Lithium Distribution and Site Disorder in Halide-Substituted Lithium Argyrodites: A Structural and Transport Study

Abstract

Lithium argyrodite superionic conductor has recently gained significant attention as a potential solid electrolyte for all-solid-state batteries because of its high ionic conductivity and ease of processing. One promising aspect of these materials is the ability to introduce halide (Li6-xPS5-xY1+x, Y = Cl and Br ) into the crystal structure, which can greatly impact the lithium distribution over the wide range of accessible sites and structural site-disorder between the S2₋ and Y₋ anion on Wyckoff 4d site, strongly influences the ionic conductivity. However, the relationship between halide substitution, structural site-disorder, and lithium distribution is not fully understood. In this study, we investigate the effect of halide substitution on lithium argyrodite and engineer site-disorder by changing the synthesis protocol. We reveal the lithium substructure and ionic transport correlations using neutron diffraction, solid-state NMR, and electrochemical impedance spectroscopy, We find that higher ionic conductivity is correlated with a negative charge on the 4d site, as replacing the S2− with Br− leads to a lowered average charge on the 4d site and weaker interactions within the Li+ “cage”, promoting a migration pathway for Li+ ions across the Li+ cage. We also identify a new T4 Li+ site, which enables an alternative jump route (T5–T4–T5) with a lower migration energy barrier. The resulting expansion of Li+ cages and increased connections between cages leads to a maximum ionic conductivity of 8.55 mS cm-1 with higher site-disorder, an improvement of 11-fold compared to lower site-disorder. Overall, this work provides a deeper understanding of the structure-transport correlations in lithium argyrodite, specifically how site-disorder and halide substitution impact the lithium substructure and transport properties.