Theoretical study on the role of magnesium chloride complexes induced by different magnesium-to-chlorine ratios in magnesium-sulfur batteries.

In magnesium-sulfur batteries, electrolyte exploration is vital for developing high-energy-density, safe, and reliable batteries. This study focused on cyclic THF and chain DME, representative solvents in ether electrolytes. MgCl, an ideal anionic salt, forms mono-nuclear (MgCl(DME)), bi-nuclear ([Mg(μ-Cl)(DME)]), and tri-nuclear ([Mg(μ-Cl)(DME)]) complexes in DME. With increasing salt concentration, these complexes sequentially form. Under lower salt concentrations, THF and MgCl form mono-nuclear complexes ([MgCl(THF)]) and continue to form bi-nuclear complexes ([Mg(μ-Cl)(THF)]). However, at higher salt concentrations, bi-nuclear complexes ([Mg(μ-Cl)(THF)]) directly form in THF. Comparing HOMO-LUMO values, [Mg(DME)] is easily oxidized. Energy gaps decrease with Cl ion addition, enhancing solution conductivity. Ratios of Mg and Cl in S-reduction complexes differ, suggesting DME is better at a low Mg/Cl ratio, and THF at a high Mg/Cl ratio. This study contributes to understanding complexes and enhancing Mg-S battery performance.