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

Antiperovskite (AP) structure compounds (X3AB, where X is an alkali cation and A and B are anions) have the potential for highly correlated motion between the cation and a cluster anion on the A or B site. This so-called “paddle-wheel” mechanism may be the basis for enhanced cation mobility in solid electrolytes. Here we show, through combined experiments and modeling, the first instance of a double paddle-wheel mechanism, leading to fast sodium ion conduction in the antiperovskite Na3-xO1-x(NH2)x(BH4). As the concentration of amide (NH2-) cluster anions is increased, large positive deviations in ionic conductivity above that predicted from a vacancy diffusion model are observed. Using EIS, PXRD, synchrotron XRD, neutron diffraction, AIMD, and NMR, we characterize the cluster anion rotational dynamics, and find that cation mobility is influenced by the rotation of both NH2- and BH4- species, resulting in sodium ion conductivity a factor of 102 higher at x = 1 than expected for the vacancy mechanism alone. Generalization of this phenomenon to other compounds could accelerate fast ion conductor exploration and design.