Promoting threshold voltage of P2-Na_0.67Ni_0.33Mn_0.67O₂ with Cu²⁺ cation doping toward high-stability cathode for...

P2-type Na_0.67Ni_0.33Mn_0.67O₂ 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 Na_0.67Ni_0.33Mn_0.67O₂ 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 (Na_0.67Ni_0.23Cu_0.1Mn_0.67O₂) 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 Na_0.67Ni_0.23Cu_0.1Mn_0.67O₂ cathode maintains the P2 phase within the voltage window of 2.5-4.3 V, whereas the P2-Na_0.67Ni_0.33Mn_0.67O₂ cathode transforms entirely into O2-type Na_0.67Ni_0.33Mn_0.67O₂ when the voltage exceeds 4.3 V. Furthermore, absolute P2-O2 phase transition of the Na_0.67Ni_0.23Cu_0.1Mn_0.67O₂ cathode occurred at 4.6 V, indicating that Cu²⁺ doping enhances the stability of the layer structure and increases the threshold voltage. The resulting Na_0.67Ni_0.23Cu_0.1Mn_0.67O₂ cathode exhibited superior electrochemical properties, demonstrating an initial reversible specific capacity of 89.1 mAh/g at a rate of 2C (360 mA g^-1) and retaining more than 78 % of its capacity after 500 cycles.