"Dual Mediator System" Enables Efficient and Persistent Regulation toward Sulfur Redox Conversion in Lithium-Sulfur Batteries.

Li-S batteries present great potential to realize high-energy-density storage, but their practical implementation is severely hampered by the notorious polysulfide shuttling and the sluggish redox kinetics. While rationally designed redox mediators can optimize polysulfide conversion, the efficiency and stability of such a mediation process still remain formidable challenges. Herein, a strategy of constructing a "dual mediator system" is proposed for achieving efficient and durable modulation of polysulfide conversion kinetics by coupling well-selected solid and electrolyte-soluble mediators. Theoretical prediction and detailed electrochemical analysis reveal the structure-activity relationships of the two mediators in synergistically optimizing the redox conversions of sulfur species, thus achieving a deeper mechanistic understanding of a function-supporting mediator system design toward sulfur electrochemistry promotion. Specifically, such a dual mediator system realizes the bridging of full-range "electrochemical catalysis" and strengthened "chemical reduction" processes of sulfur species as well as greatly suppressed mediator deactivation/loss due to the beneficial interactions between each mediator component. Attributed to these advantageous features, the Li-S batteries enable a slow capacity decay of 0.026% per cycle over 1200 cycles and a desirable capacity of 8.8 mAh cm with 8.2 mg cm sulfur loading and lean electrolyte condition. This work not only proposes an effective mediator system design strategy for promoting Li-S battery performance but also inspires its potential utilization facing other analogous sophisticated electrochemical conversion processes.