Sodium Octahydridotriborate as a Solid Electrolyte with Excellent Stability Against Sodium‐Metal Anode

Solid-state sodium metal batteries have been extensively investigated because of their potential to improve safety, cost-effectiveness, and energy density. The development of such batteries urgently required a solid-state electrolyte with fast Na-ion conduction and favorable interfacial compatibility. Herein, the progress on developing the NaB₃H₈ solid-state electrolytes is reported, which show a liquid-like ionic conductivity of 0.05 S cm^-1 at 56 °C with an activation energy of 0.35 eV after an order-disorder phase transformation, matching or surpassing the best single-anion hydridoborate conductors investigated up to now. The steady polarization voltage and significantly decreased resistance are achieved in the symmetric Na/NaB₃H₈/Na cell, indicating the great electrochemical stability and favorable interfacial contact with the Na metal of NaB₃H₈. Furthermore, a Na/NaB₃H₈/TiS₂ battery, the first high-rate (up to 1 C) solid-state sodium metal battery using the single-anion hydridoborate electrolyte, is demonstrated, which exhibits superior rate capability (168.2 mAh g^-1 at 0.1 C and 141.2 mAh g^-1 at 1 C) and long-term cycling stability (70.9% capacity retention at 1 C after 300 cycles) at 30 °C. This work may present a new possibility to solve the interfacial limitations and find a new group of solid-state electrolytes for high-performance sodium metal batteries.