Lone-Pair Driven Na-Ion Diffusion Leading to Highly Conductive Na₂(BH₄)(NH₂) Phases

Sodium (Na)-based batteries relying on solid-state electrolytes hold the potential to transform energy storage across various commercial applications. Although the ideal solid-state electrolyte has yet to be discovered, borohydrides have emerged as promising candidates. In particular, experimental studies have shown that Na2(BH4)(NH2), a sodium borohydride (NaBH4) derivative, exhibits promising Na+ conductivity in the order of 10–4 S cm–1 at 90 °C because of the naturally occurring vacant sites in its antiperovskite α-phase structure. Our computational study confirms the stability of the antiperovskite structure and identifies a second highly conductive γ phase, which may be stable at lower temperatures compared to the previously reported α-Na2(BH4)(NH2). The combination of phonon dispersion and crystallographic characterization indicates that this γ-phase is more stable than the α-phase. It is 15 meV/f.u. lower in energy compared to the α-phase, making it the ground state at zero temperature. Additionally, this work highlights the role of both [BH4]− and [NH2]− units in the diffusion mechanism as well as the crucial role of [NH2]− lone pairs in leading to a fast Na+ conductor. The latter provides additional grounds for designing advanced ionic conductors.