Designed Synthesis And Applications Of Carbon-coated Transition Metal Oxides As High-performance Anode Materials For Lithium Ion Batteries

Owing to have a lot of excellent features, such as long cycle life, high theoretical capacity, eco-friendly, no memory effect and great safety during operation, Li-ion batteries have been widely applied in modern consumer electronics and next-generation electric vehicles. However, there still have been some challenges exited in the design and assemble of Li-ion batteries since the useful technology was invented. One of challenges is how to improve the system's electrolyte diffusion, ionic transport and electronic conductivity during Li-ion charge-discharge process. Another challeange is design and synthesis of advanced electrode materials to enhance the capacity retention and improve the specific capacity in the electrochemical performances of Li-ion batteries, which frequently suffered from Li-alloying amorphization and agglomeration as well as pulverization of the active electrode material during operation. Recently, various active materials, such as metal, carbon materials, transition metal sulfides, transition metal oxides and their composites, etc., have attract widely research interest as anodes electrode materials for Li-ion batteries. Among them, the transition metal oxides have a lot of potential application for Li-ion batteries due to their unique physical and chemical properties. However, the severe active material aggregation, low conductivity and dramatic volume exchange during the charge-discharge process would lead to poor cycling stability and large irreversible capacity loss which would hinder their practical application. In this thesis, we introduce a facile approach to prepare a series of graphitized carbon coating transition metal oxides composites with very small nanoparticles encapsulated in carbon shell to form the special peapod-like or sandwich-like architectural nanostructure. The as-prepared electrode materials are characterized by Scanning Electron Microscopy?SEM?, Transmission Electron Microscopy?TEM?, X-ray Diffraction?XRD? and Brunauer-Emmett-Teller?BET?. The electrochemical performances of our samples are measurement by Cyclic Voltammetry?CV?, Galvanostatic measurement, Electrochemical impedance spectroscopy?EIS?, Rate capability and Cycling performance and so on.To improve the electrochemical performance of TiO₂-based anode material, a self-supported peapod-like mesoporous TiO₂@C array is introduced for Li-ion batteries. We successfully fabricated a novel peapod-like TiO₂@C fiber array on Ti foil with H2Ti3O7 nanotube array as precursor and sacrifice template, and glucose molecular as green carbon resource. By analyzing the charateristics of the as-prepared anode material, we can find them combines some fascinating merits, such as mesoporous nanostructure, small size of the TiO₂ peapods, large specific surface area and good graphitized carbon shell, verified an excellent energy storage performance, e.g. superior cycling performance?the specific capacity research up to 124 mAh g-1 at a current density of 10C?, high specific capacitance?162 mAh g-1 by galvanostatic charge-discharge at 1C after 200 cycles?.A two-dimensional nanosheets array formation of hollow Co3O4 nanoparticles embedded in thin graphitized carbon nanosheets is successfully achieved through using electrodeposited ?-Co?OH?₂ nanosheets as precursors. This designed binder-free composite electrode material displays many unique advantages such as high nanoporosity, large specific surface area and intimate contact between the embedded hollow Co3O4 active material and outside thin carbon shell. Because those above mentioned gratifying structure design and enhanced material properties, our sample exhibits high rate capability and outstanding reversible capacity when used as an advanced electrode material for Li-ion batteries.One-dimensional NiCo₂O₄@C composite was firstly prepared by hydrothermal method, and the comparisons between the Ni Co₂O₄@C composite array and bare NiCo₂O₄ nanowires array were investigated. The as-prepared peapod-like NiCo₂O₄@C nanorods array was directly grown on a three dimensional Ni-foam by a facile synthesis process, which includes two-steps hydrothermal reaction and consequently annealing treatments in inert and air atmospheres, respectively. Compared with other synthetic method, the sample in our experiment shown both great conductivity and high mesoporosity. All these admirable features sufficiently verify demonstrate the excellent energy storage performance in the electrochemical tests of Li-ion batteries, e.g. great rate capabilities, high specific capacities and superior cycling performance and coulombic efficiency.