Understanding the anion disorder governing lithium distribution and diffusion in an argyrodite Li6PS5Cl solid electrolyte

Abstract

Site disorder between S and Cl anions is an important feature of argyrodite Li₆PS₅Cl, which is a promising solid electrolyte for all-solid-state batteries. However, the fundamental aspects of the S/Cl disorder in Li₆PS₅Cl remain veiled. This first-principles study revealed how S/Cl disorder changes the material properties of Li₆PS₅Cl. The S/Cl disorder greatly enhances the stability of Li₆PS₅Cl, leading to the formation of the most stable Li₆PS₅Cl structure when the degree of S/Cl disorder (x), which is defined as the percentage of 4d sites occupied by Cl, is approximately 41%. The improved stability of Li₆PS₅Cl is attributed to Li redistribution by the S/Cl disorder, which enriches and depletes Li ions around S and Cl, respectively, through the movement of some Li ions from around Cl to around S, maximizing and minimizing the strong Li–S and weak Li–Cl interactions, respectively. The S/Cl disorder significantly increases Li conductivity by activating all three types of Li jumps, i.e., doublet, intracage, and intercage, resulting in the highest conductivity of 4.6 mS cm⁻¹ at x = 50%. The activation of Li jumps, especially intercage jumps, results from the site exchange between S and Cl, which causes an environment in which migrating Li ions break the weak Li–Cl bond rather than the strong Li–S bond, thereby reducing diffusion barriers.