Evidence of Superionic Na⁺ Conductivity in Partially Hydrolyzed NaBH₄

Sodium borohydride (NaBH4) has been found to display superionic Na+ conductivity upon partial oxidation. Herein, by controlling the hydrolysis of NaBH4 and by thermally stabilizing the oxide phase formed, i.e., NaB(OH)4, Na+ ionic conductivity increased from ∼10–10 S cm–1 for unmodified NaBH4 to 2.2 × 10–5 S cm–1 at room temperature and 2.6 × 10–3 S cm–1 at 75 °C for hydrolyzed NaBH4. More remarkably, this approach allowed for a comprehensive multidimensional 23Na–1H and 11B–1H solid-state NMR analysis and determination of the interplay between pristine α-NaBH4 and NaB(OH)4 in leading to high Na+ ionic conductivity. Not only NMR analysis confirmed that the superionic behavior observed in partially hydrolyzed NaBH4 was the result of Na+ hopping, but 2D NMR also revealed that the hopping mechanism was the result of the formation of a highly defective structure at the α-NaBH4/NaB(OH)4 interface, leading to high Na+ conductivity. Given the simplicity of the oxidation process reported here, this finding opens novel and facile methodologies for the development of promising solid-state electrolytes for sodium-based batteries.