Solubility Activation of Recessive Solvent Mitigates Interphase Dissolution for High‐Performance Sodium‐Ion Batteries

Abstract Solid electrolyte interphase (SEI) dissolution in sodium‐ion batteries (NIBs) triggers electrolyte decomposition and gas evolution, resulting in capacity decay and safety issues. Lowering the solvation power of electrolyte mitigates SEI dissolution but decreases ionic conductivity. Here, we report a recessive solvent activation strategy, where the recessive solvent 1,2‐epoxy‐3,3,3‐trifluoropropane (TFPO) alone dissolves NaFSI only sparingly, but its solubility is greatly enhanced by the addition of an activating solvent, diethylene glycol diethyl ether (DEE). By harnessing the tunable solvation behavior of the DEE–TFPO system, we designed a weakly solvating electrolyte (WSE) containing 24 vol% DEE, far below the bulk‐solvent fractions typical of WSEs, and a high anion‐to‐DEE ratio. This low DEE content mitigates solvent‐induced SEI dissolution, while the high anion‐to‐DEE ratio promotes anion‐dominated solvation structure, forming a robust inorganic‐rich SEI. These effects preserve high ionic conductivity while overcome the challenge of SEI dissolution. The optimized electrolyte enabled hard carbon || NaMn 0.33 Fe 0.33 Ni 0.33 O 2 full cells to retain 80.0% capacity after 500 cycles and 99.5% capacity in 1.0 Ah pouch cells after 230 cycles with suppressed gas release.