Tailoring the exposure of active facets of FeNCN towards enhanced pseudocapacitive behavior for sodium storage

Iron carbodiimide (FeNCN) anode demonstrates significant potential for rapid sodium-ion storage owing to its high reaction activity and near-metallic conductivity. However, further development of FeNCN is hindered by inherent structural instability and ambiguous structure-kinetics correlation. In this study, FeNCN crystallites with selectively exposed (002) and {010} facets were precisely engineered and synthesized. Notably, the sodium storage kinetics and electrochemical performance of FeNCN exhibit facet-dependent variations. Polyhedral-FeNCN (P-FeNCN) dominated by {010} facets exhibited a pseudocapacitance-driven storage mechanism and delivered exceptional rate capability (372 mAh/g at 5 A/g) and long cyclability (95.8 % capacity retention after 300 cycles at 0.5 A/g). In contrast, sheet-like FeNCN (S-FeNCN) with predominant (002) facet exposure displayed diffusion-limited kinetics due to sluggish ion diffusion rate. Crucially, time-resolved operando XRD analysis and DFT simulation bridge this performance gap to mechanistic origins: FeNCN as an intercalation-conversion type anode, the solid-state diffusion is the rate-determining step during charge/discharge process. Active {010} facets possess numerous broad hexagonal tunnels, coupled with a low diffusion barrier of 0.168 eV along 〈010〉 directions. This unique architectural configuration enables rapid sodium-ion transport, thereby shifting the diffusion-controlled kinetics to intercalation-pseudocapacitive behavior. This discovery establishes active facet exposure as a storage kinetic switch, offering a generalized paradigm for optimizing the rate performance and stability of sodium-ion batteries.