Fe/Ti Co-doping suppresses P2-O2 phase transition for stabilized Na_2/3 Ni_1/3 Mn_2/3 O₂ cathode

P2-type Na_2/3Ni_1/3Mn_2/3O₂ is regarded as a promising cathode material for sodium-ion batteries owing to its high specific capacity and low cost. Nevertheless, its practical application is hindered by irreversible structural degradation and rapid capacity decay associated with the detrimental P2–O2 phase transition during deep Na⁺ extraction. In this work, Fe and Ti are co-incorporated into Na_2/3Ni_1/3Mn_2/3O₂ via a sol-gel method to enhance structural stability. Rietveld refinement of X-ray diffraction (XRD) patterns indicates that Ti doping shortens TM–O bonds and enlarges Na⁺ diffusion channels by increasing Na-O distances, thereby enhancing structural stability and Na⁺ transport kinetics. In situ XRD analyses reveal that Fe/Ti co-doping regulates the phase transition pathway from the irreversible P2–O2 phase transition to a reversible P2–OP4 phase transition, effectively mitigating structural degradation during cycling. Electrochemical measurements demonstrate that optimized Fe/Ti co-doping enhances the overall electrochemical performance. The optimized composition, Na_2/3Ni_1/4Fe_1/12Mn_7/12Ti_1/12O₂, exhibits excellent cycling stability with a capacity retention of 82.33% after 50 cycles at 0.2 C, and delivers a specific capacity of 61.43 mA h g⁻¹ at a high rate of 5 C. This study demonstrates that the combination of sol-gel synthesis and co-doping engineering provides an effective strategy for designing high-performance cathode materials for sodium-ion batteries.