Exploring the Anion Site Disorder Kinetics in Lithium Argyrodites

Lithium argyrodites Li₆PS₅X (X = Cl, Br, I) are a promising class of solid-state electrolytes with the potential to achieve high conductivities (>10 mS·cm^-1) necessary for use in solid-state batteries. Previous research has shown that structural factors, in particular, site disorder between the sulfide and halide anions, can impact the ionic conductivity of lithium argyrodites. One current hypothesis for this correlation between anion site disorder and ionic transport is a connection to the lithium-ion substructure. However, as there is limited research surrounding the anion disordering process itself, this relationship has yet to be fully understood. This research explores the impact of the composition and synthesis on the anion disordering process through the Li_6+xP_1-xSi_xS₅Br (x = 0 to 0.4 in 0.1 steps) series of substitutions quenched from different annealing temperatures. Ex situ and in situ diffraction studies show that the anion site disorder within the compounds increases upon Si introduction only for samples quenched from higher annealing temperatures but remains relatively constant at lower annealing temperatures. Based on in situ diffraction measurements, we further monitor the effects of anion mobility at elevated temperatures allowing inference of slower anion disordering kinetics with changing compositional content. We complement the experimental work using nudged-elastic band calculations showing the overall preference of anions for their specific sites and the possibility of anion mobility. This work provides insight into the argyrodites and shows that the anion disordering can be monitored and that the composition has strong influences on the disordering process.