Triallyl cyanurate (TAC) and triallyl isocyanurate (TAIC) are new electrolytic additives for the Li-ion battery. Both additives are structural isomers, and are found to be flame retardant. So TAC has a triallyl structure, and TAIC a triamine structure. Both molecules include unsaturated C–C bonds, which can be good building blocks for a protective film on cathode. Flame retardants that have double bonds are able to improve both the flame retardancy of the electrolyte and the thermal protection for the cathode. The unsaturated carbon materials form a good protective film, by which the exothermic oxygen evolution reaction at the cathode is suppressed very effectively. TAC is the better thermal protector. Addition of 3wt.% TAC delays the exothermic reaction by 52°C, i.e., from 275 to 327°C. A 5wt.% TAC solution suppresses 75.1% of exothermic energy from the oxygen evolution reaction.