Resolving the conductivity puzzle of halogen-doped Li7P3S11 solid electrolyte: 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 promising trait, the ionic conductivity of LPS-type SEs falls behind LGPS-type counterparts, primarily owing to the lack of understanding on the atomic-level responses of LPS to external dopants. By a combined atomic simulation techniques of density functional theory and machine-learning-based molecular dynamics, this article reveals the role of halogen dopants in 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 on the model structures, we provide comprehensive insights on the dopant selection criteria for LPS. Our findings reveal that dopants that readily migrate into neighboring Li vacancies facilitate the structural transformation of the 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 cooperative hopping mechanism, they are less effective in LPS due to the multiple structural transformation of PSx polyhedra. Rather, 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.