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

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