We have studied a new electrolyte additive, 1,1-diphenylethane (DPE), to LiCoO2/graphite cells to reduce capacity loss during high temperature storage. It is demonstrated to be effective to reduce polarization after storage at 60 °C for one month in charged state. In half-cell tests, a LiCoO2/Li cell with DPE additive in electrolyte shows less capacity loss than that without DPE, while no significant difference is observed between DPE-added and DPE-free graphite/Li half cells. Scanning electron microscopy, X-ray diffraction, electrochemical impedance spectroscopy, time of flight-secondary ion mass spectroscopy, and hard X-ray photoelectron spectroscopy measurements are applied to elucidate the mechanism how DPE suppresses the increase in electrode resistance and polarization during the high temperature storage. All the results consistently show that less amount of surface layer is formed by electrolyte decomposition on LiCoO2 electrode surface in DPE-added electrolyte than the DPE-free one, leading to reduction of the electrode resistance and polarization.