Cooperation of Multifunctional Redox Mediator and Separator Modification to Enhance Li-S Batteries Performance under Low Electrolyte/Sulfur Ratio.

Abstract

Sluggish reaction kinetics of sulfur species fundamentally trigger the incomplete conversion of S8↔Li2S and restricted lifespan of lithium-sulfur batteries, especially under high sulfur loading and/or low electrolyte/sulfur (E/S) ratio. Introducing 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 (EtV²⁺) RM with two highly reversible redox couples (EtV²⁺/EtV⁺, EtV⁺/EtV0) is demonstrated to well match the redox chemistry of sulfur species, in terms of accelerating the electron transfer in S8 reduction, Li2S nucleation and the Li2S oxidation. When coupling with a functionalized separator with electronegative ‒SO3Li 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 (S8→Li2S4→Li2S2/Li2S→Li2S4→S8) are decreased, especially for Li2S nucleation step. The correspond lithium-sulfur batteries achieve a high specific capacity of 1006.9 mAh g-1 (0.1C; sulfur loading of 5 mg cm-2; E/S ratio of 6 μL mgs-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.