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.