Reconfiguring Electrolyte Solvation Chemistry via In Situ Solvent Polymerization for High‐Performance Sodium‐Ion Batteries

Ether‐based electrolytes exhibit excellent anode compatibility and fast ion kinetics, yet their practical application is hindered by continuous cathode interfacial decomposition due to low oxidation resistance. Traditional solvation‐regulation strategies face a trade‐off between stabilizing the interface and maintaining bulk ion transport, making synergistic optimization challenging. Herein, we propose an in situ solvent polymerization strategy to develop a liquid polymer electrolyte (NEDL) that concurrently achieves fast ion transport and high interfacial stability. Specifically, trace LiPF 6 initiates the partial ring‐opening polymerization of 1,3‐dioxolane (DOL) in diethylene glycol dimethyl ether (DEGDME), generating PDOL with abundant coordination sites to reconstruct the Na + solvation structure. This polymer‐modulated environment lowers the Na + desolvation energy barrier, accelerating interfacial kinetics, and drives the formation of an inorganic‐enriched, robust interphase that suppresses solvent decomposition. Consequently, under a 16 mg cm −2 mass loading, the Na 3 Fe 2 (PO 4 )P 2 O 7 (NFPP)||Na coin cell delivers 94.76% capacity retention after 500 cycles at 2 C. Moreover, practical NFPP||hard carbon (HC) pouch cells retain 96.59% capacity over 500 cycles at 0.5 C. This work provides fundamental insights into how polymer‐chain solvation behavior regulates ion transport kinetics and interfacial properties, offering a viable pathway for designing high‐performance sodium‐ion battery electrolytes.