Investigation of Lithium Polysulfide Reduction Associated with the Potential-Limiting Step in Lithium–Sulfur Batteries

The electrochemical reduction of sulfur has the capability to deliver high energy density in lithium-sulfur batteries; however, its effectiveness is lessened by the requirement of excessive electrolyte for high performance. The potential-limiting step of this reaction was previously proposed to be the reduction of the final soluble intermediate Li2S4 to solid Li2S2/Li2S. In this work, this step was further investigated. It was discovered that nominal Li2S4 solutions with concentrations relevant to battery operation disproportionate into Li2S6 in the solution phase and Li2S in the solid phase. Li2S6 is the more likely reactant for the observed electrochemical reduction, which manifests two discernible steps. The first reduction step occurs at 2.2 V vs Li/Li+. Its specific capacity is almost invariable with scan rate, but its relative contribution to the total reduction capacity decreases with reactant concentration. The product is a semisoluble polysulfide such as Li2S4 or Li2S3, which can be further reduced in the second step or oxidized at 2.3 V. The second reduction step, with Li2S as the product, occurs at 2.1 V and is electrochemically more irreversible than the first reduction. Its specific capacity declines significantly with increased scan rate but is less impacted by concentration.