Investigation of the Li-S Battery Mechanism by Real-Time Monitoring of the Changes of Sulfur and Polysulfide Species during the Discharge and Charge.

The mechanism of the sulfur cathode in Li-S batteries has been proposed. It was revealed by the real-time quantitative determination of polysulfide species and elemental sulfur by means of high-performance liquid chromatography in the course of the discharge and recharge of a Li-S battery. A three-step reduction mechanism including two chemical equilibrium reactions was proposed for the sulfur cathode discharge. The typical two-plateau discharge curve for the sulfur cathode can be explained. A two-step oxidation mechanism for Li2S and Li2S2 with a single chemical equilibrium among soluble polysulfide ions was proposed. The chemical equilibrium among S52-, S62-, S72-, and S82- throughout the entire oxidation process resulted for a single flat recharge curve in Li-S batteries.