Study On The Electrochemical Properties Of Lithium Insertion Into Pure And Modified TiO₂ Nanowires

Lithium ion battery is a new generation green non-pollution battery when it was used in the 1990s. It is widely used in portable electron apparatus and cars due to its highlights, such as high voltage, large specific capacity, long cycle life, low discharge rate by itself, no memory effect and pollution-free to environment, but the anode materials for lithium ion batteries are the key to constrain its whole performance. The lithium ion batteries which used early almost select carbon/graphite as the 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 discharge and charge cycle. What's more, the SEI film formation may increase the impedance of electrode/electrolyte interface, and doesn't facilitate the reversible insertion and extraction of Li-ions.The anode materials of present have reached their ultimate properties, what's more, these disadvantages of carbon materials improve the development of a new material technology becomes an urgent task for the lithium ion batteries. So 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, V₂O₅, NiO and 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 battery supplying stabilized 2-2.5 V can be constructed by TiO₂ and cathode materials providing 4 V, because of its convenient formal potential is about 1.8 V (vs. 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. Nanpomaterials 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 nanoparticles, nanotubes, nanowires, ect., have more excellent electrochemical properties than traditional TiO₂ electrode because its preparation method is simple, and it has a higher surface area and can provide more lithium intercalation location.In this thesis, on basis of the preparation of TiO₂ nanotubes as well as the condition of our lab, we synthesized TiO₂ nanowires by hydrothermal, the study has been done about the electrochemical properties of lithium insertion into unmodified and modified TiO₂ nanowires electrode as follows:1. TiO₂ nanowires were synthesized by hydrothermal on the basis of the preparation of TiO₂ nanotubes. The transformations in morphology and phase structure of the product at different treating temperature were studied. The result showed that, nanowires possessed excellent thermal stability, the morphology maintained the appearance unchanged, and the crystal phase occurred from H2TiO3 to anataseTiO₂ and TiO₂(B) when annealed at 400oC.2. The electrochemical performance of TiO₂ nanowires prepared by hydrothermal treatment was studied. The experimental results showed that, the TiO₂ nanowires exhibited excellent in capacity and rate capability in comparison to others such as nanotubular TiO₂, fibrous nanostructured TiO₂. But there is still large irreversible capacities in the first cycle.3. The TiO₂ nanowires were surface modified with gold-film by dc sputtering. The electrochemical performance of lithium insertion into un-modified and Au-modified TiO₂ nanowires was studied by comparison. It was found that the electrochemical performance of Au-modified TiO₂ nanowires improved. The Au additive increases the coulombic efficiency in the first cycle (reach about 93%), decreases the polarization of the cell, and marvelously improves the discharge and charge rate capacity.