On the Local Structure in Ordered and Disordered Closo-hydroborate Solid Electrolytes

Na+ closo-hydroborates are a heavily researched solid electrolyte class for applications in all-solid-state Na batteries. The structural characterization of these materials is notoriously challenging due to the elements involved and the fast rotational motion of hydroborate cages. The average structures obtained by Bragg diffraction have numerous atomic positions with low occupancies, complicating the determination of actual atom-atom distances. Total average scattering and derived pair distribution functions display atom-atom distances in real space, providing additional structure information to the average crystal structure. In this work, we present the pair distribution functions of the five different Na+ closo-hydroborates: Na2B10H10, Na2B12H12, NaCB11H12, and the mixtures of 1:1 Na2B12H12/Na2B10H10 and 2:1 NaCB11H12/Na2B12H12. All pair distribution functions show a fast decay of peak height with increasing atom-atom distance on the local scale, suggesting a low correlation of atom motions between hydroborate cages, as observed in various other molecular crystals. The combination of Bragg diffraction, showing the average ordering of closo-hydroborate cages, and pair distribution function analysis, providing local atom-atom distances, is a useful tool to develop a deeper understanding of the closo-hydroborates and also of other plastic crystals.