Deeping Insight of Weakly Solvating Halogen-Free Electrolyte in Enhancing the Electrochemical Performance of Magnesium–Sulfur Batteries

Magnesium-sulfur (Mg-S) batteries offer a promising energy storage system due to their high theoretical capacity. However, the sluggish conversion reaction kinetics and the shuttle effect of magnesium polysulfides hinder their practical application. The high charge density of the divalent Mg²⁺ cations leads to slow kinetics caused by significant electrostatic interactions between Mg²⁺ and its surrounding solvent and anion species. This study probes the impact of incorporating the weakly coordinating solvent tetrahydrofuran (THF) to regulate the solvation sheath's interfacial chemistry and enhance the mobility of the Mg²⁺ ions in a halogen-free electrolyte (HFE) based on Mg(NO₃)₂. An effective anode/electrolyte interface is engineered to protect the Mg anode from passivation and reduce polysulfides to shuttle. The HFE-THF electrolyte exhibits relatively goodelectrochemical performance, high ionic conductivity, and a low overpotential for Mg plating and stripping. Mg-S cells with a HFE-THF electrolyte achieve low initial specific capacities of approximately 580 mAh g^-1, high and fluctuated Coulombic efficiency at a current density of 20 μA cm^-2, and extended cycle life compared with the Mg-S cells with a blank HFE.