Copper-Doping Layered P2-Type Na0.67Ni0.25Mn0.75O2 Enable Ultrastable Cathode Material for Sodium Ion Batteries

P2-type transition metal (TM) layered oxides are commonly used as cathodes in sodium ion batteries (SIB), but their stability at high voltage (>4.0 V) remains challenging. Here, Cu-doped Na 0.67 [Ni 0.25 Mn 0.75 ] 1– x Cu x O 2 cathodes were investigated for their potential application in SIBs. Ex situ X-ray diffraction measurements demonstrate that Na 0.67 [Ni 0.25 Mn 0.75 ] 0.94 Cu 0.06 O 2 (NNMC6) maintains a stable P2 phase during the Na + de/intercalation process. In particular, the NNMC6 cathode exhibits remarkable cycling performance with an 87.4% high-capacity retention at 1 C after 200 cycles. Furthermore, the ability of NNMC6 to deliver a reversible discharge capacity of 89.7 mAh g –1 even at a higher current density of 5 °C is remarkable. These results collectively confirm that Cu doping has a synergistic effect on enhancing the performance of SIBs. This study not only provides an effective and simple method for designing P2-type cathodes with excellent structural stability but also offers valuable guidance for developing SIB cathode materials with ultralow bulk strain.