A Novel Oxidized Na₂B₁₂H₁₂–Based Electrolyte for High-Voltage All-Solid-State Sodium Batteries

Developing practical solid electrolytes is a tough challenge and an urgent necessity for future cell applications. The Na2B12H12 solid electrolyte (SE) is considered as one of the most promising candidates due to its high stability, excellent alkali metal compatibility, low density, and safe properties. However, its inherent low ionic conductivity at room temperature and narrow electrochemical stability window (ESW) limit its development. Herein, a simple method of ball milling Na2B12H12 in an oxygen atmosphere has been developed, which can significantly increase the ionic conductivity by 3 orders of magnitude and broaden the ESW. Distinctively, high ionic conductivity (1.3 × 10–3 S cm–1), wide ESW (5.2 V), favorable thermal stability (>300 °C), and good electrode adaptability have been achieved by an oxidized Na2B12H12-20 wt % SiO2 composite. The Na3V2(PO4)3O2F/oxidized Na2B12H12-20 wt % SiO2/Na cell shows a specific capacity retention of 83.7% after 100 cycles at 0.1 C and a specific capacity of 49.1 mAh g–1 at 2 C. Hence, the oxidized Na2B12H12 SE proposed in this study has the potential to function as a practical material for all-solid-state batteries (ASSBs).