Electric‐Field‐Coupled Heterointerface Engineering via Li/F Co‐Doping for Enhanced Rate Capability and Interfacial Stability of Fe/Mn‐Based Layered Sodium Cathodes

P2‐type Fe/Mn‐based layered oxide materials have long been plagued by structural degradation, air sensitivity, and interfacial instability, which severely restrict their application in sodium‐ion batteries. Herein, we propose an in situ modification strategy integrating bulk Li/F co‐doping with a surface heterojunction coating and successfully synthesize Na 0.67 Fe 0.35 Mn 0.5 Li 0.15 O 1.93 F 0.07 (LMO@NFMLOF). IDPC‐STEM reveals the heterogeneous structure comprising distinct inner and outer regions. DFT calculations confirm the presence of a built‐in electric field at the heterointerface. The experimental results demonstrate that bulk Li/F co‐doping enhances crystal stability, while the built‐in electric field generated by the surface heterojunction accelerates Na + kinetic processes, and the coating further improves air and interfacial stability. Consequently, the LMO@NFMLOF cathode delivers a discharge capacity of 110 mAh g − 1 after 200 cycles at 2C. Remarkably, it maintains a discharge capacity of 85.6 mAh g − 1 after 300 cycles at 5C, demonstrating outstanding rate capability and cycling stability. This synergistic bulk‐surface optimization provides a new approach for designing high‐rate layered oxide cathodes.