High stability of Cu-doped O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ cathode material for sodium-ion battery

In this work, Cu-doped O3-Na[Fe_1/3Ni_1/3Mn_1/3]_1-xCu_xO₂ cathodes were studied to enhance the structure stability and air stability. XPS analysis reveals that Cu-doping changes the electronic structure of Mn, thus enhancing the electrochemical performance of the NFM. The Cu-doped O3-Na[Fe_1/3Ni_1/3Mn_1/3]_0.95Cu_0.05O₂ (NFMC-0.05) cathode remains a capacity of 81.6 mAh g⁻¹ at 10 C and keeps a capacity retention of 80.7% after 200 cycles at 0.5 C, while the undoped O3-NaFe_1/3Ni_1/3Mn_1/3O₂ (NFM) can only remain a capacity of 59.9 mAh g⁻¹ at 10 C and keep a capacity retention of 68.9% after 200 cycles at 0.5 C. The 9-day air-exposed NFMC-0.05 exhibits a higher capacity of 97.3 mAh g⁻¹ than the 9-day air-exposed NFM (58 mAh g⁻¹) at 0.1 C, showing greatly enhanced air stability. Moreover, Cu-doping reduces the Rct value and the polarization, leading to a significant improvement in electronic conductivity. These results confirm that a small quantity (5 at. %) of Cu-doping improves the structural stability and air stability of the material, leading to the excellent electrochemical performance.