Sulfur Reduction Pathways and Through-thickness Distribution in Positive Composite Electrodes of All-solid-state Li–S Batteries: Elucidation of Two-stage Discharge Plateaus (Supporting Information)

Sulfur positive electrodes in liquid-type lithium–sulfur (Li–S) batteries exhibit two discharge plateaus at approximately 2.4 and 2.1 V (vs. Li). However, conventional sulfur composite electrodes in all-solid-state Li–S batteries typically do not display these two clear discharge plateaus. Furthermore, the reaction mechanisms in such frameworks remain poorly understood. In this study, the cause of these differences and reaction pathways were clarified by examining the reaction progression and distribution through the electrode thickness using hard X-ray photoelectron spectroscopy, Raman spectroscopy, and incremental capacity analysis (ΔQ/ΔV). The results indicate that the reaction proceeds from neutral bridged sulfur to Li2S via lithium polysulfides with Li-coordinated anionic sulfur, similar to liquid type ones. Furthermore, clear differences in the reaction progression across the electrode thickness are observed. In low-reactivity electrodes, the degree of reaction progress varies more significantly across the thickness compared with that in highly reactive electrodes. This suggests that in low-reactivity sulfur composite electrodes, the two-plateau reactions proceed simultaneously throughout the discharge region. In contrast, high-reactivity electrodes show clear discharge plateaus, with regions in which only the second plateau reaction occurs. These insights will accelerate further understanding of the reaction for all-solid-state Li–S batteries.