Tailoring Solution-Processable Li Argyrodites Li6+xP1–xMxS₅I (M = Ge, Sn) and Their Microstructural Evolution Revealed by Cryo-TEM for All-Solid-State Batteries

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 Li 6 PS 5 X (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 6.5 P 0.5 Ge 0.5 S 5 I) for solution-processable iodine-based Li argyrodites. A comparative investigation of three iodine-based argyrodites of unsubstituted and Ge- and Sn-substituted solution-processed Li 6 PS 5 I 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 6.5 P 0.5 Ge 0.5 S 5 I 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 LiCoO 2 electrodes tailored by the infiltration of Li 6.5 P 0.5 Ge 0.5 S 5 I–ethanol solutions are successfully demonstrated.