Operando studies of lithium–sulfur redox reactions: A review

In situ / operando techniques have been foundational for achieving a deep understanding of the complex, multistep Li–S redox reactions to enable the practical application of Li–S batteries. This includes a fundamental grasp of the traditional solid–liquid–solid Li–S transformations involved, as well as a thorough understanding of the interfacial processes under more realistic conditions. Herein, we present a selective review of Li–S redox reactions based on solid–liquid–solid and solid–solid transformation processes and under pouch cell conditions, mainly via in situ / operando tools that include in situ atomic force microscopy, operando confocal Raman imaging, and X-ray methods. While studies in model electrochemical cells provide a basic understanding of the redox reactions, pouch-cell-level, multimodal analysis provides a more detailed picture of the delicate interplay of the processes involved and sheds light on the pathways that may lead to capacity fade. We anticipate that continuous advances in in situ / operando techniques, along with calculation/simulation tools, will yield a deep and detailed understanding of Li–S reactions, with the ultimate goal of achieving high-performance batteries that can be broadly deployed. • In situ / operando studies reveal lithium–sulfur redox reactions. • Insights into the lithium–sulfur reactions following solid-liquid-solid and (quasi-)solid-solid transformations are provided. • Operando investigation inside pouch cells show the failure reasons and modes under practical conditions of cell operation.