Lithiated 3, 6-dioxa-1, 8-octane dithiol as redox mediator to manipulate polysulfides conversion for high-performance lithium-sulfur batteries

Limited by sluggish sulfur redox kinetics of lithium-sulfur (Li-S) batteries, polysulfides conversion heavily depends on the surface of conductive substrate, which will result in electrode passivation and accumulation of dead sulfur. Here, the lithiated 3, 6-dioxa-1, 8-octane dithiol (DODL) is proposed as a redox mediator to promote the bidirectional polysulfides transformation in actual operation. DODL generated by lithiation reaction has weaker acidity and is friendly to the lithium anode. During discharging, DODL can chemically reduce long-chain polysulfides, lessening the dependence of polysulfide conversion on conductive substrate and mediate 3D deposition of Li2S. Moreover, acceleration of sulfur redox kinetics can reduce the possibility of side reactions between polysulfides and lithium anode. During charging, the addition of DODL is found to decrease the voltage barrier of Li2S to polysulfides, enhancing the reversibility of sulfur redox. In particular, S-8 can dissolve in the electrolyte with DODL, which can alleviate the irreversible accumulation of dead Li2S according to comproprtionation reaction. Attributing to these properties, the Li-S cells with DODL display excellent stability of long cycles at 0.5 C, 1 C and 2 C. Under the harsh environment of low electrolyte/sulfur ratio and high sulfur loading, the cells with DODL also perform well. The proposal of DODL demonstrates the importance of manipulating polysulfide conversion path in Li-S batteries and stimulates the research enthusiasm of developing novel redox medias.