CuO-Coated and Cu²⁺-doped Co-modified P2-type Na_2/3[Ni_1/3Mn_2/3]O₂ for sodium-ion batteries

Layered P2-type CuO-coated Na2/3[Ni1/3Mn2/3]O2 (NNMO@CuO) with excellent rate capability and cycling performance was investigated as a sodium-ion battery cathode material for the first time. The NNMO@CuO cathode material combines the advantages of CuO coating and Cu2+ doping. Transmission electron microscopy (TEM) images, TEM elemental line scan analysis and ex situ scanning electron microscopy (SEM) images show that CuO has been successfully coated on the particle surface uniformly, and that this CuO layer effectively suppresses the exfoliation of the metal oxide layers and unfavorable side reactions. Furthermore, Cu2+ is also partially incorporated into the host structure, according to the X-ray diffraction (XRD) patterns and refinement results. Although incorporated Cu2+ does not take part in the redox reactions of the battery cell, the refinement results indicate that the d-spacing of the Na+-ion diffusion layer is enlarged due to Cu2+ doping in the crystal structure, which results in better Na+ kinetics. Thus, the CuO-coated cathode material shows prominent cycling performance and rate capability. We believe that this CuO-coating and Cu2+-doping co-modification strategy provides a promising approach to designing advanced cathode materials for sodium-ion batteries.