Nonaqueous electrolyte secondary battery

Details Nonaqueous electrolyte secondary battery Nonaqueous electrolyte secondary battery

2001-08-31

2004-06-08

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S231300, C429S231600, C429S231500, C429S224000, C429S223000, C429S221000, C429S218100

Reexamination Certificate

active

06746800

ABSTRACT:

TECHNICAL FIELD
The present invention relates to nonaqueous electrolyte secondary batteries as generally represented by lithium secondary batteries, and particularly to load characteristic improvements of positive electrode material after cycling.
BACKGROUND ART
In recent years, nonaqueous electrolyte batteries which use metallic lithium, alloys capable of storage and release of lithium ions or carbon materials for the negative active material and lithium-transition metal complex oxides for the positive electrode material have been noted as high-density batteries.
The use of a lithium-cobalt complex oxide (LiCoO
2
), lithium-nickel complex oxide (LiNiO
2
) or lithium-manganese complex oxide (LiMn
2
O
4
), among the lithium-transition metal complex oxides, for the positive active material results in obtaining high discharge voltages of 4 V class, particularly increasing battery energy densities.
Among the above-listed complex oxides useful for the positive active material, a spinel lithium-manganese complex oxide (LiMn
2
O
4
) is regarded as promising from viewpoints of price and stable supply of raw material.
However, there still remains a room for improvement in the use of such a lithium-manganese complex oxide (LiMn
2
O
4
) for the positive electrode material. Specifically, this spinel complex oxide shows a marked reduction in capacity with charge-discharge cycling, compared to lithium-cobalt and lithium-nickel complex oxides which do not have a spinel structure.
As one solution to this problem, M. Wakihara et al. reports that the reinforcement of a crystal structure by substitution of a dissimilar element, such as Co, Cr or Ni, for a part of Mn atoms in the spinel lithium-manganese complex oxide (LiMn
2
O
4
) improves cycle characteristics (see J.Electrochem.Soc., Vol.143, No.1, p.178 (1996)).
However, such substitution has been still insufficient to improve cycle characteristics because of the following reason. As the spinel lithium-manganese complex oxide undergoes expansion and shrinkage during every charge-discharge cycle of a secondary battery, active material particles also undergo expansion and shrinkage. This reduces a strength of the positive electrode and causes insufficient contact of the active material particles with current collector particles, resulting either in the reduced utilization of the positive electrode or in the fall-off of a cathode mix from a current collector, which both have been problems.
Japanese Patent Laying-Open No. Hei 8-45498 proposes a technique for limiting expansion and shrinkage of a cathode mix in its entirety by combining a lithium-manganese complex oxide with a lithium-nickel complex oxide, based on the finding that the lithium-manganese complex oxide undergoes crystal expansion while the lithium-nickel complex oxide undergoes crystal shrinkage when lithium ions are inserted thereinto.
Also, Japanese Patent Laying-Open Nos. Hei 11-3698 and Hei 1-54122 propose a technique for improving electronic conduction of a cathode mix as a whole and thus cycle performance characteristics by combining a lithium-nickel complex oxide, a lithium-cobalt complex oxide and a lithium-manganese complex oxide, based on the finding that the lithium-cobalt complex oxide exhibits a higher electronic conduction than the lithium-manganese complex oxide.
While such combinations achieve improvements to certain degrees, there still remains a room for improving cycle performance characteristics. The inventors of the present application have studied the reduction in capacity with cycling for a positive electrode material (active material) containing a mixture of a spinel lithium-manganese complex oxide and a lithium-nickel complex oxide and found that its load characteristics decrease with increasing cycles. That is, the capacity reduction has been observed to occur when its capacities both initially and after cycles are measured at a relatively high current, e.g., at a 1 C discharge rate, as a result of the reduced load characteristics.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a nonaqueous electrolyte secondary battery which has a high capacity retention and exhibits improved cycle performance characteristics.
A nonaqueous electrolyte secondary battery in accordance with a first aspect of the present invention is characterized as using a mixture of a first oxide and a second oxide for the positive electrode material. The first oxide is a spinel oxide consisting substantially of lithium, manganese, a metal other than manganese, and oxygen. The second oxide is different in composition from the first oxide and consists substantially of lithium, nickel, cobalt, a metal other than nickel and cobalt, and oxygen.
The first aspect of the present invention is described below.
A specific example of the first oxide is an oxide derived via substitution of other element for a part of manganese in a lithium-manganese complex oxide. A specific example of the second oxide is an oxide derived via substitution of cobalt and other element for a part of nickel in a lithium-nickel complex oxide.
The use, in combination, of the first oxide derived via substitution of other element for a part of manganese in the spinel lithium-manganese complex oxide and the second oxide derived via substitution of cobalt and other element for a part of nickel in the lithium-nickel complex oxide is effective to suppress deterioration of load characteristics with cycling. A first reason for this is considered due to the inclusion of dissimilar elements, in theform of a solid solution, that causes active material comprising the first and second oxides to undergo a change in electronic state to the extent that improves electronic conduction of the active material in its entirety. A second reason is considered due to the use, in combination, of the lithium-manganese complex oxide which undergoes crystal expansion when lithium ions are inserted thereinto and the lithium-nickel-cobalt complex oxide which undergoes crystal shrinkage when lithium ions are inserted thereinto, that is effective to maintain stable contact between particles of the first and second oxides during repetitive cycling.
Examples of first oxides include spinel lithium-manganese complex oxides represented by the compositional formula Li
x
Mn
2−y
M1
y
O
4+2
(where M1 is at least one element selected from the group consisting of Al, Co, Ni, Mg and Fe, 0≦x≦1.2, 0<y≦0.1 and −0.2≦z≦0.2).
Preferably, M1 in the compositional formula Li
x
Mn
2−y
M1
y
O
4+z
is at least one of Al and Mg.
Examples of second oxides include complex oxides represented by the compositional formula Li
a
M2
b
Ni
c
Co
d
O
2
(where M2 is at least one element selected from the group consisting of Al, Mn, Mg and Ti, 0<a<1.3, 0.02≦b≦0.3, 0.02≦d/(c+d)≦0.9 and b+c+d=1). Preferred among them are those which contain Al in the place of M2 and satisfy 0.1≦d/(c+d)≦0.5 in the compositional formula Li
a
M2
b
Ni
c
Co
d
O
2
.
The capacity is suitably maintained at high values, if the aforementioned first and second oxides are mixed in the ratio by weight of 20:80-80:20. Within the specified range, the electronic conductivity of the whole is improved and contact between particles of first and second oxides is maintained in a more stable manner, so that deterioration of load characteristics with cycling is suppressed effectively.
The first oxide in the form of a lithium-manganese complex oxide preferably has a mean particle diameter of 5-30 &mgr;m. The second oxide in the form of a lithium-nickel-cobalt complex oxide preferably has a mean particle diameter of 3-15 &mgr;m. The combination thereof is most preferred. Preferably, the first oxide has a larger mean particle diameter than the second oxide. If the mean particle diameter of each oxide is maintained within the above-specified range, contact between particles of those complex oxides is maintained at a higher degree of occurrence to thereby improve the electronic conduction of the mix in its enti

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