Enhancement of Electrochemical Properties for Monodisperse Spherical LiFePO4/C Synthesized by Ammonia Assisted Hydrothermal Route via Ni and F Co-Doping

Ni and F co-doping monodisperse spherical LiFePO4/C composites with various co-doping contents (from 0 to 0.03) were prepared by an ammonia assisted hydrothermal method. The structure, morphology and electrochemical performance of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, scanning electron microscope (SEM), charge/discharge tests and galvanostatic intermittent titration technique (GITT). The results showed that the samples of Ni and F co-doping keep the olivine structure of LiFePO4, but the microstructure and the electrochemical performance of the LiFePO4/C were improved, especially at a high rate. It has been found that LiFe0.99Ni0.01(PO4)0.99F0.03/C showed a relatively good rate capability and excellent cycling stability. The initial discharge capacity of LiFe0.99Ni0.01(PO4)0.99F0.03/C sample was 156.6 and 130.1 mAh/g as well as stable cycling performance (the capacity retention ratio was 99.6 and 96.7% till 100 cycles) at 1 C (170 mA/g) and 5 C (850 mA/g), respectively. Particularly, it still showed a high discharge capacity of over 91.6 mAh/g even at a rate of 20 C.