P2-type Na_xCu_0.15Ni_0.20Mn_0.65O₂ cathodes with high voltage for high-power and long-life sodium-ion batteries

Cu-Ni-Mn-based ternary P2-type Na_xCu_0.15Ni_0.20Mn_0.65O₂ (x = 0.50, 0.67, and 0.75) cathodes for sodium-ion batteries (SIBs) are synthesized by a co-precipitation method. We find that Na content plays a key role on the structure, morphology, and the charge-discharge performances of these materials. For x = 0.67 and 0.75, superstructure from Na⁺-vacancy ordering is observed, while it is absent in the x = 0.50 sample. Despite the same synthesis conditions, materials with x = 0.67 and 0.75 show smaller particle sizes compared to that of the x = 0.50 sample. In addition, redox potentials of the materials differ significantly even though they have the same transition metal ratios. These differences are attributed to the changes in local structures of the as-prepared materials arising from the different amount of Na and possibly oxygen in the lattice. Materials with x = 0.67 and 0.75 show excellent rate performance and cycle stability when tested as cathode material of SIBs. Average discharge potential is as high as 3.41 V versus Na-Na⁺ with capacity of 87 mAh g^-1 at 20 mA g^-1. Excellent capacity and cycle stability are maintained even when they are tested with higher current rates. For instance, a capacity of 62.3 mAh g^-1 is obtained from the x = 0.67 sample at 1000 mA g^-1 after 1000 cycles between 3.0 and 4.2 V without any decrease in capacity.