Quest for magnesium-sulfur batteries: Current challenges in electrolytes and cathode materials developments

The emerging energy-storage technologies must meet the requirement of low cost, reasonable safety, high natural abundance, and high energy density. The rechargeable magnesium-sulfur battery (Mg-S) meets these requirements as a new and emerging technology because of its high specific energy (1722 Wh. kg−1) and dendrite free plating/stripping of magnesium anode. However, its performance is restrained by the sluggish magnesium ion diffusions and unavailability of high-voltage magnesium electrolytes compatible with sulfur cathodes. As a result, Mg-S batteries suffer from poor reversible capacity and cycle life. Furthermore, the formation of magnesium polysulfide (Mg-PS) is considered as a biggest R&D challenge for Mg-S batteries. To alleviate these problems, numerous breakthroughs and achievements have been made in the past five years. This review covers the research and development to date on Mg-S systems including working mechanism, development of nucleophilic and non-nucleophilic electrolytes, inorganic electrolyte additives, fabrications of sulfur cathodes, effect of binders, polysulfide confinement via cathode or separator treatments, and in-situ characterizations of electrode/electrolyte interfaces. The electrochemical properties of newly developed Mg-S systems are debated in terms of cell configuration, composition of electrolyte, specific capacity, cycle performance, over potential and working voltages. Furthermore, the current challenges and possible research gaps in Mg-S systems are critically discussed along with future perspectives. We hope this review will revolutionize the deep understanding of Mg-S battery and open the new horizons to develop the highly efficient cathodes and electrolytes in near future.