Customized Solvation Structure for Stable and Safe Sodium‐Metal Batteries

Abstract Owing to the low cost and high abundance of sodium (Na), significant advancements are made in the field of Na‐based batteries, which are possible through the incorporation of lithium‐ion‐inspired electrodes with different electrolyte chemistries. However, the conventional carbonate electrolytes employed in such systems are flammable, thermally unstable, and prone to severe interfacial side reactions that compromise safety and shorten the overall battery cycle life. This study introduces a flame‐retardant cosolvent strategy, wherein trimethylsilyl phosphite (TMSPi) and nonafluorohexyltrimethoxysilane (NFTOS) are blended into a propylene carbonate (PC)‐based commercial electrolyte, yielding an novel electrolyte system (PCTN) with a tailor‐designed solvation structure. This PCTN electrolyte is found to self‐extinguish upon ignition, in addition to generating a robust NaSiO_x/NaF solid electrolyte interphase and a NaPO_xF_y‐dominated cathode electrolyte interphase. Consequently, the Na|NaNi_0.4Fe_0.2Mn_0.4O₂ cell incorporating PCTN retained 90.3% of its initial capacity after 450 cycles at 1 C, outperforming commercial electrolytes. Even at a high operation temperature of 70 °C, a capacity retention of 93.2% is achieved after 100 cycles using PCTN (c.f., 79.9% for a commercial electrolyte). Overall, this work demonstrates a facile, production‐compatible electrolyte design that synchronously enhances the stability of the electrode–electrolyte interface, offering a potential system for incorporation into next‐generation high‐performance Na metal batteries.