| dc.description.abstract | LiFePO4 is a promising alternative cathode material because of its low material cost, environmental friendliness, superior thermal safety, and long operational life. However, the main problem with LiFePO4 is its hard to scale up, low electronic conductivity, low tap density, and slow lithium ion mobility. In recent years, many researchers have used carbon coating to overcome the low electronic conductivity. They have tried various techniques and organic materials to optimize the effect of carbon coating on the cell performance of LiFePO4/C. In this thesis, different sizes of polystyrene (PS) spheres were employed as carbon sources to synthesize the carbon-coated LiFePO4 and to study how sphere size impacts the electrochemical properties of LiFePO4/C composites.
From SEM images, PS sphere with smallest size of 0.22 μm were more effective at preventing the aggregation of LiFePO4 particles. LiFePO4/C with 0.22 μm PS sphere as a carbon source delivered the highest first cycle discharge capacity of 145 mAh g-1 at a 0.2 C. However, it just maintained 289 cycles 80 % capacity retention. On the other hand, the composite prepared using the largest size (2.75 μm) PS sphere, with greater electronic conductivity (4.44×10-4 S cm-1) and surface area (20.58 m2 g-1), showed the lowest initial discharge capacity of 132 mAh g-1, but longest cycle life of 755 cycles. In order to understand how carbon coating influences the electrochemical properties of LiFePO4/C composite cathode materials, we analyzed them using several kinds of instruments, such as XRD, SEM, TEM/EDS/SAED, TOC, BET, Raman spectroscopy, four-point probe conductivity measurements, cyclic voltammetry, and so on. | en_US |