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
(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
x
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
x
2–. Our results can help to further understand the electrochemical
process in the Mg/S battery.