Understanding Lithium Sulfur Cells, Modelling the Mechanisms behind Voltage- and Capacity-Drop during Discharge

Abstract

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 will give an overview of our work on understanding and modelling lithium sulfur cells, and include our latest work on models that capture the essential features of lithium sulfur cell performance. We will also discuss how these models are being used to increase the useable performance of lithium sulfur cells, inform cell design and materials selection, and to create reduced order models for control and system design. 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.