Conformational Preference of Lithium Polysulfide Clusters Li2Sx (x = 4-8) in Lithium-Sulfur Batteries

Abstract

Structures are of fundamental importance for diverse studies of lithium polysulfide clusters, which govern the performance of lithium-sulfur batteries. The ring-like geometries were regarded as the most stable structures, but their physical origin remains elusive. In this work, we systematically explored the minimal structures of Li₂S_x (x = 4-8) clusters to uncover the driving force for their conformational preferences. All low-lying isomers were generated by performing global searches using the ABCluster program, and the ionic nature of the Li···S interactions was evidenced with the energy decomposition analysis based on the block-localized wave function (BLW-ED) approach and further confirmed with the quantum theory of atoms in molecule (QTAIM). By analysis of the contributions of various energy components to the relative stability with the references of the lowest-lying isomers, the controlling factor for isomer preferences was found to be the polarization interaction. Notably, although the electrostatic interaction dominates the binding energies, it contributes favorably to the relative stabilities of most isomers. The Li⁺···Li⁺ distance is identified as the key geometrical parameter that correlates with the strength of the polarization of the S_x^2- fragment imposed by the Li⁺ cations. Further BLW-ED analyses reveal that the cooperativity of the Li⁺ cations primarily determines the relative strength of the polarization.

Scheme 1. (a) Ring-like Structure of Li₂S_x (x = 4–8) and (b) Electrostatic Compression Model

Figure 1

Optimized geometries of (a–e) global minima of S_x^2– dianions (x = 4–8) with charges derived from the natural population analysis, (f–j) global minima of Li₂S_x (x = 4–8) clusters, (k–m) additional low-lying isomers of Li₂S₄, and (n, o) chain-like structures of Li₂S₇ and Li₂S₈.

Figure 2

Comparison of optimal structures of electron-delocalized (in yellow) and electron-localized states (in gray) for the lowest-lying (a) Li₂S₄, (b) Li₂S₅, (c) Li₂S₆, (d) Li₂S₇, and (e) Li₂S₈, with the corresponding RMSD values denoted.

Figure 3

Relative values of all energy components in the BLW-ED analyses for (a) Li₂X₄ and Li₂X₅, (b) Li₂X₆, (c) Li₂X₇, and (d) Li₂X₈.

Figure 4

(a) Correlation between the polarization energy and the Li···Li distance for all low-lying isomers of each Li₂S_x (x = 4–8) cluster. (b) Correlation between the polarization energy and its cooperative component and (c) correlation between cooperative components of total interaction and polarization energies of all isomers.

Figure 5

Electron density difference (EDD) maps representing electron density variations (polarizations) within the S_x^2– fragment induced by an individual Li⁺ cation for isomers (a) 7-29, (b) 8-37, and (c) 6-13 (the red color means a gain of electron density, while the blue color represents a loss of electron density with the isovalue of 0.001 e Å^–3 selected).