Ca$^{2+}$ ‐Driven Enhancement of Anodic Performance and Sulfur Utilization for Magnesium–Sulfur Batteries

Magnesium–sulfur (Mg–S) batteries are emerging as promising energy storage systems due to their cost-effectiveness, safety, and high theoretical volumetric energy density. However, their practical implementation is hindered by sluggish sulfur redox kinetics with Mg$^{2+}$ and severe polysulfide shuttling. Here, a double-divalent Mg–Ca hybrid electrolyte is introduced, where a small amount of Ca$^{2+}$ additive significantly enhances sulfur redox kinetics, leading to higher sulfur utilization. Notably, Ca$^{2+}$ primarily facilitates the solid-to-solid conversion of disulfide to sulfide. In addition to the cathode reaction, the Mg–Ca hybrid electrolyte also contributes to the anode reaction; it enables smoother Mg plating and reduces overpotential with the long cycle (>1000 cycles). For mitigating the polysulfide shuttling, the glass fiber separator with ultrasmall α-MnO$_2$ nanoparticles is modified to adsorb polysulfide. This synergistic strategy of electrolyte and separator engineering enables the Mg–S battery to achieve an initial capacity exceeding 1000 mAh g$^{-1}$ and extended cycling stability. These findings highlight the potential of Mg–Ca hybrid electrolytes and nanosized α-MnO2-modified separators in the development of high-performance Mg–S batteries.