A High-Rate and Long-Life Sodium Metal Battery Based on a NaB₃H₈ ⋅ xNH₃@NaB₃H₈ Composite Solid-State Electrolyte

All-solid-state sodium metal batteries are promising for large-scale energy storage applications owing to their intrinsic safety and cost-effectiveness. However, they generally suffer from sodium dendrite growth or rapid capacity fading, especially at high rates, mainly due to poor wettability, sluggish ionic transport, or low interfacial stability of the solid electrolytes. Herein, we report a novel composite, NaB₃H₈ ⋅ xNH₃@NaB₃H₈ (x<1), as a new class of solid electrolyte for high-rate batteries. NaB₃H₈ ⋅ xNH₃@NaB₃H₈ is obtained from the sticky NaB₃H₈ ⋅ NH₃ after removal of NH₃ partially at room temperature. It delivers an ionic conductivity of 0.84 mS cm^-1 at 25 °C and reaches 20.64 mS cm^-1 at 45 °C after an order-disorder phase transformation. It also reveals a good capability of dendrite suppression and remarkable stability against sodium metal. These performances enable the all-solid-state Na//TiS₂ battery with a high capacity of 232.4 mAh g^-1 (97.2 % of theoretical capacity) and long-term cycling stability at 1 C. Notably, this battery shows superior long-life cycling stability even at 5 and 10 C, which has been rarely reported in all-solid-state sodium metal batteries. This work opens a new group of solid electrolytes, contributing to fast-charging or high-power-density sodium metal batteries.