Particle size effects of carbon sources on electrochemical properties of LiFePO4/C composites

Carbon-coated LiFePO4 cathode materials were prepared by a solid-state method incorporating different sizes of polystyrene (PS) spheres as carbon sources. In scanning electron microscope images, small PS spheres appear more effective at preventing aggregation of LiFePO4 particles. From transmission electron microscopy images, it was found that the LiFePO4 particles were completely uniformly coated with 5-nm carbon layer when the carbon source was 0.22 mu m PS spheres. When the size of PS sphere was increased to 2.75 mu m, a network of carbon was formed and wrapped around the LiFePO4 to create a conductive web. Raman spectroscopy and four-point probe conductivity measurement showed that using larger sizes of PS spheres as carbon sources leads to greater conductivity of LiFePO4/C. The LiFePO4 precursor sintered with 0.22 mu m PS spheres delivered an initial discharge capacity of 145 mAh g(-1) at a 0.2 C rate, but it only sustained 289 cycles at 80% capacity. When the diameter of PS spheres was increased to 2.75 mu m, the discharge capacity of LiFePO4/C decreased, but the cycle life reached 755 cycles, the highest number in this work probably due to the network formation of carbon wrapping around LiFePO4 particles.