Abstract: A series of Cu-doped layered P2-type Na_0.67Ni_0.33-xCu_xMn_0.67O₂ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25) with regular morphology and smooth surface were synthisized by sol-gel method. The morphology, structure and composition of those materials were characterized using SEM, XRD, EDS and XPS technologies, and their electrochemical properties were investigated by cyclic voltammetry and constant current charge-discharge methods through using them as cathode materials for sodium ion batteries. The optimum doping ratio was obtained. It is found that doping did not change the layered structure and morphology of the materials. Those materials exhibit good cycling stability and multiplicative performance due to the introduction of highly electrochemically active Cu²⁺ as a substituent and the increase of the surface active sodium storage sites. In the voltage range of 2.0 V to 4.3 V and a multiplicity of 0.1 C, the initial discharge specific capacity of Na_0.67Ni_0.18Cu_0.15Mn_0.67O₂ (with Cu doped ratio of 0.15) has an initial discharge specific capacity of 126.74 mAh/g and a capacity retention rate of 79.10% after 100 cycles, which is 50.92% higher than that of the undoped material. The enhanced electrochemical properties of the material are attributed to the insertion of Cu²⁺ into the transition metal layer, and the radius of Cu²⁺ (0.73 ) is larger than that of Ni²⁺ (0.69 ), which widens the transition metal layer spacing and provides channels for Na⁺ diffusion, and in turn increases the Na⁺ diffusion rate. When charged to high voltage, the Na⁺ vacancy generation during Na⁺ de/embedding process is inhibited, thus the crystal structure of the material is stabilized, the P2-O2 phase transition is suppressed, and the cycling stability is improved.

Key words: cathode material for sodium ion battery, sol-gel method, Cu doping, Na_0.67Ni_0.33Mn_0.67O₂, layered oxide, cycling stability