Built-In Electric Field Enhanced Sodium Storage in FeO/Fe Homogeneous Heterojunction Confined by N-Doped Carbon Shells.

Iron oxide (FeO) 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. 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 FeO/Fe@N-doped carbon (FeO/Fe@CN) nanostructure is rationally designed, which integrates FeO/Fe homogeneous heterojunctions with a uniform nitrogen-doped carbon shell. The built-in electric field at the Fe-FeO 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 FeO/Fe@CN anode delivers a high reversible capacity of 336.9 mA h g at 0.1 A g, excellent rate capability with 244.7 mA h g at 2 A g, and remarkable cycling stability, retaining 76.4% capacity after 500 cycles. Furthermore, a full cell assembled with a NaV(PO) cathode exhibits a high energy density of 112.67 Wh·kg at 51.42 W·kg and outstanding cycling performance. This study offers a versatile strategy to unlock the potential of FeO for high-performance SIB anodes through heterojunction and interfacial engineering.