Lithium/sulfur batteries are considered as one of the most promising post-lithium-ion battery systems, because the high specific capacity of sulfur drastically outranges the capacities of Li-ion insertion cathodes. However, at the Li metal anode, dendrite formation and a high reactivity towards the electrolyte lead to lifetime and safety concerns.[1] In order to circumvent these issues while maintaining high specific capacities, magnesium/sulfur batteries are increasingly studied.[2] In contrast to Li, the Mg metal anode enables homogeneous Mg deposition and dissolution at up to 100% Coulombic efficiency. In addition, Mg is much cheaper and highly abundant. In Li/S and Mg/S cells, charge and discharge of the sulfur cathode proceeds through a cascade of bivalent S_x^2- and radical S_y^•- polysulfide intermediates. The individual polysulfide chemistries thus determine the accessibility and performance of both battery systems. However, as dissolved polysulfides can migrate through the electrolyte and react at the metal anode, they are also related to the capacity fading and self-discharge of Li/S and Mg/S batteries.

For the first time, both the electrochemical behavior as well as the solubility and stabilization of Mg polysulfides were studied. In order to reveal the influence of Li⁺ and Mg²⁺, their polysulfide solutions were systematically compared in a broad variety of solvents. At first, the polysulfide disproportionation and dissociation equilibria in solutions of “Li₂S₈“ and “MgS₈” in DMSO, DMF, ACN, THF, DME, TEGDME, and Pyr_1,4TFSI were investigated in UV/Vis spectroscopy.[2] Afterwards, the influence of additional LiTFSI and MgTFSI₂, respectively, was examined and the red/ox processes of these polysulfide-containing electrolytes were studied in cyclic voltammetry experiments. In addition, quantum mechanical calculations of the stability of the individual polysulfide species were conducted.

It could be shown that Mg polysulfides form similar disproportionation and dissociation equilibria as Li polysulfides. The stabilization of different polysulfide species is not only determined by the solvents’ relative dielectric permittivity and donor number, but also depends on the coordination by either Li or Mg cations in a mutual interplay with the solvent. Adding LiTFSI/MgTFSI₂ intensifies the differences between the Li and Mg polysulfide solutions. As a consequence, the red/ox behavior of these solutions differ strongly. It is demonstrated that the typically high overvoltages of Mg/S cells are related to the electrochemistry of the Mg polysulfides.

The achieved insights into the fundamentals of the Li/S and Mg/S cell chemistries might lead the way towards new electrolyte designs and play a key role in increasing the reversibility and lowering the overvoltages of Mg/S cells.

Acknowlegdements:

The authors wish to thank the BMBF for funding this work within the projects ‘ACHiLiS’ (03XP0037A) and ‘MgMeAnS’ (03XP0140). The Heinrich Böll Foundation is acknowledged for funding through a PhD scholarship.

References:

[1] G. Bieker, M. Winter, P. Bieker, Physical Chemistry Chemical Physics, 2015, 17, 8670-8679.

[2] Z. Zhao-Karger, M. Fichtner, MRS Communications, 2017, 1-15.

[3] G. Bieker, J. Wellmann, M. Kolek, K. Jalkanen, M. Winter, P. Bieker, Physical Chemistry Chemical Physics, 2017, 19, 11152-11162.