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

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||Na₃V₂(PO₄)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.