Stability and Semiconducting Behavior of Magnesium Polysulfides by Nonequivalent sp³ Hybridizations

The Mg/S battery has attracted enormous interest in recent years due to its high theoretical capacity, low cost, and high security. However, the understanding of many intermediate magnesium polysulfides in the Mg/S battery remains elusive. Combining extensive structural search and first-principles calculations, we investigate the phase stability, structural character, and electronic structure of magnesium polysulfides in a wide range from MgS to MgS₈. The pyrite-type MgS₂ (space group: Pa3̅) is predicted to be stable. Five magnesium polysulfides, MgS (x = 3, 4, 5, 6, and 8), are found to be metastable, with formation enthalpies slightly above the convex hull. S₂ dimer, "V"-like S₃, and highly distorted S chains are found for the polysulfides with bond lengths close to or slightly longer than S₈ and bond angles similar to S₈. A wide range of band gaps (0.77-2.82 eV) are revealed for the polysulfides due to the contribution of the nonequivalent sp³ hybridization of the S atoms in S^2-. Our results can help to further understand the electrochemical process in the Mg/S battery.