Optical Spectroscopy to Characterize Advanced Metal–Sulfur Battery Materials and Reveal Mechanistic Insights: A Review

Developing new generation, cost-effective battery technology such as metal–sulfur is urgently required to meet the growing energy demands. Unlike lithium-ion batteries that undergo solid → solid transitions, the metal–sulfur batteries involve complex working mechanism where the cathode transitions through solid → liquid → solid phases during cycling. Such frequent and multiple phase transitions during the operation results in the shuttling of polysulfides that induces premature battery failure and cause low Coulombic efficiency. Over the last few decades, optical spectroscopy has emerged as a powerful, informative, and noninvasive analytical tool in advancing our understanding of the operational mechanism in different types of metal–sulfur battery. This review outlines diverse applications of optical spectroscopy for enhancing the efficiency of metal–sulfur batteries. Various types of optical spectroscopy measurements, such as ex situ and in situ and their contribution in revealing the reaction kinetics that aided in the development of key battery components, such as cathodes, electrolytes, separators, and additives, are emphasized. Advantages, limitations, and mitigation strategies of this technique in in situ and ex situ measurement conditions are also outlined. Overall, we highlight potential role of optical spectroscopy tool in developing new materials to advance the performance of emerging battery technologies.