Synthesis, Characterization And Electrochemical Properties Of Iron-based Material And Its Carbon Composites

Entering the21st century, with the rapid development of economy and the improvement of society needs, energy global reduction and severe environmental degradation. The serious energy and environmental crisis make people aware of the limitations of traditional energy in terms of resources and conversion, so researchers in an effort to improve traditional energy use. At the same time, researches also began to notice new energy materials with lowcost, rich storage and small pollution. Lithium ion battery, as one of the new energy sources, has large specific energy, stable output voltage, low self-discharge, no memory effect, green environmental protection as well as the durability strong advantage and so on. Materials with micro/nano size will produce a lot of novel properties which are related to their size, shape. How to choose appropriate micro/nano materials as an electrode material of lithium ion battery, has become a very popular research topic. The purpose of this paper is to obtain nanoparticles with good crystallinity and high purity by high temperature reaction. Meanwhile, synthesis3d micro/nano composite materials with better electrochemical performance which composed of iron oxides nanoparticles and carbon materials. We discuss the influence of dosage, time, etc.to the size and morphology of the product. According to the characteristics of the materials, we study its electrochemical behavior used in the lithium ion batteries, and discuss the effect of the different three-dimensional structures on the properties of materials. The main content can be summarized as follows:1. Fe3O4/C composites have been prepared by sucrose calcining with Fe3O4particles obtained from ferrous oxalate decomposition. The scanning electron microscopy (SEM) images show that Fe3O4nanoparticles (Fe3O4NPS) with average size of200nm are embedded in the three-dimensional (3D) carbon-framework. As an anode material for rechargeable lithium-ion batteries, the Fe3O4/C composite delivers a reversible capacity of773mAh g-1at a current density of924mA g-1after200cycles, higher than that of the bare Fe3O4NPS which only retain a capacity of350mAh g-1. When the current density rises to1848mA g-1, Fe3O4/C material still remains670mAh g-1even after400cycles. The enhanced high-rate performance can be attributed to the3D carbon-framework, which improves the electric conductivity, relaxes the strain stress and prevents the aggregation of Fe3O4particles during the charge/discharge process.2. Carbon network-supported Fe3O4@C composites as an active material are prepared by solvothermal decomposition and heat treatment. The structure and morphology of the Fe3O4@C composites and carbon network are characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscope analysis. The composites consist of～200nm Fe3O4nanoparticles uniformly embedded in a three dimensional carbon network to form a micro-nanoarchitecture. As an anode material for rechargeable lithium-ion batteries, the Fe3O4/C composites present attractive properties with a high specific capacity of730mAh g-1at2C after300cycles, exhibiting excellent electrical stability and superior electrochemical performance.3. In order to further research whether this polymer can also be applied to other iron oxide particles, we synthetized the composite material composed of iron oxide and carbon network. After testing, we found that the enhanced cycle performance of the new composite. The results showed that the polymer has flexibility properties. The formation of the carbon network not only can improve conductivity of the composites, but also maintain the structure stability of the active materials.4. Various LiFePO4microstructures were synthesized via hydrothermal or solvothermal routes using different additives. In an aqueous solution, LiFePO4spindles whose length was about2μm were obtained with the assistance of polyvinyl pyrrolidone (PVP). As PVP and P2O74-added in water, ellipsoidal LiFePO4particles which composed of nanoparticles around100nm in diameter were obtained. If the additive was cetyltrimethyl ammonium bromide (CTAB), sheet-like LiFePO4crystals with the width of100nm were prepared. In the mixed solvents of water together with ethanol or acetylacetone, when adding CTAB or polyethylene glycol (20000), LiFePO4plates or nanoparticles were obtained. The ellipsoidal LiFePO4had the best electrochemical properties among all these products. It is found that the annealed samples were significantly better than the corresponding unannealed ones. Take the ellipsoidal LiFePO4for example, the initial discharge capacity of annealed (161mAh g-1) was much higher than the unannealed ones (85mAh g-1) at0.1C and the former cell still could deliver a capacity of143mAh g-1after30cycles.