Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
2001-11-02
2004-09-21
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S323000, C429S300000, C429S231400, C429S231800, C429S162000, C429S163000
Reexamination Certificate
active
06794089
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-054937, filed Feb. 28, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonaqueous electrolyte and a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
Nowadays, a lithium ion secondary battery is put on the market as a nonaqueous electrolyte secondary battery for a portable apparatus such as a portable telephone. As an example of such a lithium ion secondary battery is a secondary battery comprising a positive electrode containing lithium cobalt oxide (e.g., LiCoO
2
), a negative electrode containing a graphitized material or a carbonaceous material, a nonaqueous electrolyte containing as a main component an organic solvent having a lithium salt dissolved therein, and a porous film used as a separator. A nonaqueous solvent having a low viscosity and a low boiling point is used as the solvent of the electrolyte.
Jpn. Pat. Appln. KOKAI Publication No. 4-14769 discloses an organic solvent containing as a main component a mixed solvent consisting of propylene carbonate, ethylene carbonate and &ggr;-butyrolactone. It is taught that it is possible to improve the low temperature discharge characteristics of a cylindrical nonaqueous electrolyte secondary battery by using a nonaqueous electrolyte prepared by using the mixed solvent noted above, which contains 10 to 50% by volume of &ggr;-butyrolactone.
However, the lithium ion secondary battery comprising a nonaqueous electrolyte disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-14769 quote above gives rise problems. Specifically, a large amount of gas is generated from the negative electrode in the initial charging step. Alternatively, where the lithium ion secondary battery is stored under very high temperatures not lower than 60° C., the positive electrode reacts with the nonaqueous electrolyte so as to decompose the nonaqueous electrolyte by oxidation, thereby bringing about a gas generation. It follows that, if the wall thickness of the case housing the positive electrode, the negative electrode, the separator and the nonaqueous electrolyte is decreased in order to decrease the thickness of secondary battery, the case is swollen by the generated gas so as to be deformed. If the case is deformed, the secondary battery fails to be incorporated in an electronic apparatus or tends to bring about a malfunction of the electronic apparatus. Further, the lithium ion secondary battery gives rise to additional problems that the self-discharge tends to proceed under a high temperature environment, and that the secondary battery is not satisfactory in respect of the charge-discharge cycle life characteristics.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 11-97062 teaches a nonaqueous electrolyte prepared by dissolving lithium borofluoride (LiBF
4
) in a solvent containing 100% by volume of &ggr;-butyrolactone. It is taught that use of the particular nonaqueous electrolyte makes it possible to prevent the positive electrode containing a lithium cobalt composite oxide as an active material from being oxidized and decomposed by the nonaqueous electrolyte.
Also, a lithium ion polymer secondary battery comprising a polymer gel electrolyte is reported on pate 23 of “Summaries of Lectures in 67
th
Meeting of Electrochemical Society (published on Mar. 28, 2000). The polymer gel electrolyte is manufactured as follows. Specifically, ethylene carbonate and &ggr;-butyrolactone are mixed first at a volute ratio of 2:3 so as to prepare a mixed solvent, followed by dissolving LiBF
4
or LiPF
6
as a salt in the resultant mixed solvent so as to obtain an electrolyte. Further, a mixed solution consisting of the electrolyte thus obtained and a polyfunctional acrylate monomer is polymerized, followed by bringing about a chemical crosslinking reaction within the polymer thus obtained so as to obtain the polymer gel electrolyte.
However, in the secondary battery disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-97062 and reported in the literature quoted above, the electrolyte or the gel electrolyte tends to react with the negative electrode so as to be reduced and decomposed, with the result that a current concentration tends to take place in the negative electrode. It follows that problems are brought about that the lithium metal is precipitated on the surface of the negative electrode, that the impedance is increased in the interface of the negative electrode, and that the charge-discharge efficiency of the negative electrode is lowered so as to lower the charge-discharge cycle characteristics.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2000-235868 is directed to a nonaqueous electrolyte secondary battery using a nonaqueous electrolyte containing a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent. This prior art teaches that the nonaqueous solvent includes &ggr;-butyrolactone (BL), ethylene carbonate (EC) and at least one kind of a third solvent selected from the group consisting of propylene carbonate (PC), vinylene carbonate (VC), trifluoro propylene, diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and an aromatic compound, the amount of &ggr;-butyrolactone being larger than 50% by volume and not larger than 95% by volume.
This prior art also teaches that, if the ratio of at least one kind of the solvent selected from DEC, MEC, PC and VC exceeds 10% by volume based on the entire nonaqueous solvent, it is difficult to suppress sufficiently the decomposition of the nonaqueous electrolyte by oxidation under a high temperature environment. Alternatively, it is also taught that the viscosity of the nonaqueous electrolyte is increased so as to make it possible for the ionic conductivity to be lowered. Such being the situation, this prior art teaches that it is desirable for the volume ratio of at least one kind of the solvent selected from the group consisting of DEC, MEC, PC and VC to be not higher than 10% by volume based on the entire nonaqueous solvent.
However, the secondary battery disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-235868 is defective in that it is difficult to obtain a long charge-discharge cycle life under a high temperature environment.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a nonaqueous electrolyte and a nonaqueous electrolyte secondary battery capable of improving the charge-discharge cycle characteristics.
According to a first aspect of the present invention, there is provided a nonaqueous electrolyte comprising a nonaqueous solvent and a solute dissolved in the nonaqueous solvent, wherein:
the nonaqueous solvent contains ethylene carbonate (EC), propylene carbonate (PC), &ggr;-butyrolactone (BL), and a fourth component, which is a solvent other than the EC, PC and BL; and
the mixing ratio x (% by volume) of ethylene carbonate based on the total amount of the nonaqueous solvent falls within a range of between 15 and 50, i.e., 15≦x≦50, the mixing ratio y (% by volume) of propylene carbonate based on the total amount of the nonaqueous solvent falls within a range of between 2 and 35, i.e., 2≦y≦35, the mixing ratio z (% by volume) of &ggr;-butyrolactone based on the total amount of the nonaqueous solvent falls within a range of between 30 and 85, i.e., 30≦z≦85, and the mixing ratio p (% by volume) of the fourth component based on the total amount of the nonaqueous solvent is larger than 0 and is not larger than 5, i.e., 0≦p≦5.
According to a second aspect of the present invention, there is provided a nonaqueous electrolyte comprising a nonaqueous solvent and a solute dissolved in the nonaqueous solvent, wherein:
the nonaqueous solvent contains ethylene carbonate (EC), propylene carbonate (PC), &ggr;-butyrolactone (BL), vinylene carbonate (VC) and a fifth component, which is a solvent other than the EC, PC, BL and VC; a
Fujiwara Masashi
Hasebe Hiroyuki
Kato Makoto
Momma Jun
Oguchi Masayuki
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Weiner Laura
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