Improving the Na_0.67Ni_0.33Mn_0.67O₂ Cathode Material for High-Voltage Cyclability via Ti/Cu Codoping for Sodium-Ion Batteries

P2-type Na0.67Ni0.33Mn0.67O2 cathode materials that show high discharge voltage and theoretical specific capacity have attracted extensive research, although the problem of rapid capacity decay under high voltage needs to be overcome. Here, a series of Na0.67Ni0.33–xCuxMn0.67–yTiyO2 cathode materials were synthesized that had good cyclic stability and rate performance at a high voltage of 4.5 V. The combined analyses of Rietveld refinement X-ray diffractometer (XRD), X-ray photoelectron spectroscope (XPS), Raman, and transmission electron microscope (TEM) showed that Ti4+ and Cu2+ had been successfully incorporated into the material crystal lattice. The Ti/Cu dual-doping materials operated at a high mid-voltage of ∼3.2 V vs Na/Na+ and exhibited a reversible capacity of 93 mA·h·g–1 at 5C. The voltage step of Ti4+/Cu2+-doped materials at ∼4.2 V was clearly suppressed with increased Cu content, and NNMT-0.14Cu materials exhibited an initial discharge capacity of 153.2 mA·h·g–1 owing to Cu contributing to reversible capacity based on Cu2+/Cu3+. Galvanostatic intermittent titration technology measurements showed that’ the Na+ mobility of NNMT-0.14Cu materials was improved.