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

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, S˙, S, [LiS] and LiS (n = 1-8) were calculated at the CCSD(T)/cc-pVTZ//MP3/cc-pVDZ level. The heats of reduction reactions of S and LiS 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 LiS (n = 2-6 and 8) from S normalized as a one electron reaction is more exothermic than that for the formation of LiS directly from S, while the reduction reactions for the formation of LiS from LiS are slightly less exothermic than that for the formation of LiS directly from S. If the reduction reactions with large exotherm occur first, these results suggest that the reduction reactions forming LiS (n = 2-6 and 8) from S occur first, then LiS is formed, and therefore, a two-step discharge-curve is observed.