A two-step solid state synthesis of LiFePO4/C cathode with varying carbon contents for Li-ion batteries

Olivine structured LiFePO4/carbon composites with different amounts of carbon were prepared by a modified two-step solid-state reaction. The iron citrate used as both iron and carbon sources in the reaction resulted in the formation of carbon coatings on the olivine particles as the mixed precursors were heated at three different initial-step temperatures of 200, 300 and 400 °C followed by a second-step annealing at moderate temperatures of 700 °C. The obtained final powders with varying carbon contents were systematically analyzed by characterization techniques of thermo-gravimetric analysis, X-ray powder diffraction, field-emission scanning electron microscopy, and field-emission transmission electron microscopy prior to electrochemical testing in order to determine the structural and calcination effects on the electrochemical properties of the composites. The field-emission transmission electron microscopy images revealed that the morphology of the LiFePO4 composites consist of agglomerated particles surrounded by carbon as a conductive material. Among the prepared samples, the LiFePO4/carbon composite calcined at initial-step temperature of 300 °C showed the highest discharge capacity and the best rate capability in the voltage range of 2.5–4.2 V.