Closo-Hydroborate Sodium Salts as an Emerging Class of Room-Temperature Solid Electrolytes

Na-based hydroborates, such as NaCB9H10, Na2B10H10, NaCB11H12, and Na2B12H12, are known for their high ionic conductivity above a phase transition, usually far from room temperature. Here, a systematic investigation to suppress this phase transition and thus to stabilize novel crystal structures is conducted by means of anion mixing. The obtained compounds crystallize either with hexagonal and cubic close packed (hcp and ccp) or body-centered cubic (bcc) arrangement of anions without polymorphic evolution in the temperature range 100 < T < 700 K. The high symmetry and the configurational ionic disorder, which characterize all crystals, give room-temperature Na+ conductivity close or above 1 mS cm−1. Finally, high oxidative electrochemical stability between 3 and 4 V versus Na+/Na results from the aromatic nature of the boron cage, making Na-based hydroborates potentially suitable as efficient solid electrolytes for next-generation, high-voltage, Na-ion batteries. Various drawbacks limit the development of solid electrolytes for sodium-based batteries as cheap and efficient alternatives to conventional Li-ion batteries. Brighi et al. present a strategy to stabilize hydroborate-based compounds with high sodium mobility, large electrochemical and thermal stability, and excellent mechanical properties.