Na 2 B 11 H 13 and Na 11 (B 11 H 14 ) 3 (B 11 H 13 ) 4 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 Na 2 B 11 H 13 and Na 11 (B 11 H 14 ) 3 (B 11 H 13 ) 4 is demonstrated. Na 2 B 11 H 13 presents an ionic conductivity in the order of 10 -7 S cm -1 at 30 °C, but undergoes an order-disorder phase transition and reaches 10 -3 S cm -1 at 100 °C, close to that of liquids and the solid-state electrolyte Na-β-Al 2 O 3 . The formation of a mixed-anion solid-solution, Na 11 (B 11 H 14 ) 3 (B 11 H 13 ) 4 , partially stabilises the high temperature structural polymorph observed for Na 2 B 11 H 13 at room temperature and it exhibits Na + conductivity higher than its constituents (4.7 × 10 -5 S cm -1 at 30 °C). Na 2 B 11 H 13 and Na 11 (B 11 H 14 ) 3 (B 11 H 13 ) 4 exhibit an oxidative stability limit of 2.1 V vs. Na + /Na.