Pressure dependence of ionic conductivity in site disordered lithium superionic argyrodite Li6PS5Br

Abstract

The understanding of transport in Li⁺ solid ionic conductors is critical for the development of solid-state batteries. The influence of activation volumes on ion transport in solid electrolytes has recently garnered renewed research interest, due to the need to control the ion dynamics that influence the ionic conductivity in solid electrolytes. Microscopic activation volumes are believed to correspond to the volume change in the atomic structure of a material that occurs during an ion jump and can be determined thermodynamically from pressure dependent conductivity measurements. However, it remains unknown if and how this external pressure can affect the structure and transport properties of Li⁺ solid electrolytes. The lithium argyrodites Li₆PS₅Br have shown high ionic conductivities, influenced by their Br⁻/S²⁻ site disorder, which is associated with more spatially diffuse lithium-ion distributions. Herein, impedance spectra were acquired over a pressure range of 0.1 GPa to 1.5 GPa and revealed the activation volumes for Li⁺ migration. Specifically, activation volumes for Li⁺ migration increase with increasing degrees of Br⁻/S²⁻ site disorder in Li₆PS₅Br and with more spatially distributed lithium-ions. Furthermore, estimations of the corresponding migration volumes, which are thought to be a constant of the diffusing mobile ion in the structure are here found to change significantly among different Br⁻/S²⁻ site disorders. These observations motivate further investigations on how the thermodynamic activation volume in superionic Li⁺ conductors may provide novel insights to the influences of structure on ion transport in fast ionic conductors.