Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-12-17
2002-07-02
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S340000, C429S341000
Reexamination Certificate
active
06413677
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel lithium secondary battery electrolyte capable of providing a lithium secondary battery superior in the battery cycle characteristics and in battery characteristics such as the electric capacity and storage characteristics and to a lithium secondary battery using the same.
2. Description of the Related Art
In recent years, lithium secondary batteries have come into wide use as sources for driving compact electronic devices. A lithium secondary battery is mainly composed of a cathode, a non-aqueous electrolyte, and an anode. In particular, a secondary battery having a lithium complex oxide such as LiCoO
2
as its cathode and a carbonaceous material or lithium metal as its anode is preferably used. As the electrolyte for this lithium secondary battery, carbonates such as ethylene carbonate (EC), propylene carbonate (PC) are preferably used.
However, a secondary battery having more superior battery cycle characteristics and battery characteristics such as the electric capacity, is being sought.
In a lithium secondary battery using, for example, LiCoO
2
, LiMn
2
O
4
, LiNiO
2
, or the like as the cathode active material, the solvent in the non-aqueous electrolyte is locally partially oxidized and decomposed at the time of charging. The decomposed product inhibits the desirable electrochemical reaction of the battery, and therefore, causes the reduction in the battery performance. This is believed to be due to the electrochemical oxidation of the solvent at the interface of the cathode material and non-aqueous electrolyte.
Further, in a lithium secondary battery using, for example, highly crystallized carbonaceous material such as natural graphite, artificial graphite, as the anode active material, exfoliation of the carbonaceous material is observed. Depending on the extent of the phenomenon, the capacity sometimes becomes irreversible. This exfoliation occurs due to the decomposition of the solvent in the electrolyte at the time of charging and is due to the electrochemical reduction of the solvent at the interface of the carbonaceous anode material and the electrolyte.
Further, in general, a carbonaceous anode has a capacity called an “irreversible capacity” in which part of the lithium inserted in the carbonaceous anode at the time of the first charging will not disassociate at the time of discharging. To prepare a battery of a predetermined capacity, it is necessary to use an excess amount of the cathode active material corresponding to the amount of the irreversible capacity, and therefore, there are the problems that the battery weight becomes heavier by that amount or extra cost is required.
As mentioned above, the battery cycle characteristics and battery characteristics such as the electric capacity are not necessarily satisfactory at the present time.
SUMMARY OF INVENTION
The objects of the present invention are to solve the above-mentioned problems relating to lithium secondary battery electrolytes in the prior art and to provide a lithium secondary battery electrolyte capable of constructing a lithium secondary battery superior in battery cycle characteristics and further superior in battery characteristics such as the electric capacity and storage characteristics in a charged state and a lithium secondary battery using the same.
In accordance with the present invention, there is provided a lithium secondary battery electrolyte comprising a non-aqueous solvent and a lithium salt dissolved therein, said electrolyte containing at least one compound having the formula (I):
wherein, R
1
and R
2
independently indicate, a C
1
to C
12
alkyl group, a C
3
to C
6
. cycloalkyl group, or an aryl group, x indicates an oxygen atom or a sulfur atom, and n indicates an integer of 1 or 2.
In accordance with the present invention, there is also provided a lithium secondary battery comprising a cathode, an anode, and an electrolyte comprising a non-aqueous solvent and a lithium salt dissolved therein, said electrolyte containing at least one compound having the formula (I):
wherein, R
1
and R
2
independently indicate, a C
1
to C
12
alkyl group, a C
3
to C
6
cycloalkyl group, or an aryl group, wherein X indicates an oxygen atom or a sulfur atom, and n indicates an integer of 1 or 2.
In accordance with the present invention, there is further provided a lithium secondary battery electrolyte comprising a non-aqueous solvent and a lithium salt dissolved therein, said electrolyte containing at least one thiol salt having the formula (II):
R
3
—S—M (II)
wherein, R
3
indicates a C
1
to C
15
alkyl group or a C
3
to C
12
cycloalkyl group which may be substituted with at least one C
1
to C
4
alkyl group, a C
7
to C
15
benzyl group which may be substituted with at least one C
1
to C
4
alkyl group, or a C
6
to C
15
aryl group, which may be substituted with at least one C
1
to C
4
alkyl group, M indicates an alkali metal, and R
3
may be substituted with at least one halogen atom.
In accordance with the present invention, there is further provided a lithium secondary battery comprising a cathode, an anode, and a lithium salt comprising a non-aqueous solvent and an electrolyte dissolved therein, said electrolyte containing at least one thiol salt having the formula (II):
R
3
—S—M (II)
wherein, R
3
indicates a C
1
to C
15
alkyl group or a C
3
to C
12
cycloalkyl group which may be substituted with at least one C
1
to C
4
alkyl group, a C
7
to C
15
benzyl group which may be substituted with at least one C
1
to C
4
alkyl group, or a C
6
to C
15
aryl group, which may be substituted with at least one C
1
to C
4
alkyl group, M indicates an alkali metal, and R
3
may be substituted with at least one halogen atom.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The compound having the general formula (I) contained in the electrolyte is reduced and decomposed before the organic solvent in the electrolyte at the carbon anode surface at the time of charging. Part of the decomposed product forms a passivation film at the carbon anode surface made highly crystalline by activity such as natural graphite or artificial graphite to thereby, it is believed, prevent in advance the reduction and decomposition of the organic solvent in the electrolyte.
Further, part of the decomposed product is oxidized and decomposed before the organic solvent in the electrolyte at the slight overvoltage portion where the potential of the surface of the cathode material becomes excessively high to thereby, it is believed, prevent in advance the oxidation and decomposition of the organic solvent in the electrolyte.
Due to the above phenomina, it is believed that there is the effect of suppressing the decomposition of the electrolyte, without impairing from the normal reaction of the battery.
In the compound contained in the electrolyte composed of a non-aqueous solvent and a lithium salt dissolved therein, the R
1
and R
2
in the compound having the general formula (I) preferably independently represent a C
1
to C
12
alkyl group such as a methyl group, ethyl group, propyl group, butyl group, pentyl group, and hexyl group. The alkyl group is preferably a branched alkyl group such as an isopropyl group and isobutyl group. Further, the cyclopropyl group may be a C
3
to C
6
cycloalkyl group such as a cyclohexyl group. Further, it may contain a C
6
to C
12
aryl group such as a phenyl group, benzyl group, or p-tolyl group. Further, X represents an oxygen atom or sulfur atom, wherein n indicates an integer of 1 or 2.
As specific examples of the compound having the general formula (I), for example, when X is an oxygen atom, S-methyl O-methyl thiocarbonate (R
1
=methyl group, R
2
=methyl group, n=1), S-ethyl O-methyl thiocarbonate (R
1
=ethyl group, R
2
=methyl group, n=1), S-butyl O-methyl thiocarbonate (R
1
=butyl group, R
2
=methyl group, n=1), S-cyclohexyl O-methyl thiocarbonate (R
1
=cyclohexyl group, R
2
=
Abe Koji
Hamamoto Toshikazu
Matsumori Yasuo
Miyoshi Kazuhiro
Takai Tsutomu
Kalafut Stephen
Morgan & Lewis & Bockius, LLP
UBE Industries Ltd.
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