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For lithium-sulfur batteries, porous carbon/sulfur composite cathodes are the primary solution to compensate the non-conductive nature of sulfur. The composition and structure of this class of cathodes are crucial to the electrochemical performance, achieved energy density and the stability of the cell. Electrochemical impedance spectroscopy is employed to investigate and correlate the electrochemical performance of lithium-sulfur batteries to the composition and microstructure of differently fabricated carbon/sulfur composite cathodes. A transmission line model is applied to identify different underlying electrochemical processes appearing in the impedance response of a range of porous carbon/sulfur cathodes. The integration of a lithium ring serving as a counter electrode coupled with advanced wiring has allowed an artifact-free recording of the cathode impedance at different states of charge with the aim to investigate the evolution of impedance during discharge/charge and the kinetics of charge transfer depending on the infiltration method and the utilized carbon host. It is shown that impedance response of this class of cathodes is highly diverse and the plausible underlying processes are discussed in details. To this end, quasi-solid-state and various polysulfide-based charge transfer mechanisms are identified and their time constants are reported.

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, such as Lithium-sulfur or Lithium-air, 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 term can be directly used as anode material due to its 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 and 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]. In order to harvest the conceptual similarities between lithium and magnesium sulfur batteries we formulate a common framework for metal sulfur batteries. In a multiscale approach we describe the processes in sulfur host materials (e.g. meso/mricoporous carbons) and on cell level (1+1D). The transport of dissolved species is modeled by the Nernst-Planck equation and sulfur red/ox kinetics are described by a reduced mechanism which is able to reproduce the key experimental results for Me-S batteries [3]. Therefore, the model is able to capture the kinetics of sulfur redistribution in the cell during cycling driven by the polysulfide shuttle. By taking into account side reactions at the negative electrode we are able to describe the experimentally observed decrease in coulombic efficiency and capacity. In our presentation we focus on the investigation of common properties of Li-S and Mg-S batteries, and more importantly key differences between the two. The simulation results will be compared to in-house experimental data measured on sulfur/carbon composite electrodes, such as galvanostatic cycling data. For these experiments the Mg-S and Li-S cells under investigation are identical with respect to dimensions, cathode material and electrolyte solvent. The insights gained from these are used to focus the simulations on aspects specific for Mg-S batteries. In close collaboration with the experimental groups at DLR and HIU 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

Modern Lithium-ion batteries based on intercalation chemistry hold more than twice as much energy by weight and are ten times cheaper as the first commercial versions sold by Sony. But today they are near its limits. Metal anodes for ‘beyond Li-Ion’ batteries, such as Lithium-sulfur or Lithium-air, promise higher energy density and lower cost. In recent years magnesium-sulfur batteries are discussed as an attractive next-generation energy storage system. Magnesium can be directly used as anode material due to its dendrite-free deposition and thus increases the safety as well as energy density of such a cell. Two electrons are stored per Mg atom which compensates the rather low discharge potential of magnesium-sulfur cells of 1.7 V and 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]. In order to harvest the conceptual similarities between lithium and magnesium sulfur batteries we formulate a common framework for metal sulfur batteries. In a multiscale approach we describe both the processes in sulfur host materials (e.g. meso/mricoporous carbons) and on cell level (1+1D). The transport of dissolved species is modeled by the Nernst-Planck equation and sulfur red/ox kinetics are described by a reduced mechanism which is able to reproduce the key experimental results for Me-S batteries [3]. Therefore, the model is able to capture the kinetics of sulfur redistribution in the cell during cycling driven by the polysulfide shuttle. By taking into account side reactions at the negative electrode we are able to describe the experimentally observed decrease in coulombic efficiency and capacity. In our presentation we focus on the investigation of common properties of Li-S and Mg-S batteries, and more importantly key differences between the two. The simulation results will be compared to in- house experimental data measured on sulfur/carbon composite electrodes, such as charge and discharge curves as well as electrochemical impedance spectra. For these experiments the Mg-S and Li-S cells under investigation are identical with respect to dimensions, cathode material and electrolyte solvent. In close collaboration with the experimental groups at DLR and HIU 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

Sulfur is one of the most promising cathode materials due to its high theoretical capacity of 1672 mAh g-1, its low cost and environmental friendliness. In combination with a negative lithium electrode (Li-S) a cell voltage of 2.4 V and an energy density of 2800 Wh l-1 can be theoretically achieved. Yet the use of lithium metal suffers from safety and cost issues due to dendrite formation and limited world occurrence, respectively. In contrast magnesium features significant advantages as it is abundant and shows no dendrite formation during cycling, which ensures superior safety. Furthermore the combination of a negative magnesium electrode and a positive sulfur electrode (Mg-S) offers a higher theoretical energy density of 3200 Wh l-1 at a cell voltage of 1.77 V. Contrary to the Li-S technology the Mg-S system is still in a very early stage of research (first report by Kim et al. in 2011) and suffers from fast capacity decay in the first cycles and poor overall cycle stability. Especially the search for a suitable electrolyte emerged to be difficult as it inter alia has to be non-nucleophilic and capable of reversible Mg deposition/stripping. Very recently Zhao-Karger et al. published a Mg(BH4)2-derived electrolyte with enhanced cycling properties which allows further detailed investigation of Mg-S cells. As both systems rest upon similar electrochemical reactions they display many related properties but also significant differences in the underlying processes. Ensuring the comparability by an identical setup, Li-S and Mg-S cells are investigated by means of galvanostatic cycling and cyclic voltammetryto identify the origin of the fast degradation of magnesium-sulfur batteries.

Efficient and sustainable batteries are a key challenge to pave the way for the availability of energy from renewable sources. Despite the fact that Li-based systems provide a good energy density, they still suffer from some disadvantages concerning cost and safety issues. To overcome these drawbacks alternative systems are under intensive research. Currently sulfur is one of the most promising cathode materials due to its high energy density of 1672 mAh g−1, its low cost and environmental friendliness. Magnesium features significant advantages as it is abundant and shows no dendrite formation during cycling, which ensures superior safety. Above all magnesium offers a high theoretical volumetric and gravimetric capacity of 3832 mAh cm−3 and 2230 mAh g−1 respectively. Thus the combination of a negative magnesium electrode and a positive sulfur electrode (Mg-S) represents a promising cell technology. Until now Mg-S cells suffer from fast capacity decay in the first few cycles and poor cycle stability. To understand the underlying processes and to identify the origin of the rapid degradation, different approaches regarding the synthesis and characterization of S-C electrodes in Mg2+-based electrolyte systems are investigated. The cathodes prepared are characterized with cell cycling experiments and optimized regarding their electrochemical performance. In Addition electrochemical impedance spectroscopy was applied to examine internal processes.