All-Solid-State Lithium–Sulfur Batteries Enhanced by Redox Mediators

Redox mediators (RMs) play a vital role in some liquid electrolyte-based electrochemical energy storage systems. However, the concept of redox mediator in solid-state batteries remains unexplored. Here, we selected a group of RM candidates and investigated their behaviors and roles in all-solid-state lithium–sulfur batteries (ASSLSBs). The soluble-type quinone-based RM (AQT) shows the most favorable redox potential and the best redox reversibility that functions well for lithium sulfide (Li₂S) oxidation in solid polymer electrolytes. Accordingly, Li₂S cathodes with AQT RMs present a significantly reduced energy barrier (average oxidation potential of 2.4 V) during initial charging at 0.1 C at 60 °C and the following discharge capacity of 1133 mAh g_s^–1. Using operando sulfur K-edge X-ray absorption spectroscopy, we directly tracked the sulfur speciation in ASSLSBs and proved that the solid–polysulfide–solid reaction of Li₂S cathodes with RMs facilitated Li₂S oxidation. In contrast, for bare Li₂S cathodes, the solid–solid Li₂S–sulfur direct conversion in the first charge cycle results in a high energy barrier for activation (charge to ～4 V) and low sulfur utilization. The Li₂S@AQT cell demonstrates superior cycling stability (average Coulombic efficiency 98.9% for 150 cycles) and rate capability owing to the effective AQT-enhanced Li–S reaction kinetics. This work reveals the evolution of sulfur species in ASSLSBs and realizes the fast Li–S reaction kinetics by designing an effective sulfur speciation pathway.