Fe-Single-Atom Incorporated Wood-Derived Anode with Fe─N─C/Fe₃C Structural Unit and Hollow Diffusion Sites for Enhanced Sodium-Ion Storage

Sluggish diffusion kinetics of Na⁺ drastically restrain the rate capability and capacitance of the anode for sodium-ion batteries (SIBs). Herein, a Fe single-atom strategy is employed to construct Fe─N₄─O₂ active sites closely coupled with Fe₃C species, establishing strong electronic interactions and, more importantly, an optimized coordination environment through precise tuning of their composition ratio with wood-derived nanoporous carbon (WNC) support. The charging Na⁺ through nanoporous carbon of Fe─N₄─O₂–WNC anode is revealed by electrochemical capacitive and charge–discharge studies to establish a reversible conversion and diffusion of Na⁺ supported by theoretical calculation of Na⁺ migration energy (eV) against the diffusion path. Fe─N₄─O₂–WNC anode, assembled with sodium foil as counter electrodes in a coin cell, exhibits a significant discharge-specific capacity of 318 mAh g⁻¹ at a current density of 50 mAg⁻¹. The electrochemical analysis support the role of Fe─N bonding in modulating the electronic environment of Na⁺ diffusion sites. The incorporation of Fe─N₄─O₂ in WNC results in 1) faster Na⁺ diffusion through hollow (H) sites, 2) stretching of the Fe─N bond during discharge cycles. In addition, Fe─N₄─O₂–WNC anode promises for the manufacturing of advanced SIBs from a renewable material and thereby enhancing the investigation of sodiophilic Fe─N sites.