In this work, we propose a novel electrolyte additive phenyl trifluoromethyl sulfide (PTS) to improve the interfacial stability of high voltage lithium nickel manganese oxide (LiMn1.5Ni0.5O4) cathode/electrolyte. At 1C rate, addition of 0.5% PTS improves the capacity retention of LiNi0.5Mn1.5O4 from 65% to 84% after 450 cycles at room temperature, and from 64% to 95% after 100 cycles at evaluate temperature. Density functional theory calculation and experimental characterization demonstrate that PTS oxidizes preferentially to the carbonate base electrolyte, generating a SEI film, which effectively suppresses the continuous decomposition of carbonate base electrolyte and the structural destruction of LiMn1.5Ni0.5O4.