| dc.description.abstract | This dissertation contains five chapters, introduction and conclusions are presented in Chapter 1 and Chapter 5, respectively. All results and discussion are divided into the rest of chapters.
In Chapter 2, we prepared the pristine LiCoVO4 powder and La2O3-coated LiCoVO4 materials by a citric acid-urea polymeric method, and La2O3-coated LiCoVO4/carbon composite cathode materials by a solid state high temperature method. A citric acid-urea polymeric method was successfully applied to synthesize nanocrystalline LiCoVO4 cathode materials. Raman and XPS analyses confirmed the presence of carbon and La2O3 coating on the surface of LiCoVO4 powders. FTIR spectral results confirmed the complete removal of organic residues at a low temperature and the formation of LiCoVO4. TEM images revealed that the particles were a uniform nanosize of about 95 nm and the coated layer was about 15 nm thick on the LiCoVO4 material. The electrochemical performance of the Li/LiCoVO4 cell demonstrated good capacity retention when charged to a high voltage of 4.5V. The cell performance of pristine LiCoVO4 synthesized by a citric acid-urea process sustained 10 mAhg-1 for 30 cycles and 0.5 wt.% La2O3-coated LiCoVO4 sustained 37 mAhg-1 for 110 cycles. However, the sample obtained from a 0.5 wt.% La2O3-coated LiCoVO4 calcined with 60 wt.% malonic acid demonstrated the best cell performance and thermal stability of the materials we studied. It had an initial capacity of 71 mAhg-1 and reached 60 mAhg-1 at 30 cycles. The onset temperature of thermal decomposition of this composite cathode material was 475 K compared to 452 K for bare LiCoVO4. The total reaction heat was reduced significantly by La2O3 and malonic acid coatings and its heat evolution was 35 Jg-1 vs. 176 Jg-1 for bare LiCoVO4, after they were charged to 4.5 V at a 0.1 C-rate and then potentiostated at 4.5 V versus Li+ for 10 h. These results demonstrate a remarkable improvement of the LiCoVO4 cathode material in terms of capacity, cycle life and thermal stability.
In Chapter 3, we synthesized LiFe1-xLaxPO4/C composite materials through lattice doping with La3+ cation and non-lattice doping with carbon. The conductivity of LiFePO4 was enhanced significantly via carbon coating and La-doping. The physical and electrochemical properties of La-doped LiFePO4 cathode materials synthesized via a high temperature solid-state method were systematically investigated. The La doping did not affect the structure of the cathode material, but considerably improved its capacity performance and cyclic stability. Among the materials studied, the LiFe0.99La0.01PO4/C composite demonstrated the best cell performance with a maximum discharge capacity of 156 mAhg-1 cycled between 2.8 and 4.0 V at a 0.2 C-rate, compared to 104 mAhg-1 for pure LiFePO4. This composite electrode can sustain 497 cycles based on 80% charge retention. Such a significant improvement was mainly attributed to enhanced electronic conductivity (from 5.88×10-6 to 2.82×10-3 Scm-1) and high Li+ mobility in the doped samples.
In our carbon coating process, we found that carbon coating thickness and structure play important roles in determining the capacity of LiFePO4 cathode materials. Therefore, in chapter 4, we investigated the effect of carbon coating thickness and its homogeneity (uniformity) on the electrochemical properties of LiFePO4/C composite materials prepared by a carbon vapor deposition technique, and used both polystyrene and malonic acid as carbon sources. The physical, structural, and Li-ion diffusion kinetics of LiFePO4/C composites were systematically investigated. Using a carbon vapor deposition technique, we have shown that the amount of carbon and its coating thickness and uniformity in LiFePO4/C materials are all crucial parameter in determining the electrochemical performance. LiFePO4 coated with a thin and uniform carbon film can deliver maximum discharge capacity of 151 mAhg-1 at a 0.2 C-rate and sustain 415 cycles at 80% of capacity retention. In order to minimize the polarization, a carbon coating layer must be uniformly distributed around each active particle, because the lower polarization leads to a higher reversible capacity.
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