Insights on the operation of metal-sulfur batteries: a modeling perspective

A major driving force for the development of electrochemical storage technology with high energy density is the electrification of the mobility sector. High expectations rest on the development of beyond Li-Ion battery systems [1]. Especially, lithium-sulfur batteries (Li/S) are believed to be a promising candidate already in the near future. Recently, promising results have also been published on the magnesium-sulfur (Mg/S) system [2]. Mg is non-toxic, inexpensive and provides an even higher volumetric energy density compared to Li. Initial discharge curves show comparable features to Li/S batteries which are a hint for a similar reduction mechanism of sulfur. However, the cycle life of Mg/S batteries is still rather poor. A thorough understanding of the fundamental processes in both Li/S and Mg/S batteries will be a key factor for future developments and the overall success of metal-sulfur batteries (Me/S). In our contribution we will present a systematic study of sulfur/carbon composite electrodes cycled against both Li and Mg metal anodes. S/C composite electrodes are prepared in a wet process either by melt-infiltration or by simply mixing the two components. The data is used as input for continuum models of Me/S batteries developed in our group [3], [4]. Our framework describes the reaction and transport of solid and dissolved sulfur species on particle as well as on cell level. This multi-scale approach allows tracking the concentration and volume fraction of sulfur species during cycling and gives the opportunity for a systematic study of the polysulfide shuttle in Me/S batteries. In particular we are able to elucidate the degradation behaviour of the different electrode composites investigated in this work. In the case of Li/S batteries an additional focus is set on the redistribution of the solid end-products S8 and Li2S during charge and discharge. We propose a detailed model for the nucleation and growth of S8 and Li2S particles and keep track of particle size distributions at various positions in the cathode, separator and close to the anode. Depending on the operating conditions we observe variations in the final particle size which results at high C-rates in a passivation of the electrode surface and the occurrence of a characteristic feature in the cell voltage upon recharge. Our approach provides mechanistic insights for the operation of Me/S batteries and will contribute to the understanding and, therefore, improvement of next-generation batteries. [1] P. G. Bruce, S. A. Freunberger, L. J. Hardwick, and J.-M. Tarascon, “Li–O2 and Li–S batteries with high energy storage,” Nat. Mater., vol. 11, no. 2, pp. 172–172, 2011. [2] Z. Zhao-Karger, X. Zhao, D. Wang, T. Diemant, R. J. Behm, and M. Fichtner, “Performance improvement of magnesium sulfur batteries with modified non-nucleophilic electrolytes,” Adv. Energy Mater., vol. 5, no. 3, pp. 1–9, 2015. [3] T. Danner, G. Zhu, A. F. Hofmann, and A. Latz, “Modeling of nano-structured cathodes for improved lithium-sulfur batteries,” Electrochim. Acta, vol. 184, pp. 124–133, Dec. 2015. [4] A. F. Hofmann, D. N. Fronczek, and W. G. Bessler, “Mechanistic modeling of polysulfide shuttle and capacity loss in lithium–sulfur batteries,” J. Power Sources, vol. 259, pp. 300–310, Aug. 2014