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

Highlights: • 3d MPc theoretically perform better than H₂Pc as sulfur host in LSB. • The anchoring of Li₂S_x on MPc is largely depends on the formation of M–S bond. • TiPc has the best anchoring effect owing to the large energy overlap between Ti_d and S_p orbitals. • The formation of M–S bond weakens the strength of Li–S bond inside Li₂S_x. • The decomposition barrier of Li₂S on TiPc is the lowest compared with other H₂Pc/MPc. 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.