Cooperation of Multifunctional Redox Mediator and Separator Modification to Enhance Li‐S Batteries Performance under Low Electrolyte/Sulfur Ratios

Abstract Sluggish reaction kinetics of sulfur species fundamentally trigger the incomplete conversion of S 8 ↔Li 2 S and restricted lifespan of lithium‐sulfur batteries, especially under high sulfur loading and/or low electrolyte/sulfur (E/S) ratios. Developing redox mediators (RMs) is an effective strategy to boost the battery reaction kinetics, yet their multifunctionality and shuttle inhibition are still not available. Here, a unique ethyl viologen (EtV 2+ ) RM with two highly reversible redox couples (EtV 2+ /EtV + , EtV + /EtV 0 ) is demonstrated to well match the redox chemistry of sulfur species, in terms of accelerating the electron transfer in S 8 reduction, Li 2 S nucleation and the Li 2 S oxidation. When coupling with a functionalized separator with electronegative ‐SO 3 Li groups, a synergetic chemistry is established to ensure the substantial inhibition of the shuttle effect and the acceleration of charge transfer. As a result, the activation energies during sulfur species conversion (S 8 →Li 2 S 4 →Li 2 S/Li 2 S 2 →Li 2 S 4 →S 8 ) are decreased, especially for Li 2 S nucleation step. The correspond lithium‐sulfur batteries achieve a high specific capacity of 1006.9 mAh g− 1 (0.1 C; sulfur loading of 5 mg cm −2 ; E/S ratios of 6 μL mg s −1 ), and an outstanding cycling stability. This study provides a paradigm of solving complex problems via multifunctional molecule engineering and strategic cooperation towards Li−S batteries and other battery communities.