Rich Se Vacancies Mo_xV_1-xSe₂ Nanosheets with Synergistic Optimization of Electronic and Lattice Structures for Accelerating Sulfur Redox Kinetics in Mg─S Batteries

Rechargeable magnesium–sulfur (Mg─S) batteries are considered promising next-generation energy storage solutions because of their high volumetric energy density. However, they often suffer from severe performance degradation due to the well-known polysulfide shuttle effect and sluggish reaction kinetics. Defective materials are widely employed in metal-sulfur battery systems due to their unique adsorptive and catalytic properties, which effectively address the challenges of polysulfide shuttle and sluggish conversion kinetics during charge-discharge processes. Nevertheless, studies systematically correlating defect concentration with the adsorptive-catalytic properties of electrodes remain scarce. In this study, Mo_xV_1-xSe₂ (x = 0-0.1) with tunable selenium-vacancy concentrations is employed as a model system to modulate its electronic structure and enhance catalytic performance. A quantitative correlation is further established between selenium-vacancy concentration and adsorption-catalytic properties to regulate sulfur redox kinetics. Experimental and theoretical findings indicate that a higher density of selenium vacancies effectively provides additional active sites and promotes electron accumulation, leading to reduced energy barriers for MgS nucleation/decomposition as well as faster kinetics in polysulfide conversion reactions. Thus, the Mo_0.075V_0.925Se₂ with abundant selenium vacancies concentrations exhibits exceptional performance as a sulfur host, delivering the highest reversible capacity (1127 mAh g⁻¹) and remarkable cycling stability (200 cycles with ∼99.7% capacity retention). This study contributes to advancing the practical implementation of defect engineering with quantitative control for application in Mg─S batteries.