Cu-doped layered P2-type Na0.67Ni0.33-xCuxMn0.67O2 cathode electrode material with enhanced electrochemical performance for sodium-ion batteries

A series of Cu-doped layered P2-type Na0.67Ni0.33-xCuxMn0.67O2 (x = 0, 0.05, 0.10, 0.15, 0.20, 0.33) were fabricated using a convenient solid-state method and studied as cathode materials for sodium-ion batteries. The microstructure and morphology of the cathode materials were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques. The electrochemical characteristics of Na0.67Ni0.33-xCuxMn0.67O2 samples have been investigated systematically and it shows good capacity retention, cycling stability and rate performance by introducing electrochemically active Cu2+ ions as substituents. When x= 0.15, the sample delivers an initial discharge capacity of 120 mAh g−1 at 0.1 C in the voltage range 2–4.3 V with a capacity retention of 78% after 200 cycles, and a reversible capacity of 62 mAh g-1 can be obtained at a high current rate of 20 C. Compared with the pristine compound, the enhanced electrochemical performance can be attributed to the Cu2+ inserted into the transition metal (TM) layer, which stabilizes the P2-phase structure against P2-O2 phase transition when charging to high voltage. Meanwhile, the presence of copper also contributes to the reversible capacity based on the Cu2+/Cu3+ redox reaction. This strategy can improve the cyclability and rate performance by enhancing the stability between TM layers.