Role of the Metal Atom in a Carbon-Based Single-Atom Electrocatalyst for Li-S Redox Reactions (vol 18, 2200395, 2022)

Carbon-based single metal atom catalysts (SACs) are being extensively investigated to improve the kinetics of the Li-S redox reaction, which is greatly important for batteries with cell-level energy densities >500 W h kg(-1). However, there are contradictory reports regarding the electrocatalytic activities of the different metal atoms and the role of the metal atom in Li-S chemistry still remains unclear. This is due to the complex relationship between the catalytic behavior and the structure of carbon-based SACs. Here, the catalytic behavior and active-site geometry, oxidation state, and the electronic structure of different metal centers (Fe/Co/Ni) embedded in nitrogen-doped graphene, and having similar physicochemical characteristics, are studied. Combining X-ray absorption spectroscopy, density functional theory calculations, and electrochemical analysis, it is revealed that the coordination-geometry and oxidation state of the metal atoms are modified when interacting with sulfur species. This interaction is strongly dependent on the hybridization of metal 3d and S p-orbitals. A moderate hybridization with the Fermi level crossing the metal 3d band is more favorable for Li-S redox reactions. This study thus provides a fundamental understanding of how metal atoms in SACs impact Li-S redox behavior and offers new guidelines to develop highly active catalytic materials for high-performance Li-S batteries.