Design and Validation of a Computational Model of the Lithium/Sulfur Cell

The lithium-sulfur (Li/S) battery is among the most promising next-generation systems for electrochemical energy storage. Its key benefits are improved specific energy and volumetric energy density as well as cheap, safe, and environmentally benign raw materials. Recent advancements in rate capability and cycle life bring the system even closer to practical applications [1]. However, the complex redox chemistry of sulfur, the large number of soluble intermediates and the dissolution and precipitation of solid phases, still present a major challenge to understanding and mastering Li/S electrochemistry.

The goal of this work is to contribute to the understanding and design of future Li/S batteries by investigating the internal states of a battery via simulations with a physically-based electrochemical model of the cell. The computational model used for this work is based on our previous model [2] of the Li/S cell including a multi-step reaction mechanism, a detailed model of the evolution of solid phases [3] as well as multi-component mass and charge transport in the liquid electrolyte.

To parameterize and refine the model, experimental data were obtained from Li/S coin cells operated under various conditions. The cells used for this study are composed of Li metal anodes, an ionic liquid based electrolyte and positive electrodes made from a carbon/binder/lithium sulfide composite, similar to ref. [4].

The techniques used for analysis include electrochemical methods, i.e., short and long term cycling at different charge/discharge rates, cyclic voltammetry, impedance spectroscopy (also during cycling) as well as ex situ methods, e.g. SEM imaging at various states of charge and states of health. Along with prior knowledge about the materials used and the geometrical properties of the cells, these data were used to determine most parameters required by the model.

Using the refined model, experimental results can be reproduced with good agreement. Validated in such a way, the model is used to simulate properties of the Li/S cell that are not easily accessible by experiments, e.g., lithium and polysulfide concentration gradients within the electrolyte and electrode volume expansion during cycling.

Results include simulations of charge/discharge profiles and electrochemical impedance spectra as well as distribution of solids and concentration profiles. The results confirm that the discharge behavior for the most common type of Li/S cell is governed by the presence of solid reactant and product phases in exchange with the dissolved sulfur poly-anions. The first and second discharge stages are characterized by the presence of solid S₈ and Li₂S, respectively, while all sulfur compounds are dissolved in the electrolyte during an intermediate stage. Simulated electrochemical impedance spectra indicate that the contributions of various processes to the cell's overpotential are significantly different for different stages of charge.

These findings help to understand what is happening in the cell during operation, which in turn allows for the identification of optimized operating conditions as well as the improvement of cell design on the electrode and material level.

References:

[1] M.-K. Song, E. J. Cairns, Y. Zhang, Nanoscale, 5 (2013) 2186–2204

[2] D. N. Fronczek, W. G. Bessler, J. Power Sources, 244 (2013) 183–188

[3] J. P. Neidhardt, et al., J.Electrochem. Soc. 159 (2012) A1528–A1542

[4] K. Cai et al., Nano Lett. 12 (2012) 6474–6479