Insight into Mg-S Battery Mechanism
Authors: Vizintin Alen , Robba Ana , Bitenc Jan , Arcon Iztok , Kavcic Matjaz , Mali Gregor , Randon-Vitanova Anna , Dominko Robert 
Published: February 2020

Magnesium batteries are a promising technology as future energy storage system. Due to non-dendritic deposition of magnesium, Mg metal anode can be used, which significantly lowers weight and size of the batteries. Mg metal has a higher volumetric specific capacity (3833 mAh cm-3)1 in comparison to lithium (2062 mAh cm-3). Additional advantages of magnesium are its abundance and low price, since it is one of the ten most abundant elements in Earth’s crust. Nowadays, the biggest challenges are development of high-capacity cathode materials and electrolytes, compatible with both anode and cathode. Sulfur as a cathode material is of great interest, due to its high capacity, low price and natural abundance. Even though the concept of metal-sulfur batteries is relatively old, the first reports on magnesium-sulfur (Mg-S) batteries were only published in 2011.2 Non-nucleophilic magnesium electrolytes were needed to enable use of electrophilic sulfur cathode and magnesium anode. Problems, present in first batteries, such as self-discharge, capacity fade in initial cycles and high polarization, are still challenges that remain to be addressed. Even though some improvements were proposed, lack of fundamental understanding of the system hinders development of this battery system. Here we present an application of different analytical techniques, which were employed to obtain better insight into the mechanism of Mg-S battery and processes occurring on the cathode. The electrochemical reaction of sulfur with magnesium proceeds through two well-defined plateaus, which correspond to the equilibrium between sulfur and magnesium polysulfides (MgSx) in the high-voltage plateau and magnesium polysulfides (MgSx) and magnesium sulfide (MgS) in the low-voltage plateau as presented in figure 1. The use of operando X-ray absorption near edge structure (XANES), resonant inelastic X-ray scattering (RIXS), and ex situ
25Mg MAS NMR studies, have shown that the end discharge product involves MgS with magnesium atoms in a tetrahedral environment resembling the wurtzite structure, while synthesized MgS crystallizes in the rock-salt structure with magnesium atoms in a octahedral coordination.3 Our results represent a step further towards better understanding of the fundamentals of Mg-S system. These advances will enable us to tackle challenges such as polysulfide diffusion from cathode, high polarization and self-discharge more systematically and successfully.

References

(1) Yoo, H. D.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.; Aurbach, D. Mg Rechargeable Batteries: An on-Going Challenge. Energy Environ. Sci.
2013, 6, 2265–2279.

(2) Kim, H. S.; Arthur, T. S.; Allred, G. D.; Zajicek, J.; Newman, J. G.; Rodnyansky, A. E.; Oliver, A. G.; Boggess, W. C.; Muldoon, J. Structure and Compatibility of a Magnesium Electrolyte with a Sulphur Cathode. Nat. Commun.
2011, 2, 427.

(3) Robba, A.; Vizintin, A.; Bitenc, J.; Mali, G.; Arčon, I.; Kavčič, M.; Žitnik, M.; Bučar, K.; Aquilanti, G.; Martineau-Corcos, C.; Vitanova, A. R.; Dominko, R.; Mechanistic Study of Magnesium–Sulfur Batteries. Chem. Mater.
2017, 29, 9555–9564.

Figure 1