Hydridoborate solid electrolytes: opportunities and challenges

Hydridoborates have emerged as a distinct class of inorganic solid electrolytes with exceptional potential for solid-state batteries. Their lithium and sodium salts with polyhedral closo- and closo-carba-hydridoborate anions offer low crystallographic density, mechanical softness suitable for cold pressing, and broad electrochemical stability, enabling integration with alkali metal anodes and high-voltage cathodes. Superionic transport arises from order-disorder transitions and the resulting rotational dynamics of the cage anions, which create a highly connected and dynamically accessible network of Li⁺ and Na⁺ migration pathways. So far, electrolyte synthesis is costly, due to the close chemical relationship among boron-hydrogen clusters that leads to low selectivity and often produces mixtures of hydridoborates that are difficult to separate. Most reported routes are multistep procedures involving elevated temperatures, extended reaction times, solvent handling, and purification steps. Synthetic routes based on inexpensive NaBH₄ precursors, and direct synthesis of mixed-anion electrolytes instead of pure hydridoborate salts showcase promising paths toward scalable cost-effective synthesis. Finally, recently discovered mechanisms of hydridoborate oxidation and reduction are outlined, and their integration into solid-state batteries is summarized. By linking structural chemistry, transport mechanisms, and device-level behavior, this Feature Article outlines key design principles and future directions for hydridoborate solid electrolytes in next-generation solid-state batteries.