Synergistic Solvation and Interphase Engineering for Low‐Temperature Quasi‐Solid Sodium Metal Batteries

ABSTRACT Low‐temperature sodium metal batteries demand electrolytes capable of simultaneously promoting fast ion transport and establishing stable electrode‐electrolyte interphases. However, conventional electrolyte systems suffer from sluggish charge‐transfer kinetics and fragile interphases under subzero conditions, leading to rapid capacity fading and cell failure. Here, we present a rationally engineered quasi‐solid polymer electrolyte designed to precisely tailor solvation structures while constructing durable interphases at both anode and cathode. The electrolyte incorporates an in situ polymerized copolymer framework combined with a multifunctional fluorinated diamide solvent, N,N‐diethyl‐2,2,2‐trifluoroacetamide (FDEA). The distinctive molecular characteristics of FDEA enable the formation of fluorine‐enriched, highly stable interphases, while simultaneously enhancing Na + transport kinetics and facilitating efficient desolvation processes. As a result, the electrolyte exhibits a high ionic conductivity of 0.82 mS cm −1 at −20°C and supports Na 3 V 2 (PO 4 ) 3 ||Na cells with exceptionally stable cycling performance, even at temperatures as low as ‐30°C. Our findings highlight that advanced solvent engineering, beyond conventional polymer matrix optimization, is also crucial for achieving high‐performance quasi‐solid sodium metal batteries capable of reliable operation at low temperatures.