Engineering the Site‐Disorder and Lithium Distribution in the Lithium Superionic Argyrodite Li₆PS₅Br

Lithium argyrodite superionic conductors, of the form Li₆PS₅X (X = Cl, Br, and I), have shown great promise as electrolytes for all‐solid‐state batteries because of their high ionic conductivity and processability. The ionic conductivity of these materials is highly influenced by the structural disorder of S²⁻/X⁻ anions; however, it is unclear if and how this affects the Li distribution and how it relates to transport, which is critical for improving conductivities. Here it is shown that the site‐disorder once thought to be inherent to given compositions can be carefully controlled in Li₆PS₅Br by tuning synthesis conditions. The site‐disorder increases with temperature and can be "frozen" in. Neutron diffraction shows this phenomenon to affect the Li⁺ substructure by decreasing the jump distance between so‐called "cages" of clustered Li⁺ ions; expansion of these cages makes a more interconnected pathway for Li⁺ diffusion, thereby increasing ionic conductivity. Additionally, ab initio molecular dynamics simulations provide Li⁺ diffusion coefficients and time‐averaged radial distribution functions as a function of the site‐disorder, corroborating the experimental results on Li⁺ distribution and transport. These approaches of modulating the Li+ substructure can be considered essential for the design and optimization of argyrodites and may be extended to other lithium superionic conductors.