Modulating the 3d Electron Density of Transition Metal Sites via Atomic Engineering for Advanced Lithium−Sulfur Batteries

Abstract Precisely constructing atom skeletal editing engineering of organometallic macrocyclic molecules and thus accelerating sulfur redox kinetics is promising but challenging in lithium−sulfur (Li−S) batteries. The understanding of the metal center and the peripheral ligand function is largely restricted, especially for the correlation between the electronic structure of the metal and reaction mechanism. The transition metal sites from the organometallic molecules provide the catalyst platforms with tunable electronic structures. In this work, the well-defined cobalt-based complexes are designed and synthesized for Li−S batteries with remarkably improved performance. Through substituting the carbon with a nitrogen atom in the molecular skeleton, the microenvironment of Co−N4 sites is modulated at the atom level. The molecular structures of Co-based complexes are demonstrated by a series of characterization, when Coulombic attraction from the introduced nitrogen atoms on skeletons drives more charge transfer from the Co center to peripheral skeletons. Consequently, the decreased 3d electron density of Co−N4 sites enables strong d−p hybridization between Co 3d and S 3p orbitals, thereby accelerating sulfur redox kinetics. Cells with optimized Co−N4 sites show impressive electrochemical performance. This work serves as a pioneering step toward enhancing the fundamental comprehension and facilitating the development of metal−N4 sites in Li−S batteries.