Ultrastable Cycling of Sodium Metal Battery at Rates Exceeding 20 C Achieved with a NaB3H8-SiO2 Composite Solid Electrolyte

Solid-state sodium metal batteries are promising candidates for grid-scale energy storage due to their inherent safety and low cost. However, the development of solid electrolytes that support high-rate (>20 C) cycling for fast-charging applications remains a major challenge. Herein, we explore how adding SiO2 affects the ionic conductivity and dendrite suppression of NaB3H8, to develop superior solid electrolytes for high-rate batteries. The composites featuring an amorphous passivating layer with B-O bonds were fabricated via ball milling. Among them, NaB3H8-40%SiO2 exhibits the best conductivity, reaching 0.62 mS cm-1 at 25 °C, 2 orders of magnitude higher than that of pristine NaB3H8. Crucially, by reducing electronic conductivity and enhancing mechanical strength, it possesses a high critical current density of up to 6.5 mA cm-2, significantly higher than that of other reported hydridoborates. The Na/NaB3H8-40%SiO2/TiS2 batteries exhibit superior rate capability (216.4 mAh g-1 at 1 C and 143.1 mAh g-1 at 20 C) and long-term cycling stability (nearly 100.0% capacity retention after 2000 cycles at 20 C) at 30 °C. This work offers a promising approach for the development of innovative solid electrolytes in fast-charging or high-power-density sodium metal batteries.