Structural reversibility and charge-discharge cycle of Li2S-V2S3-LiI positive electrodes for all-solid-state lithium batteries

All-solid-state batteries with sulfur-based positive electrode active materials have been attracting much attention regarding their safety and long cycle life. The Li₂S−V₂S₃−LiI system with high ionic and electronic conductivity is a promising positive electrode material for sulfide-based all-solid-state batteries. Such cells using Li₂S−V₂S₃−LiI in the positive electrode layer operate without conductive carbons and solid electrolytes. In particular, cells using 90(0.75Li₂S·0.25V₂S₃)·10LiI (mol %) exhibit a high capacity and cycle durability even after 100 cycles. To clarify the charge-discharge mechanism of Li₂S−V₂S₃−LiI, we investigated microstructural changes during charge-discharge cycles via transmission electron microscopy (TEM). The microstructure of 90(0.75Li₂S·0.25V₂S₃)·10LiI before charge-discharge measurement was characterized by LiVS₂ and Li₂S−LiI nanocrystallites in an amorphous matrix. In the Li₂S−LiI domain, the Li₂S−LiI nanocrystallites with an antifluorite-type crystal structure amorphized after charging and reprecipitate as Li₂S−LiI nanocrystallites after discharging. As for LiVS₂, Li deintercalation and intercalation occurred during the charge-discharge processes. Ex-situ TEM observations demonstrated that the structural reversibility of LiVS₂ and Li₂S−LiI in an amorphous matrix contributes to high cycle performance.