Amorphous-Nanocrystalline Fluorinated Halide Electrolytes with High Ionic Conductivity and High-Voltage Stability

All-solid-state sodium-ion batteries (ASSSIBs) offer a cost-effective, scalable alternative to rechargeable lithium-ion batteries, but their advancement requires solid electrolytes with high ionic conductivity, wide electrochemical stability, and robust interfacial compatibility. Here, a fluorine-doped halide solid electrolyte (2NaF-ZrCl₄, 2-NFZC) featuring an amorphous-nanocrystalline structure with high ionic conductivity (2.35 × 10^-4 S cm^-1 at 25 °C) and good high-voltage stability is presented. Fluorine doping in 2-NFZC promotes Zr-F bonding with limited Na-F interaction, which facilitates fast Na-ion transport through disordered regions and the NaF/amorphous phase interface. Paired with a NaNi_1/3Fe_1/3Mn_1/3O₂ cathode, a Na₁₅Sn₄ anode, and a Na₃PS₄ anode interlayer, the all-solid-state cell with the 2-NFZC electrolyte demonstrates a discharged capacity of 137.1 mAh g^-1, 81.1% capacity retention over 600 cycles, and suppressed interfacial side reactions. These findings highlight the potential of fluorine doping in designing advanced solid electrolytes for high-performance all-solid-state Na-ion batteries.