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

Seiji Tsuzuki, Tomoaki Kaneko, Keitaro Sodeyama, Yasuhiro Umebayashi, Wataru Shinoda, Shiro Seki, Kazuhide Ueno, Kaoru Dokko, Masayoshi Watanabe

Phys Chem Chem Phys, 23 6832-6840 (2021).

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_n^2-, [LiS_n]^- and Li₂S_n (n = 1-8) were calculated at the CCSD(T)/cc-pVDZ//MP3/cc-pVDZ level. The heats of reduction reactions of S₈ and Li₂S_n with Li in solid phase were estimated from the calculated energies and sublimation energies. The estimated heats of redox reactions show that there are several redox reactions with nearly identical heats of reactions, 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 reactions for the formation of Li₂S_n (n = 2-6 and 8) from S₈ normalized as 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. This suggests that a two-step discharge curve, where the reduction reactions for the formation of Li₂S_n (n = 2-6 and 8) from S₈ occur first, followed by the formation of Li₂S, is observed, if reduction reactions with large heat generation are favored.