Tailoring Solution-Processable Li Argyrodites Li_{6+ x}P_{1- x}M_xS₅I (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 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 (Li6.5P0.5Ge0.5S5I) 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 Li6.5P0.5Ge0.5S5I 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 Li6.5P0.5Ge0.5S5I-ethanol solutions are successfully demonstrated.