Zirconium and copper dual-doping strategy in NaNiFeMnO2: Advancing the electrochemical stability and capacity for sodium-ion batteries

O3-type layered transition-metal oxide cathode materials are considered one of the most promising cathode materials for sodium ion batteries due to their high theoretical capacity and optimal operating potential. However, it encounters significant challenges, such as poor cycling stability and limited reversible capacity, primarily due to structural instability. Herein, we have successfully synthesized dual cation doped O3-type Na(Ni_0.3Fe_0.3Mn_0.3)_0.87Cu_0.12Zr_0.01O₂ (NaCuZrFNM). The co-doping of Zr/Cu into NaFNM layered structure led to an expansion of the sodium layer, enabling enhanced sodium ion mobility during charge/discharge processes compared to NaFNM. Therefore, sodium ions demonstrated faster diffusion in NaCuZrFNM than NaFNM. It was found that the Zr/Cu dual-doped NaFNM electrode deliverers a reversible capacity of 135.2 mA h/g at 0.1C as well as 121.5 mA h/g initial discharge capacity with remarkable capacity retention of 81.8 % at 1C after 250 cycles. Furthermore, it also exhibits the good rate performance of 82 mA h/g at high current density of 10C with 74.4 % capacity retention after 1000 cycles, indicating excellent structural stability. Our results demonstrate that Cu/Zr dual-doping in O3-type cathode materials is viable strategy for improving the long-term performance of sodium-ion batteries.