Stabilizing lattice oxygen interface with dual-functional NiFe2O4 coating and Ni/Fe co-doping strategy towards high-performance sodium-ion layered oxide cathodes

P2-Na 0.67 Mn 0.8 Fe 0.1 Ni 0.1 O 2 (MNF811) has garnered considerable attention due to its low cost and high energy density, while the poor structural and air stability and lattice oxygen evolution lead to undesirable electrochemical performance. Here, a NiFe 2 O 4 surface coating triggering a Ni/Fe co-doping strategy has been reported to suppress lattice oxygen evolution, eliminate poor air stability and mitigate the detrimental P2-OP4 phase transition. Furthermore, the NiFe 2 O 4 coating layer with air stability protects P2-Na 0.67 Mn 0.8 Fe 0.1 Ni 0.1 O 2 (MFN811@NF) cathode from reacting with moist air, and endows the P2-Na 0.67 Mn 0.8 Fe 0.1 Ni 0.1 O 2 electrode with three-dimensional fast sodium-ion diffusion channels and stable cathode/electrolyte interface (CEI), benefiting from the NiFe 2 O 4 coating layer. Ni/Fe co-doping by increasing the hybridization of the O(2p)- transition metal (TM) (3d-eg*) states, stabilizes excited state lattice oxygen and suppresses the irreversible migration of TM ions and the Jahn-Teller effect, mitigating adverse phase transitions and inhibiting structural degradation. In comparison to the Na 0.67 Mn 0.8 Fe 0.1 Ni 0.1 O 2 electrode, the initial discharge capacity of the 10 % NiFe 2 O 4 -coated P2-Na 0.67 Mn 0.8 Fe 0.1 Ni 0.1 O 2 (MFN811@NF-10) cathode was 114.4 mA h g −1 in the voltage range of 1.5–4.3 V at a high current density of 5C, with a capacity retention rate of 66.7 % after 1000 cycles. Eventually, the ex-situ X-ray Powder Diffraction (XRD) and ex-situ X-ray photoelectron spectroscopy (XPS) revealed that NiFe 2 O 4 coating layer and Ni/Fe co-doping can prevent structural/chemical transformations and stabilize excited state lattice oxygen to realize high specific capacity. Density functional theory (DFT) calculations also confirm that Ni/Fe co-doping stabilizes lattice oxygen through the strengthening of transition metal–oxygen bonds. This dual modification strategy may provide crucial insights for the development of high-performance sodium-ion battery cathode materials.