Fe‐Fe ₃ O ₄ /Fe ₇ S ₈ @CNSs Heterostructure Boosts Fast Reaction Kinetics for Ultrahigh‐Performance Sodium‐Ion Battery Anodes

Abstract Iron‐based oxides and sulfides are attractive anode materials for sodium‐ion batteries (SIBs) due to their high theoretical capacities and natural abundance. Nevertheless, their practical application remains hindered by significant volume expansion and sluggish charge transport kinetics during electrochemical cycling. Herein, a rationally designed Fe‐modified Fe ₃ O ₄ /Fe ₇ S ₈ heterostructure anchored on MOF‐derived N, S‐doped carbon (Fe‐Fe ₃ O ₄ /Fe ₇ S ₈ @CNSs) is synthesized via a scalable surface sulfidation strategy. Kinetic analyses and density functional theory (DFT) calculations demonstrate that the heterostructure and Fe modified synergistically enhance Na⁺ adsorption energy, accelerate charge transfer, and promote ultrafast redox reactions. When evaluated as anode materials, Fe‐Fe ₃ O ₄ /Fe ₇ S ₈ @CNSs exhibits exceptional performance, including an ultrahigh rate capability (423 mAh g ⁻¹ at 50 A g ⁻¹ ) and remarkable long‐term cyclability (326 mAh g ⁻¹ after 1000 cycles at 10 A g ⁻¹ ). In situ X‐ray diffraction (XRD) analysis demonstrates a highly reversible conversion reaction mechanism between Fe ₃ O ₄ and Fe ₇ S ₈ phases during Na ⁺ (de)intercalation. Impressively, a full‐cell assembled with Na ₃ V ₂ (PO ₄ ) ₃ cathode delivers a reversible capacity of 149.8 mAh g ⁻¹ after 2000 cycles at 10 A g ⁻¹ , underscoring its practical energy storage applications. This work not only presents a scalable synthesis of heterostructured anodes with ultrafast kinetics but also deciphers the mechanistic role of interfacial engineering in achieving ultrahigh‐performance SIBs.