Exploring the anchoring effect and catalytic mechanism of 3d transition metal phthalocyanine for S8/LiPSs: A density functional theory study

The strong anchoring effects for lithium polysulfides (LiPSs), as well as fast redox kinetics, are of great significance and necessity for the commercial development of lithium-sulfur batteries (LSBs). Metal phthalocyanines (MPc), with special M-N4 moieties, are a class of macrocyclic compounds that have the potential to be employed as sulfur host materials in LSBs. Herein, a series of 3d transition metal phthalocyanines (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) were systematically investigated for the anchoring effects and catalytic conversion activities for S8/LiPSs. The binding energy analysis demonstrates that most of MPc (except NiPc and CuPc) have stronger binding strength for S8/LiPSs than metal-free phthalocyanine (H2Pc), mainly due to the strong interaction between transition metals and S atoms. Meanwhile, the formation of the M-S bond shows a weakening effect on the Li-S bonds of Li2S then boosts the decomposition of Li2S, and MPc also possesses a relatively lower lithium diffusion barrier than H2Pc. Moreover, MPc can also accelerate the multi-step conversions from S8 to Li2S by reducing the free energy of the rate-limiting reaction (Li2S2 to Li2S) in sulfur reduction reactions (SRR). Among these MPc, TiPc has the best performance in anchoring LiPSs, accelerating the decomposition of Li2S, and promoting SRR.

Automated summary

Metal phthalocyanines (MPc) can effectively anchor LiPSs, accelerate the decomposition of Li2S, and promote sulfur reduction reactions. Among these, TiPc performs the best in anchoring LiPSs, accelerating the decomposition of Li2S, and promoting sulfur reduction reactions.