Lithium (Poly)sulfide Phase Conversion in Working Lithium-Sulfur Batteries: The Insight from the Equilibrium Potential Model

The commercialization of lithium-sulfur (Li-S) batteries is currently hindered by the electrochemical instability and capacity loss, due to the ultracomplex phase conversion of lithium polysulfides (LPSs) and lithium sulfide (Li₂S). Revealing the mechanism is critical to rational regulation of the cathode reaction process so as to improve the cycling stability and capacity retention. Herein, we develop an equilibrium potential model to reveal the two conversion pathways under different electrolyte/sulfur ratios of working Li-S batteries. The model predicts that the supersaturation of Li₂S and the precipitation of LPSs are responsible for the potential drop at the onset of the second plateau and the emergence of the third plateau, respectively, deviating from the typical discharge profiles. Furthermore, quantitative guidance for electrolyte screening and battery assembling is presented to selectively control LPS precipitation. The description of equilibrium potential could be used as a foundation for mechanism study and battery monitoring system development.