Correlation between Li-ion migration and local structural distortion in Li superionic conductors

The all-solid-state Li-ion batteries, overcoming the shortages of liquid electrolyte, have emerged to be promising for the next-generation energy storage system, where the development of new solid-state electrolytes (SSE) with high ionic conductivity is critical. In this study, we aim to reveal the correlation between activation energy barriers, volume, local structural distortion (characterized by continuous symmetric measure, CSM) of the migrating Li-ions by first-principles calculations. Three typical superionic conductor systems, Li₃MX₆ (M= La, Sc, Y, X= Cl, I, Br), garnet Li₇La₃Zr₂O₁₂, and Li₁₀XP₂S₁₂ (X = Ge, Si, Sn) are studied. By analyzing the energy barrier, CSM, and local volume of migrating Li-ion along different paths, we note that the Li site with smaller volume is usually accompanied by higher structural distortion, whose site energy is also more sensitive to the variation of structural distortion. Thus, for the Li migration involving tetrahedral and octahedral sites, it is more effective to tune the tetrahedral site's structural distortion to achieve low activation energy, e.g. reducing the tetrahedral transition Li site distortion or increasing the tetrahedral initial Li site distortion.