Decoupling Static and Dynamic Disorder in a Polyanionic Na-Ion Conductor

Abstract Solid-state Na electrolytes promise safer and more efficient batteries with increased resource availability and simplified supply chains. Recently, antiperovskite Na2[NH2][BH4] emerged as a candidate where internal polyanion dynamics promotes Na transport, but experimental validation of the predicted mechanisms of atomic motion remains scarce. To this end, we map local atomic disorder in Na2[NH2][BH4] with total scattering and compare it to ab initio molecular dynamics (AIMD) trajectories to discriminate static from dynamic contributions. The resulting disorder maps reproduce AIMD predictions of polyanion rotation and Na+ translation, including that rotation in NH2– anions is less constrained in space than BD4–. Critically, the analysis uncovers significant translational displacement of BD4– anions, which was overlooked in prior work. BD4– tetrahedra exhibit orientational static disorder distinct from the dynamic disorder driven by rotation. These findings extend the prevailing rotational picture of ion transport in polyanionic conductors by establishing rotor-translator effects and static disorder as additional, independently important degrees of freedom that are unique to a specific polyanion. Design strategies for rotor-based electrolytes should be considered for polyanion translation and static disorder, rather than optimizing rotational freedom in isolation. The combination of total scattering experiments with AIMD provides a plausible route to screen potential polyanion-based candidates for favorable rotational, translational, and static disorder within a pipeline of discovery of new phases with high ionic conductivity.