F-Doping improves the electrochemical performance of Na2VTi(PO4)3 as the cathode for sodium-ion battery

•A promising NASICON-structured Na2VTi(PO4)3 material is modified by fluorine doping.•F doping greatly improves the dynamics and enhances the ion/electron transmission.•The optimal composites show ultra-long cycle life and excellent rate capability. Herein, Na super ionic conductor-structured Na2-2xVTi(PO4)3-xFx/C (denoted as NVTP-Fx, x = 0.01, 0.03 and 0.05) is achieved through a simple sol–gel method. It is the first time that fluorine is introduced in NVTP material, which enhanced the ion/electron transmission of materials, leading to the better rate performance and outstanding cycle life. Proper fluorine doping demonstrates no significant impact on morphology of composites. But the electrochemical performance of the Na2VTi(PO4)3 material has been significantly improved. The best NVTP-F0.03 sample achieves high discharge specific capacities of 134.3, 82.1, and 75.4 mAh g−1 at 0.1, 5, and 10C (1C = 125 mA g−1), respectively. The capacity retention is as high as 96.5% at 0.5C over 100 cycles, equivalenting to only 0.035% capacity loss per cycle. At a higher current density of 2C, it retains 81.3% capacity after 500 cycles, higher than that of the pristine sample (51.7%). The electrical analysis results show that the Na+ diffusion kinetics has been greatly improved after fluorine doping, and the carbon coating effectively promotes the charge transfer of electrode. The results reveal that fluorine doping is a simple and valid strategy to improve the electrochemical performance of NVTP material.