Magnesium Sulfur Batteries
Authors: Sievert Brigitta , Friedrich K. Andreas , Wagner Norbert 
Published: February 2020

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.