Lithium secondary battery

Details Lithium secondary battery Lithium secondary battery

2002-04-18

2004-10-05

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S341000, C429S245000, C429S231950, C429S324000, C429S326000, C429S332000, C429S233000

Reexamination Certificate

active

06800400

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lithium secondary battery, particularly to an improvement in a nonaqueous electrolyte in a lithium secondary battery using an electrode formed by depositing a thin film composed of active material capable of lithium storage and release on a current collector.
2. Related Art
In recent years, lithium secondary batteries have been actively developed. In the lithium secondary batteries, the battery characteristics thereof, such as charge-discharge voltage, charge-discharge cycle life characteristics, and storage characteristics, depend largely on the type of an electrode active material used therein.
We have found that an electrode formed by depositing a thin film of active material capable of lithium storage and release, such as an amorphous or microcrystalline silicon thin film, on a current collector exhibits high charge-discharge capacities and superior charge-discharge cycle characteristics. In such an electrode, the thin film is divided into columns by gaps formed therein in a manner to extend in its thickness direction and the bottoms of the columnar portions are adhered to the current collector. In the electrode having such a structure, spaces are formed around the columnar portions, and stress caused by expansion and shrinkage during charge-discharge cycle is relaxed by the spaces to suppress the stress that causes separation of the thin film of active material from the current collector. Therefore, superior charge-discharge cycle characteristics can be obtained.
However, in the lithium secondary battery having such an electrode, relationship between a nonaqueous electrolyte and charge-discharge cycle characteristics is not sufficiently investigated.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a lithium secondary battery using an electrode formed by depositing a thin film of active material capable of lithium storage and release on a current collector, having further improved charge-discharge cycle characteristics.
A first aspect of the present invention is a lithium secondary battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode or the negative electrode is an electrode formed by depositing a thin film of active material capable of lithium storage and release on a current collector, the thin film of active material is divided into columns by gaps formed therein in a manner to extend in its thickness direction, and the bottoms of the columnar portions are adhered to the current collector, and wherein the nonaqueous electrolyte contains at least one selected from phosphate ester, phosphite ester and borate ester.
In the first aspect of the present invention, the nonaqueous electrolyte contains at least one selected from phosphate ester, phosphite ester and borate ester; therefore, coating films containing phosphorus and/or boron are selectively formed on the side faces of the columnar portions of the thin film of active material. It can be considered that by the thus-formed coating films, the columnar structure of the thin film of active material is stabilized so that the deterioration or collapse of the columnar portions is suppressed. It can also be considered that by suppressing the deterioration or the collapse of the columnar portions, the state that the bottoms of the columnar portions are adhered to the current collector is kept so that charge-discharge cycle characteristics can be improved.
The phosphate ester used in the present invention is preferably phosphate ester represented by the general formula (1), and the phosphite ester used in the present invention is preferably phosphite ester represented by the general formula (2):
wherein R
1
, R
2
, R
3
, R
4
, R
5
and R
6
