Superionic Conduction of Sodium and Lithium in Anion‐Mixed Hydroborates Na₃BH₄B₁₂H₁₂ and (Li0.7Na0.3)₃BH₄B₁₂H₁₂

Novel mixed-anion compounds Na3BH4B12H12 and (Li0.7Na0.3)3BH4B12H12 show superionic conduction corresponding to highly disordered cations and continuous migration paths in the structure. Na-rich Na3BH4B12H12 with a room-temperature conductivity of 0.5 × 10−3 S cm−1 provides a light-weight solution for the electrolyte in Na-based, all solid-state batteries. (Li0.7Na0.3)3BH4B12H12 favors ionic conductivity for both Li and Na cations, but only above 500 K.