Lithium/Sulfur Cell Discharge Mechanism: An Original Approach for Intermediate Species Identification

The lithium/sulfur battery is a promising electrochemical system that has a high theoretical capacity of 1675 mAh g(-1), but its discharge mechanism is well-known to be a complex multistep process. As the active material dissolves during cycling, this discharge mechanism was investigated through the electrolyte characterization. Using high-performance liquid chromatography, UV-visible absorption, and electron spin resonance spectroscopies, we investigated the electrolyte composition at different discharge potentials in a TEGDME-based electrolyte. In this study, we propose a possible mechanism for sulfur reduction consisting of three steps. Long polysulfide chains are produced during the first reduction step (2.4-2.2 V vs Li+/Li), such as S-8(2-) and S-6(2-), as evidenced by UV and HPLC data. The S-3(center dot-) radical can also be found in solution because of a disproportionation reaction. S-4(2-) is produced during the second reduction step (2.15-2.1 V vs Li+/Li), thus pointing out the gradual decrease of the polysulfide chain lengths. Finally, short polysulfide species, such as S-3(2-), S-2(2-), and S2-, are produced at the end of the reduction process, i.e., between 2.1 and 1.9 V vs Li+/Li. The precipitation of the poorly soluble and insulating short polysulfide compounds was evidenced, thus leading to the positive electrode passivation and explaining the early end of discharge.