Influence of Structural Distortion and Lattice Dynamics on Li-Ion Diffusion in Li3OCl1-xBrx 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.

Automated summary

In this study, the influence of substituting Cl with Br in Li3OCl1-xBrx on the local structure, lattice dynamics, and Li-ion conductivity was investigated through first-principles calculations. The results showed that the incorporation of Br softens the overall lattice stiffness of Li3OCl1-xBrx, but the accompanying local structural distortion is the main factor in altering the activation energy for Li-ion diffusion.