Metastable sodium closo-hydridoborates for all-solid-state batteries with thick cathodes

All-solid-state batteries (ASSBs) featuring a thick cathode layer paired with a high-capacity alloy anode offer enhanced energy density and reliable performance, even at subzero temperatures, and can outperform their liquid-based counterparts. Enabling such technology requires a solid electrolyte with high ionic conductivity, mechanical formability, and excellent electrochemical stability. Here, we demonstrate that a kinetically stable orthorhombic Na₃(B₁₂H₁₂)(BH₄) phase exhibits a superionic conductivity of 4.6 mS cm⁻¹ at 30°C alongside excellent reduction stability. High-throughput molecular dynamic simulations reveal that the propensity for anion motion significantly enhances the population of highly mobile Na⁺ without affecting the activation energy. By leveraging its high conductivity across a wide temperature range, this material enables the development of all-solid-state sodium-ion batteries with ultra-thick cathodes, delivering reliable functionality at room temperature and in subzero environments. This study expands our understanding of hydridoborate-based solid electrolytes, highlighting their potential in next-generation energy storage systems.