Revealing the superior rate performance and cycle stability of the La and F Co-doping LiFePO4/C

In this work, the La substitution of iron and replacing the oxygen with fluorine are both adopted to expectantly enhance the rate performance and structure stability of LiFePO 4 during working. The characterizations of X-ray diffraction, scanning electron microscope , transmission electron microscope, charging-discharging tests, electrochemical impedance spectroscopy and safety tests are used to investigate the influences of La 3+ and F − doping on the LiFePO 4 . The results indicate the cathodes after La 3+ doping not only deliver the larger lattice parameters and unit cell volume to broaden the lithium ions migration pathway and accelerate the Li  +  diffusion speed, but also strengthen the whole bond energy of LiFePO 4 owing to the high bond energy of La–O. In addition, the F replacement of O could increase the length of Li–O bonds and decrease the length of P–O bonds, further be helpful to enhance the Li  +  diffusion between LiFePO 4 phase and FePO 4 phase due to the weakening of Li–O bonds. Finally, the La 3+ and F − co-doped cathode delivers the superior discharge capacity of 139.3 mAh g −1 at 5C high rate and 127.2 mAh g −1 at −20 °C, with the high discharging capacity of 160.1 mAh g −1 after 100 cycles at 45 °C.