Diphenyl disulfide (DPDS) is evaluated as a new bifunctional electrolyte additive to improve the high-voltage performance of LiCoO2/graphite batteries. With the addition of DPDS in the electrolyte, the cell with 2.0 wt% DPDS exhibits enhanced performance in the normal voltage range of 3.0 V–4.2 V. In particular, when the cut-off potential is increased from 4.2 V to 4.4 V, the cell with 1.0 wt% DPDS also exhibits improved discharge capacity and cycle performance. Linear sweep voltammetry and cyclic voltammetry indicate that the DPDS can be reduced prior to the solvent and that the oxidative decomposition of the electrolyte can also be suppressed. In addition, X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy analyses demonstrate that the solid electrolyte interface (SEI) film is produced primarily on the graphite anode via the decomposition of DPDS at normal voltage and that the SEI films induced by DPDS can be formed simultaneously on the two electrodes at higher potentials. It is hypothesized that these compact SEI films covering the electrode surface provide protection for the LiCoO2 and graphite materials and accordingly improve the cyclic performance of battery in the voltage range of 3.0 V–4.4 V.