Delineating the Impact of Diluent on High‐Concentration Electrolytes for Developing High‐Voltage LiNi _0.5 Mn _1.5 O ₄ Spinel Cathode

Abstract LiNi _0.5 Mn _1.5 O ₄ (LNMO) is a high‐voltage spinel cathode with low nickel content, making it an attractive candidate for next‐generation lithium‐ion batteries (LIBs). However, its application is limited by interfacial instability with conventional carbonate‐based electrolytes at high voltages. In this work, a localized saturated electrolyte (LSE) capable of stably operating up to 4.85 V is investigated. Molecular dynamics simulations and Fourier transform infrared spectroscopy reveal that adding “non‐solvating” 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropyl ether diluent in the saturated electrolyte, more PF ₆ ⁻ anions are present in the first solvation shell of Li ⁺ , at the expense of solvent molecules. This tailored solvation environment promotes the formation of a robust, LiF‐rich cathode‐electrolyte interphase that mitigates transition metal dissolution and parasitic side reactions. The optimized LSE enables excellent cycling performance, with 95% capacity retention in Li|LNMO half‐cells after 100 cycles and 94% retention in Li ₄ Ti ₅ O ₁₂ |LNMO full cells after 250 cycles, even at a practically relevant LNMO cathode loading of ≈15 mg cm ⁻² . These results highlight the benefits of electrolyte engineering and solvation structure control in advancing high‐voltage LIB technologies.