Electrochemical processes in all-solid-state Li-S batteries studied by electrochemical impedance spectroscopy

Understanding the electrochemical processes in all-solid-state lithium‑sulfur batteries is essential for designing high-performance devices. Here, the evolution of the electrochemical impedance spectra of an all-solid-state lithium‑sulfur battery during one charge/discharge cycle was quantitatively investigated with a reasonably established equivalent circuit. The impedance spectra could be decomposed into several parts as follows: the bulk resistance of solid electrolyte and current collector at highest frequency, three semicircles from high to low frequency corresponding to the dynamic processes at different interfaces and a 45° straight line followed by an inclined small tail at low frequency. It is found that the middle frequency semicircle, corresponding to charge transfer at the cathode, varied obviously during the charge/discharge cycle. Moreover, the 45° straight line, which is attributed to the Li diffusion in the cathode material LixS based on qualitative analysis, dominates the impedance spectra, revealing that the main kinetics-limiting factor is the Li diffusion in the active materials. Finally, an electrochemical model of the battery is proposed which provides insight for the designing of high-performance all-solid-state lithium‑sulfur batteries.