Abstract
Lithium-sulfur batteries could deliver significantly higher gravimetric energy density and lower cost than Li-ion batteries. Their mass adoption, however, depends on many factors, not least on attaining a predictive understanding of the mechanisms that determine their performance under realistic operational conditions, such as partial charge/discharge cycles. This work addresses a lack of such understanding by studying experimentally and theoretically the response to partial cycling. A lithium-sulfur model is used to analyze the mechanisms dictating the experimentally observed response to partial cycling. The zero-dimensional electrochemical model tracks the time evolution of sulfur species, accounting for two electrochemical reactions, one precipitation/dissolution reaction with nucleation, and shuttle, allowing direct access to the true cell state of charge. The experimentally observed voltage drift is predicted by the model as a result of the interplay between shuttle and the dissolution bottleneck. Other features are shown to be caused by capacity fade. We propose a model of irreversible sulfur loss associated with shuttle, such as caused by reactions on the anode. We find a reversible and an irreversible contribution to the observed capacity fade, and verify experimentally that the reversible component, caused by the dissolution bottleneck, can be recovered through slow charging. This model can be the basis for cycling parameters optimization, or for identifying degradation mechanisms relevant in applications. The model code is released as Supplementary material B.
Highlights
Lithium sulfur (LiS) batteries have the potential to provide a step change in performance, compared to Li-ion batteries, with an expected practical energy density of 700 Wh kg−1 compared to that of the intercalation Li-ion batteries, of 210 Wh kg−1.1,2 Added benefits, such as a potential low cost due to the abundance of the active materials, low toxicity and relative safety,[3] make them an attractive energy storage solution for a wide variety of applications, such as space exploration[4] and low temperature energy delivery.[5]
The cells contain a sulfone-based solvent and their electrolyte/sulfur ratio has been optimized in order to deliver maximum cell-level specific energy density, indicating a relatively high sulfur and low electrolyte loading compared to the cells in most other published results
Cycling of LiS cells is shown experimentally to lead to voltage drift and apparent end of life, even when the Coulombic efficiency appears at its highest
Summary
Of the voltage envelope is the voltage drift: despite seeing the same charge throughput during charge and discharge, the cell voltage hits the lower and the higher voltage cutoff. Once the latter occurs, a sharp decrease in charge throughput leads to apparent cell death. As measured by charge throughput, are equal and constant, illustrating the fact that both charge and discharge can occur for the pre-set time, without causing the cell to reach either voltage cutoff. The discharge voltage curves for cycles number 3 and 4 in Figure 2 indicate that the capacity delivered in the high
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