On Sulfur Reduction Mechanism in Li-S Cells. Formation of Li2S8
Authors: Khamitov Edward , Kuzmina Elena , Kolosnitsyn Vladimir 
Published: February 2020

The results of quantum-chemical simulation of the first stage of the electrochemical reduction of sulfur S8 to Li2S8 are presented. The thermal effects of the formation of all possible intermediates for the electrochemical reduction of sulfur to lithium octasulfide are estimated. The sequence of these elementary reactions is established. The structure and properties of the resulting intermediates are characterized in AIMAll programm.

Quantum chemical calculations were performed using the Gaussian09 software package. The search for equilibrium geometrical parameters and the calculation of the energy characteristics of sulfur S8 and possible intermediate products of its reduction were carried out without taking into account solvation. The optimization procedure and the calculation of the vibrational problem were performed using the TPSS/aug-cc-pVTZ method for the correct description of anions. Electronic correlation was taken into account by a single point calculation at the level of perturbation theory of combined CCSD clusters (CCSD/aug-cc-pVTZ). Structures with an unpaired electron were calculated using the unlimited Hartree Fock/Kohn Sham method.To construct the initial structures of the intermediates of the sulfur reduction reaction to lithium octasulfide containing lithium cations, quantitative estimates of the electrostatic potential were carried out in MultiWFN software package.

Two parallel processes with the participation of the S8 molecule was simulated: electron gain and the formation of complexes with lithium cations. According to the calculations, a deformation of the S8 structure is observed when electrons are added. First electron gain reaction leads to DS8
- anion formation. S2-S8 and S4-S5 bond lengths increases compared to S8 molecule by 0.285 Å; S1-S2 and S5-S6 bonds are shortened by 0.022 Å; S3-S4 and S7-S8 bonds are shortened by 0.017 Å; S1-S3 and S6-S7 increase by 0.014 Å. When a second electron is added, the tendency to change bond lengths remains, but the structure becomes more symmetrical. Thus, in the DS8
- anion there are 4 pairs of equivalent bonds, and in the SS8
-2 and TS8
-2 dianions, one pair of enlarged bonds and 4 pairs of bonds shortened compared to the S8 molecule. In the case of dianions, the final product is a complex of two S4
- anions.

The addition of lithium cations to the four states of sulfur SS8, DS8
-, SS8
-2 and TS8
-2 leads to the following changes: when adding the first Li+ cation to SS8, the initial symmetry of the sulfur ring does not change, the S-S bond length increases by 0.010 Å. The lithium atom forms four equivalent bonds with sulfur (Li-S) of 2.692 Å. The subsequent addition of the second lithium cation leads to the deformation of the structure and the violation of symmetry. The addition of lithium cations to the DS8
- and SS8
-2 anions leads to the breaking of one of the S-S bonds and the formation of Li-S bonds, while the polysulfide chain is not broken. In contrast to the DS8
- and SS8
-2 anions, in the TS8
-2 anion, changes in the initial sulfur ring are more significant - when the first lithium cation is added, two S-S bonds break to form a complex (S3-Li-S5)-2, and with the addition of two cations, a complex (S4-Li2-S4)-2 is formed.

According to AIM analysis, the properties of the bonds of all the particles under study are established, in particular, the electron density (ρ) and the bond delocalization index (DI). Thus, in the S8 molecule, all bonds are equivalent since ρ and DI for all bonds are 0.131401 and 1.3, respectively. When an electron is accepted as a sulfur molecule, the strain of the S–S bonds is observed and, in addition to the bond lengths, the ρ and DI values change in a similar way. It is noteworthy that in the triplet form of the dianion, despite the fact that the length of the breaking S-S bond is longer than in the singlet form, DI is 0.2 more than in the singlet form, indicating a stabilizing factor.

In the framework of the ideal gas model, the most probable reaction path for sulfur reduction was established based on thermodynamic calculations: the first act is the interaction of sulfur S8 with the Li+ cation; the second is the electron gain by the particle SLiS8
+; the third step is the electron gain by the DLiS8
- and the final stage is the formation of the Li2S8 molecule.

This work was financially supported by the Russian Science Foundation (project No 17-73-20115 ), Russia.

Figure 1