Effect of sintering time on the physical and electrochemical properties of LiFePO4/C composite cathodes

LiFePO4/C composite materials were synthesized at 700 °C by an in situ solid-state reaction. The effect of the sintering time on its structure, surface morphology and electrochemical performance was investigated by X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), Raman spectroscopy analyses, cyclic voltammogram (CV), and galvanostatically charge-discharge techniques. It was found that, all prepared materials for various sintering time show the single olivine structure. The powder morphology change is negligible with extending sintering time from 5 to 40 h. Moreover, the ID/IG ratio of carbon in Raman shift remains almost invariant (∼0.96) with the increase of sintering time. Electrochemical tests show that the discharge capacity remains at 153 mA h g−1 with medium-rate (0.5 C), while increases from about 120 to 130 mA h g−1 at high-rate (1 C) with increasing sintering time, and the possible reasons are discussed. The LiFePO4/C composite synthesized at 700 °C for 30 h demonstrates a best electrochemical performance, delivering a discharge capacity of 110 mA h g−1 (1.5 C) after 50 cycles.