Designing a descriptor for the computational screening of argyrodite-based solid-state superionic conductors: uniformity of ion-cage size

The use of solid electrolytes is a promising way to improve the energy density of lithium-ion batteries, and Li argyrodites make up a promising family of solid-state electrolytes with comparable Li-ion conductivities to liquid electrolytes. However, the vast number of compositions potentially available from the periodic table (Li 7+ y − x [(A y B 1− y )S 4 ]S 2− x X x (0 ≤ x ≤ 1), where A, B and X are elements of group IV, V, and VI, respectively) poses an overwhelming challenge for researchers to find the best chemical combinations and structures. Therefore, it is essential to design a simple descriptor for the computational screening of Li argyrodite-based superionic conductors. Recently, it was suggested that altering the halogen distribution in Li argyrodites during synthesis could increase the Li-ion conductivity of these materials due to site disorder of S 2− /X − single anions. Inspired by this work, we systematically investigated the "composition-structure-property" relationship in Li 6− x PS 5− x X 1+ x (0 ≤ x ≤ 1 and X = Cl, Br or I) model structures. Our results show a close correlation between the Li-ion conductivity and the cage-like Li sublattice structure around the S 2− /X − single anions. We particularly found that the size of the Li-ion cage becomes uniform with increasing the halogen doping level, and the inter-cage diffusion of Li ions is accelerated to increase Li-ion conductivity. Therefore, we propose a standard deviation (STD) of Li-cage size around S 2− /X − single anions as a descriptor for the screening of argyrodite-based superionic conductors. Furthermore, this work provides a correction method for accurate bulk ionic conductivity calculations considering all possible site disorder configurations and crystallinities of Li argyrodite materials. Our results will provide a novel approach for tuning the compositional change of Li argyrodites based on "composition-structure-property" relationships that accelerate inter-cage diffusion to increase Li-ion conductivity. The Li-ion conductivity is closely correlated with the uniformity of the 4a- and 4c-cages in Li argyrodites. This is a novel approach for tuning the composition of Li argyrodites with "composition-structure-property" relationships.