may be identical to or different from one another, and each of them represents an alkoxy group, a hydrocarbon group, a hydrocarbon group containing an ether bond, or a hydrocarbon group containing a carbonyl group, and at least one hydrogen atom therein may be substituted with a halogen atom.
When the substituents R
1
to R
6
in the general formulas (1) and (2) are alkoxy groups, the alkoxy groups are preferably alkoxy groups each having 1 to 5 carbon atoms.
Specific examples of the phosphate ester include phosphate ester compounds having substituents Rs shown in Table 1.
TABLE 1
R
Compound
R
1
, R
2
, R
3
= —OCH
3
Trimethyl phosphate
R
1
, R
2
, R
3
= —OCH
2
CH
3
Triethyl phosphate
R
1
, R
2
, R
3
= —OCH
2
CH
2
CH
3
Tripropyl phosphate
R
1
, R
2
, R
2
= —O(CH
2
)
3
CH
3
Tri-n-butyl phosphate
R
1
, R
2
, R
3
= —OCH
2
CH═CH
2
Triallyl phosphate
R
1
, R
2
, R
3
= —OCH
2
OCH
3
Trimethoxymethyl phosphate
R
1
, R
2
, R
3
= —0C
6
H
5
Triphenyl phosphate
R
1
, R
2
= —OC
6
R
5
, R
3
= —CH
2
CH
3
Diphenyl ethylphosphonate
R
1
, R
2
= —OCH
2
CH
3
, R
3
= —CHF
2
Diethyl difluoromethyl
phosphonate
R
1
, R
2
= —OCH
3
, R
3
=—CH
2
COCH
3
Dimethyl 2-oxopropyl
phosphonate
R
1
, R
2
= —OCH
2
CH
3
,
Diethyl
R
3
= —CH
2
COO(CH
2
)
2
CH
3
propoxycarbonylmethyl-
phosphonate
Specific examples of the phosphite ester include phosphite ester compounds having substituents Rs shown in Table 2.
TABLE 2
R
Compound
R
4
, R
5
, R
6
= —OCH
3
Trimethyl phosphite
R
4
, R
5
, R
6
= —OCH
2
CH
3
Triethyl phosphite
R
4
, R
5
, R
6
= —OCH(CH
3
)
2
Triisopropyl phosphite
R
4
, R
5
, R
6
= —O(CH
2
)
3
CH
3
Tri-n-butyl phosphite
R
4
, R
5
, R
6
= —O(CH
2
)
2
OCH
3
Tris (methoxyethyl) phosphite
R
4
, R
5
, R
6
= —OCH
2
CF
3
Tris (2,2,2-trifluoroethyl)
phosphite
The borate ester used in the present invention is preferably borate ester represented by the general formula (3):
wherein R
7
, R
8
and R
9
may be identical to or different from one another, and each of them represents an alkoxy group or a hydrocarbon group and at least one hydrogen atom therein may be substituted with a halogen atom.
When the substituents R
7
to R
9
in the general formula (3) are alkoxy groups, the alkoxy groups are preferably alkoxy groups each having 1 to 5 carbon atoms.
Specific examples of the borate ester include borate ester compounds having substituents Rs shown in Table 3.
TABLE 3
R
Compound
R
7
, R
8
, R
9
= —OCH
3
Trimethyl borate
R
7
, R
8
, R
9
= —OCF
3
Tris (trifluoromethyl)
borate
R
7
, R
8
, R
9
= —OCH
2
CH
3
Triethyl borate
R
7
, R
8
, R
9
= —OCH
2
CH
2
CH
3
Tripropyl borate
R
7
, R
8
, R
9
= —OCH(CH
3
)
2
Triisopropyl borate
R
7
, R
8
, R
9
= —OC(CH
3
)
3
Tri-t-butyl borate
R
7
, R
8
, R
9
= —O(CH
2
)
4
CH
3
Tri-n-pentyl borate
R7 = —OCH
3
, R
8
, R
9
= —CH
2
CH
3
Diethylmethoxy borane
R7 = —CH
3
, R
8
, R
9
= —OCH(CH
3
)
2
Diisopropoxymethyl borane
R7 = —(CH
2
)
4
CH
3
, R
8
, R
9
= —OCH(CH
3
)
2
Isopropoxypentyl borane
A second aspect of the present invention is a lithium secondary battery comprising a positive electrode, a negative electrode and a nonaqueous electrolyte, wherein the positive electrode or the negative electrode is an electrode formed by depositing a thin film of active material capable of lithium storage and release on a current collector, the thin film of active material is divided into columns by gaps formed therein in a manner to extend in its thickness direction, the bottoms of the columnar portions are adhered to the current collector, and the nonaqueous electrolyte contains carboxylic ester represented by the general formula (4):
wherein R
10
represents a fluoroalkyl group having 1 to 4 carbon atoms, and R
11
represents an alkyl group having 1 to 5 carbon atoms.
Specific examples of the carboxylic ester include carboxylic ester compounds having substituents Rs shown in Table 4.
TABLE 4
R
Compound
R
10
= —CHF
2
, R
11
= —CH
3
Methyl difluoroacetate
R
10
= —CHF
2
, R
11
= —C
2
H
5
Ethyl difluoroacetate
R
10
= —CHF
2
,

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