Asymmetric Sulfur Redox Paths in Sulfide-Based All-Solid-State Lithium–Sulfur Batteries

Sulfide-based all-solid-state lithium–sulfur batteries (ASSLSBs) have shown promise in next-generation energy storage devices. Nevertheless, controversy surrounds the redox pathway and the mechanism of the sulfur cathode. In this study, through galvanostatic intermittent titration technique tests, we first disclose that the difference between the discharge curves at room temperature and 60 °C originates from the sluggish kinetics at RT. Furthermore, we unravel the asymmetric sulfur redox paths and show the species evolution during the charging/discharging process at 60 °C through electrochemomechanical-based differential pressure versus voltage (dP/dV) analysis and spectroscopy studies. Our findings reveal that the sulfur cathode undergoes a two-step conversion from S8 to Li2S4 and finally to Li2S during discharge, while a one-step process from Li2S to S8 occurs during charge. Density functional theory calculations suggest that the slow diffusion of polysulfide Li2S4 and/or S42– anions in the solid-state electrolyte and the high intermediate phase dissociation energy barrier are mainly responsible for the observed asymmetry in the discharge/charge curves. In this case, the sulfur cathode undergoes a thermodynamic ground state path during discharge and a kinetically driven path during charging. This work sheds light on the debated sulfur redox mechanism and contributes to high-performance design in sulfide-based ASSLSBs.