Fundamental Mechanistic Insights on the Peripherally Substituted Iron Phthalocyanine Selectively Catalyzing the Sulfur Redox Reactions

Abstract

The microenvironment of nitrogen-coordinated single metal (M−N_x) sites significantly impacts the electronic properties and the kinetics of sulfur species in lithium−sulfur (Li−S) batteries. However, accurately designing the M−N_x materials remains challenging, which is crucial for investigating the structure-function relationship and developing high-performance electrocatalysts. Compared with the traditional pyrolyzed M−N_x catalysts, the single-atom metal sites with precise microenvironment can be fabricated with molecularly dispersed MPc loaded on matrix. Herein, we modulate the d-band electronic states by tailoring the molecularly dispersed iron phthalocyanine (FePc) by means of donating/withdrawing (tetraamino, TA/tetranitro, TN) groups with amino-functionalized carbon nanotube (ACNT) as matrix. The static and dynamic properties between FePc derivatives and LiPSs are investigated by in-situ Raman spectra and quasi-in-situ XPS methods. Density functional theory (DFT) calculations further reveal the enhanced orbital hybridization of 3d_π-2p_x/y between Fe and S for FeTNPc@ACNT, which improves the reduction of long-chain polysulfides and the dissociation of Li₂S. Consequently, cells with FeTNPc@ACNT exhibit a high specific capacity of 1000.9 mA h g⁻¹ at 2 C, along with a decay rate of 0.041% after 1000 cycles. This study uncovers that peripheral ligand structure regulation selectively steers the redox kinetics in Li−S batteries.