Metal (Li, Mg) sulfur batteries can be considered as important energy storage devices in the future due to abundance of sulfur and high theoretical gravimetric energy density. In the last decade different groups showed progress in the electrochemical stability during cycling and in the understanding of complicated conversion reactions connected with unpredictable equilibrium states of sulfur in different solvents. Vast of reports was connected with application of different types of host matrices for sulfur impregnation and recently also with additives which can adsorb polysulfides. Much less work has been performed on development of novel electrolytes, ion selective separators and on protection of the negative electrode.

Sulfur and polysulfides are soluble in the classical electrolytes based on glyme solvents and this future enables reduction of sulfur to metal sulfide and its reoxidation. But on the same time, in the liquid cells with high excess of electrolyte, soluble sulfur species diffuse/migrate to the negative electrode where can be further reduced and even deposited on the surface of the negative electrode.

Although sulfur conversion in Li-S and Mg-S batteries is very similar, their electrochemical stability is completely different what can be connected with passivation of the negative electrode. Recent progress in understanding of both systems and their electrochemical stability will be discussed in this presentation.

Acknowledgement: This work is supported by the HELiS project which receives funding from the European Union‘s Horizon 2020 research and innovation program under Grant Agreement No 666221. The financial support from the Slovenian Research Agency (research core funding No. P2-0393) is fully acknowledged.

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