Modelling Li-S Cells: Mechanisms behind Voltage- and Capacity-Drop during Discharge

The lithium-sulphur (Li-S) cell could provide the next step-change in battery technology with a promising practical energy density of 500-600 Wh/kg. However, a lack of understanding of the complex electrochemical, transport, and phase-change phenomena in Li-S cells is arguably holding back development. Acquiring this knowledge requires experimental characterizations in tandem with mechanistic modelling. In this presentation, we point out that existing Li-S models do not sufficiently capture the voltage- and capacity-drop mechanisms of Li-S cells during discharge. We first demonstrate that introducing a concentration dependence of the electrolyte conductivity is necessary to retrieve the experimentally documented trends in electrolyte resistance, which contributes to a major voltage-loss mechanism for high-energy density Li-S cells. We further illustrate the existence of an often overlooked potential drop mechanism – the ‘precipitation overpotential’ – which originates from the limited rate of lithium sulphide precipitation. In addition, we propose that the rate capability of high energy-density Li-S cells is mainly limited by the slow transport of ionic species, as is evident from the good agreement between experimental and model-predicted capacity loss at high discharge currents as well as a cell capacity recovery phenomenon that we report for the first time. References: T. Zhang, M. Marinescu, M. Wild, L. O’Neill, and G. Offer, Phys. Chem. Chem. Phys. 17 , 22581 (2015). M. Marinescu, T. Zhang, G. Offer, Phys. Chem. Chem. Phys., 2015, advance article.