Spatial Solvation Regulation by a Swollen Polymer Interphase Enables Ultrastable Sodium Metal Batteries

ABSTRACT Sodium (Na) metal batteries are considered promising candidates for next‐generation electrochemical energy storage because of their low costs and high energy densities. However, their development is hindered by a fundamental trade‐off in electrolyte design: strong Na + solvation enhances conductivity but aggravates undesirable anode degradation. Herein, we construct a swellable artificial polymer interphase rich in F─(Si─O─) n on the anode via a competitive coordination reaction involving fluoroethylene carbonate (FEC), ethyl trifluoroacetate, and (3‐aminopropyl)triethoxysilane. Employing in situ atomic force microscopy, in situ attenuated total reflection infrared spectroscopy and X‐ray absorption near‐edge structure spectra, we demonstrate that the interphase selectively attracts weakly solvating solvents while effectively excluding the highly polar tris(ethyl) phosphate (TEP) from the anode surface. This results in a gradient transition from a strong solvation configuration in the bulk electrolyte to a weak one at the interface, thereby enhancing the overall ionic conductivity, reducing the Na + desolvation energy barrier, and improving interfacial stability. Consequently, Na||Na 3 V 2 (PO 4 ) 3 cells deliver stable long‐term cycling, remarkable fast‐charging performance, and operate over a wide temperature range. The practical applicability of this strategy is further validated by pouch‐cell tests. This work paves the way for rational design of high‐performance and safe sodium metal batteries through advanced interfacial chemistry.