Abstract: The outstanding performance of O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM111) at both high and low temperatures coupled with its impressive specific capacity makes it an excellent cathode material for sodium-ion batteries. However, its poor cycling, owing to high-pressure phase transitions, is one of its disadvantages. In this study, Cu/Ti was introduced into NFM111 cathode material using a solid-phase method. Through both theoretically and experimentally, this study found that Cu doping provides a higher redox potential in NFM111, improving its reversible capacity and charge compensation process. The introduction of Ti would enhance the cycling stability of the material, smooth its charge and discharge curves, and suppress its high-voltage phase transitions. Accordingly, the NaNi_0.27Fe_0.28Mn_0.33Cu_0.05Ti_0.06O₂ sample used in the study exhibited a remarkable rate performance of 142.97 mAh·g^–1 at 0.1 C (2–4.2 V) and an excellent capacity retention of 72.81% after 300 cycles at 1 C (1 C = 150 mA·g^–1).