Synthesis and electrochemical analyses of vapor-grown carbon fiber/pyrolytic carbon-coated LiFePO₄ composite

A vapor-grown carbon fiber/pyrolytic carbon-coated LiFePO₄ (VGCF/PCLFP) composite has been prepared in one step through a solid-state reaction accompanied by a gas-phase decomposition process. This method leads to the formation of a conductive network composed of pyrolytic carbon layer and in situ vapor-grown carbon fiber in the composite. The amount of carbon in the composite has been determined by a modified formula based on thermogravimetric analysis to be around 3.0 wt%. The optimized electrode of VGCF/PCLFP composite can deliver 150 mAhg⁻¹ at 0.5 C rate, 137 mAhg⁻¹ at 1.0 C rate and 132 mAhg⁻¹ at 3.0 C rate. And its discharge capacity loses only ~4% at a higher rate of 3.0 C after 100 cycles. The area-specific impedance of a cell fabricated with VGCF/PCLFP composite is lower than that made of only pyrolytic carbon-coated LiFePO₄, reported here for the purpose of comparison. In comparison to the electrode made of carbon black/LiFePO₄ composite (10 wt% carbon), the charge transfer resistance of the VGCF/PCLFP composite electrode decreases from 165 to 91 Ω. This technique presents an attractive way to produce high-performance LiFePO₄ cathode material through a low-cost high-efficiency process.