Double Paddle-Wheel Enhanced Sodium Ion Conduction in an Antiperovskite Solid Electrolyte

摘要

Antiperovskite structure compounds (X_3AB, where X is an alkali cation and A andB are anions) have the potential for highly correlated motion between the cationand a cluster anion on the A or B site. This so-called “paddle-wheel” mechanismmay be the basis for enhanced cation mobility in solid electrolytes. Throughcombined experiments and modeling, the first instance of a double paddlewheelmechanism, leading to fast sodium ion conduction in the antiperovskiteNa_(3?x)O_(1?x)(NH_2)_x(BH_4), is shown. As the concentration of amide (NH_2~?) clusteranions is increased, large positive deviations in ionic conductivity above thatpredicted from a vacancy diffusion model are observed. Using electrochemicalimpedance spectroscopy, powder X-ray diffraction, synchrotron X-ray diffraction,neutron diffraction, ab initio molecular dynamics simulations, and NMR, thecluster anion rotational dynamics are characterized and it is found that cationmobility is influenced by the rotation of both NH_2~? and BH_4~? species, resultingin sodium ion conductivity a factor of 10~2 higher at x = 1 than expected for thevacancy mechanism alone. Generalization of this phenomenon to other compoundscould accelerate fast ion conductor exploration and design.