Particle size effects of carbon sources on electrochemical properties of LiFePO ₄/C composites

Abstract Carbon-coated LiFePO ₄ 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 LiFePO ₄ particles. From transmission electron microscopy images, it was found that the LiFePO ₄ particles were completely uniformly coated with 5-nm carbon layer when the carbon source was 0.22 μm PS spheres. When the size of PS sphere was increased to 2.75 μm, a network of carbon was formed and wrapped around the LiFePO ₄ 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 LiFePO ₄/C. The LiFePO ₄ precursor sintered with 0.22 μm 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 μm, the discharge capacity of LiFePO ₄/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 LiFePO ₄ particles.