Structure Optimization And Electrochemical Performance Of The Anode Materials Of Ti And Sn Oxides

Compared with the traditional lead-acid and nickel-hydride batteries, lithium-ion batteries, which do not contain lead, cadmium, mercury and other heavy metals, known as "green chemistry power", have a high capacity, a high power, and a high cycle performance, etc., and thus, they exhibit a very broad application prospect in the mobile digital electronic, electric vehicles, spacecraft and other high-power fields. It has been well documented that the anode is an important part of the lithium-ion battery, which is commercially made of carbon material in the present. However, the carbon materials are with a low theoretical capacity, which limits the development of the battery capacity. Recently, a number of new non-carbon anode materials, such as Si, Sn, alloys, oxides, etc., were developed due to their high specific capacity, which are considered to be the promising alternative anode materials of carbon. However, for these new anode materials, a very large volume deformation will happen in the charge-discharge cycle process due to the enhancing lithiation capability, which may result in the material cracking or breaking out, and eventually degrade the electrochemical performance of lithium-ion batteries. Therefore, many efforts have been made to optimize the structure of the anode materials through nano-technology and composites to relieve the volume expansion for the improvement of the cycle performance of the alloys.In light of it, in the present thesis, to improve the rate and cycling of metal oxide anode materials for lithium-ion batteries, two kinds of metallic oxides, TiO₂ and SnOx were taken as the research objects, and the microstructures of the materials were optimized by doping and composite methods, the titanium and tin oxide anode materials were prepared by ball milling method, electrospinning method, hydrothermal method and plating-anodizing method. Furthermore, the microstructures and electrochemical properties of the anode materials, including macro and microstructures, chemical compositions, morphologies, electrochemical impedance spectroscopy and charge-discharge behaviors, etc., were characterized using XRD, SEM, TEM, BET, XPS and electrochemical tests. The main innovative results are as follows:1. Using ball milling and electrospinning methods, the mesoporous TiO₂/CNTs composites and TiO₂ nanofibers were successfully prepared, respectively. It found that the two kinds of anode materials exhibit the improved specific capacity and rate performance. It is attributed to the big specific surface areas of the mesoporous TiO₂ and TiO₂ nanofibers which can promote the active substances to participate in the reaction. In the mean time, the structures of the anode materials can be used as the bracket supporting the negative electrode materials and the channels for the lithium delithiation in charging-discharging processes, which is conducive to deintercalation of lithium ions.2. By the electrospinning and hydrothermal methods, two kinds composites of SnO₂/TiO₂ and SnO₂/RGO with high specific capacity were successfully prepared, respectively. It is found that the cycling performance and the specific capacity of the two composites are significantly improved in comparison with pure SnO₂, especially for SnO₂/RGO anode material. The initial discharge capacity of SnO₂/RGO is up to 1696 mAh g-1, which maintains 600 mAh g-1 after 50 cycles. It is attributed to the introduction of RGO and TiO₂ into the SnO₂ substrate, which may effectively alleviate the volume expansion caused by the insertion and extraction of the lithium ion. Moreover, the specific capacity and conductivity of the composite anodes are enhanced, which is benefit to the further improvement of the cycle performance of the lithium battery.3. Based on the integration design idea of layer composite materials, a mesoporous integrated-layered structure of SnO₂/SnO/Sn composite were successfully prepared by electroplating-anodization on the copper substrate directly. To further improve the specific capacity and cycling performance, the electrode of CNTs@SnO₂/SnO/Sn integrated-layered composite material were also prepared. It is found that the first discharge capacity of the composite material is up to 1895 mAh g-1, which remains 927 mAh g-1 after 50 cycles. So that by the comprehensive design of composite, integration and doping, the electrochemical properties of the electrode material can be well improve the integration design idea is a good way for the improvement of the electrochemical properties of lithium batteries.4. To take the foam nickel as the substratethe, tin oxidere with CNTs dopants integrated composite electrode was prepared by eletroplating-anodizion method, and the effects of the electroplating parameters and oxidation time on the electrochemical properties of the composite electrode were investigated. It is found that the composite exhibits a high coulombic efficiency, which reaches 98%, and the first discharge capacityis up to 1260 mAh g-1, which remains 393.4 mAh g-1 after 50 cycles. In addition, the microstructures and surface morphologies of the electrodes under the different charging states were studied, and the degradation mechanisms of the electrochemical properties of the lithium ion battery during in the cycling processes were discussed.