In situ optical spectroscopy characterization for optimal design of lithium-sulfur batteries.

The lithium-sulfur (Li-S) battery is one of the most promising high-energy-density secondary battery systems. However, it suffers from issues arising from its extremely complicated "solid-liquid-solid" reaction routes. In recent years, enormous advances have been made in optimizing Li-S batteries via the rational design of compositions and architectures. Nevertheless, a comprehensive and in-depth understanding of the practical reaction mechanisms of Li-S systems and their effect on the electrochemical performance is still lacking. Very recently, several important in situ optical spectroscopic techniques, including Raman, infrared and ultraviolet-visible spectroscopies, have been developed to monitor the real-time variations of the battery states, and a bridge linking the macroscopic electrochemical performance and microscopic architectures of the components has been set up, thus playing a critical role in scientifically guiding further optimal design of Li-S batteries. In this tutorial review, we provide a systematic summary of the state-of-the-art innovations in the characterization and optimal design of Li-S batteries with the aid of these in situ optical spectroscopic techniques, to guide a beginner to construct in situ optical spectroscopy electrochemical cells, and develop strategies for preventing long-chain polysulfide formation, dissolution and migration, thus alleviating the shuttle effect in Li-S batteries and improving the cell performances significantly.