Lithium-rich manganese oxide (LRMO) is a promising high-energy-density material for high-voltage lithium-ion batteries, but its performance is hindered by interfacial side reactions, transition metal dissolution, and oxygen release. To address these issues, we propose a high-voltage electrolyte strategy that utilizes cosolvent and additive synergy to create stable dual interphases at both the cathode and anode. Specifically, lithium difluoro(oxalato)borate (LiDFOB) additive sacrificially decomposes to form a uniform yet stable cathode-electrolyte interphase (CEI) layer, while cosolvent of bis(2,2,2-trifluoroethyl) carbonate (BTFEC) effectively adjusts the solvation structure and synergistically stabilizes the solid-electrolyte interphase (SEI) on the anode, ultimately achieving ultrahigh cycle stability and fast-charging feasibility. The presence of B-F, LiB