First-principles investigation on multi-magnesium sulfide and magnesium sulfide clusters in magnesium-sulfide batteries

Abstract

Because of the abundance of magnesium and sulfur and their low cost, the development of magnesium sulfur batteries is very promising. In particular, the battery performance of nanoscale (MgS)_n clusters is much better than that of bulk sized MgS. However, the structures, stability, and properties of Mg_xS_y and (MgS)_n clusters, which are very important to improve the performance of Mg–S batteries, are still unexplored. Herein, the most stable structures of Mg_xS_y (x = 1–8, y = 1–8) and (MgS)_n (n = 1–10) are reliably determined using the structure search method and density functional theory to calculate. According to calculation results, MgS₃ and Mg₆S₈ may not exist in the actual charging and discharging products of magnesium sulfide batteries. The (MgS)_n (n ≥ 5) clusters exhibit intriguing cage-like structures, which are favorable for eliminating dangling bonds and enhancing structural stability. Compared to the MgS monomer, each sulfur atom in the clusters is coordinated with more magnesium atoms, thus lengthening the Mg–S bond length and decreasing the Mg–S bond activation energy. Notably, with the increase of dielectric constant of electrolyte solvent, compared to the DME (ε = 7.2), THF (ε = 7.6) and C₂H₄Cl₂ (ε = 10.0), Mg_xS_y and (MgS)_n clusters are most stable in the environment of C₃H₆O (ε = 20.7). It can delay the transformation of magnesium polysulfide to the final product MgS, which is conducive to improving the performance of Mg–S batteries. The predicted characteristic peaks of infrared and Raman spectra provide useful information for in situ experimental investigation. Our work represents a significant step towards understanding (MgS)_n clusters and improving the performance of Mg–S batteries.