In Mg–S batteries, a double-divalent Mg–Ca hybrid electrolyte accelerates sulfur redox kinetics and significantly enhances capacity. On the anode side, it promotes smoother Mg plating and lowers overpotential, resulting in stable cycling performance. This electrolyte-based strategy offers valuable insights for Mg–S batteries and is broadly applicable to other Mg-based battery systems.

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 Mg²⁺ and severe polysulfide shuttling. Here, a double-divalent Mg–Ca hybrid electrolyte is introduced, where a small amount of Ca²⁺ additive significantly enhances sulfur redox kinetics, leading to higher sulfur utilization. Notably, Ca²⁺ 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-MnO₂ 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.