Promoting threshold voltage of P2-NaNiMnO with Cu cation doping toward high-stability cathode for sodium-ion battery.

P2-type NaNiMnO has attracted considerable attraction as a cathode material for sodium-ion batteries owing to its high operating voltage and theoretical specific capacity. However, when the charging voltage is higher than 4.2 V, the NaNiMnO cathode undergoes a detrimental irreversible phase transition of P2-O2, leading to a drastic decrease in specific capacity. To address this challenge, we implemented a Cu-doping strategy (NaNiCuMnO) in this work to stabilize the structure of the transition metal layer. The stabilization strategy involved reinforcing the transition metal-oxygen (TMO) bonds, particularly the MnO bond and inhibiting interlayer slip during deep desodiation. As a result, the irreversible phase transition voltage is delayed, with the threshold voltage increasing from 4.2 to 4.4 V. Ex-situ X-ray diffraction measurements revealed that the NaNiCuMnO cathode maintains the P2 phase within the voltage window of 2.5-4.3 V, whereas the P2-NaNiMnO cathode transforms entirely into O2-type NaNiMnO when the voltage exceeds 4.3 V. Furthermore, absolute P2-O2 phase transition of the NaNiCuMnO cathode occurred at 4.6 V, indicating that Cu doping enhances the stability of the layer structure and increases the threshold voltage. The resulting NaNiCuMnO cathode exhibited superior electrochemical properties, demonstrating an initial reversible specific capacity of 89.1 mAh/g at a rate of 2C (360 mA g) and retaining more than 78 % of its capacity after 500 cycles.