Tailoring Solution-Processable Li Argyrodites Li_6+<i>x</i>P_1-<i>x</i>M_<i>x</i>S₅I (M = Ge, Sn) and Their Microstructural Evolution Revealed by Cryo-TEM for All-Solid-State Batteries.

Abstract

Owing to their high Li⁺ conductivities, mechanical sinterability, and solution processability, sulfide Li argyrodites have attracted much attention as enablers in the development of high-performance all-solid-state batteries with practicability. However, solution-processable Li argyrodites have been developed only for a composition of Li₆PS₅X (X = Cl, Br, I) with insufficiently high Li⁺ conductivities (∼10^-4 S cm^-1). Herein, we report the highest Li⁺ conductivity of 0.54 mS cm^-1 at 30 °C (Li_6.5P_0.5Ge_0.5S₅I) for solution-processable iodine-based Li argyrodites. A comparative investigation of three iodine-based argyrodites of unsubstituted and Ge- and Sn-substituted solution-processed Li₆PS₅I with varied heat-treatment temperature elucidates the effect of microstructural evolution on Li⁺ conductivity. Notably, local nanostructures consisting of argyrodite nanocrystallites in solution-processed Li_6.5P_0.5Ge_0.5S₅I have been directly captured by cryogenic transmission electron microscopy, which is a first for sulfide solid electrolyte materials. Specifically, the promising electrochemical performances of all-solid-state batteries at 30 °C employing LiCoO₂ electrodes tailored by the infiltration of Li_6.5P_0.5Ge_0.5S₅I-ethanol solutions are successfully demonstrated.