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

Abstract

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. MgCl2, an ideal anionic salt, forms mono-nuclear (MgCl2(DME)2), bi-nuclear ([Mg2(μ-Cl)2(DME)4]2+), and tri-nuclear ([Mg3(μ-Cl)4(DME)5]2+) complexes in DME. With increasing salt concentration, these complexes sequentially form. Under lower salt concentrations, THF and MgCl2 form mono-nuclear complexes ([MgCl2(THF)4]) and continue to form bi-nuclear complexes ([Mg2(μ-Cl)3(THF)6]+). However, at higher salt concentrations, bi-nuclear complexes ([Mg2(μ-Cl)3(THF)6]+) directly form in THF. Comparing HOMO-LUMO values, [Mg(DME)3]2+ is easily oxidized. Energy gaps decrease with Cl- ion addition, enhancing solution conductivity. Ratios of Mg2+ 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.