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

LiNi_0.5Mn_1.5O₄ (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 PF₆ ^- 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 Li₄Ti₅O₁₂|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.