Liquid Metal Mediated Heterostructure Fluoride Solid Electrolytes of High Conductivity and Air Stability for Sustainable Na Metal Batteries

Fluoride-based solid electrolytes (SEs) have emerged as a promising component for high-energy-density rechargeable solid-state batteries (SSBs) in view of their wide electrochemical window, high air stability, and interface compatibility, but they still face the challenge of low ion conductivity and the lack of a desired structure for sodium metal SSBs. Here, we report a sodium-rich heterostructure fluoride SE, Na₃GaF₆-Ga₂O₃-NaCl (NGFOC-G), synthesized via in situ oxidation of liquid metal gallium and in situ chlorination using low-melting GaCl₃. The distinctive features of NGFOC-G include single-crystal Na₃GaF₆ domains within an open-framework structure, composite interface decoration of Ga₂O₃ and NaCl with a concentration gradient, exceptional air stability, and high electrochemical oxidation stability. By leveraging the penetration of gallium at NaF grain boundaries and the in situ self-oxidation to form Ga₂O₃ nanodomains, the solid-phase reaction kinetics of NaF and GaF₃ is activated for facilitating the synthesis of main component Na₃GaF₆. The introduction of a small amount of a chlorine source during synthesis further softens and modifies the boundaries of Na₃GaF₆ along with Ga₂O₃. Benefiting from the enhanced interface ion transport, the optimized NGFOC-G exhibits an ionic conductivity up to 10^-4 S/cm at 40 °C, which is the highest level reported among fluoride-based sodium-ion SEs. This SE demonstrates a "self-protection" mechanism, where the formation of a high Young's modulus transition layer rich in NaF and Na₂O under electrochemical driving prevents the dendrite growth of sodium metal. The corresponding Na/Na symmetric cells show minimal voltage hysteresis and stable cycling performance for at least 1000 h. The Na/NGFOC-G/Na₃V₂(PO₄)₃ cell demonstrates stable capacity release around 100 mAh/g at room temperature. The Na/NGFOC-G/FeF₃ cell delivers a high capacity of 461 mAh/g with an excellent stability of conversion reaction cycling.