Preparation Of TiO₂ Nanocrystals Based On Ionic Liquids And Research On The Lithium Ion Intercalation-Deintercalation Properties

Along with the prominent energy and environmental problems, electric vehicle (EV) and hybrid electric vehicle (HEV) are getting more and more attention by researchers worldwide. The main problems for lithium-ion battery technology applied in EV and HEV are the rapid charge-discharge ability and security issues, while the main reason for the restriction of its rapid charge-discharge performance is the low lithium-ion diffusion rate in the solid phase. TiO₂ electrode materials can achieve rapid charge-discharge and meet safety requirements due to their higher charge-discharge voltage, high chemical stability, zero volume strain during the charge-discharge process, and open channel for lithium-ion insertion and extraction. Materials with nano-structure can effectively shorten the lithium-ion deffusion path and increase the specific surface with electrochemical activity, which greatly enhance the material properties of the fast Li-ion insertion and extraction.In recent years, the application of ionic liquids (ILs) in the synthesis of nano-materials has been increasing, which shows excellent advantages compared to the traditional synthetic methods. In view of this, this thesis has studied the function of ionic liquids in the hydrolysis and the sol-gel synthesis of TiO₂ nanocrystals, and the effecting factors such as temperature, auxiliary solvent, anion species and carbon chain length are researched. The electrochemical Li-ion insertion-extraction performance has also been studied. In addition, in order to improve the electronic and ionic conductivity of TiO₂, we have prepared TiO₂/Graphene (GE) composites and F-doping TiO₂.Firstly, we synthesized TiO₂ nanocrystals by hydrolysis using ILs. Rutile nanocrystalline TiO₂ with large specific surface area were fabricated in the high halogen ion and high acidity conditions. This material showed a excellent reversible capacity of 216mAh/g, and the capacity kept 127mAh/g after 200 cycles at 5C. After adding auxiliary solvent, anatase TiO₂ nanocrystals were obtianed, while its specific surface area increased greatly. This material also showed a high reversible capacity of 192mAh/g and maintained 119mAh/g after 200 cycles at 5C. It was also found that BF4-can be more effective in promoting crystallization and grain growth, while Long carbon-chain ionic liquids showed superior template effect. The dynamics of the lithium-ion intercalation-deintercalation of TiO₂ electrode was controlled by the Li-ion diffusion in the solid phase.Secondly, Mesoporous nanocrystalline anatase TiO₂ with high surface area, well distributed pore size and high thermal stability was prepared by sol-gel method based on ILs. Long carbon-chain ionic liquids showed superior function of template and stabilizing agent. The anatase TiO₂ showed a high reversible specific capacity of 226mAh/g and kept 127mAh/g after 200 cycles at 5C.Finally, Rutile TiO₂/GE nano-composite was prepared by in-situ growth method and F-doping rutile TiO₂ was fabricated by taking TiF₄ as fluoride source. Test results showed that the electrochemical properties of rutile TiO₂, especially the rapid charge and discharge capacities were greatly improved.