Thermodynamic aspect of sulfur, polysulfide anion and lithium polysulfide: plausible reaction path during discharge of lithium–sulfur battery

Abstract

The elucidation of elemental redox reactions of sulfur is important for improving the performance of lithium–sulfur batteries. The energies of stable structures of S_n, S_n˙⁻, S_n²⁻, [LiS_n]⁻ and Li₂S_n (n = 1–8) were calculated at the CCSD(T)/cc-pVTZ//MP3/cc-pVDZ level. The heats of reduction reactions of S₈ and Li₂S_n with Li in the solid phase were estimated from the calculated energies and sublimation energies. The estimated heats of the redox reactions show that there are several redox reactions with nearly identical heats of reaction, suggesting that several reactions can proceed simultaneously at the same discharge voltage, although the discharging process was often explained by stepwise reduction reactions. The reduction reaction for the formation of Li₂S_n (n = 2–6 and 8) from S₈ normalized as a one electron reaction is more exothermic than that for the formation of Li₂S directly from S₈, while the reduction reactions for the formation of Li₂S from Li₂S_n are slightly less exothermic than that for the formation of Li₂S directly from S₈. If the reduction reactions with large exotherm occur first, these results suggest that the reduction reactions forming Li₂S_n (n = 2–6 and 8) from S₈ occur first, then Li₂S is formed, and therefore, a two-step discharge-curve is observed.