Preparation Electrochemical Performance Of Transition Metal Oxide Composites As Anode Materials For Lithium Ion Batteries

With the rapid development of economy worldwide, fossil fuels have been in largely exploitation and application, leading to some issues, such as energy crisis, environmental pollution, greenhouse effect, and climate change. All these situations have brought great challenges to human’s modern life. Therefore, developing clean and sustainable energy resources has attracted more and more attention. At present, many being developed clean energy, such as solar, wind, tide and geothermal energy, are usually intermittent and regional, which made energy srorage has become an important topic for people. Due to its high voltage, high energy density, small self-discharge, light wight, long cycle life and environmental friendliness, lithium ion battery (LIB) is considered to be an ideal energy storage and conversion device. In recent years, LIB has been widely used in laptops, mobile phones, digital camera, video camera and portable electronic devices. Moreover, LIB also shows wide application prospects in many fields, such as electric bicycle, hybrid cars, electric tools, smart grid and other.The electrochemical performance of LIB largely depends on its electrode materials. The current LIB industry has been primarily dominated by graphite as the anode material. However, due to its shortcoming, such as low theoretical capacity (372 mAh g-1), low energy density, potential safety hazard, graphite has been unable to meet the needs of developing high performance, high safety of lithium ion battery. Therefore, the research and development of novel and high-performance anode material has become a very important work. Among numerous alternative materials, transition metal oxides have attracted considerable attention since reported by Tarascon group in 2000, owning to their high theoretical capacity, rich resources, environmental friendly, high security, etc. However, the poor electrical conductivity, low initial coulombic efficiency and fast attenuation of reversible capacity, have brought a bottleneck in development and application of transition metal oxide anode materials. In this paper, carbon coating and hybridization methods have been used to prepare transition metal oxide composites with nanoporous structure. The results of their electrochemical performance test show that the synthesized transition metal oxide composites all have the high reversible specific capacities, good rate performances and long lifespan, which provide a strong guarantee for their application as anode materials in lithium ion battery. The main contents are summaried as following:(1) First of all, MnCO3 precursor was prepared by solvothermal method, with manganese acetate and urea as raw material, diethylene glycol as solvent, polyvinyl-pyrrolidone (PVP) as surfactant. By adjusting the PVP was added to the control the size and morphology of the precursor distribution, the MnCO3 microspheres with a size of 2-3μm and excellent dispersion have been successfully synthesized through adjusting the additive of PVP.Then the porous Mn2O3 microspheres were obtained through annealing the MnCO3 precursor in air. Finally, the carbon-coated MnO porous microspheres (MnO@C) were prepared by carbonization of pyrrole coated porous Mn2O3. XRD, SEM, HRTEM, TG andnitrogen absorption-desorption measurements were performed to investigate the microscopic morphology, structure, phase and components of the products. Meanwhile, the electrochemical performances were tested through the cyclic voltammogram and galvanostatic charge-discharge curves. The as-prepared MnO@C composite obtained a reversible capacity of 527.5 mAh g-1 after 100 cycles at the current density of 100 mA g"1, which is ca.2.5 times than the porous Mn2O3 microspheres tested in the same conditions. The experimental results indicated that carbon coating is an effective way to improve the electric conductivity of the transition metal oxides. The enhanced electrochemical performances of MnO@C is attributed to the carbon coating obtained by pyrolyzing pyrrole, which not only existed on the surface of particles, but also distributed in the spaces and the pores of the composite; and can effectively relieve the volume change of MnO@C microspheres during the 1 ithiation/delithiation, decrease of the agglomeration of MnO phase, improve the activity of MnO components, enhance the electronic conductivity of the transition metal oxide electrode owning to its excellent conductivity. As comparing with the reported MnO/C powders and MnO/C hybrid microspheres, the prepared porous MnO@C also exhibited obvious superiority, which is attributed to the high specific surface area and porosity, which can facilitate an efficient contact of the internal active materials with electrolyte, leading to a fast transportation of Li+ ions and favorably alleviating the volume variation during the Li+insertion/extraction, resulting in a relatively high reversible capacity and cycling stability. In summary, the special porous structure, large specific surface area and unique method for preparing porous MnO@C composite resulted in its excellent electrochemical performance, which provides a good application prospect for the as-prepared porous MnO@C microspheres used as a promising anode material in LIBs.(2) TBT was added to the mixed solution of DMF and IPA, the TiO2 porous microspheres were prepared through solvothermal and pyrolysis method. Porous TiO2/Mn3O4 or TiO2/Fe2O3 nanocomposite microspheres have been successfully fabricated through impregnating Mn2+ or Fe3+ ions repectively into the lab-made porous TiO2, followed by an annealing process. XRD, XPS, SEM, HRTEM, TG andnitrogen absorption-desorption measurements were performed to investigate the microscopic morphology, structure, phase and components of the products. Meanwhile, the electrochemical performances and mechanism of the Li+ insertion/extraction were tested through the cyclic voltammogram, galvanostatic charge-discharge curves and electrochemical impedance spectra. The electrochemical measurements demonstrated that the as-prepared porous ternary TiO2/MnTiO3@C and TiO2/FeTiO3@C hybrid microspheres all exhibitd superior cycling and rate performances, and long cycle life, comparing with the TiO2, TiO2/Mn3O4 or TiO2/Fe2O3, and TiO2@C microspheres with similar porous microstructural. The main reasons are rooted in the synergistic effect that generated by little volume variation of TiO2 matrix, high capacity of MnTiO3 or FeTiO3, and good electrical conductivity of carbon coating (with a thickness of 2-5 nm deposited on the surface and inner wall of pores) during the charge/discharge processes. In addition, as comparing with the recent reported research on TiO2-based composite anodes, the as-prepared porous ternary TiO2/MnTiO3@C and TiO2/FeTiO3@C hybrid microspheres displayed obvious superiority, owning to the special mesoporous structure, reasonable design of composition and unique composite materials. The excellent electrochemical performances provide a good application prospect for the as-prepared porous ternary TiO2/MnTiO3@C and TiO2/FeTiO3@C hybrid microspheres as a promising anode material in LIBs. Meanwhile, a unique synthetic method is also proposed for modification of TiO2 anode materials.(3) With the assistance of PVP, the porous iron titanium oxide (Fe2TiO5) microparticulates have been successfully synthesized via a facile hydrothermal route followed by a subsequent calcination process. Experimental results reveal that PVP plays a pivotal role in controlling the size, uniformity and inducing mesoporous structure of the porous Fe2Ti05 microparticulates. XRD, XPS, SEM, HRTEM, TG andnitrogen absorption-desorption measurements were performed to investigate the microscopic morphology, structure, phase and components of the products. Meanwhile, the electrochemical performances and mechanism of the Li+ insertion/extraction were tested through the cyclic voltammogram, galvanostatic charge-discharge curves and electrochemical impedance spectra. The electrochemical measurements demonstrated that the as-prepared porous porous Fe2TiO5 microparticulates exhibitd superior cycling and rate performances, and long cycle life, attributed to the synergistic effect of TiO2 and Fe2O3 in Fe2TiO5, as well as the mesoporous structure and high specific surface area.