Herein we report a magnesium powder anode with medium capacity retention and recovery of potential plateaus over reversible cycling. Furthermore powder anodes showed an optimized behavior during charging leading to an improved coulomb efficiency compared to magnesium foil anodes. Battery cells with a powder anode showed pronounced voltage plateaus and no problem to reach a 2.8 V cutoff voltage. Within the compaction pressure range investigated a magnesium powder anode prepared by relatively low pressures showed advantageous properties and was further investigated by electrochemical impedance spectroscopy.

In the presented study, a sulfur infiltrated ultra-microporous carbon aerogel as a composite cathode for lithium sulfur batteries is developed and investigated.

No abstract

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 coating process. The S/C composite is made either by melt-infiltration or by simply mixing the two components during slurry preparation. 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 behavior 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.

Demands in energy storage require a significant improvement in the energy density of battery systems. Therefore innovative systems with a substantially higher specific energy and improved cycle life have to be developed. The combination of Magnesium and Sulfur in a galvanic element addresses several advantages, such as natural abundance, operational safety and a high volumetric capacity. However the research so far was hindered because a stable electrolyte for Mg-S-Batteries was missing [1]. Muldoon et. al. developed a stable and reversibly working non-nucleophilic electrolyte based on a recrystallized Mg2+ salt complex [Mg(µ-Cl)3(THF)6][HMDSAlCl3] (HMDS=hexamethyldisilazid) [2]. This crystalline electrolyte salt though could only be obtained in THF, revealing a disadvantageous overcharging and fast capacity fading [2]. A break-through in Mg/S battery performance was obtained by Fichtner et. al. They at first synthesized a one-step routine between [(HMDS)2Mg]=magnesium bis(hexamethyldisilazid) and AlCl3in different ethers, enhancing solvent choice and possibility of neglecting aforementioned disadvantages [3]. Furthermore a magnesium-anode enhances safety of Mg-S-battery, due to the missing tendency of dendrite formation [4]. Safety as well as capacity and scalability of fabrication of a battery are important factors to assess market opportunities of these secondary cells. Therefore we are focusing on a cell design that is "easy-to-fabricate" with a realistic material loading. Based on the multiannual experience in the development of lithium-sulfur batteries, we extend our activities to the field of magnesium sulfur-battery-development. In a first attempt S-C-cathodes with Sulfur content of 50 wt.% and 70 wt.% were evaluated with a 1.2 M Mg2+-salt electrolyte dissolved in a mixture of 2:1 by volume of bis(2-methoxyethyl)ether and an ionic liquid. The influence of the cathode composition was minimal at this early stage of development. Both types of batteries showed a similar discharge capacity of 600 mAh/g(Sulfur) in the first cycle. Furthermore different approaches for a working Mg-graphite-composite-anode are presented. Such composite-anodes were prepared via die-pressing and could only be obtained in specific composition. The obtained charge-discharge profiles showed remarkably well defined plateaus, which have not been obtained elsewhere, jet (see Figure 1). The preparation routine of the composite-anode was found to have the highest impact on the obtained performance. References [1] R. Mohtadi and F. Mizuno, “Magnesium batteries: Current state of the art, issues and future perspectives.,” Beilstein J. Nanotechnol., vol. 5, pp. 1291–311, Jan. 2014. [2] J. Muldoon, H. S. Kim, T. S. Arthur, G. D. Allred, J. Zajicek, J. G. Newman, A. E. Rodnyansky, A. G. Oliver, and W. C. Boggess, “Structure and compatibility of a magnesium electrolyte with a sulphur cathode.,” Nat. Commun., vol. 2, p. 427, Jan. 2011. [3] J. M. F. Z., Zhao-Karger, X. Zhao, D. Wang, T. Diemant, R. Behm, “Performance Improvement of Magnesium Sulfur Batteries with Modified Non-Nucleophilic Electrolytes,” Adv. Energy Mater., vol. 5, no. 3, p. 1401155, 2015. [4] T. D. Gregory, R. J. Hoffman, and R. C. Winterton, “Nonaqueous Electrochemistry of Magnesium Applications to Energy Storage,” J. Electrochem. Soc., vol. 137, no. 3, p. 775, 1990.