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. The fundamental thermodynamic principles of sulfur redox reactions in Li–S batteries are not fully understood. A ternary phase diagram is obtained after equilibrium between sulfur, lithium sulfide and dissolved polysulfides, which accurately describes the system evolution and predicts the behavior of Li–S batteries at an arbitrary given state.