Lithium/nickel/cobalt composite oxide, process for preparing...

Details Lithium/nickel/cobalt composite oxide, process for preparing... Lithium/nickel/cobalt composite oxide, process for preparing...

1999-02-12

2002-05-28

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Oxygen or compound thereof

Metal containing

C423S599000, C423S600000

Reexamination Certificate

active

06395250

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a process for preparing a lithium nickel cobalt complex oxide having a high purity, a high crystallinity, a high battery capacity and stable structure so that the degree of decrease in the capacity is little even with increasing the number of charging and discharging cycles, and to a positive electrode active material for a secondary battery which contains said lithium nickel cobalt complex oxide as an effective ingredient.
BACKGROUND ART
As electronic appliances have been rendered small and portable in recent years, there has been increased a demand for a lithium ion secondary battery having a light weight and a high energy density in place of nickel/cadmium battery and nickel hydrogen battery.
As active materials of a positive electrode for this lithium ion secondary battery, there are known LiNiO
2
and LiCoO
2
which are layered compounds capable of intercalating and deintercalating lithium ions. Of them, LiNiO
2
is preferred due to its higher electrical capacity than LiCoO
2
.
However, LiNiO
2
has not yet been put to practical use because it has problems in the charging and discharging cycle characteristics, the storage stability and the stability at a high temperature. Only LiCoO
2
has been practically used as the positive electrode active material.
Although various attempts have been made to improve the above faults of LiNiO
2
for its utilization as the positive electrode active material for a secondary battery, there has not yet been realized one wherein all of the above faults have been solved.
That is, in case of LiNiO
2
it is known that when many lithium ions are liberated therefrom (during charge), the structure becomes unstable owing to the two dimensional structure and therefore the cycle property, storage stability and high temperature stability of the lithium ion secondary battery are poor [for example, see J. Electrochem. Soc., 140 [7] p. 1862-1870 (1993), Solid State Ionics, 69 p. 265-70 (1994)]. Although many attempts have been made to stabilize the structure by replacing a portion of Ni with other components (Co, Mn, Fe, Ti, V etc.) for the purpose of securing the structure stability with elimination of the above faults, it was difficult to obtain highly purified and completely doped crystals as a solid solution on an industrial scale because there have been practically applied dry blending and heating processes.
Also, an attempt has been made to control to certain specific levels of the physical properties such as the shape and size of LiNiO
2
particles and its doped product with other components as solid solution. However, satisfactory results could not be achieved. For example, Japanese Patent Laid-open No. 151998/1993 proposes an improvement wherein the particle size distribution is specified to such extent that 10% cumulative size is 3~15 &mgr;m, 50% cumulative size 8~35 &mgr;m and 90% cumulative size 30~80 &mgr;m. However, it is very difficult to adjust the particle size distribution to such an extent by grinding the positive electrode active material.
Usually, LiNiO
2
has been prepared by mixing lithium components (LiOH, Li
2
CO
3
, LiNiO
3
etc.) with nickel components (hydroxide, carbonate etc.) in a dry state and thereafter subjecting the mixture to the reaction, and hence the heating at an elevated temperature for a long time was required. Consequently, the crystal growth proceeds but some of lithium is evaporated off and NiO as a by-product is formed, thereby lowering the purity.
Therefore, it was difficult to prepare highly purified product by the dry process in cases where the primary particle size is small. On the other hand, in cases where the primary particle size is large, a considerable lattice defect in the structure is caused resulting in a lowering of the purity. It was impossible to adjust crystal size as desired while keeping crystallinity and purity at high levels by the dry process.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a process for preparing a lithium nickel cobalt complex oxide which has improved properties with respect to the above faults of the hitherto known LiNiO
2
and its related complex oxide, namely which has a high purity, a high crystallinity, a high battery capacity and stable structure so that the degree of decrease in the capacity is little even by increasing the number of charging and discharging cycles.
Another object of the present invention is to provide a process for preparing said lithium nickel cobalt complex oxide via wet process which is different from the hitherto known dry process whereby the size of the formed spherical and secondary and primary particle may be set to a desired size.
A further object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery containing as an effective component said lithium nickel cobalt complex oxide.
As a result of having studied ardently to achieve the above objects, the present inventors have found that a complex oxide which may be represented by the following general formula (I) and which may be prepared at the first time by a wet process described later coincides with the above objects;
Li
y
N
1−x
Co
x1
M
x2
O
2
  (I)
(wherein M represents at least one element selected from the group consisting of Al, Fe, Mn and B. y represents 0.9≦y≦1.3, x
1
+x
2
=x, x represents 0<x≦0.5, x
1
represents 0<x
1
<0.5; when M is at least one element among Al, Fe and Mn, x
2
represents 0<x
2
≦0.3; when M is B, x
2
represents 0<x
2
<0.1 and when M is a combination of B and at least one element among Al, Fe and Mn, x
2
represents 0<x
2
<0.3 wherein the proportion occupied by B is in the range of being larger than 0 but being smaller than 0.1).
The complex oxide which may be obtained by the process of the present invention has the following features.
The first feature lies in the composition represented by the above general formula (I).
Holding as high a battery capacity as LiNiO
2
, the disclosed composition has improved cycle properties (i.e. lowered deterioration of discharge capacity when increasing the number of cycles), high temperature stability, and uses less expensive cobalt.
The second feature of the complex oxide lies in having a high crystallinity and a high purity as identified by its X-ray diffraction pattern. That is, it is highly purified complex oxide to such extent that an X-ray diffraction pattern shows that a ratio in the peak intensity of the face (003) to the face (104) i.e., (003)/(104) is 1.2 or higher and a ratio in the peak intensity of the face (006) to the face (101) i.e., (006)/(101) is 0.13 or lower, said face being defined by Miller indices hkl, the proportion of (Ni
3−
+Co
3+
) to the total (Ni+Co) being 99% by weight or higher, a BET specific surface area being 0.1~2 m
2
/g, an average secondary particle size D being in the range of 5~100 &mgr;m with 10% of the particle size distribution being 0.5D or higher and 90% 2D or lower, the surface of the spherical secondary particle being uneven as observed with a scanning electron microscope (SEM) and the primary particle constituting the spherical secondary particle being in the range of 0.2~30 &mgr;m in terms of long diameter with the average diameter of 0.3~30 &mgr;m as observed with a SEM.
In case of LiNiO
2
and its related complex oxide, when a part of the Ni is intended to be doped with other component(s) as the solid solution it is difficult to dope them homogeneously by the hitherto known dry process because the homogeneity is lowered in proportion to the amount added of other component(s) whereby not only is the battery capacity lowered but also the improvement to be achieved in the cycle property, the heat resistance and the electrolytic solution resistance are insufficient.
The lithium nickel cobalt complex oxide which may be obtained by the process of the present invention can be kept in high purity, in spite of being one doped with at least one

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