Engineering the Site-Disorder and Lithium Distribution in the Lithium Superionic Argyrodite Li_6PS_5Br

摘要

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.