Ab initio crystal structure prediction of magnesium (poly)sulfides and calculation of their NMR parameters

Abstract

Ab initio prediction of sensible crystal structures can be regarded as a crucial task in the quickly-developing methodology of NMR crystallography. In this contribution, an evolutionary algorithm was used for the prediction of magnesium (poly)sulfide crystal structures with various compositions. The employed approach successfully identified all three experimentally detected forms of MgS, i.e. the stable rocksalt form and the metastable wurtzite and zincblende forms. Among magnesium polysulfides with a higher content of sulfur, the most probable structure with the lowest formation energy was found to be MgS, exhibiting a modified rocksalt structure, in which S anions were replaced by S dianions. Magnesium polysulfides with even larger fractions of sulfur were not predicted to be stable. For the lowest-energy structures, Mg quadrupolar coupling constants and chemical shift parameters were calculated using the density functional theory approach. The calculated NMR parameters could be well rationalized by the symmetries of the local magnesium environments, by the coordination of magnesium cations and by the nature of the surrounding anions. In the future, these parameters could serve as a reference for the experimentally determinedMg NMR parameters of magnesium sulfide species.

Mechanistic Study of Magnesium–Sulfur Batteries

“…The observed isotropic chemical shift is characteristic for octahedrally coordinated Mg species and thus confirms that the material adopts the rock-salt crystalline form, as observed by the Rietveld analysis shown in Figure 6. 30,31 This form is the most stable crystalline form of MgS, more stable than the zincblende and wurtzite forms, and the only crystalline form in which Mg is octahedrally coordinated by S. In the MgS−rock-salt structure, 25 Mg nuclei should experience zero chemical shift anisotropy and zero electric quadrupolar interaction. The spinning sidebands, which can be seen in the 25 Mg MAS NMR spectrum of MgS chem in Figure 7, suggest that in this material MgS 6 octahedra are actually slightly distorted, leading to weak electric quadrupolar interaction (with the magnitude of the quadrupolar coupling constant below 100 kHz).…”

“…The maximum position clearly shows that Mg in this sample is tetrahedrally coordinated. 31 Because of the relatively poor quality of the spectrum (poor signal-to-noise ratio even though the spectrum was scanned for ∼2.5 days), one cannot distinguish whether broadening of the NMR signal is due to electric quadrupolar interaction or chemical shift anisotropy. Nevertheless, it is clear that the tetrahedral MgS 4 environment in this sample is more similar to the tetrahedral environment within the MgS−wurtzite form than to the tetrahedral environment of the MgS−zincblende form.…”

Insight into the Discharge Products and Mechanism of Room-Temperature Sodium–Sulfur Batteries: A First-Principles Study

“…The first-principles study of solid (crystalline) Na 2 S n ( n ≤ 3) consists of two parts: the prediction of crystal structures and the density functional theory (DFT) calculation based on those structures; already a large number of works have been done in this way. − The structural prediction approach in our work was performed using the structure swarm global optimization algorithms through the CALYPSO package, and a number of significant and meaningful works have recently been performed by using it, − especially in the fields of lithium-ion batteries and sodium-ion batteries. − The structures were searched with simulation cell sizes of 1–4 formula units; in each generation, 60% of them with lower enthalpies are selected to produce the next-generation structures by PSO, and 40% of the structures in the new generation are randomly generated. For most of the cases, the structure searching simulation for each calculation was stopped after the generation of 900–1500 structures (about 30–50 generations).…”

Thermodynamics and Kinetics of Sulfur Cathode during Discharge in MgTFSI2–DME Electrolyte

“…The phase transition from long‐chain polysulfide to MgS 2 (Equation ) may explain the potential plateau. Despite no Mg/S binary compounds with high sulfur content (MgS x , x = 2–8) are found experimentally, MgS 2 with modified rock salt structure is predicted by ab initio calculation while other polysulfides seem only exist in amorphous state . The liquid (long‐chain polysulfide) to solid (MgS 2 ) phase transition may also explain the potential plateau Stage III : Since no S 2 2− or S 2− can be found in the electrolyte, the entire reaction occurs in the solid state.…”