Lithium secondary cell

Details Lithium secondary cell Lithium secondary cell

1999-08-20

2001-05-08

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S218100, C429S231950, C429S245000

Reexamination Certificate

active

06228532

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a secondary cell, particularly to a lithium polymer secondary cell.
2. Description of the Related Art
Recently, lithium secondary cells are drawing attention among other secondary cells, since they have high energy densities and can realize lightening of the cells.
The lithium secondary cell has, for example, a structure which comprises; a positive electrode having a positive electrode layer such as of lithium cobaltate (LiCoO
2
) formed on a positive electrode collector; and a negative electrode having a negative electrode layer such as of graphite formed on a negative electrode collector, both the electrodes are disposed in a sealed vessel filled with an organic electrolyte solution to be spaced from each other by a separator.
Meanwhile, there are developed lithium secondary cells employing polymeric positive electrode layer materials such as polyanilines (hereinafter abbreviated sometimes as PAn), in place of LiCoO
2
used as the positive electrode layer material, for the purpose of weight reduction and for safety's sake. And also there are developed lithium secondary cells containing additionally in the positive electrode layer a thiol compound having a great theoretical capacity such as 2,5-dimercapto-1,3,4-thiadiazole (hereinafter abbreviated as DMcT), which increases storable energy density.
In positive electrode layer materials, thiol compounds each having a mercapto group (—SH) in the molecule such as DMcT undergo reversible formation and dissociation of the disulfide bonds (—SS—) as redox reactions take place, so that those compounds which have two —SH groups in the molecule like DMcT each liberate two electrons per molecule in the oxidation reaction to undergo a polymerization reaction and form a polydisulfide bond, whereas they each undergo a depolymerization in the reduction reaction to return to the DMcT monomer. Further, DMcT has insulating properties both in the form of oxidant and in the form of reductant. Accordingly, it is necessary to combine DMcT with a conductive material so that it can be used as an electrode material. Such conductive materials include carbon powders or fibers such as graphite powders, graphite fibers and acetylene black (hereinafter abbreviated sometimes as AB) powders, and conductive polymers such as polypyrroles excluding PAn.
OBJECT AND SUMMARY OF THE INVENTION
Since it is necessary to increase the redox reaction rate in such positive electrode layer materials for lithium polymer secondary cells, it has conventionally been attempted to improve charge and discharge characteristics of cells by combining the positive electrode layer materials with various types of collectors.
For example, in a polymer secondary cell, there is proposed a capability of use of a copper collector in combination with a DMcT-PAn composite. When a copper collector is used in such polymer secondary cells, the copper collector itself serves also as an electrode active material, and redox reactions of copper take place at the positive electrode during charging and discharging. Therefore, Cu ions are discharged from the copper constituting the positive electrode into the solid electrolyte during charging, whereas lithium ions are converted to metal lithium at the negative electrode to reduce the lithium ions in the solid electrolyte. Reversed reactions take place during discharging. Accordingly, with the timing that the solid electrolyte has run dry of lithium ions during charging, the cell becomes non-chargeable. That is, in a polymer secondary cell composed essentially of a copper collector and a DMcT-PAn composite, the number of lithium ions contained initially in the solid electrolyte limits chargeable capacity. Thus, in order to obtain a sufficient capacity per unit surface area, the film thickness of the solid electrolyte must be increased.
The present invention, which was accomplished in view of the problems as described above, is directed to providing a lithium secondary cell in which the film thickness of the solid electrolyte can be reduced.
The lithium secondary cell according to the present invention comprises a composite electrode, as a positive electrode, having a positive electrode layer containing a mercaptide compound, in which the hydrogen atom in at least one mercapto group is substituted with a lithium atom, and a conductive material carried on a collector; a polyelectrolyte; and a lithium negative electrode.
The lithium secondary cell of the present invention is characterized in that the mercaptide compound is lithium thiocyanurate.
The lithium secondary cell of the present invention is characterized in that the conductive material contains a polyaniline.
The lithium secondary cell of the present invention is characterized in that the collector is a carbon film.
The lithium secondary cell of the present invention is characterized in that the polyelectrolyte contains at least one of ethylene carbonate and propylene carbonate.


REFERENCES:
patent: 6057056 (2000-05-01), Kim et al.

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