Study On The Electrochemical Properties Of Lithium Insertion Into Pure And Doped TiO₂ Nanotubes

Lithium-ion batteries are widely used in portable electronic devices and electric vehicles, because of high energy storage density, high voltage, little self-discharge, large specific capacity, long cycle life, no memory effect, pollution-free to environment, and so on. The electricity storage ability of Li-ion rechargeable batteries critically depends on the selection of electrode material, in particularly the anode material. In present commercial lithium-ion batteries, various carbon materials, such as graphite, coke, are widely used as an anode material. However, as the voltage of lithium intercalation into carbon materials closes to lithium metal, some Li-ions may deposit on the surface of the anode leading to lithium dendrite and hence safety concerns. On the other hand, SEI (Solid Electrolyte Interface) Film is essential to form for carbon electrode on the first charge cycle, which leads to a large irreversible capacity loss. And also, the SEI film formation may increase the impedance of electrode/electrolyte interface, and don't facilitate the reversible insertion and extraction of Li-ions.In order to overcome these disadvantages of carbon materials, looking for new type anode materials with better safety performance, higher specific capacity and longer cycle life, becomes hotspot in the research for lithium-ion batteries. Transition metal oxides, such as WO3, MoO3, TiO₂, have been extensively studied as anode materials for lithium-ion batteries. Among these oxides, TiO₂ is paid more attention because of its advantages such as high theoretical specific capacity (335 mAh·g-1), low cost, no toxicity. Lithium-ion batteries supplying stabilized 2～2.5 V can be constructed by TiO₂ and cathode materials providing 4 V, because of its convenient formal potential is around 1.8 V (versus Li+/Li). The voltage plateau of TiO₂ is higher than that of carbon electrode, which can avoid the deposition of lithium, and hence improve the safety character. In recent years, nanomaterials have attracted widespread attention with the development of nanotechnology and new materials technology. Nanomaterials as electrochemical lithium-ion insertion materials, the large specific surface area can buffer the process of charging and discharging volume changes, reduce the distance over which Li+ must diffuse in the material, facilitate the reversible insertion and extraction of Li-ions, better to release the strain during lithium insertion and extraction, prolong the cycle life, and the larger electrode/electrolyte contact area increases the charge/discharge rate. Comparatively, nanostructure TiO₂ electrode materials, such as nanowires, nanoparticles, nanotubes, ect., have more excellent electrochemical properties than traditional TiO₂ electrode material. Nanotube TiO₂ is a very promising anode material for lithium-ion batteries, because its preparation method is simple, and it has a higher surface area and can provide more lithium intercalation location.In this thesis, on the basis of TiO₂ nanotubes prepared by different ways, the study has been done about the electrochemical properties of lithium insertion into undoped and doped TiO₂ nanotubes electrode as follows:1. Anatase TiO₂ nanotubes were synthesized by hydrothermal and sol-gel-hydrothermal respectively. The transformations in morphology and phase structure of the product at different treating temperature were studied. The result showed that, nanotubes possessed excellent thermal stability, the morphology maintained the appearance unchanged, and the crystal phase occurred from H2TiO3 to anatase when annealed at 400℃. With increasing the heat treatment temperature, the tubular structure was gradually destroyed, and crystal phase transformed from anatase to rutile.2. The electrochemical performances of TiO₂ nanotubes prepared by hydrothermal treatment and sol-gel-hydrothermal are studied respectively. The experimental results showed that the anatase TiO₂ nanotubes synthesized through the different way have obvious difference from each other on lithium insertion and extraction.3. Carbon-doped TiO₂ nanotubes and Fe-doped TiO₂ nanotubes were prepared by sol-gel-hydrothermal method respectively. The electrochemical performances of lithium insertion into un-doped TiO₂ nanotubes, carbon-doped TiO₂ nanotubes and Fe-doped TiO₂ nanotubes were studied by comparison. It was found that the electrochemical performance of doped TiO₂ nanotubes improved, and the electrochemical performance of Fe-doped TiO₂ nanotubes was better than that of carbon-doped TiO₂ nanotubes.