Lithium secondary battery

Details Lithium secondary battery Lithium secondary battery

2000-12-18

2003-03-25

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S307000, C252S062200

Reexamination Certificate

active

06537697

ABSTRACT:

BACKGROUND OF THE INVENTION
This application claims the Paris convention priority of Japanese Patent Application No. 11-363692/1999 filed on Dec. 22, 1999, which is incorporated herein by reference.
The present invention relates to a lithium secondary battery using a nonaqueous electrolyte including an electrolytic salt dissolved in a nonaqueous solvent, and more particularly, it relates to improvement of a nonaqueous electrolyte for the purpose of providing a lithium secondary battery exhibiting better charge-discharge cycle performance than a lithium secondary battery using a conventional nonaqueous electrolyte.
A conventional water reactive lithium secondary battery uses, as an electrolyte, a nonaqueous electrolyte including an electrolytic salt dissolved in a nonaqueous solvent.
Examples of the conventionally used nonaqueous solvent are ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, 1,2-dimethoxyethane and a mixed solvent including any of these solvents. An example of the conventionally used electrolytic salt is a lithium salt such as LiPF
6
, LiBF
4
, LiCF
3
SO
3
, LiN(CF
3
SO
2
)
2
or LiN(C
2
F
5
SO
2
)
2
.
In order to improve the characteristics, such as the charge-discharge cycle performance in particular, of a lithium secondary battery, a variety of improvements have been proposed with respect to not only a positive electrode active material and a negative electrode active material but also a nonaqueous electrolyte.
For example, it has been reported that the charge-discharge cycle performance can be improved by using both LIPF
6
and LIBF
4
as the electrolytic salt of the nonaqueous electrolyte in a lithium secondary battery using Li
x
MO
2
(wherein M is one or more transition metals; and 0.05≦x≦1.10) as the positive electrode active material (Japanese Laid-Open Patent Publication No. 8-17468/1996).
As a result of examination made by the present inventors, however, it has been found that the charge-discharge cycle performance, at a high temperature of approximately 50 through 60° C. in particular, cannot be largely improved by using both LiPF
6
and LIBF
4
. This is probably because P—F bonds and B—F bonds with low bonding strength are cut through repeated charge and discharge, resulting in releasing fluorine ions degrading the charge-discharge cycle performance or lowering the concentration of the electrolytic salt in the nonaqueous electrolyte.
Accordingly, an object of the invention is providing a lithium secondary battery exhibiting better charge-discharge cycle performance than a lithium secondary battery using a conventional nonaqueous electrolyte. This object is achieved by using a specific lithium salt as the electrolytic salt of the nonaqueous electrolyte as described in detail below.
SUMMARY OF THE INVENTION
The lithium secondary battery of this invention (present battery) comprises a positive electrode, a negative electrode and a nonaqueous electrolyte including an electrolytic salt dissolved in a nonaqueous solvent, and a part or whole of the electrolytic salt is lithium tetrakis(pentafluorophenyl)borate.
As a result, the present battery can exhibit better charge-discharge cycle performance than a lithium secondary battery using a conventional nonaqueous electrolyte.


REFERENCES:
patent: A817468 (1996-01-01), None

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