The specific capacity of lithium-ion battery with lithium cobalt oxide as cathode depends on the upper limitation voltage for charge/discharge cycling, but this oxide tends to be destructed structurally when it is cycled in carbonate-based electrolyte under high voltage. We report a novel electrolyte additive, fumaronitrile (FN, CNCHCHCN), which can maintain the structural integrity of lithium cobalt oxide. Electrochemical measurements indicate that lithium cobalt oxide exhibits poor cyclic stability when it is cycled under 4.5 V (vs. Li/Li+) and the charged cathode suffers serious self-discharge in a base electrolyte, 1.0 mol L−1 LiPF6 in EC/EMC/DEC (3:5:2, by weight). These issues can be overcome effectively by adding 0.5% FN into the base electrolyte. Physical and chemical characterizations demonstrate that the poor cyclic stability and self-discharge of lithium cobalt oxide result from its structural destruction caused by HF formed from electrolyte decomposition, and FN yields a protective cathode interphase film which maintains the structural integrity of lithium cobalt oxide.