Cu-doping improves the cyclic stability of P2-type Na_0.67Ni_0.33Mn_0.67O₂

P2-type Na_0.67Ni_0.33Mn_0.67O₂ garners significant interest in the research community owing to its impressive electrochemical performance, including a high operating voltage of 3.8 V and a substantial cyclic specific capacity of 173 mAh g⁻¹. Despite its promising electrochemical properties, the operational viability of P2–Na_0.67Ni_0.33Mn_0.67O₂ cathodes is significantly constrained by voltage-activated crystalline reconfiguration between P2 and O2 polymorphs. To address this issue, this study prepares a series of Cu-doped cathode materials, Na_0.67Ni_0.33Mn_0.67-xCu_xO₂ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10), using a co-precipitation method. X-Ray Diffraction shows that Cu-doping enlarges the unit cell, shortens TM-O but lengthens Na–O bonds. Other material phase characterization indicate that Cu is evenly doped in the material structure. The Na_0.67Ni_0.33Mn_0.61Cu_0.06O₂ material exhibits higher cyclic stability (while the undoped sample retains only 14.15 % capacity after 200 cycles at 100 mA g⁻¹, the modified material maintains 73.51 % of its initial capacity under identical conditions). Electrochemical characterization through charge-discharge cycling and Cyclic Voltammetry tests reveals that Cu doping stabilizes the structure by hindering the P2→O2 phase change. Meanwhile, Electrochemical Impedance Spectroscopy tests indicate that Cu doping effectively improves the charge exchange kinetics of the material. In-situ XRD demonstrates that Cu substitution kinetically suppresses the voltage-induced polymorphic transformation during electrochemical cycling.