How multi-length scale disorder shapes ion transport in lithium argyrodites

Abstract

The rate performance of all-solid-state batteries can be limited by the low conductivity of the solid electrolyte in the composite cathode. A conductivity of 10 mS cm⁻¹ is required, which exceeds that of many solid electrolytes. This limitation can be attributed to intra- and inter-grain ion transport. Understanding the limitations of ion transport is a multi-length scale problem ranging from single bond hops to particle–particle transport. Here we show that spark plasma sintering of Li₆PS₅Cl not only enhances ion transport on the macroscopic length scale but also on the microscopic scale. On the macroscopic length scale, greater densification improves particle-to-particle contact. On the nanoscale, short-range order (SRO) of the neighbouring 4a/4a and 4d/4d Wyckoff sites present in the cold-pressed Li₆PS₅Cl produces unfavourable Li ion pathways through the cell. Spark plasma heating removes the SRO, creating a connected network of microscopic pathways for the Li to migrate. Finally, on the atomistic level, spark plasma heating increases the amount of Cl⁻ residing on the 4d site and S²⁻ on the 4a site. By understanding the limitations of ion mobility across a range of length scales, one can target methods to produce solid-state argyrodite electrolytes with higher ionic conductivities.