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 Na 2 (BH 4 )(NH 2 ), a sodium borohydride (NaBH 4 ) 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-Na 2 (BH 4 )(NH 2 ). 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 [BH 4 ] − and [NH 2 ] − units in the diffusion mechanism as well as the crucial role of [NH 2 ] − lone pairs in leading to a fast Na + conductor. The latter provides additional grounds for designing advanced ionic conductors.