Electrochemical analysis of the reaction mechanism of sulfur reduction as a function of state of charge

Although being crucial for developing high capacity and long lasting lithium-sulfur batteries, the discharge chemistry of the sulfur electrode is debated. In this study we examine a three electrode system with a glassy carbon working electrode immersed in electrolyte solution containing elemental sulfur as reactant and 1,3-dioxolane: 1,2-dimethoxyethane with bis-triflourmethansulfonimid lithium salt electrolyte. We present essential new quantitative insights into the reaction steps occurring at the sulfur electrode of a lithium-sulfur battery during discharge. The conducted sequence of electrochemical experiments allow to study the complex phenomena with classical electrochemical methods of cyclic voltametry, open circuit potential measurements and discharge. Our experimental analysis at different states of charge reveals the important role of chemical disproportionation to cell relaxation. The reduction reactions, their reversibility and the electrode performance strongly depend on the chemical equilibrium between the polysulfides. These results allow to formulate surround experimentally validated nonformal reaction kinetic models. These give detailed information of the complex interaction of chemical and electrochemical reactions. (c) 2018 Published by Elsevier Ltd.