title: 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 S(8)(2-) and S(6)(2-), as evidenced by UV and HPLC data. The S(3)(•-) 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 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.
authors.affiliation: French Atomic Energy and Alternative Energy Agency, Laboratory of Innovation for New Energy Technologies and Nanomaterials, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France. celine.barchasz@cea.fr
authors.forename: Céline
authors.initials: C
authors.lastname: Barchasz
authors.forename: Florian
authors.initials: F
authors.lastname: Molton
authors.forename: Carole
authors.initials: C
authors.lastname: Duboc
authors.forename: Jean-Claude
authors.initials: JC
authors.lastname: Leprêtre
authors.forename: Sébastien
authors.initials: S
authors.lastname: Patoux
authors.forename: Fannie
authors.initials: F
authors.lastname: Alloin