d–d/p Orbital Hybridization in Symmetry-Broken Co–Y Diatomic Sites Enables Efficient Na–S Battery

Despite advances of single-atom catalysts (SACs) in sodium–sulfur (Na–S) batteries, their symmetric coordination geometry (e.g., M–N4) fundamentally restricts orbital-level modulation of sulfur redox kinetics. Herein, we demonstrate that hetero-diatomic Co–Y sites with Co–N4–Y–N4 coordination on N-doped carbon (Co–Y/NC) break the M–N4 symmetry constraint through d–d orbital hybridization, which is confirmed by an implementation of advanced characterizations, including the high-angle annular dark-field scanning transmission electron microscopy and x-ray absorption fine structure spectroscopy. In practical operation, the Co–Y/NC@S cathode with 61% sulfur mass fraction delivers a superior capacity (1,109 mAh/g) at 0.2 A/g, outperforming that of Co or Y SAC and further setting a new benchmark of diatomic catalysts for Na–S battery systems. Furthermore, the theoretical calculations show a hybridization-induced d-band splitting energy (ΔE = 0.5 eV), which induces electron-deficient Y sites for polysulfide adsorption (Na2S6) and electron-rich Co sites for S–S scission (barrier energy = 0.28 eV) via the d-p orbital hybridization of an asymmetric configuration. Our work establishes a strategy based on rare-earth-transition metal orbital hybridization to design asymmetric active sites for promoting multielectron sulfur redox reactions.