An electrolyte includes a lithium salt, a non-aqueous organic solvent, gamma-butyrolactone and halogenated toluene represented by the following formula 1: wherein X represents at least one element selected from the group consisting of F, Cl, Br and I, and n represents an integer of 1 to 5. The lithium ion secondary battery including the electrolyte provides improved safety under overcharge and high-temperature storage conditions.