DFT Equilibrium Thermodynamics of Lithium Polysulfides Transformations in Sulfolane Solution

The structure and UV/Vis-absorption spectra of Li₂S_n (n = 2–8) in sulfolane solution (Sl), as well as the thermodynamic parameters of their com- and disproportionation, have been studied using DFT to gain a deeper understanding of the nontrivial mechanistic steps underlying the S-shaped discharge voltage curve of the Li-S electrochemical system. Comproportionation of Li₂S_n was established using PBE0/6–311 + G(d,p) approximation with PCM to be thermodynamically favorable: the greater the difference in the oxidation states of the interacting forms, the more significant the Gibbs free energy drop. The transformations of Li₂S₃ and Li₂S₆ drop out of the general pattern: dis- and comproportionation for them are allowed equally, which can lead to a smooth change in the concentrations of Li₂S₈, Li₂S₄, and Li₂S₂ and, thus, provide a relatively constant potential in the low-voltage part of the Li-S battery discharge curve. UV/Vis-absorption spectra of Li₂S_n obtained using TD-DFT [TPSS/6–311 + G(d,p)] with PCM + explicit solvation were found to present allowed electronic transitions located from 30 000 to 50 000 cm⁻¹. It has also been observed that Li₂S_n self-association in Sl is relevant mainly for short-chain structures (n = 2–4); there is a dynamic equilibrium between different forms of Li₂S_n, and the solution is usually presented by a mixture of these forms.