Effect of sintering time on the physical and electrochemical properties of LiFePO 4/C composite cathodes

Abstract

LiFePO 4/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 I D/ I G 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 LiFePO 4/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.