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Synthesis and characterization of lithium iron phosphate as cathode material for lithium ion batteries

Authors Wong, Feray Chi Hung
Issue Date 2011
Summary Air pollution is a global problem. Transportation is one of the major contributors. Using electric vehicles is one of the choices for a green community. Rechargeable batteries are usually made of lead-acid, nickel cadmium, nickel metal hydride, lithium ion, and lithium ion polymer. Among them, lithium ion battery has more advantages. It has the best energy-to-mass ratio and has a very slow loss of charge. It has a longer service life and safer when operating at relatively high temperature. LiCoO2, LiMnO2, LiNiO2, LiFePO4 and LiCo1/3Ni1/3Mn1/3O2 are the most common types of cathode materials for lithium ion batteries. LiFePO4 is relatively cheap and safe. Hence, it is the most popular material for electric vehicles. LiFePO4 can be made by solid state reaction, sol-gel, and hydrothermal synthesis. Hydrothermal synthesis is easier to control and cheaper to produce commercially. In this thesis, LiOH, FeSO4, citric acid, phosphoric acid, isopropanol and double deionized water were used as raw materials. Nano-LiFePO4 was synthesized by hydrothermal method. The optimal synthesis conditions were found to be reaction temperature 200℃, reaction time 6 hours, 5:1 ratio of isopropanol to DDI water, and slow mixing under absolute inert gas atmosphere protection. The particles formed were less than 50nm in cubic-like granules with very high purity. The particles were confirmed with XRD as LiFePO4. The product was then coated with carbon by PVA/CA and characterized by TEM. Under performance tests, both types of cells were charged/discharged at 0.1C. The average specific capacity among 15 cells was calculated as 144mAh/g for PVA coating and 127mAh/g for CA coating. Compared with the best results, 155mAh/g for PVA and 134mAh/g for CA, the capacities of the other samples was not fluctuated greatly. The little fluctuation was contributed to the constant thickness of the activated material layers. And the average capacity drop for the CA was explained by the thickness of the carbon layer coated and different carbon source used. The voltage plateau was at 3.45V and no significant drop of voltage after thousands of cycles at 5C.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2011
Language English
Format Thesis
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