Influence of Structural Distortion and Lattice Dynamics on Li-Ion Diffusion in Li₃OCl_1–xBr_x Superionic Conductors

The antiperovskite superionic conductors Li3OCl and Li3OBr show great differences in ionic conductivity. The more polarizable Li3OBr shows a lower ionic conductivity than Li3OCl, contradicting the idea that a more polarizable framework is beneficial for ionic conduction. In this work, we study the influence of substituting Cl with Br on the local structure, lattice dynamics, and Li-ion conductivity of Li3OCl1–xBrx, based on first-principles calculations. We find that the incorporation of Br does soften the overall lattice stiffness of Li3OCl1–xBrx, but the accompanying local structural distortion plays a dominant role in changing the activation energy for Li-ion diffusion, which could increase the site energy as well as the Li-ion vibration frequency. We suggest that more distorted initial octahedral sites and less distorted saddle trigonal-plane sites can lead to better ionic transport. In addition, the correlation between activation energy and the pre-exponential factor of diffusivity extracted from the Arrhenius plot of Li3OCl1–xBrx is found to be nonlinear, which is due to the decreasing migration entropy with increasing activation energy.