Altering the reaction mechanism to eliminate the shuttle effect in lithium-sulfur batteries

Lithium-sulfur (Li-S) battery is one of the most promising energy storage devices. However, the development of Li-S battery is seriously hindered by the shuttle effect of polysulfides. Up to now, almost in all the researches related to sulfur cathode, the polysulfide motion restricting strategy is used to suppress the shuttle effect. However, this issue still cannot be thoroughly solved. Here, we report a new polysulfide generation restricting strategy to eliminate the shuttle effect in Li-S batteries by turning the S-8 molecules into stably adsorbed small sulfur species with suitable solid redox mediators (RMs) to generate an S-2(sigma-)-RM sigma+ at the very beginning. In this way, the mediators (S-2(sigma-)-RM sigma+) are reduced into Li2S2 and Li2S directly without soluble polysulfide forming during the discharging process. Therefore, the shuttle effect of polysulfides (Li2S8, Li2S6, and Li2S4) is absolutely eliminated. This new polysulfide generation restricting strategy is realized by using a TiOxNy-TiO2 quantum dots@carbon composite (TiONQDs@C) as a sulfur host. The TiONQDs@C is proved to be an efficient RM to convert S-8 molecules into stably adsorbed S-2(sigma-)-RM sigma+ species, eliminating the formation of lithium polysulfide completely. Owing to the new mechanism, the Li-S battery with TiONQDs@C host achieves a capacity of 869 mA h g(-1) (96% of the initial capacity) after 200 cycles with a low capacity decay of 0.02% per cycle. This strategy provides a new way to thoroughly solve the shuttle effect in Li-S batteries.