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

Abstract

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 LiFePO4 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 La3+ and F− doping on the LiFePO4. The results indicate the cathodes after La3+ 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 LiFePO4 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 LiFePO4 phase and FePO4 phase due to the weakening of Li–O bonds. Finally, the La3+ 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.