Balancing Electronic Spin State via Atomically-Dispersed Heteronuclear Fe–Co Pairs for High-Performance Sodium–Sulfur Batteries

Room-temperature sodium-sulfur (Na-S) batteries are emerging as a promising next-generation energy storage technology, offering high energy densities at low cost and utilizing abundant elements. However, their practical application is hindered by the shuttle effect of sodium-polysulfides and the sluggish kinetics of sulfur redox reactions. In this study, we demonstrate a heteronuclear diatomic catalyst featuring Fe and Co bimetallic sites embedded in nitrogen-doped hollow carbon nanospheres (Fe-Co/NC) as an effective sulfur host at the cathode of Na-S batteries. Aberration-corrected high-angle annular dark field scanning transmission electron microscopy demonstrates the presence of isolated Fe-Co atomic pairs, while synchrotron radiation X-ray absorption fine structure analysis confirms the (Fe-Co-N₆) coordination structure. Density functional theory calculations show that the introduction of Fe atoms induces electron delocalization in Co(II), shifting the electronic configuration from a low-spin to a higher-spin state. This shift enhances the hybridization of the Co dz² orbitals with the antibonding π orbitals of sulfur atoms within the sodium sulfide species that accelerates their catalytic conversion. As a result, Fe-Co/NC-based cathodes exhibit excellent cycling stability (378 mAh g^-1 after 2000 cycles) and impressive rate performance (341.1 mAh g^-1 under 5 A g^-1).