Toward Robust Anion‐Derived Interphases: A Novel Ternary‐Solvent Electrolyte for High‐Voltage Fast‐Charging Sodium Batteries

Conventional carbonate electrolytes suffer from severe oxidative decomposition and unstable interphases for NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NaNFM) cathode under high‐voltage and fast‐charging condition. Herein, an anion‐derived interphase strategy is designed via synergistic solvation structure regulation utilizing sulfolane (SL), dimethyl carbonate (DMC), and ethoxy(pentafluoro)cyclotriphosphazene (PFPN) solvents. In the novel electrolyte (denoted as SDP), SL competitively coordinates with Na + to suppress DMC oxidation, while PFPN acts as a free co‐solvent for interphase stability. Critically, the synergistic interactions promote anion (PF 6 − ) entry into the primary solvation sheath, inducing preferential decomposition of anion and solvents to form an inorganic, anion‐rich cathode–electrolyte interphase (CEI) with high mechanical modulus (19.967 GPa) and uniform thickness (∼7 nm). Consequently, SDP delivers favorable cycling stability (85.72% capacity retention after 300 cycles at 3 C, 2–4.1 V), high initial coulombic efficiency (92.03%), and superior rate capacity (57.83 mAh g −1 at 20 C). Furthermore, SDP effectively mitigates particle cracking, transition metals dissolution, and gas evolution (H 2 and CO 2 ), while demonstrating broad compatibility with various layered oxide and polyanionic cathode materials. This work provides a rational electrolyte design strategy to construct a stable anion‐derived CEI by manipulating the electrolyte structure of anion and solvents.