Effects of fluorine-substitution on the electrochemical behavior of LiFePO4/C cathode materials

The effects of fluorine substitution on the electrochemical properties of LiFePO4/C cathode materials were studied. Samples with stoichiometric proportion of LiFe(PO4)1−xF3x/C (x = 0.025, 0.05, 0.1) were prepared by adding LiF in the starting materials of LiFePO4/C. XRD and XPS analyses indicate that LiF was completely introduced into bulk LiFePO4 structure in LiFe(PO4)1−xF3x/C (x = 0.025, 0.05) samples, while there was still some excess of LiF in LiFe(PO4)0.9F0.3/C sample. The results of electrochemical measurement show that F-substitution can improve the rate capability of these cathode materials. The LiFe(PO4)0.9F0.3/C sample showed the best high rate performance. Its discharge capacity at 10 C rate was 110 mAh g−1 with a discharge voltage plateau of 3.31–3.0 V versus Li/Li+. The LiFe(PO4)0.9F0.3/C sample also showed obviously better cycling life at high temperature than the other samples.