Patent ID: 8247112

Claim:
An electrolyte solution for lithium-ion secondary batteries characterized by comprising: a tetraalkylphosphonium salt represented by general formula (1): wherein R 1 represents a linear, branched or alicyclic alkyl group having 2 to 6 carbon atoms and R 2 represents a linear, branched or alicyclic alkyl group having 1 to 14 carbon atoms, provided that R 1 and R 2 are different from each other and the total number of carbon atoms in the phosphonium cation is 20 or less; and X represents an anion selected from the group consisting of SO 3 CF 3 , B(C 2 O 4 ) 2 , and N(SO 2 CF 3 ) 2 ; and a lithium compound, wherein the lithium compound is one or more selected from the group consisting of LiPF 6 , LiBF 4 , LiSO 3 CF 3 , LiB(C 2 O 4 ) 2 , and LiN(SO 2 CF 3 ) 2 .