Insight into lithium-sulfur batteries: Elementary kinetic modeling and impedance simulation

Abstract

We present a model of the lithiumesulfur (Li/S) battery based on a multi-step, elementary sulfur reduction mechanism including dissolved polysulfide anions. The model includes a description of the evolution of solid phases in the carbon/sulfur composite cathode as well as multi-component (Li+, PF6-, S8, S8--, S6--, S4--, S2--, S--) mass and charge transport in the liquid electrolyte. The chemical reaction mechanism consists of a Li/Li+ oxidation reaction at the anode and a six-step polysulfide reduction mechanism at the cathode. The modeling framework allows for the simulation of charge and discharge profiles as well as electrochemical impedance spectra. The latter are obtained via a voltage step/current relaxation technique based on the physicochemical model without the need for applying equivalent circuit models. The results indicate that the discharge behavior of this Li/S cell is governed by the presence of solid reactant and product phases in exchange with the dissolved polysulfide anions. The first and last stages of the discharge are characterized by the presence of solid S8 and Li2S, respectively, while the intermediate stage corresponds to a situation where all chemical compounds are dissolved in the electrolyte.