Dual-Anion Mixed Sodium Halide Solid Electrolytes with High Ionic Conductivity and High-Voltage Stability for All-Solid-State Sodium Batteries

Sodium halide solid electrolytes are promising for all-solid-state sodium batteries because of their expected oxidative stability and compatibility with high-voltage cathodes, yet their room-temperature ionic conductivity is often insufficient for practical application. Here, we report a dual-anion mixing strategy to enhance Na + transport in halide-based solid electrolytes through O 2- incorporation into NaNbCl 6 . Compared with crystalline NaNbCl 6 , O 2- doping yields an amorphous-dominated NaNb 1.07 Cl 4.35 O solid electrolyte with substantially improved transport properties. The optimized composition exhibits a room-temperature ionic conductivity of 1.02 mS cm -1 , which increases to 1.17 mS cm -1 after heat treatment and remilling, together with a low activation energy of 0.23-0.25 eV. In addition, the electrolyte shows a low electronic conductivity of 4.57 × 10 -10 S cm -1 and an oxidative stability up to 4.3 V vs. Na/Na + . Structural characterization by X-ray diffraction, Raman spectroscopy, and solid-state nuclear magnetic resonance indicates that O 2- incorporation disrupts the highly ordered parent framework and modifies the local Na environments. The improved ionic transport is attributed to the dual-anion design and the resulting amorphous-dominated structure, which together facilitate Na + migration. Machine-learning-assisted energetic analysis further suggests that anion mixing reduces the energetic disparity among Na sites and lowers the Na + migration barrier. To demonstrate practical applicability, the developed electrolyte was integrated into an all-solid-state sodium battery using a doped-Na 2/3 Ni 1/3 Mn 2/3 O 2 composite cathode, Na 3 PS 4 anolyte, and Na 2 Sn anode. The cell delivered an initial capacity of 95 mAh g -1 and exhibited excellent long-term cycling stability, retaining 80% of its capacity after 500 cycles and 73% after 1000 cycles at 0.3 C. These results show that dual-anion mixing is an effective strategy for developing sodium halide solid electrolytes with both fast Na + transport and robust high-voltage stability for solid-state battery applications.