Metastable Sodium closo-Hydroborates for Low Temperature All-Solid-State Battery with Thick Cathode

All-solid-state batteries featuring a thick cathode layer paired with a high-capacity alloy anode offer enhanced energy density1,2 and reliable performance, even at subzero temperatures, can outperform their liquid-based counterparts. Enabling such technology requires a solid electrolyte with high ionic conductivity, mechanical formability, and excellent electrochemical stability3. While non-close-packed frameworks offer lower symmetry and irregular coordination between mobile ions and anions due to distortion, resulting in higher ionic conductivity4,5, fast ionic diffusion in hydroborate chemistry is often associated with close-packed or cubic anion frameworks6-9. Here, we demonstrate that a metastable, non-close-packed orthorhombic Na3(B12H12)(BH4) phase possesses superionic conductivity of 4.6 mS cm−1 at 30 °C, three orders of magnitude improvement over its precursors, 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 hydroborate-based solid electrolytes, highlighting their potential for high ionic conductivity and broad electrochemical stability windows in next-generation energy storage systems.