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 S₈^2– and S₆^2–, as evidenced by UV and HPLC data. The S₃^•– radical can also be found in solution because of a disproportionation reaction. S₄^2– 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 S₃^2–, S₂^2–, and S^2–, 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.