Understanding the anchoring effects of metal ions and organic ligands in two-dimensional conductive metal-organic frameworks used for lithium-sulfur batteries

High performance cathode materials are crucial for the fabrication of lithium-sulfur (Li-S) batteries which possess ultrahigh theoretical capacity and energy density. An ideal cathode of Li-S batteries should be conductive and effectively prevent the dissolution of intermediate lithium polysulfides into the electrolyte solvents. Here, we explored the two-dimensional (2D) conductive metal-organic frameworks (MOFs) with a chemical formula of M3(C6X6)2 (M = Co, Ni, Cu; X = NH, O, S) as promising anchoring materials of Li-S batteries by using firstprinciples theory. The results suggest that three hexaaminobenzene-derived M3(C6N6H6)2 nanosheets (M = Co, Ni, and Cu) can adsorb Li2Sn clusters via the Li-N and S-M interactions in a moderate binding strength, with binding energies ranging from -1.29 to -2.49 eV. For the rate-determining step of sulfur reduction reaction, Co3(C6N6H6)2 and Ni3(C6N6H6)2 exhibit lower Gibbs free energies (0.76-0.77 eV) than that of Cu3(C6N6H6)2 (1.23 eV). Furthermore, we found that organic ligands of MOFs also significantly affect the anchoring of Li2Sn clusters. The Cu3(C6O6)2 film strongly binds with Li2Sn that causes a structure decomposition, whereas the Cu3(C6S6)2 film has a weak interaction with soluble Li2Sn. Based on the results of anchoring strength, catalytic performance, and conductivity, we can conclude that Co3(C6N6H6)2 and Ni3(C6N6H6)2 films are effective sulfur host materials, which can prevent the shuttle effect and promote the reaction of sulfur reduction. These results demonstrate that both the metal atoms and organic ligands can serve as active anchoring sites, offering new insights for the design of cathodes of Li-S batteries.