Stable Crystalline Forms of Na Polysulfides: Experiment versus Ab Initio Computational Prediction

Abstract

For the design of light-metal-sulfur batteries and for the understanding of their performance, knowledge on the stable crystalline polysulfides is very important. We confronted experimental and ab initio crystal structure prediction studies on the stability of Na polysulfides. The selected evolutionary-based structure-prediction algorithm was able to quickly and correctly predict the thermodynamically stable crystalline forms of Na polysulfides with small unit cells. For Na polysulfides with large unit cells, the algorithm correctly proposed short unbranched polysulfide chains to be energetically favorite structural motifs, but could not find proper three-dimensional structures in the limited number of generations. Experimentally, the polysulfides were studied by X-ray diffraction and Na solid-state NMR spectroscopy. Complemented by calculations of the isotropic chemical shifts and quadrupolar coupling constants, NMR spectroscopy proved to be an excellent tool for the examination of Na polysulfides, because it allowed easy distinction and quantification of components in the samples.

Pressure‐Induced Stable Binary Compounds of Magnesium and Germanium

“…To search all potential stable ground‐state structures of the Mg–Ge system, an ab initio variable‐composition evolutionary method USPEX was performed at 50, 100, 150, and 200 GPa . Its success was demonstrated by determining the stable structures of the various compounds . All structural optimizations, calculations of the enthalpies, electronic calculations, and other structural property calculations were performed with the Vienna ab initio simulation package (VASP) .…”

“…[17][18][19] Its success was demonstrated by determining the stable structures of the various compounds. [20][21][22][23][24] All structural optimizations, calculations of the enthalpies, electronic calculations, and other structural property calculations were performed with the Vienna ab initio simulation package (VASP). [25] The Perdew-Burke-Ernzerhof generalized gradient approximation (GGA) [26] was chosen as the exchange-correlation functional, and projector-augmented wave (PAW) [27] potentials were adopted to describe the ionic potentials.…”

A First-Principles Exploration of NaxSy Binary Phases at 1 atm and Under Pressure

“…They found that the inclusion of dispersion, as implemented in Grimme's PBE-D2 functional, did not substantially influence their structural parameters nor their energies, in contrast to the results for elemental sulfur [19]. DFT calculations have concluded that, based upon the energy alone, β-Na 2 S 2 is somewhat more stable than α-Na 2 S 2 [19,20]. Crystal structure prediction (CSP) techniques coupled with DFT calculations have been employed to search for new ambient pressure phases [20,21].…”

“…The advantages of the Na-S battery include the fact that it is made from inexpensive materials, has a long cycle lifetime, and it can deliver high-energy densities [15][16][17][18]. Density functional theory (DFT) calculations [19][20][21] and experiments [20] have been carried out to study the ambient pressure phases with Na 2 S n , n > 1. Momida and co-workers only considered the known phases.…”

“…DFT calculations have concluded that, based upon the energy alone, β-Na 2 S 2 is somewhat more stable than α-Na 2 S 2 [19,20]. Crystal structure prediction (CSP) techniques coupled with DFT calculations have been employed to search for new ambient pressure phases [20,21]. Mali and co-workers computed the enthalpies of formation of the novel Cmme-Na 2 S 3 and C2-Na 2 S 6 phases, and found that they lay only slightly above the convex hull, suggesting these crystalline lattices may be metastable.…”