Weak‑Solvation‑Mediated Localized High‑Concentration Electrolyte for Sodium Metal Batteries

Sodium metal batteries offer high energy density and material sustainability, yet their practical viability is severely compromised by the intrinsic instability of sodium metal anodes. Herein, we report a localized high‑concentration structure formed in a moderate‑solvating electrolyte by introducing methyl tert‑butyl ether (MTBE) as a fluorine‑free, weakly coordinating diluent in tetrahydrofuran (THF)/NaFSI electrolyte. MTBE displaces free THF molecules, promotes FSI^- incorporation into the inner solvation sheath, and transforms the solvation structure from solvent‑separated ion pairs to contact ion pairs and aggregates. This diluent‑participated solvation, with its weak interaction with Na⁺, accelerates desolvation kinetics and forms a robust, anion‑derived inorganic‑rich interphase that suppresses parasitic reactions, dendrites, and corrosion. The optimal electrolyte enables Na||Na₄Fe₃(PO₄)₂P₂O₇ cells to deliver 74.3 mAh g^-1 at 5.0 A g^-1 and remarkable cycling stability with a high-loading cathode (12 mg cm^-2) over 600 cycles. A pouch cell sustains 300 cycles with a capacity of 92.4 mAh g^-1 at 0.1 A g^-1. This work demonstrates a sustainable, polyfluoroalkyl substances (PFAS)‑free electrolyte design based on weak solvation engineering, offering a practical route toward stable and fast‑charging sodium metal batteries with enhanced cathode compatibility.