Synergistic Hybrid Interfaces for Ultrastable Sodium Anode via Spontaneous Conversion of Tin Fluoride in Polymer Matrix

We report a synergistic hybrid organic–inorganic artificial solid–electrolyte-interphase (SEI) engineered to stabilize sodium metal anodes for high-performance sodium metal batteries, achieved via the spontaneous conversion of tin fluoride (SnF2) in a polymer matrix. A composite protective layer is fabricated by integrating poly(ethylene oxide) (PEO) and tin fluoride (SnF2) in a dimethyl carbonate solution, forming a hybrid interphase that synergistically combines the flexibility of organic polymers with the mechanical robustness of inorganic fluorides. The optimized coating, particularly at an 11 wt % PEO:SnF2 (10:1) ratio, develops a uniform and chemically stable barrier interphase that effectively suppresses dendrite formation, minimizes electrolyte decomposition, and maintains a low interfacial impedance. Comprehensive morphological and chemical analysis confirm that this protective layer undergoes spontaneous conversion to generate a NaF-rich phase and a Na–Sn alloy network, both of which facilitate uniform sodium deposition and enhance interphase stability. Atomistic ReaxFF molecular-dynamics simulations of the PEO/SnF2/Na interface further reveal the in situ formation of NaF clusters and a Na–Sn alloy network that homogenizes Na+ flux and fortifies the hybrid layer, providing mechanistic insight into the observed electrochemical stability. Most importantly, electrochemical testing in symmetric cells with such artificial SEI layer demonstrates a dramatic extension of cycle life from approximately 190 h for bare sodium to nearly 2000 h (at a current density of 0.25 mA/cm2 in 1 M NaPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC), and from less than 100 h for bare sodium to over 2000 h at a current density of 1.0 mA/cm2 in 1 M NaPF6 in tetraethylene glycol dimethyl ether, representing a (10–20×) in enhancement in cycling performance and stability. This work presents a promising strategy for enabling safer, longer-lasting, and higher energy density sodium metal batteries through the design of hybrid interfaces with spontaneous inorganic conversion.