P2-Type NaxCu0.15Ni0.20Mn0.65O2Cathodes with High Voltage for High-Power and Long-Life Sodium-Ion Batteries

Cu–Ni–Mn-based ternary P2-type NaxCu0.15Ni0.20Mn0.65O2(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–1at 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–1is obtained from the x= 0.67 sample at 1000 mA g–1after 1000 cycles between 3.0 and 4.2 V without any decrease in capacity.