Dual‐Interface Fluorination Stabilizing Rechargeable Na‐NASICON Solid‐State Batteries

ABSTRACT Solid‐state metal batteries promise next‐generation energy storage with inherent safety and high energy density. However, instability at grain boundaries and surfaces critically hinders practical deployment in solid polycrystalline ion conductors. This work proposes a dual‐interface fluorination strategy through the incorporation of fluoride‐based grain boundary phases in solid electrolytes to simultaneously address these issues. The resulting NZSP‐MgF 2 electrolyte exhibits a low electronic conductivity of 6.0 × 10 −9 S cm −1 and a widened bandgap, effectively suppressing the formation and growth of internal dendrites. Meanwhile, the interfacial contact impedance of the ceramic electrolyte against the metallic Na electrode is significantly reduced to 11 Ω cm 2 . Crucially, the fluorinated polycrystalline Na 3 Zr 2 Si 2 PO 12 ceramic electrolyte retains exceptional stability even after 30 days of exposure to air: symmetric sodium metal cells still exhibit low interfacial contact impedance (12 Ω cm 2 ), high critical current density (1.0 mA cm −2 ), and stable cycling over 1100 h at 0.2 mA cm −2 . Furthermore, full cells paired with Na 3 V 2 (PO 4 ) 3 cathode demonstrate outstanding electrochemical performance with a capacity retention of 98.1% after 2000 cycles at 2 C. This work provides a general strategy to enhance air stability, dendrite suppression, and electrode compatibility of polycrystalline electrolyte, promoting the practical realization of solid‐state metal batteries.