Lithium/sulfur cell discharge mechanism: An original approach for intermediate species identification

Abstract

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 S82- and S62-, as evidenced by UV and HPLC data. The S3•- radical can also be found in solution because of a disproportionation reaction. S42- 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 32-, S22-, 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. © 2012 American Chemical Society.