Polyanion Dynamics Leads to High Ionic Conductivities in Site-Exchanged Antiperovskites

Sodium anti-perovskite conductors (APs) are a promising class of solid-state electrolytes attributing to their high structural tolerance and good formability. However, limited APs have been synthesized experimentally, pursuing the exploration of the other potential chemical spaces. Herein, through combined particle swarm optimization algorithm, high-throughput first-principles calculations, ab initio molecular dynamics and long timescale machine-learning molecular dynamics, the strategies based on site-exchanging and anion clusters are shown to simultaneously enhance the thermal stability and the sodium diffusivity in the designed APs. Among these APs, the highest ionic conductivity of 39.05 mS/cm is achieved in Na₃BrSO₄ at room temperature, due to the strong coupling of cluster rotation and sodium migration. We highlight not only the rotation dynamics but also its coupling with Na diffusion, as confirmed by the proposed rotational tolerance factor and local difference frequency center to evaluate rotation possibility and coupling degree, respectively. Particularly, according to this simple descriptor, rotational tolerance factor, the anion rotation possibility in APs can be predicted from the lattice structure, which can be applied for screening of superionic conductors with cluster rotation dynamics.