P2-type Na0.67Ni0.33-x Cu x Mn0.67O2 as new high-voltage cathode materials for sodium-ion batteries

P2-Na0.67Ni0.33-x Cu (x) Mn0.67O2 (x = 0, 0.02, 0.04, 0.06, 0.08) cathode materials have been synthesized via acetate decomposition method. The elementary composition and crystal structure of the powders are studied in detail using inductively coupled plasma-atomic emission spectrometry (ICP-AES) and X-ray diffraction (XRD). XRD results demonstrate that Cu2+ ions have been incorporated into the crystal structure successfully and the P2-type structure remains unchanged after substitution. According to XPS data, Cu substitution does not change the valence states of Ni and Mn, whose predominant oxidation states in Na-Ni-Mn-O structure remains +2 and +4. The introduction of Cu2+ can effectively suppress P2-O2 phase transformation when charging to 4.5 V, and significantly improve rate performance and cyclic stability compared to the undoped material. The P2-Na0.67Ni0.27Cu0.06Mn0.67O2 sample can deliver an initial discharge capacity of 211.6 mAh g(-1) at 10 mAh g(-1) between 1.5 and 4.5 V, and a capacity retention of 93.9% after 10 cycles. Moreover, it can also deliver a discharge capacity of 115.2 mAh g(-1) at 100 mAh g(-1). In addition, electrochemical impedance spectroscopy (EIS) reveals that P2-Na0.67Ni0.27Cu0.06Mn0.67O2 cathode exhibits a higher electronic conductivity and faster sodium ion diffusion velocity than that of undoped sample. These results show that P2-Na0.67Ni0.27Cu0.06Mn0.67O2 is a promising high-voltage cathode material for sodium-ion batteries.