A framework for the simulation of next-generation magnesium-sulfur batteries

Abstract

Modern Lithium-ion batteries based on intercalation chemistry hold more than twice as much energy by weight and are ten times cheaper than the first commercial versions sold by Sony. But today they are near its limits. Metal anodes for ‘beyond Li-Ion’ batteries promise higher energy density and lower cost. However, the formation of dendrites is a major security risk and prevented a commercialization of the technology in large scales. Magnesium in turn can be directly used as anode material due to ist dendrite-free deposition and thus increases the safety as well as energy density. Two electrons are stored per Mg atom which compensates the generally lower discharge potential of magnesium-based cell chemistries. In recent years magnesium-sulfur batteries are discussed as an attractive next-generation energy storage system which provides a high capacity of 3832 mAh/cm3 and 2230 mAh/g with an energy density of over 3200 Wh/l [1]. Such an energy density is beyond that of lithium-sulfur batteries and is, therefore, very promising for automotive and stationary applications. Furthermore, magnesium and sulfur are both naturally abundant, low in price and non-toxic. However, magnesium-sulfur batteries are in a very early stage of research and development. The system suffers from similar problems like the early Li-S batteries which are a low coulombic efficiency and cycle life, mainly associated with the well-known polysulfide shuttle. Moreover, the reactions at both the positive and negative electrode are not yet fully understood but similar sulfur reduction mechanisms are generally assumed [2]. Our aim is to provide more insight in the operation of Mg-S batteries and to support the experimental progress. Therefore, we develop a framework for metal-sulfur cells and focus in our presentation on the investigation of novel Mg-S batteries. In a multiscale approach we describe the processes in sulfur host materials (e.g. meso/mricoporous carbons) and on cell level (1+1D). Combined with the transport of dissolved species modeled by the Nernst-Planck equation the model is able to capture the kinetics of sulfur redistribution in the cell during cycling driven by the polysulfide shuttle. The sulfur red/ox kinetics are described by a reduced mechanism which is able to reproduce the key experimental results [3]. By taking into account side reactions at the negative electrode we are able to describe the experimentally observed decrease in coulombic efficiency and capacity. The simulation results will be compared to in-house experimental data measured on sulfur/carbon composite electrodes, such as galvanostatic cycling data. In close collaboration with the experimental groups at DLR and HIU/KIT [2] we aim at guiding new developments of the Mg-S system.

References: 1. H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour, D. Aurbach, Energy Environ. S. 6 (2013), 2265 2. Z. Zhao-Karger, X. Zhao, D. Wang, T. Diemant, R. J. Behm, M. Fichtner, Adv. Energy Mater. 5 (2015), 140155 3. T. Danner, G. Zhu, A. F. Hofmann, A. Latz, Electrochimica Acta 184 (2015), 124-133