Grain Boundary Transport in the Argyrodite-Type Li6PS5Br Solid Electrolyte: Influence of Misorientation and Anion Disorder on Li Ion Mobility

To realize efficient solid-state batteries, many efforts are directed toward maximizing the bulk Li⁺ conductivity of sulfide superionic conductors, as demonstrated for the argyrodite-type materials Li₆PS₅Cl and Li₆PS₅Br. Notably, in these archetype materials, the fast Li⁺ transport benefits from considerable anion disorder on the halide and sulfur sublattices. To further improve the Li⁺ conductivity, however, one must consider not only the bulk properties of the solid electrolyte (SE) but also microstructural aspects. It is, however, controversially discussed whether grain boundary (GB) transport is generally detrimental for the overall ion conductivity in agyrodite-type SEs. Thus, by means of atomistic computer simulations, the Li⁺ ion transport is studied in twist and tilt GBs of Li₆PS₅Br, revealing that the Br/S site exchange determines whether the presence of GBs deteriorates the ionic conductivity: Whereas the material with 0% Br/S site exchange only shows locally limited bulk diffusion but enhanced GB conductivity, at higher degrees of site exchange, GBs deteriorate Li⁺ diffusion. These results show that the interplay of GB transport directly depends on the degree of site exchange in argyrodite-type materials.