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 degrees C at a rate of 2 C, respectively. The capacity retentions over 200 cycles at -10 degrees 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. (C) 2021 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.

Automated summary

By employing a co-doping strategy of Ti4+/F-, the electrochemical properties of P2-Na0.67Ni0.33Mn0.67O2 cathode material for sodium-ion batteries were improved, leading to enhanced cycling stability and high-rate capability.