Synthesis Of Nano/Micro Composite Structured Copper,Iron,Vanadium Oxides And Their Applications For Lithium-ion Batteries

The prospect of applications of lithium-ion batteries (LIBs) in electric vehicles and hybrid electric vehicles increase the need for high capacity and high power electrode materials. As nanotechnology is rapidly developed in many fields, nanostructured electrode materials become to be advantageous for high-performance LIBs.Compared with larger-scale-structured materials, nanostructured electrode materials have a lot of unique physical and chemical properties, such as high surface area, high electrochemical activities, and short ion diffusion length. Nanomaterials used for lithium-ion batteries can enhance the specific capacity and high current charge/discharge capability. But they also have some drawbacks, such as easy formation of aggregate during the charge/discharge process, more severe side reactions with the electrolyte which will lead to capacity fading and thus bad cycling performance. Despite of their lower electrochemical activities, micron-size transition metal oxides usually have better cycling performance than nano-size particles. In this thesis, we use many methods to design nanostructured micron/sub-micron size particles which can combine both the advantages from nanomaterials and micron-size materials with high specific capacity and good cycling performance.Chapter 1 is a general introduction of lithium-ion batteries including the development of lithium-ion batteries, working priciple, and components. Then, the synthesis methods for nanostructured materials and thermal test methods used for lithium-ion batteries are given in detail.Chapter 2 presents the experimental reagents and test equipments we used in this thesis.In Chapter 3, the dandelion-like hollow CuO microspheres have been synthesized by a hydrothermal method at low temperatures. The as-prepared CuO can exhibit very high and stable capacity when used as an anode material for lithium-ion batteries. At the current of 0.0645 mA cm2, they can deliver more than 600 mAh g-1 discharge capacity which is higher than literature data. Then, we conclude that nanostructured microspheres materials have better electrochemical performance.In Chapter 4, nanobelt-tangled CuO spheroids have been synthesized by a self-sacrificed template method in a solution at room-temperature. The size of the product can be controlled by controlling the particle size of templates used and it is an energy-saving method because the whole synthesis process is carried out at room temperature. The templates can easily be removed with a highly concentrated solution of sodium hydroxide. The CuO nanoabelts exhibit very high specific capacity and very low reversible capacity loss compared with dandelion-like CuO microspheres inChapter 3. Also, it has excellent C-rate performance when charge/discharge at high current densities. At 7 C rate, the as-prepared CuO can deliver a discharge capacity of 410 mAh g-1 which is higher than the theoretical capacity of the commonly used graphite electrode.Chapter 5 presents the synthesis of nanostructured iron oxide (Fe2O3) and magnetite (Fe3O4) and their electrochemical performance as anode materials for lithium-ion batteries. Ethylene glycol is used as both a solvent and a reducing agent to synthesize monodispersed flower-like Fe3O4 sub-micron spheres by a solvothermal method. Also,α-Fe2O3 powders with different morphologies are synthesized by changing the concentration of the precursors. We demonstrate again that electrode materials with nano/micro composite structure have better cycling stability.Chapter 6 mainly concerns with the synthesis and growth mechanism of monodispersed porous vanadium oxides. Here, we use a very simple alkoxide hydrolysis method to obtain size-controlled sub-micron spheres, which can be converted in the subsequent thermal treatment to porous structures. This type of structure is beneficial for the high C-rate capability and cycling performance. Also, we demonstrate that the porous vanadium oxide spheres can have even higher low-temperature electrochemical performance than nanorod V2O5 reported in literature.In order to ultimately produce transition metal oxide materials used as anode materials for practical application, it is essential to obtain accurate heat generation data of transition metal oxide electrodes. Such information is hardly seen in literature. In Chapter 7, the thermal behavior of transition metal oxides has been studied in in-situ process of charging/discharging by Isothermal Microcalorimetry (IMC) machine. With dandelion-like CuO microsphere and flower-like Fe3O4 as the model materials to test the heat generation rate during the charge/discharge process, we have found that the exothermic peaks observed are corresponding to the reduction/oxidation reactions during the in-situ charge/discharge process, which is actually another support for the "conversion reaction" mechanism. Finally, In Chapter 8, the author gives an overview on the achievements and the deficiency in this thesis. Some prospects and suggestions of the possible future research directions are pointed out.