ELECTROLYTE FOR LITHIUM ION SECONDARY BATTERY, LITHIUM ION SECONDARY BATTERY, AND MODULE

An electrolyte solution for a lithium ion secondary battery, containing a compound (1) represented by the following formula (1): wherein Rand Rare each independently a C1-C4 alkyl group optionally containing an ether bond, and wherein chloride ion is present at a concentration of 1.0×10to 1.0×10ppm. Also disclosed is an electrolyte solution for a lithium ion secondary battery, containing an imidazolium cation (1-1) represented by the following formula (1-1), a bis(oxalato)borate anion, and a hexafluorophosphate anion, the formula (1-1) being: wherein Rand Rare each independently a C1-C4 alkyl group optionally containing an ether bond, and wherein chloride ion is present at a concentration of 1.0×10to 1.0×10ppm.