Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-02-14
2001-11-20
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231500, C429S218100
Reexamination Certificate
active
06319633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rechargeable lithium battery including a positive electrode, a negative electrode and a non-aqueous electrolyte, and more particularly to a rechargeable lithium battery incorporating the improved active material, for use in the positive or negative electrode, which undergoes a reversible electrochemical reaction with a lithium ion.
2. Description of Related Art
In recent years, intensive efforts have been made to develop rechargeable lithium batteries. Rechargeable lithium batteries are known as relying their battery performances, such as charge-discharge voltages, charge- discharge cycle life characteristics and storage capability, largely on the particular electrode active materials used. Thus extensive searches for improved electrode materials have been conducted to achieve improvements in performance of batteries.
In Japanese Patent Laying-Open No. Hei 6-275315 (1994), a rechargeable lithium battery is disclosed which employs a combination of transition metal sulfide active material and a lithium-ion conducting solid electrolyte to improve the battery performances. However, the use of tungsten disulfide (WS
2
) for a positive electrode material results in the reduced charge-discharge cycle life performance, because a crystal structure of WS
2
, serving as the positive active material, is significantly affected by insertion and release of lithium ions.
SUMMARY OF THE INVENTION
The present invention is directed toward solving the above-described problem and its object is to provide a rechargeable lithium battery which exhibits excellent charge-discharge cycle characteristics, such as a cycle life of several tens of cycles practically required for secondary batteries.
A rechargeable lithium battery of the present invention includes a positive electrode, a negative electrode and a non-aqueous electrolyte. Characteristically, the active material of the positive or negative electrode is comprised of tungsten complex sulfide, either with or without addition of lithium thereto, which contains tungsten, sulfur, and at least one transition metal selected from Cu, V, Cr, Mn, Fe, Co and Ni, and has substantially the same crystal structure as WS
2
.
Specifically, the rechargeable lithium battery in accordance with the present invention uses, as positive or negative active material, tungsten complex sulfide, either with or without addition of lithium thereto, which is represented by the formula M
x
W
1-x
S
2
where M is at least one selected from Cu, V, Cr, Mn, Fe, Co and Ni, and x satisfies the relationship 0<x<0.48, and which has substantially the same crystal structure as WS
2
.
In the present invention, a crystal structure of the active material has been stabilized by the substitution of the metallic element M (Cu, V, Cr, Mn, Fe, Co or Ni) for a part of tungsten (W) at the W site of tungsten disulfide (WS
2
). The use of such tungsten complex sulfide for active material thus leads to successful improvement of charge- discharge cycle life characteristics of rechargeable lithium batteries.
Its crystal structure can be confirmed by X-ray diffractometry (XRD) as being substantially identical to that of WS
2
.
In the present invention, specified as the metallic element M are Cu, V, Cr, Mn, Fe, Co and Ni which have been found to effectively improve charge-discharge cycle life characteristics of rechargeable lithium batteries. These metallic elements are known as each forming a stable compound, when combined with sulfur (S), which can be decomposed at a temperature over 1,000° C. (See, for example, binary phase diagrams for M-S in “Binary Alloy Phase Diagrams”, American Society for Metals, Vol.2, (1986)). That is, any of these metallic elements tends to form a relative strong chemical bond with sulfur (S) so that it occupies a part of a crystal lattice of WS
2
to stabilize the crystal structure. Therefore, other elements, such as Cd, In, Mo, La, Ce, Sm and Pt, which form compounds with S, are also expected to serve to improve cycle lives as analogously to the present invention.
In the above-defined formula, x (stoichiometry of the metallic element M) is specified as being below 0.48. This is because, for x≧0.48, a single phase or sulfide phase of M may be caused to separate from the crystal structure to result in reducing the effect of active material to improve the cycle life performance capability.
An electrolyte solvent for use in rechargeable lithium batteries according to the present invention may be a mixed solvent of cyclic carbonate and chain carbonate, for example. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate and butylene carbonate, and examples of chain carbonates include dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate. The electrolyte solvent may alternatively be a combination of the aforementioned cyclic carbonate and an ether solvent.
Examples of ether solvents include 1,2-dimethoxyethane and 1,2-diethoxyethane.
A useful electrolyte solute may be LiPF
6
, LiBF
4
, LiCF
3
SO
3
, LiN(CF
3
SO
2
)
2
, LiN(C
2
F
5
SO
2
)
2
, LiN(CF
3
SO
2
)(C
4
F
9
SO
2
) LiC(CF
3
SO
2
)
3
, LiC(C
2
F
5
SO
2
)
3
and any combination thereof, for example.
Other applicable electrolytes include gelled polymer electrolytes wherein a liquid electrolyte is impregnated in polymers such as polyethylene oxide and polyacrylonitrile, and inorganic solid electrolytes such as LiI and Li
3
N.
Any electrolyte can be used for rechargeable lithium batteries in accordance with the present invention, so long as a lithium compound, as its solute used to realize an ionic conductivity, as well as its solvent used to solubilize and hold the Li compound, are hardly decomposed at voltages applied during charge, discharge and storage.
In the case where the active material of the present invention is used for a positive electrode, a suitable active material for a negative electrode may be selected from carbon materials which are capable of electrochemical storage and release of Li, such as graphite (either natural or synthetic), coke, and calcined organics; Li alloys such as Li—Al, Li—Mg, Li—In, Li—Al—Mn alloys; and metallic Li.
In such instances, an end-of-charge voltage of about 3.4 V and discharge voltage of about 2.9 V will be given. The contemplated effect of improving cycle life performances becomes more significant when the carbon materials, among those active materials, are used for a negative electrode. This is because the carbon materials are contrary in property to the Li alloys and metallic Li which, during charge and discharge, are likely to be accompanied by the growth of treelike dendrites that could cause internal short circuits, and which have a tendency to react with sulfur (S) slightly dissolved in a liquid electrolyte to form, on a negative electrode surface, a compound that could cause deactivation, such as Li
2
S (See, for example, a binary alloy phase diagram for Li—S in “Binary Alloy Phase Diagrams”, American Society for Metals, Vol.2, p.1500 (1986)).
On the other hand, in the case where the active material of the present invention is used for a negative electrode, and lithium-containing transition metal oxide, such as LiCoO
2
, LiNiO
2
, LiMn
2
O
4
, LiMnO
2
, lithium-containing MnO
2
, LiCo
0.5
Ni
0.5
O
2
, or LiNi
0.7
Co
0.2
Mn
0.1
O
2
, is used for a positive electrode, a battery can be constructed which exhibits an end-of-charge voltage of about 1.2 V and a discharge voltage of about 0.7 V. The same level of cycle life improving effect as contemplated in the present invention is obtained for this case.
The tungsten complex sulfide in accordance with the present invention can be synthesized by calcining a mixture of simple substances of respective constituent elements, a mixture of compounds of respective constituent elements, or a mixture of simple substance of one or more constituent elements and compounds of remaining constituent elements, for example. Preferably, a calcining temperature used in the calcining process is controlled between 400° C.
Fujimoto Masahisa
Fujitani Shin
Ikeda Hiroaki
Brouillette Gabrielle
Fasse W. F.
Fasse W. G.
Sanyo Electric Co,. Ltd.
Wills M.
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