Ni-doped LiFePO4/C as high-performance cathode composites for Li-ion batteries

摘要

LiFe1-xNixPO4/C composites were successfully synthesized by a simple hydrothermal method. X-ray diffraction (XRD) data indicate that doped Ni can enhance the crystallinity of the composite and retain the olivine lattice type of LiFePO4. The scanning electron microscopy (SEM) results show that a suitable amount of doped Ni can promote the formation of a uniform and small morphology. The high-resolution transmission electron microscopy (HRTEM) results verify that the Ni-doped sample is well covered by a carbon layer and has a high crystallinity. Charge/discharge data reveal that an appropriate amount of doped Ni can improve electrochemical performance. The LiFe0.97Ni0.03PO4/C composite has the strongest intensity of the XRD peaks, the smallest particle size, highest capacity and best cycling performance. Its specific capacity is 169.5 mAh-g(-1) at 0.2C and can reach 122.9 mAh-g(-1 )at a 5C rate. Even at 10C, the capacity retention ratio is still 93.9% after 200 cycles. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data indicate that the electrode polarization and electrochemical impedance of LiFe0.97Ni0.03PO4/C are reduced compared with pure LiFePO4. Its specific capacity at 0.2C can be 175.8 mAh-g(-1) after sufficient activation, which is higher than the theoretical value of 170 mAh-g(-1).