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

Abstract

P2-NaNiMnO cathode material with high specific capacity and high operating voltage is favored by researchers. However, the complex phase transition (P2-O2) 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 P2-type NaNiCuMnO (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) cathodes are synthesized. The NaNiCuMnO cathode achieves a high initial discharge capacity of 133.6 mAh g 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 NaNiCuMnO 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 Cu in NaNiMnO can successfully regulate the ratio of Na/Na, with enhanced cell performance when Na occupies more Na sites compared to Na sites. In-situ XRD confirms that the Cu substitution for Ni stabilizes the P2 structure during the charge–discharge process and inhibits the unfavorable P2-O2 phase transition at high voltage. In addition, Cu-substituted cathode materials exhibit a good effect on improving air stability, attributed to the higher Cu/Cu 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.