Achieving Efficient Magnesium-Sulfur Battery Chemistry via Polysulfide Mediation

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 approximate to 1500 mAh g(-1) and decreases the sulfur overpotential from >600 to approximate 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.