Sulfur electrochemistry and metal-sulfur batteries

Lithium–sulfur batteries promise high energy density storage but show poor stabilities owing to uncontrolled polysulfide dissolution. Although limiting polysulfide solvation to establish quasi-solid-state sulfur reaction can decouple electrode reactions from the electrolyte volume, this approach suffers from slow reaction kinetics. Here I will present our work on a surface-localized polysulfide-solvation strategy to mediate the reaction of ‘quasi-solid’ polysulfide by leveraging an organic phase mediator with a weakly solvating electrolyte. This electrolyte restricts polysulfide dissolution globally while the phase mediator complexes with the surface polysulfide, promoting polysulfide solvation at the surface and facilitating fast surface-localized solution-phase sulfur reactions. Furthermore, with promises for high specific energy, high safety and low cost, the all-solid-state lithium–sulfur battery (ASSLSB) is ideal for next-generation energy storage. However, the poor rate performance and short cycle life caused by the sluggish solid–solid sulfur redox reaction (SSSRR) at the three-phase boundaries remain to be solved. I will also discuss our work on a fast SSSRR enabled by lithium thioborophosphate iodide (LBPSI) glass-phase solid electrolytes (GSEs). On the basis of the reversible redox between I− and I2/I3−, the solid electrolyte (SE)—as well as serving as a superionic conductor—functions as a surficial redox mediator that facilitates the sluggish reactions at the solid–solid two-phase boundaries, thereby substantially increasing the density of active sites.