Facile Self‐Forming Superionic Conductors Based on Complex Borohydride Surface Oxidation

Abstract Complex hydrides have attracted significant attention as better inorganic solid‐state electrolytes owing to their lightweight and good compatibility with metal anodes (Li, Na, and/or Mg) for all solid‐state batteries. However, high ionic conductivity is usually observed at high temperatures upon the stabilization of adequate crystalline phases enabling fast ionic mobility. Here, an extremely simple strategy to significantly increase the ionic conductivity of complex borohydrides is reported. By exposing complex borohydrides to oxygen, the rearrangement of surface atoms upon the oxidation of borohydride particles and the resulting defects lead to extremely high ionic conductivity. NaBH₄ and LiBH₄ exposed to 5% O₂ show an ionic conductivity of ≈10⁻³ S cm⁻¹ at 35 °C. Similarly, oxidized Mg(BH₄)₂ displays a conductivity of ≈10⁻⁶ S cm⁻¹ at 25 °C instead of 9.63 × 10⁻¹³ S cm⁻¹. To the best of the authors' knowledge, this is to date, the simplest approach to tune the properties of borohydrides toward high ionic conductivity at room temperature as it does not rely on the difficult synthesis of large cage boron based anions to substitute BH₄⁻ and allow better ionic conduction paths. Owing its simplicity, the finding has the potential to enable new avenues toward the realization of viable complex borohydride based solid‐state electrolytes.