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

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