High-Performance Li-S Batteries Boosted by Redox Mediators: A Review and Prospects

Lithium-Sulfur (Li-S) batteries are considered as the next generation of energy storage systems due to their high theoretical energy density. However, the insulation nature of solid sulfur species and the high activation barrier of lithium polysulfides (LiPSs) lead to the slow sulfur redox kinetics. Therefore, it is of critical importance to develop efficient strategies to accelerate the formation/conversion kinetics of LiPSs from initial S8 or Li2S. In recent years, redox mediators (RMs) have been explored to catalyze sulfur redox reactions and have proved to be an advanced strategy for promoting the further development of Li-S cells. This review timely summarizes the operating principles of reported RMs, and classifies them according to their distinct reaction mechanisms and key functions on the S redox electrochemistry. In this review, both the solid and soluble RMs are described in view of similar fundamental function mechanisms of them and the significant advances are highlighted involving of molecule structure optimization, novel reaction pathway proposition and the development of new catalytic molecules. Finally, the selection criteria and general design principles as well as the current insufficiency and futher reaearch trend for RMs are outlined. We believe that this review will provide timely and comprehensive information on the future research directions of RMs to boost high-performance Li-S batteries.