NaBH and Na(BH)(BH) as potential solid-state electrolytes for Na-ion batteries.

Solid-state sodium batteries have attracted great attention owing to their improved safety, high energy density, large abundance and low cost of sodium compared to the current Li-ion batteries. Sodium-boranes have been studied as potential solid-state electrolytes and the search for new materials is necessary for future battery applications. Here, a facile and cost-effective solution-based synthesis of NaBH and Na(BH)(BH) is demonstrated. NaBH presents an ionic conductivity in the order of 10 S cm at 30 °C, but undergoes an order-disorder phase transition and reaches 10 S cm at 100 °C, close to that of liquids and the solid-state electrolyte Na-β-AlO. The formation of a mixed-anion solid-solution, Na(BH)(BH), partially stabilises the high temperature structural polymorph observed for NaBH at room temperature and it exhibits Na conductivity higher than its constituents (4.7 × 10 S cm at 30 °C). NaBH and Na(BH)(BH) exhibit an oxidative stability limit of 2.1 V Na/Na.