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Title: Investigation of novel nanostructured LiFePO₄ cathode materials
Other Titles: Xin ying na mi jie gou lin suan tie li zheng ji cai liao de yan jiu
Authors: Lu, Zhouguang (盧周廣)
Department: Department of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2009
Publisher: City University of Hong Kong
Subjects: Lithium cells -- Materials.
Nanostructured materials.
Notes: CityU Call Number: TK2945.L58 L8 2009
x, 186 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2009.
Includes bibliographical references.
Type: thesis
Abstract: Olivine LiFePO4 is widely considered as the most competitive cathode material for the next generation Li-ion batteries, particularly used as power sources for electric vehicles. However, LiFePO4 suffers from poor rate capability substantially due to its intrinsic low electronic and Li+ ions conductivity. The objective of this dissertation is to find method(s) to improve the rate performance of LiFePO4 cathode material by employing advanced novel nanotechnology. To achieve this goal, a research program comprising three parts was performed: (1) preparation of nano thin-film type electrodes; (2) synthesis and electrochemical characterization of LiFePO4 crystallites possessing various morphologies and the subsequent study of the correlation between particle morphology and electrochemical performance; (3) investigation of growth mechanism of the novel hollow structured LiFePO4 crystallites that exhibits advantageous rate capability. Firstly, a pulse laser deposition (PLD) method was employed to deposit olivine LiFePO4 thin film electrodes on the Si/SiO2/Ti/Pt substrates. Experimental parameters including deposition time, working pressure, substrate-target distance, content of codeposited carbon and silver, and post-deposition annealing, were optimized to prepare LiFePO4 thin films with smooth surfaces and uniform thickness. The electrochemical performance of the deposited LiFePO4 thin-film electrodes was evaluated by cyclic voltammetry (CV) and galvanostatic measurements. Investigation was focused on the effects of film thickness and co-deposition of conductive carbon and silver on the electrochemical performance of the deposited thin-film electrodes. The optimum film thickness was found to be around 200nm. The co-deposition of small amounts of conductive carbon and silver was found to considerably improve the electrochemical activity, in particular the high rate capability. Secondly, a facile hydrothermal technique was employed to synthesize olivine phase-pure LiFePO4 crystallites. Efforts were focused on the shape and size control of the LiFePO4 crystallites. Special attention was paid to the investigation of the effects of additives which act as shape controller, such as ethylene glycol (EG), polyvinylprrolidone (PVP), and citric acid (CA), on the particle morphology of the LiFePO4 crystallites (large bundles, spindle-like assemblies, nanorods, hexagonal platelets, irregular particles, and hollow structures, etc.). Based on a combination of Xray diffraction (XRD) and electron microscopy, the particle shape, particle size, crystallographic structure, as well as their correlations with the electrochemical performance (specific rate capability) were investigated in detail. It was found that the hollow particles outperformed other particles in the rate performance characterization. Thirdly, the formation mechanism of the novel hollow particles was systematically studied. Evolution of crystal phase as well as particle shape and size with respect to the experimental parameters, such as pH value, precursor concentration, hydrothermal temperature, and hydrothermal reaction time was elucidated in detail. It was found that the presence of ammonia and citric acid plays a vital role in the formation of the hollow structure. Olivine LiFePO4 crystallites with unique cubic and drum-like morphology were experimentally obtained for the first time.
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