Resolving conflicting interpretations of ionic conductivity in halogen-doped Li7P3S11 solid electrolytes: atomistic insights from Li and polyhedral dynamics

Abstract

Among bulk solid electrolytes (SEs) without elemental substitution, Li₇P₃S₁₁ (LPS) is recognized for its remarkably high ionic conductivity. Despite such a promising trait, the ionic conductivity of LPS-type SEs falls behind the LGPS-type counterparts, primarily owing to the lack of understanding on the atomic-level responses of LPS to external dopants. Using combined atomic simulation techniques of density functional theory and machine-learning-based molecular dynamics, this study reveals the role of halogen dopants in promoting the ionic conductivity of LPS. Specifically, we first generate model atomic structures that reproduce the experimental ionic conductivity of halogen-doped LPS. Through systematic examinations of the model structures, we provide comprehensive insights into the dopant selection criteria for LPS. Our findings reveal that dopants that readily migrate into neighboring Li vacancies facilitate the structural transformation of polyhedral structures, thereby diversifying Li diffusion pathways and enhancing Li-ion transport. Although Li interstitials, often unintentionally introduced during synthesis, are generally known to promote ionic conductivity via a cooperative hopping mechanism, they are less effective in LPS due to the multiple structural transformations of the PS_x polyhedra. In contrast, Li interstitials reduce the number of available Li vacancies for dopant migration, ultimately hindering Li diffusion. This study quantitatively elucidates the correlation between structural evolution and ionic transport behavior in halogen-doped LPS solid electrolytes, providing a fundamental theoretical basis for the rational design of high-performance sulfide electrolytes.