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).