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 Li 2 S 4 to solid Li 2 S 2 /Li 2 S. In this work, this step was further investigated. It was discovered that nominal Li 2 S 4 solutions with concentrations relevant to battery operation disproportionate into Li 2 S 6 in the solution phase and Li 2 S in the solid phase. Li 2 S 6 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 Li 2 S 4 or Li 2 S 3, which can be further reduced in the second step or oxidized at 2.3 V. The second reduction step, with Li 2 S 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.