Revealing the Flexible 1D Primary and Globular Secondary Structures of Sulfur-Rich Amorphous Transition Metal Polysulfides

Sulfur-rich transition metal polysulfides MS₅ (M=Mo, W) are synthesized by a low-temperature solution method from corresponding carbonyls M(CO)₆ and elemental sulfur. Extensive characterization reveals that all sulfur atoms are assembled into disulfide ligands (S−S) within the structure of the amorphous spherical particles. Their thermodynamic stabilities are estimated for the first time using density functional theory (DFT) calculations, indicating two stable chain models composed either of binuclear [M₂S₈] or trinuclear [M₃S₁₂] fragments linked through S−S units. Molecular dynamics (MD) DFTB simulation proves that the S−S bridges predetermine the supreme flexibility of the polysulfide chains as primary structures of MS₅ and their globular secondary arrangements. Interestingly, this type of structural organization is reminiscent of that for classical polymers. Thus, the reasons for MS₅ forming exclusively as amorphous phases are uncovered, which may be extended to many other sulfur-rich polysulfides. The potential of these materials as increased capacity cathodes for lithium-ion batteries is shown.