Enabling Long‐Life All‐Solid‐State Sodium Metal Batteries via in situ Construction of a Stable Solid Electrolyte Interphase

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Suppressing the interface deterioration and sodium dendrites growth is crucial for achieving long‐life polyethylene oxide (PEO)‐based all‐solid‐state sodium metal batteries. Herein, we systematically screen Sb 2 S 3 for use as a PEO‐based solid‐state electrolyte (PSE) additive through theoretical calculations, and in situ construct a highly stable solid electrolyte interphase (SEI) enriched with Na 2 S and Na 3 Sb. This SEI, characterized by its low reduction reaction activity, high ionic conductivity, and strong Na affinity, significantly inhibits interfacial side reactions, accelerates ion transport, and facilitates smooth Na + deposition. Moreover, the incorporation of Sb 2 S 3 effectively enhances the mechanical robustness, ionic transference number, and ionic conductivity of the composite solid‐state electrolyte film (Sb 2 S 3 @PSE), thereby mitigating the sodium dendrites formation. Consequently, remarkable electrochemical performances for the Sb 2 S 3 @PSE symmetric battery (achieving 5200 h at 0.1 mA cm −2 , 520 times longer than that of pristine PSE), and the Na 3 V 2 (PO 4 ) 3 |Sb 2 S 3 @PSE|Na full battery with high‐capacity retention of 91% after 1000 cycles, are demonstrated. This work, which emphasizes the in situ construction of a stable SEI, provides significant guidance to suppress interface degradation for long‐life solid‐state metal batteries.