Insights into the enhanced structure stability and electrochemical performance of Ti4+/F− co-doped P2-Na0.67Ni0.33Mn0.67O2 cathodes for sodium ion batteries at high voltage

P2-Na0.67Ni0.33Mn0.67O2 is considered as a promising cathode material for sodium-ion battery (SIBs) because of its high capacity and discharge potential. However, its practical use is limited by Na+/vacancy ordering and P2-O2 phase transition. Herein, a Ti4+/F− co-doping strategy is developed to address these issues. The optimal P2-Na0.67Ni0.33Mn0.37Ti0.3O1.9F0.1 exhibits much enhanced sodium storage performance in the high voltage range of 2.0–4.4 V, including a cycling stability of 77.2% over 300 cycles at a rate of 2 C and a high-rate capability of 87.7 mAh g−1 at 6 C. Moreover, the P2-Na0.67Ni0.33Mn0.37Ti0.3O1.9F0.1 delivers reversible capacities of 82.7 and 128.1 mAh g−1 at −10 and 50 °C at a rate of 2 C, respectively. The capacity retentions over 200 cycles at −10 °C is 94.2%, implying more opportunity for practical application. In-situ X-ray diffraction analysis reveals that both P2-O2 phase transitions and Na+/vacancy ordering is suppressed by Ti4+/F− co-doping, which resulting in fast Na+ diffusion and stable phase structure. The hard carbon//P2-Na0.67Ni0.33Mn0.37Ti0.3O1.9F0.1 full cell exhibits a high energy density of 310.2 Wh kg−1 and remarkable cyclability with 82.1% retention after 300 cycles at 1 C in the voltage range of 1.5–4.2 V. These results demonstrate that the co-doping Ti4+/F− is a promising strategy to improve the electrochemical properties of P2-Na0.67Ni0.33Mn0.67O2, providing a facile tactic to develop high performance cathode materials for SIBs.