LITHIUM ION SECONDARY BATTERY, SECONDARY BATTERY SYSTEM INCLUDING THE SAME, NONAQUEOUS ELECTROLYTIC SOLUTION FOR LITHIUM ION SECONDARY BATTERY

摘要

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having battery performance (for example, initial capacity) equivalent to the conventional ones, and having high safety capable of complying with such requirements as increased capacity/output, and a secondary battery system including the lithium ion secondary battery.SOLUTION: A lithium ion secondary battery according to the present invention includes a nonaqueous electrolytic solution containing an electrolyte comprising a lithium compound, a nonaqueous solvent, and a flame retarder. A lithium concentration in the nonaqueous electrolytic solution is 0.5 mol/L or more and 2.5 mol/L or less, the flame retarder includes trimethyl phosphite and dimethyl phosphite, and the total content of the trimethyl phosphite and the dimethyl phosphite is 1 mass% or more and 60 mass% or less relative to the amount of the nonaqueous electrolytic solution.