Rotational Cluster Anion Enabling Superionic Conductivity in Sodium-Rich Antiperovskite Na₃OBH₄

Abstract

Sodium superionic conductors are keys to develop high safety and low cost all-solid-state sodium batteries. Among developed sodium ionic conductors, antiperovskite-type ionic conductors have attracted vast interest due to their high structural tolerance and good formability. Herein, we successfully synthesize Na₃OBH₄ with cubic antiperovskite structure by solid-state reaction from Na₂O and NaBH₄. Na₃OBH₄ exhibits ionic conductivity of 4.4 × 10^-3 S cm^-1 at room temperature (1.1 × 10^-2 S cm^-1 at 328 K) and activation energy of 0.25 eV. The ionic conductivity is 4 orders of magnitude higher than the existing antiperovskite Na₃OX (X = Cl, Br, I). It is shown that such enhancement is not only due to the specific cubic antiperovskite structure of Na₃OBH₄ but also because of the rotation of BH₄ cluster anion. This work deepens the understanding of the antiperovskite structure and the role of cluster anions for superionic conduction.