FeCoN₆ Sites Unlock Superior Sodium-Ion Storage Through Synergizing Capture-Release and Orbital-Mediated Charge Delocalization

Sodium-ion hybrid capacitors (SHCs) are severely hindered by sluggish sodiation kinetics in conventional anodes. Single-atom sites, though promising, suffer from localized d-orbitals that induce overly strong Na⁺ binding, creating a kinetic bottleneck. Herein, we design heteroatomic FeCo dual-atom sites (FeCoN₆) that unlock a dual-mechanism synergy for fast and durable Na⁺ storage. Theoretical-experimental evidence confirms that strong d–d orbital coupling induces an orbital-mediated charge-delocalization (OMCD) effect, which downshifts the d-band center to moderate Na⁺ binding affinity. Meanwhile, the intrinsic heteroatomic nature of the FeCoN₆ sites provides a stepped energy landscape for optimized Na⁺ capture-release pathways. This synergy between electronic OMCD modulation and the kinetic capture-release model significantly lowers the diffusion barrier. Consequently, the FeCo dual-atom nitrogen-doped carbon anode exhibits dominant pseudocapacitive kinetics, superior rate capability (225 mAh g⁻¹ at 10 A g⁻¹), and exceptional durability. The full SHC delivers a high energy density of 165 Wh kg⁻¹ at 23 W kg⁻¹, retains 108 Wh kg⁻¹ at 9424 W kg⁻¹, and achieves 90% capacity retention over 10,000 cycles. This study establishes that engineering heteroatomic sites to leverage both intrinsic functional heterogeneity and electronic delocalization is a powerful strategy to overcome kinetic limitations in energy storage.