Highly stable Na0.67(Ni0.25Mn0.75)1−xCuxO2 cathode substituted by Cu for sodium-ion batteries

P2-type Na 0.67 Ni 0.25 Mn 0.75 O 2 is considered as one of the most promising cathode materials for sodium-ion batteries due to its high initial capacity and operating voltage based on the Ni 2+ /Ni 4+ redox reaction. However, it suffers from rapid capacity decay caused by P2–O2 phase transition when the cutoff voltage is above 4.2 V (vs. Na + /Na). Herein, a high-voltage phase transition from P2 to an amorphous phase with poor crystallinity has been proved to be the main cause of material deterioration and Mn redox process is another cause. Cu-substituted Na 0.67 (Ni 0.25 Mn 0.75 ) 1−x Cu x O 2 ( x  = 0, 0.05, 0.1, 0.2) with active redox couples of Ni 2+ /Ni 4+ and Cu 2+ /Cu 3+ has been fabricated for solving these issues. It is proved that Cu substitution can reduce the Mn 3+ /Mn 4+ reaction below 3 V (vs. Na + /Na) and suppress the high-voltage phase transition effectively at deep de-sodiated state, resulting in the significantly enhanced cycle life and rate capability of Na 0.67 Ni 0.25 Mn 0.75 O 2 .