Origin of Fast Ion Conduction in Li10GeP2S12, a Superionic Conductor

The discovery of additional sites has revealed important insights into the possibility of cross-channel ionic diffusion in Li10GeP2S12 (LGPS). Here, we present an ab initio study of a Li diffusion mechanism in LGPS. The presence of fractional occupancy for Li sites leads to different c channel population of Li and different local environment for Li hop both along the c-directional channels and in the ab plane. We perform a rigorous computation of the energy barriers with different population of Li and different environment of Li surrounding the c channels. It is found that the diffusion energy barrier in LGPS depends significantly on channel population, local environment, and the type of hop which leads to a range of relative diffusivity along different directions. A novel mechanism of correlated diffusion between the c channel and the ab plane is found with significantly low energy barrier. The relative frequency of occurrence of the novel hop mechanism critically controls the degree of anisotropy in LGPS. Moreover, low activation barrier associated with this hop leads to high diffusivity which is sustained even after repeated cycling due to the presence of cross-channel pathways