Phase equilibrium thermodynamics of lithium–sulfur batteries

The unique conversion chemistry of sulfur endows lithium−sulfur batteries with a high theoretical energy density. However, the basic principles of the sulfur conversion chemistry remain unclear. In this work, phase equilibrium analysis is conducted to update the thermodynamic understanding on lithium−sulfur batteries. A ternary phase diagram is plotted following the equilibrium between sulfur, lithium sulfide and dissolved polysulfides. The diagram accurately describes the existing form of different polysulfides and the solid–liquid−solid phase transitions. Quantitative analysis further reveals the stoichiometric ratio of 1.0:4.5 between the two discharge plateaus and identifies the intrinsic insufficient liquid−solid deposition as the main limitation. The relationship between system point and equilibrium potential is established so that the ternary phase diagram can predict the lithium−sulfur thermodynamics at an arbitrary state.