A method for producing an electrolyte solution for a lithium ion battery involving reacting a lithium halide selected from the group consisting of lithium fluoride, lithium chloride, lithium bromide, lithium iodide and a mixture of at least two of these, with phosphorus pentachloride and hydrogen fluoride in a nonaqueous organic solvent, thereby producing lithium hexafluorophosphate as an electrolyte of the electrolyte solution.