Built‐In Electric Field Enhanced Sodium Storage in Fe₃O₄/Fe Homogeneous Heterojunction Confined by N‐Doped Carbon Shells

Iron oxide (Fe₃O₄) has attracted significant attention as a promising anode material for sodium-ion batteries (SIBs) due to its natural abundance, environmental benignity, and high theoretical capacity of 926 mA h g^-1. Nevertheless, its practical application is limited by intrinsic drawbacks, including low electrical conductivity, sluggish Na⁺ diffusion kinetics, and severe volume variation during cycling, leading to rapid capacity fading and poor rate capability. To address these issues, a novel Fe₃O₄/Fe@N-doped carbon (Fe₃O₄/Fe@CN) nanostructure is rationally designed, which integrates Fe₃O₄/Fe homogeneous heterojunctions with a uniform nitrogen-doped carbon shell. The built-in electric field at the Fe-Fe₃O₄ interface promotes charge redistribution and accelerates electron/ion transport, while the N-doped carbon shell enhances electrical conductivity and buffers mechanical stress during sodiation/desodiation processes. Benefiting from this synergistic structure, the Fe₃O₄/Fe@CN anode delivers a high reversible capacity of 336.9 mA h g^-1 at 0.1 A g^-1, excellent rate capability with 244.7 mA h g^-1 at 2 A g^-1, and remarkable cycling stability, retaining 76.4% capacity after 500 cycles. Furthermore, a full cell assembled with a Na₃V₂(PO₄)₃ cathode exhibits a high energy density of 112.67 Wh·kg^-1 at 51.42 W·kg^-1 and outstanding cycling performance. This study offers a versatile strategy to unlock the potential of Fe₃O₄ for high-performance SIB anodes through heterojunction and interfacial engineering.