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

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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 NaB 3 H 8 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/NaB 3 H 8 /Na cell, indicating the great electrochemical stability and favorable interfacial contact with the Na metal of NaB 3 H 8 . Furthermore, a Na/NaB 3 H 8 /TiS 2 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.