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.