Revealing the complex sulfur reduction mechanism using cyclic voltammetry simulation

Abstract

Understanding the complex reaction mechanism and kinetics of sulfur reduction is prerequisite to design well performing lithium-sulfur batteries. Decoupling of individual polysulfide species is problematic when only using experimental methods. Model supported electrochemical analysis together with HPLC–MS analysis of the reacting species is used to compensate for this missing insight, as it enables to analyze the underlying species, transport, kinetic and thermodynamic processes. Concentration measurements confirm a strong prevalence of chemical equilibrium reactions that cause many species to be present over a wide range of SoC. An EEC-mechanism represents the top-down view and yields the simplest generic mechanism, that is able to reproduce the electrochemical behavior. Reduction of sulfur is performed by two consecutive electron transfer reactions while a chemical reaction accounts for the decreasing cathodic-to-anodic peak ratio with decreasing scan rate. A physically motivated E3C4-mechanism is shown to yield convincing results besides transport and kinetic parameters. Circular routes and chemical equilibrium are included to reproduce all characteristics of the experimental cyclic voltammogram. Influence of kinetic and transport parameters are elucidated with a global sensitivity analysis. Peak currents are almost exclusively influenced by electrochemical kinetics, while diffusion limited currents largely depend on transport parameters. In addition, the E3C4-mechanism reveales the prominent role of chemical equilibrium between polysulfides in the range of transport limitation. The revealed mechanistic complexity leads to complex, non-intuitive behavior of sulfur electrodes and Li-S batteries. Thus it is highly recommended to use the presented E3C4 kinetics and model for a more reliable interpretation of experimental behavior, including the dynamic relaxation behavior, and for improved electrode and cell design that takes chemical disproportionation into account.