Effect of Polysulfide Speciation on Mg Anode Passivation in Mg–S Batteries

Mg-S batteries are a promising next-generation system for beyond conventional Li-ion chemistry. The Mg-S architecture pairs a Mg metal anode with an inexpensive, high-capacity S₈ cathode. However, S₈-based cathodes exhibit the "polysulfide shuttle" effect, wherein soluble partially reduced S_x^2- species generated at the cathode diffuse to and react with the anode. While dissolved polysulfides may undergo reactions to form Li⁺-permeable layers in Li-S systems, the interfaces on Mg anodes are passivating. In this work, we probe the reactivity of various Mg polysulfide solutions at the Mg anode interface. Mg polysulfide solutions are prepared without any chelating agents to closely mimic conditions in a Mg-S cell. The polysulfides are synthesized by reacting Mg metal and S₈ in electrolyte, and the speciation is controlled by varying the Mg:S precursor ratio. S-poor precursor ratios produce magnesium polysulfide solutions with a higher proportion of short-chain polysulfides that react at the Mg anode faster than the longer-chain analogues. Anode passivation can be slowed by shifting the polysulfide equilibria toward longer-chain polysulfides through addition of S₈.