Synergistic effect of lithium ions for fast diffusion in Li2ZrCl6 solid electrolytes

Rare-earth metal-free Li₂ZrCl₆ has recently attracted much attention as a promising solid electrolyte for all-solid-state batteries due to its cost-effectiveness. This first-principles study using van der Waals correction clarifies the Li diffusion mechanism in trigonal α- and monoclinic β-Li₂ZrCl₆. In the most stable α- and β-Li₂ZrCl₆ structures obtained from systematic investigations, Li and Zr ions prefer to exist in the same layer, forming two-dimensional Zr/Li layers with Li ions interspersed between ZrCl₆ octahedra. When Li ions diffuse in α- and β-Li₂ZrCl₆, they escape from the Zr/Li layer along the out-of-plane direction and move to the Zr-free layer, where they diffuse freely along the in-plane direction. Thus, Li diffusion in Li₂ZrCl₆ is characterized by interlayer diffusion between the Zr/Li and Li layers followed by intralayer diffusion in the Li layer. The interlayer/intralayer diffusion, resulting in the highest conductivity of 5.4 × 10⁻⁵ S cm⁻¹ for α-Li₂ZrCl₆, is enabled by a synergistic effect of two Li ions, where one Li ion already present in the Li layer promotes the other adjacent Li ion to escape from the Zr/Li layer. Individual Li ions vibrate at the stable octahedral site for a long time and then quickly jump to another octahedral site via the intermediate tetrahedral site, which is identified as the atomic-scale Li jump process in Li₂ZrCl₆.