DFT Equilibrium Thermodynamics of Lithium Polysulfides Transformations in Sulfolane Solution

The structure and UV/Vis‐absorption spectra of Li 2 S n ( n = 2–8) in sulfolane solution (Sl), as well as the thermodynamic parameters of their com ‐ and dis proportionation, 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. Com proportionation of Li 2 S 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 Li 2 S 3 and Li 2 S 6 drop out of the general pattern: dis ‐ and com proportionation for them are allowed equally, which can lead to a smooth change in the concentrations of Li 2 S 8 , Li 2 S 4 , and Li 2 S 2 and, thus, provide a relatively constant potential in the low‐voltage part of the Li‐S battery discharge curve. UV/Vis‐absorption spectra of Li 2 S 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 Li 2 S n self‐association in Sl is relevant mainly for short‐chain structures ( n = 2–4); there is a dynamic equilibrium between different forms of Li 2 S n , and the solution is usually presented by a mixture of these forms.