Cu-substituted Na0.75Ni0.17Cu0.08Mn0.75O2 cathode with suppressing P2-O2 phase transition and air-stable for high-performance sodium-ion batteries

P-2-Na0.75Ni0.25Mn0.75O2 cathode material with high specific capacity and high operating voltage is favored by researchers. However, the complex phase transition (P-2-O-2) at high voltage and the rapid capacity decay caused by Na+/vacancy ordering seriously restrict its application. In this study, a series of Cu-substituted P-2-type Na0.75Ni0.25-xCuxMn0.75O2 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) cathodes are synthesized. The Na0.75Ni0.17Cu0.08Mn0.75O2 cathode achieves a high initial discharge capacity of 133.6 mAh g(-1) and remains 80.5 % of this capacity after 150 cycles at 0.1C, outperforming the performance of other compositions. Investigations into the superior electrochemical performance of Na0.75Ni0.17Cu0.08Mn0.75O2 through a multi-technique approach, including in-situ X-ray diffraction (XRD), ex-situ X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT), elucidate the underlying mechanisms. The results show that the introduction of Cu2+ in Na0.75Ni0.25Mn0.75O2 can successfully regulate the ratio of Naf/Nae, with enhanced cell performance when N-a+ occupies more Nae sites compared to Naf sites. In-situ XRD confirms that the Cu substitution for Ni stabilizes the P-2 structure during the charge-discharge process and inhibits the unfavorable P-2-O-2 phase transition at high voltage. In addition, Cu-substituted cathode materials exhibit a good effect on improving air stability, attributed to the higher Cu2+/Cu3+ redox potential. This unique substitution mechanism offers a novel perspective for understanding the structure-performance relationship of P2-type cathode materials and provides important support for the design of air-stable, high-performance cathode materials.