Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-04-25
2002-06-04
Dunn, Tom (Department: 1725)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231800
Reexamination Certificate
active
06399251
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to lithium secondary batteries.
Currently, decreasing carbon dioxide, suppression of energy consumption, and the like are strongly required in view of environmental requirement. Accordingly, electric power storage systems, electric vehicles, and the like are receiving attention as new environmental technology. Lithium secondary batteries using non-aqueous electrolyte have been developed remarkably, because of their high battery voltage and high energy density, and the lithium secondary batteries are practically used for information apparatus such as computers, portable telephones, and the like.
However, because industrial batteries of high input, high output, and a large capacity require a large amount of active material, Co group materials and Ni group materials, which have been used for the information apparatus, can not be used practically for the industrial batteries in view of cost and resources. Therefore, spinel type Mn group materials are expected to solve these problems. However, the spinel type Mn group materials had problems such as low cycle life at high temperature, which is the most important issue for the industrial batteries, undesirable output characteristics, and undesirable input characteristics.
In order to apply the lithium secondary battery as power sources for electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like, a life at least 1000 cycles (at least 70% of capacity maintaining rate) at a high temperature higher than 50° C., and an output power at least 500 W/kg are required for the lithium secondary batteries. However, conventional Mn group materials could not achieve such a long life nor high output power density.
Hitherto, many trials for extending the life have been performed. For instance, in accordance with JP-A-6-187993 (1994), extension of the life by increasing a composition ratio of Li and Mn, i.e. Li/Mn ratio, has been tried. However, decreasing its capacity of approximately several per cent was occurred after only 10 cycles of charge-discharge cycle even at room temperature. The cycle life of the lithium secondary battery is significantly influenced by environmental temperature, and, in particular, the life is remarkably shortened at a high temperature higher than 50° C. Accordingly, it is difficult to obtain the cycle life longer than 1000 cycles at a high temperature higher than 50° C. by only increasing the Li/Mn ratio.
In accordance with JP-B-8-24043 (1996), the extension of life has been tried by increasing the Li/Mn ratio similarly, and calcining the material at a temperature in the range of 430-510° C. so as to obtain a material having a lattice constant smaller than 8.22 Å. However, only a life of approximately 200 cycles at room temperature could be obtained, and any prospect to obtain a cycle life of at least 1000 cycles at a high temperature higher than 50° C. could not be obtained. In accordance with JP-A-7-282798 (1995), the extension of life has been tried by using a material having a large Li/Mn ratio, i.e. Li[Mn
2−x
Li
x
]O
4
(0.020≦x≦0.081). However, in a case when x was made 0.081 (Li/Mn ratio=0.58), decreasing the capacity of 5% was observed even in room temperature after approximately 100 cycles, and the cycle life longer than 1000 cycles could not be expected at a high temperature higher than 50° C.
The reason of short cycle life is in disintegration of crystals of the positive electrode active material by repeating expansion and shrinkage of the positive electrode active material with plural cycles of charging and discharging operations, which makes it prohibit reversal absorption and desorption of lithium. Additionally, Mn ions are readily dissolved into the electrolyte at a high temperature, and the crystal of the positive electrode active material is more readily disintegrated than at room temperature. The dissolved ions are precipitated on the negative electrode, the charge and discharge reactions at the negative electrode is disturbed, and the life of the negative electrode is shortened.
The reason of low output characteristics, and low input characteristics with the lithium secondary battery is in low diffusion velocity relating to intercalation and deintercalation of lithium ions, because an organic solvent having a lower ion conductivity than aqueous solution is used as the electrolyte. In particular, a coating film is generated at the surface of the negative electrode by a reaction of the lithium ions, which are isolated by the presence of excessive amount of lithium ions for the reaction at the surface of the negative electrode, with the organic electrolyte, and the diffusion velocity of the lithium ions are further decreased by the presence of the coating film to make the output characteristics and the input characteristics worse. The ion conductivity of the organic solvent, i.e. the electrolyte, is decreased significantly at a low temperature, and the output characteristics and the input characteristics are getting further worse.
SUMMARY OF THE INVENTION
One of the objects of the present invention is to solve the above problems, and to provide a lithium secondary battery having a long life by using materials of long life at a high temperature, which is capable of receiving or supplying electric power rapidly corresponding to variation in power sources and power demands.
In accordance with the lithium secondary battery of the present invention, a material containing amorphous carbon is used for the negative electrode, and a complex oxide containing Li and Mn, which has a spinel type crystalline structure, is used for the positive electrode.
The complex oxide as the positive electrode of the present invention necessitates Li and Mn as indispensable elements, but a small amount of other elements such as transient element other than Mn, and elements in IIa group and IIIb group can be contained. For instance, these elements are Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Be, Mg, Ca, Sr, Ba, Ra, B, Al, Ga, In, Ti, and the like. Hereinafter structures of the positive electrode and the negative electrode are explained in details.
The positive electrode active material of the present invention is characterized in Li/Mn atomic ratio of the complex oxide in the range of larger than 0.55 and smaller than 0.80. When the Li/Mn atomic ratio is equal to or smaller than 0.55, the cycle life is short, because Mn ions are dissolved into the electrolyte and the crystalline structure of the complex oxide is disintegrated by repeating the charge and discharge cycles at a temperature higher than 50° C. When the Li/Mn atomic ratio is equal to or larger than 0.80, the discharging capacity is small, and an objective lithium secondary battery for mounting on power sources of electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like can not be obtained.
The spinel type crystalline of the complex oxide of the present invention is characterized in its lattice constant in the range larger than 8.031 Å and smaller than 8.230 Å. If the lattice constant is larger than 8.230 Å, the cycle life is short, because Mn ions are dissolved into the electrolyte and the crystalline structure of the complex oxide is disintegrated by repeating the charge and discharge cycles at a temperature higher than 50° C. If the lattice constant is smaller than 8.031 Å, the discharging capacity is small, and an objective lithium secondary battery for mounting on power sources of electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like can not be obtained.
The crystalline of the complex oxide of the present invention is further characterized in its half value width of 2&thgr; angle at (400) peak in the x-ray diffraction pattern in the range smaller than 0.20°. In the measurement of the x-ray diffraction pattern, the Cu-k
&agr;
ray was used as the radiation
Gotoh Akihiro
Honbo Kyoko
Hotta Yoshiji
Kasai Masahiro
Antonelli Terry Stout & Kraus LLP
Dunn Tom
Edmondson Lynne Renee
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