is from 1.4 to 4.3% and the rate of change of μ h is from 44.9 to 61.2%.
9. The cathode active material of claim 1 , having a porosity of 23.2 to 22.6%.
10. The active cathode material of claim 1 , having a volume capacity density of from 615 to 624 mA h /cm 3 .
11. A method for producing the cathode active material for a lithium ion secondary battery as defined in claim 1 , which comprises mixing
a powder P which is a collection of plural particles having different particle sizes and which is made of large particle size lithium composite oxide, wherein a particle size distribution function I(x) of a particle size x of a particle contained in the powder P and having a particle size frequency I has a relation of the formula 3 (provided that in the formula 3, the median size μ i is 10 μm≤μ i ≤30 μm and the standard deviation σ i is 1.16<o≤σ i ≤1.65), with respect to a powder P′ after the powder P is pressed by 1.92 t/cm 2 , a particle size distribution function N(x) of a particle size x of a particle contained in the powder P′ and having a particle size frequency N has a relation of the formula 4 (provided that in the formula 4, in the collection of the large particles, the median size μ′ i is 10 μm≤μ′ i ≤30 μm and the standard deviation σ′ i is 1.16≤σ′ i ≤1.65), and the rate of change of the median size μ′ i relative to μ i is at most 10%, and
a powder Q which is a collection of plural particles having different particle sizes and which is made of small particle size lithium composite oxide, wherein a particle size distribution function J(x) of a particle size x of a particle contained in the powder Q and having a particle size frequency J has a relation of the formula 5 (provided that in the formula 5, the median size μ j is 0.1 μm≤μ j ≤10 μm and the standard deviation σ j is 1.16≤σ j ≤1.65), with respect to a powder Q′ after the powder Q is pressed by 1.92 t/cm 2 , a particle size distribution function L(x) of a particle size x of a particle contained in the powder Q′ and having a particle size frequency L has a relation of the formula 6 (provided that in the formula 6, in the collection of the small particles, the median size μ′ j is 0.1 μm≤μ′ j ≤10 μm and the standard deviation σ′ j is 1.16≤σ′ j ≤1.65), and the rate of change of the median size μ′ j relative to μ j is at least 20%, so that their weight mixing ratio would be within a range of from 90:10 to 50:50,
I
(
x
)
=
1
2
π
σ
i
x
exp
{
-
(
log
x
-
μ
i
)
2
2
σ
i
2
}
Formula
3
N
(
x
)
=
1
2
π
σ
i
′
x
exp
{
-
(
log
x
-
μ
i
′
)
2
2
σ
i
′2
}
Formula
4
J
(
x
)
=
1
2
π
σ
j
x
exp
{
-
(
log
x
-
μ
j
)
2
2
σ
j
2
}
Formula
5
L
(
x
)
=
1
2
π
σ
j
′
x
exp
{
-
(
log
x
-
μ
j
′
)
2
2
σ
j
′2
}
.
Formula
6
12. A method for producing the cathode active material for a lithium ion secondary battery as defined in claim 1 , which comprises mixing
a large particle size raw material C which comprises at least one compound selected from a nickel compound, a cobalt compound and a manganese compound and which is a collection of many large particles having different particle sizes, wherein a particle size distribution function K(x) of a particle size x of a particle contained in the large particle size raw material C and having a particle size frequency K has a relation of the formula 7 (provided that in the formula 7, the median size μ k is 10 μm≤μ k ≤30 μm and the standard deviation σ k is 1.16≤σ k ≤1.65), a particle size distribution function R(x) of a particle size x of a large particle size raw material C′ after the large particle size raw material C is pressed by 1.92 t/cm 2 and having a particle size frequency R has a relation of the formula 8 (provided that in the formula 8, the median size μ′ k is 10 μm≤μ′ k ≤30 μm and the standard deviation σ′ k is 1.16≤σ′ k ≤1.65), and the rate of change of the median size μ′ k relative to μ k is at most 10%, and
a small particle size raw material D which comprises at least one compound selected from a nickel compound, a cobalt compound and a manganese compound and which is a collection of many small particles having different particle sizes, wherein a particle size distribution function M(x) of a particle size x of a particle contained in the small particle size raw material D and having a particle size frequency M has a relation of the formula 9 (provided that in the formula 9, the median size μ m is 0.1 μm≤μ m ≤10 μm and the standard deviation σ m is 1.165≤σ m ≤1.65), a particle size distribution function S(x) of a particle size x of a particle contained in a small particle size raw material D′ after the small particle size raw material D is pressed by 1.92 t/cm 2 and having a particle size frequency S has a relation of the formula 10 (provided that in the formula 10, the median size μ′ m is 0.1 μm≤μ′ m ≤10 μm and the standard deviation σ′ m is 1.16≤σ′ m ≤1.65), and the rate of change of the median size μ′ m relative to μ m is at least 20%,
so that their weight mixing ratio would be within a range of from 90:10 to 50:50, then mixing a lithium compound thereto to obtain a mixture, and firing the mixture at a temperature of from 700 to 1,050° C.,
K
(
x
)
=
1
2
π
σ
k
x
exp
{
-
(
log
x
-
μ
k
)
2
2
σ
k
2
}
Formula
7
R
(
x
)
=
1
2
π
σ
k
′
x
exp
{
-
(
log
x
-
μ
k
′
)
2
2
σ
k
′2
}
Formula
8
M
(
x
)
=
1
2
π
σ
m
x
exp
{
-
(
log
x
-
μ
m
)
2
2
σ
m
2
}
Formula
9
S
(
x
)
=
1
2
π
σ
m
′
x
exp
{
-
(
log
x
-
μ
m
′
)
2
2
σ
m
′2
}
.
Formula
10
13. A lithium ion secondary battery, comprising the cathode active material of claim 1 .
14. The lithium ion secondary battery of claim 13 , wherein the organic solvent comprises one or more of ethylene carbonate and diethyl carbonate, and the cathode active material further comprises LiPF 6 .