Rechargeable magnesium (Mg) batteries are a sustainable option for electrochemical energy storage of renewable energy due to the inherent merits associated with Mg such as natural abundance, operational safety and high theoretical volumetric capacity (3,832 mAh cm^-3), with a reduction potential of − 2.356 V versus normal hydrogen electrode (NHE).¹ Among different types of cathode materials, the combination of Mg with a sulfur cathode and a non-nucleophilic electrolyte is very promising due to its large theoretical energy density of about 3200 Wh l⁻¹, based on a two-electron conversion reaction expressed as, Mg²⁺ + S + 2e ⁻ ↔ MgS, which is beyond that of a Li/S battery (2800 Wh l^-1). In addition, these battery systems are reversible and using low cost materials. Compared to the progress made on Li/S batteries, the Mg/S battery is still in a very early stage of research and development due to number of reasons such as low kinetics of Mg, lack of suitable electrolytes, polysulfide dissolution, insulating nature of sulfur, capacity fading etc.

Herein, we demonstrate a reversible Mg dissolved polysulfide cell using reduced graphene oxide (rGO) as the cathode host matrix and a non-nucleophilic Mg electrolyte in tetraglyme solution as the electrolyte. The electrolyte was prepared by mixing a non-nucleophilic base of magnesium-bis(hexamethyldisilazide) [(HMDS)₂Mg] with the Lewis acid, AlCl₃ (1:2 molar ratio) and dissolving the reaction product in a tetraglyme (C₁₀H₂₂O₅) ethereal solvent . The liquid Mg polysulfides cell delivers better reversible capacity (~750 mAh g^-1, Figure. 1) and cyclability as compared to previous Mg/S cell reports. The high surface area, porous structure and presence of oxygen functional groups over rGO give simultaneous physical and chemical adsorption of Mg polysulfides with the host matrix. Further, the working mechanism of liquid Mg polysulfides within Mg/S cell is investigated by Raman spectroscopy.

References

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[2] B. P. Vinayan, Z. Zhao-Karger, T. Diemant, V. S. K. Chakravadhanula, N. I. Schwarzburger, M. A. Cambaz, R. J. Behm, C. Kubel and M. Fichtner, Nanoscale, 2016, DOI: 10.1039/C5NR04383B.