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, NaGaF-GaO-NaCl (NGFOC-G), synthesized via in situ oxidation of liquid metal gallium and in situ chlorination using low-melting GaCl. The distinctive features of NGFOC-G include single-crystal NaGaF domains within an open-framework structure, composite interface decoration of GaO 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 GaO nanodomains, the solid-phase reaction kinetics of NaF and GaF is activated for facilitating the synthesis of main component NaGaF. The introduction of a small amount of a chlorine source during synthesis further softens and modifies the boundaries of NaGaF along with GaO. Benefiting from the enhanced interface ion transport, the optimized NGFOC-G exhibits an ionic conductivity up to 10 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 NaO 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/NaV(PO) cell demonstrates stable capacity release around 100 mAh/g at room temperature. The Na/NGFOC-G/FeF cell delivers a high capacity of 461 mAh/g with an excellent stability of conversion reaction cycling.