Patent ID: 7341807

Claim:
A non-flammable, nonaqueous electrolyte solution having a lithium salt dissolved in a solvent containing a phosphate ester, comprising at least one additive compound selected from each of the three compound groups consisting of: (a) vinylene carbonate compounds represented by formula (I): wherein R 1 and R 2 each independently represents a hydrogen atom or a straight-chain or branched-chain alkyl group having 1 to 4 carbon atoms, (b) a compound selected from the groups consisting of vinyl acetate compounds represented by formula (II), alkyl methyl carbonate compounds represented by formula (III), and vinyl ethylene carbonate compounds of formula (IV): wherein R 3 , R 4 and R 5 each independently represents a hydrogen atom or a straight-chain or branched-chain alkyl group having 1 to 4 carbon atoms, wherein R 6 represents a hydrogen atom or a straight-chain or branched-chain alkyl group having 1 to 4 carbon atoms, wherein R 7 , R 8 , R 9 , R 10 , R 11 and R 12 each independently represents a hydrogen atom or a straight-chain or branched-chain alkyl group having 1 to 4 carbon atoms; and (c) a compound selected from the group consisting of 2-pyrrolidinone compounds represented by formula (V), cyclic alkyl compounds represented by formula (VI), and cyclic pentanone compounds represented by formula (VII): wherein R 13 represents a hydrogen atom or a straight-chain or branched-chain alkyl group having 1 to 4 carbon atoms, wherein R 14 represents a straight-chain or branched-chain alkylene group having 2 to 4 carbon atoms, wherein R 15 represents a straight-chain or branched-chain alkylene group having 1 to 3 carbon atoms; wherein each of compounds (a), (b), and (c) is present in an amount from 1 to 12% by weight and the total amount of compounds (a), (b), and (c) is from 5 to 20% by weight; and further wherein the total concentration of the lithium salt in the phosphate ester-based solvent is in the range of 1.5 to 2.5 mol/dm 3 ; and further wherein the solvent comprises a phosphate ester selected from the group consisting of the chain phosphate esters of formula (IX) and the cyclic phosphate esters of formula (X), wherein at least either one of the chain phosphate ester represented by formula (IX) and the cyclic phosphate ester represented by formula (X) is contained in the phosphate ester-based solvent in an amount ranging from 50 to 100% by volume: wherein R 16 , R 17 and R 18 each independently represents an alkyl group having 1 or 2 carbon atoms, in which each of the hydrogen atoms may be substituted with fluorine, wherein R 19 represents an alkyl group having 1 or 2 carbon atoms in which each of hydrogen may be substituted with fluorine, and R 20 represents an alkylene group having 2 to 4 carbon atoms.