Mg Anode Passivation Caused by the Reaction of Dissolved Sulfur in Mg-S Batteries

As Li-ion battery optimization approaches theoretical limits, interest has grown in designing next-generation batteries from low-cost earth-abundant materials. Mg-S batteries are promising candidates, exhibiting widespread abundance of elemental precursors and a relatively large theoretical energy density albeit at lower cell voltage. However, Mg-S batteries exhibit poor reversibility, in part due to interactions between dissolved polysulfides and the Mg anode. Herein, we employ electrochemical experiments using Ag₂S quasi-reference electrodes to probe the interactions between Mg anodes and dissolved polysulfides. We show that Mg²⁺ reduction (charging) is impeded in the presence of polysulfides, while Mg metal oxidation (discharging) remains facile. Large reduction overpotentials arise due to the formation of a passivation layer on the anode surface, likely composed primarily of MgS. The passivation layer is removed under oxidative conditions but quickly reforms during reduction. We discover that dissolved S₈ influences the rate of MgS formation by shifting the polysulfide disproportionation equilibria. Shorter-chain polysulfides react more readily than longer-chain polysulfides at the Mg electrode, and thus, film formation is mediated by the electrochemical generation of shorter-chain polysulfide species.