In Situ Characterization Techniques for Investigating the Reaction Mechanisms of Lithium–Sulfur Batteries: Progress, Application, and Future

Lithium-sulfur (Li-S) batteries remain constrained by polysulfide shuttling, sluggish redox kinetics, and Li dendrite growth under practical conditions. Introducing catalysts at the three-phase interface of cathode/electrolyte/sulfur can markedly accelerate polysulfide redox reactions, thereby suppressing the shuttle of soluble intermediates and improving the utilization of sulfur active species. Modifying the Li anode can concomitantly alleviate Li dendrites to enhance cycling stability. Despite these advances, the interaction between catalytic centers and reactive sulfur species and the mechanistic evolution of polysulfide species during real-time charge/discharge remains far from fully resolved. Therefore, in situ characterization techniques are crucial for the real-time study of phase transitions and interface reactions. However, a systematic review that establishes a reaction-interface/research-question-driven framework for technique selection remains lacking, particularly one that integrates diverse operando characterizations with the multi-electron and multistep electrochemical mechanisms of Li-S batteries. Against this backdrop, this review focuses on three key mechanisms in Li-S batteries: cathode catalysis, polysulfide shuttle behavior, and anode SEI evolution. It establishes a framework that matches research objectives with in situ characterization techniques. Our aim is to help researchers quickly and rationally select and combine appropriate in situ characterization techniques in studies of different mechanisms, thereby more accurately revealing the nature of the reactions and guiding the material and system design of high-performance Li-S batteries.