Theoretical Design and Study of a Single-Atom Catalyst in Lithium–Sulfur Batteries: Edge-Type FeN4 Active Site Electron Density Redistribution Driven by Heteroatoms

Lithium-sulfur (Li-S) batteries are considered to be the most promising next-generation high energy density storage systems. However, they still face challenges, such as the shuttle effect of lithium polysulfides (LiPSs) and slow sulfur oxidation-reduction kinetics. In this work, heteroatom (P and S)-doped edge-type Fe single-atom catalytic materials (FeN₄S₂/P₂-DG) for sulfur reduction reactions (SRRs) and sulfur oxidation reactions in Li-S batteries are investigated using density functional theory calculations. Theoretical analysis suggests that compared to planar Fe-N₄ fragments, the charge density accumulation around edge-type Fe-N₄ fragments in S- or P-doped structures is higher. Furthermore, the doping of P or S reduces the electron filling state of Fe_3d orbitals, leading to a decrease in electron occupancy in the antibonding orbitals, which is beneficial for the formation of d-p orbital hybridization, strengthening the anchoring strength of FeN₄P₂/S₂-DG for S₈/LiPSs. Specifically, FeN₄P_1,2-DG showed the lowest free energy barriers (0.57 eV) for SRRs and reduced the dissociation energy barrier of Li₂S from 1.85 eV (for planar FeN₄-G) to 0.96 eV during the charging process, demonstrating excellent catalytic ability. Additionally, this theoretical study provides further insights into the application of graphene-supported single-atom catalyst materials as anchoring materials for Li-S batteries.