Conductivity mechanism of lithium argyrodite solid-state electrolytes

Abstract

Solid-state argyrodite electrolytes have the potential to offer safer, more enduring and more efficient electrolyte properties than traditional liquid electrolytes. We performed multivariate statistical analysis of 175 argyrodite conductivity measurements to show that the conductivity of argyrodite correlates strongly with the isotropy level of the ionic potentials between 4a and 4d Wyckoff sites. At room temperature, the conductivity of argyrodite drops by up to four orders of magnitude when the distribution of sulfur and halide ions is completely anisotropic between the 4a and 4d Wyckoff sites. The activation energy drops by more than 30% between the least and the most isotropic samples, which correlates with Li transportation kinetics. The effect of isotropy is sub-second order. These results reconcile previous studies showing that halide substitution in the 4d site exerted a strong effect on argyrodite conductivity, which could not explain the high conductivity observed for halide-free argyrodites. The newly proposed argyrodite conductivity mechanism has great practical importance for redesigning and improving the conductivity of argyrodite electrolytes, as well as other materials currently being developed as solid-state battery electrolytes.