The origin of the two-plateaued or one-plateaued open circuit voltage in Li–S batteries

Li–S batteries have been actively pursued as a storage device of high energy density. Interestingly, most of the discharging curves for Li–S batteries show either one plateau or two plateaus. Understanding the mechanism and conditions of the one-plateaued discharging curves can provide guidance for Li–S battery design to prevent the Li-polysulfide (Li-PS) dissolution. In this article, we first calculated a solvent-dependent open circuit voltage (OCV) using density functional theory (DFT) when both crystals (S, Li₂S, Li) and dissolved Li-PS molecules (such as Li₂S₄) are involved in the discharging reactions. We successfully predicted the two-plateaued OCV with fully solvated Li-PS and the one-plateaued OCV with non-solvated Li-PS, in agreement with experimental observations. Furthermore, if Li-PS changes from partial or non-solvated state to fully solvated state, its formation energy increases, leading to a transition from the one-plateaued to the two-plateaued OCV. The partially solvated Li-PS can be realized either by a highly-concentrated electrolyte or by minimizing the pore size in the carbon/sulfur composites to limit the number of solvents transported into the pores, which changes the complex liquid-solid reactions of Li–S chemistry to a single solid-state reaction. The solid-state reaction allows Li–S batteries to operate in lean electrolytes with less Li excess. A new Li-PS dissolution mitigation strategy based on this synergetic effect is therefore proposed by modeling and verified by experiments.