Simulating the polysulfide shuttle effect: a thermodynamically consistent, fully reversible, numerical Li/S-battery model

Abstract

Lithium-Sulfur (Li/S) cells are promising candidates for a next generation of safe and cost-effective high energy density batteries for mobile and stationary applications. At present, Li/S cells still suffer from poor cycleability, capacity loss under high current densities and self-discharge. Furthermore, the underlying chemical mechanisms of the general discharge/charge behavior as well as Li/S-specific phenomena like the (polysulfide) shuttle are not yet fully understood. Here we present a thermodynamically consistent and fully reversible continuum model of a Li/S cell with simplified four-step electrochemistry, based on a multi-phase modeling framework [1]. The model reproduces experimentally obtained discharge curves from literature [2] at various current densities with fairly high accuracy. While being instructively simple, the presented model can still reproduce distinct Li/S-cell features like seemingly infinite charging at low charge current-densities (shuttle effect), voltage spikes before the plateau-phases of the charging process, as well as voltage drops and capacity loss due to cycling. This not only fosters understanding of possible mechanisms for the shuttle effect, but also makes the presented model a suitable tool for investigation and interpretation of Li/S cell experimental data and a powerful base model for further, more complex model studies focusing on specific details of Li/S cells.