Achieving Efficient Magnesium–Sulfur Battery Chemistry via Polysulfide Mediation

Abstract Magnesium–sulfur batteries promise a higher theoretical volumetric energy density, improved safety, and lower cost compared to lithium–sulfur batteries. However, Mg–S batteries suffer from poor cycle life and low energy efficiency. Here, it is revealed that Mg–S reactions are dominated by “solid–solid” reactions due to much lower polysulfide solubility in the presence of Mg²⁺ compared to that of Li⁺ in 1,2‐dimethoxyethane (DME)‐based electrolyte, leading to sluggish kinetics and poor reversibility. The polysulfide solubility is increased by using high‐donor‐number solvents (e.g., dimethyl sulfoxide (DMSO)), which increases the discharge capacity from 660 to ≈1500 mAh g⁻¹ and decreases the sulfur overpotential from >600 to ≈200 mV at 0.1 C (energy efficiency over 90%). Based on this strategy, an Mg–S cathode with DMSO‐based electrolyte demonstrates a reversible capacity of 700 mAh g⁻¹ at 0.4 C over 300 cycles. This work reveals a reaction limitation of Mg–S batteries and provides critical insights into the electrolyte design for high‐energy and reversible Mg–S batteries.