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 Na4B(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 [B36H34](4-), contributing to the fast Na+ conduction in Na4B36H34. Remarkably, the mixed-anion Na4B36H34-7Na(2)B(12)H(12) SE demonstrates high ionic conductivity (1.02 x 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/Na4B36H34-7Na(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 Na3V2(PO4)(2)O2F/Na battery with good cycling stability was fabricated using Na4B36H34-7Na(2)B(12)H(12) as SE. The proposed Na4B36H34-based hydroborate electrolyte turns out to be a promising SE candidate for the practical application of high-voltage and high-energy-density ASSBs.