Local structure and anion-cation dynamic coupling in sodium closoborate solid electrolytes for solid-state batteries

The objective of the experiments proposed here is to elucidate the local structure of sodium closoborates, molecular-ionic crystals composed of mobile Na+ cations and rotating cage like polyanions (e.g. B12H122-). To that end, we propose temperature-dependent diffraction (total scattering) experiments appropriate for pair distribution function (PDF) analysis in direct space. Structural analyses from Bragg diffraction show increasing rotational disorder of the polyanions and correlated increase in Na+ diffusivity with increasing temperature. However, Bragg diffraction elucidates only theaverage structure and cannot elucidate whether such orientational disorder is static or dynamic in nature and whether/how it aids ion conduction. It is now time to answer this long outstanding fundamental question of the coupling between anion and cation dynamics in ion conductors, in view of its practical interest for the development of solid electrolytes for next-generation solid state batteries.