Development of high-performance cathode materials is one of the key challenges in the practical application of sodium-ion batteries. Among all the cathode materials, layered sodium transition-metal oxides are particularly attractive. However, undesired phase transitions are often reported and have detrimental effects on the structure stability and electrochemical performance. Cu substitution of zinc in the P2-type Na0.6Mn0.7Ni0.15Zn0.15-xCuxO2 (x = 0, 0.075, and 0.15) composites was investigated in this study for mitigating the biphase transition and enhancing the electrochemical performance of sodium-ion batteries. The coupling effect of Zn and Cu enables an excellent capacity retention of 96.4% of the initial discharge capacity after 150 cycles at 0.1 C in the Na/Na0.6Mn0.7Ni0.15Zn0.075Cu0.075O2 cell. The biphase transition that occurred in the high voltage range has been significantly suppressed after the incorporation of Cu in Na0.6Mn0.7Ni0.15Zn0.15O2, which was confirmed by in situ X-ray diffraction studies. Moreover, the substitution of the inert element Zn with electrochemically active Cu leads to the suppression of anionic redox and the occurrence of Cu2+/3+ redox reaction, and the electrolyte decomposition is impeded after the introduction of electrochemically active Cu in the Na0.6Mn0.7Ni0.15Zn0.15-xCuxO2 composite cathode. The enhanced electrochemical performance in the Na0.6Mn0.7Ni0.15Zn0.075Cu0.075O2 electrode can be ascribed to the coexistence of Zn and Cu and alleviated volumetric change as well as suppressed electrode/electrolyte side reaction after Cu substitution.
Keywords: anionic redox; cathode material; copper substitution; phase transition; sodium layered oxide.