Electrolytes for Rechargeable Magnesium-Sulfur Batteries

Rechargeable magnesium-sulfur batteries have advantages of good safety, low cost, and high theoretical energy density. Currently, electrolyte is the key factor to restrict the development of magnesium-sulfur batteries. Generally, traditional Grignard reagents and magnesium ion electrolytes are nucleophilic and incompatible with sulfur. In this paper, the electrolytes for magnesium-sulfur batteries are divided into four categories, containing organomagnesium+AlCl₃, MgCl₂+organoaluminum halide, MgCl₂+AlCl₃ and magnesium bis(trifluoromethane sulfonyl)imide (Mg(TFSI)₂). Among them, the organomagnesium+AlCl₃ electrolyte is relatively promising for magnesium-sulfur batteries, and HMDSMgCl+AlCl₃/THF electrolyte is used to demonstrate a proof of concept for the first rechargeable Mg/S battery. The Mg(TFSI)₂ or MgCl₂+AlCl₃ based electrolyte for magnesium-sulfur batteries do not contain organic metal salts, and the synthesis methods are simple. However, the batteries using these two kinds of electrolytes exhibit low specific capacity and discharge voltage. As for MgCl₂+organoaluminum halide electrolyte, the compatibility with the sulfur electrode has been proved, while the specific capacity, the discharge voltage and other electrochemical properties have not been reported. It is generally believed that the electrolytes for magnesium-sulfur batteries contain monomer Mg, dimeric Mg, trimeric Mg species and other Mg complexes at the same time, and there is a thermodynamically equilibrium among them regulating electrolyte ion conductivity, diffusion coefficient and so on. The effect of anions on the electrolyte is mainly reflected in the anodic stability of electrolytes and the reversibility of Mg deposition-dissolution. Electrolyte additives in the magnesium-sulfur battery electrolyte make a contribution to reduce dissolution of sulfur species and the reoxidation kinetic barrier of low-order MgPS (PS is polysulfides), restrain shuttle effect, and improve the electrolyte conductivity. At last, the research trends of electrolyte for magnesium sulfur batteries are proposed.