Laboratory Courses

Halide argyrodite solid-state electrolytes of the general formula Li₆PS₅X exhibit complex static and dynamic disorder that plays a crucial role in ion transport processes. Here, we unravel the rich interplay between site disorder and dynamics in the plastic crystal argyrodite Li₆PS₅CN and the impact on ion diffusion processes through a suite of experimental and computational methodologies, including temperature-dependent synchrotron powder X-ray diffraction, ⁷Li solid-state NMR, and machine learning-assisted ab initio molecular dynamics simulations. Sulfide and (pseudo)halide site disorder unilaterally improves long-range lithium diffusion in the argyrodite family irrespective of the (pseudo)halide identity, which demonstrates the crucial role of site disorder in dictating bulk ionic conductivity in the argyrodite family. Furthermore, we find that anion site disorder dictates the extent and timescales of cyanide rotational dynamics. Ordered configurations of anion order enable fast, quasi-free rotations of cyanides that occur on substantially faster timescales than lithium hopping as probed by solid-state NMR. In contrast, cyanide dynamics are slow or frozen in anion-disordered Li₆PS₅CN, presumably due to strong elastic dipole interactions between neighboring cyanides that impedes free rotations. Through this study, we find that anion disorder plays a decisive role in dictating the extent and timescales of both lithium and cyanide dynamics in Li₆PS₅CN and presents an exciting materials design strategy for harnessing coupled motions in complex ion transport mechanisms in the next generation of solid-state electrolytes.