Promoting threshold voltage of P2-Na0.67Ni0.33Mn0.67O2 with Cu2+ cation doping toward high-stability cathode for sodium-ion battery.

[Display omitted] • Improving the bond energy of TM O bonds (especially that of Mn O bonds) and suppresses the formation of O O bonds. • Delaying the P2-O2 phase transition voltage from 4.2 to 4.4 V. • The Na 0.67 Ni 0.23 Cu 0.1 Mn 0.67 O 2 cathode retained specific capacity of 69.4 mAh/g (2C) even after 500 cycles. P2-type Na 0.67 Ni 0.33 Mn 0.67 O 2 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 0.67 Ni 0.33 Mn 0.67 O 2 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 0.67 Ni 0.23 Cu 0.1 Mn 0.67 O 2) in this work to stabilize the structure of the transition metal layer. The stabilization strategy involved reinforcing the transition metal–oxygen (TM O) bonds, particularly the Mn O 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 0.67 Ni 0.23 Cu 0.1 Mn 0.67 O 2 cathode maintains the P2 phase within the voltage window of 2.5–4.3 V, whereas the P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 cathode transforms entirely into O2-type Na 0.67 Ni 0.33 Mn 0.67 O 2 when the voltage exceeds 4.3 V. Furthermore, absolute P2-O2 phase transition of the Na 0.67 Ni 0.23 Cu 0.1 Mn 0.67 O 2 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 0.67 Ni 0.23 Cu 0.1 Mn 0.67 O 2 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. [ABSTRACT FROM AUTHOR]