A Practical Nonflammable Na₄B₃₆H₃₄-Based Hydroborate Electrolyte for High-Voltage All-Solid-State Sodium Batteries

High-voltage solid electrolytes (SEs) are highly desired for the development of safe and high-energy-density all-solid-state batteries (ASSBs). Herein, a nonflammable conjuncto -hydroborate of Na 4 B 36 H 34 is developed with a wide electrochemical stability window (ESW) of up to 6.9 V. Density functional theory calculations reveal an enlarged Na-ion diffusion pathway and fast dynamics of [B 36 H 34 ] 4–, contributing to the fast Na + conduction in Na 4 B 36 H 34 . Remarkably, the mixed-anion Na 4 B 36 H 34 -7Na 2 B 12 H 12 SE demonstrates high ionic conductivity (1.02 × 10 –3 S cm –1 ), wide ESW (5.5 V), high Na + transference number (0.97), low electronic conductivity, low density (1.1709 g cm –3 ), good compressibility, decent mechanical strength, nonflammability, and solubility. Moreover, the Na/Na 4 B 36 H 34 -7Na 2 B 12 H 12 /Na symmetrical cell can maintain long-term cyclability over 200 h at 0.1 mA cm –2 . In addition, a 4.5 V high-voltage Na 3 V 2 (PO 4 ) 2 O 2 F/Na battery with good cycling stability was fabricated using Na 4 B 36 H 34 -7Na 2 B 12 H 12 as SE. The proposed Na 4 B 36 H 34 -based hydroborate electrolyte turns out to be a promising SE candidate for the practical application of high-voltage and high-energy-density ASSBs.