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 S₈↔Li₂S 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 (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→Li₂S/Li₂S₂→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 (0.1 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.