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 Na4B36H34 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 Na4B36H34-7Na2B12H12 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/Na4B36H34-7Na2B12H12/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)2O2F/Na battery with good cycling stability was fabricated using Na4B36H34-7Na2B12H12 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.