| Lithium ion battery has a variety of applications due to its high energy density and no memory effect. It can not only be used in portable electronic devices, but also be promising in electric vehicle and energy storage devices. At present the mainly commercialized anode material is graphite. Graphite has a low theoretical capacity and safety problems, so researchers are seeking new materials with higher theoretical capacity and more safety. Oxide anode materials owing high theoretic capacity, good cycleability and good safety, are ideal candidates as anode mterials for lithium ion batteries. However, the disadvantages of poor conductivity, high irreversible capacity and dramatic volume change during cycling process hinder their practical applications. Studies indicate that nanocrystallization, carbon coating, morphology controlling can improve the conductivity and buffer the volume expansion during cycling process. In this Thesis, we prepare oxide nanomaterials by precipitation and hydrothermal method, and treat oxide nanomaterials by carbon coating. The as-prepared materials are characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The electrochemical performance is studied by Cyclic Voltammetry, a.c. impedance spectroscopy and galvanic charge and discharge. The relationship between morphologies and electrochemical performances is also discussed. The main contents and results listed below:CO3O4nanoparticles are prepared by simple chemical deposition method using urea as precipitating agent. SEM and TEM images display that the diameter of these particles is less than200nm. Compared with the large sized particles, nanometer sized particles have large specific surface which provide more channels for lithium ions and more stable structure, so they may have better electrochemical properties. The prepared CO3O4nanoparticles deliver initial discharge capacity of802mAh g-1at a current density of100mA g-1, and retain580mAh g-1over100cycles, showing good cycling performance. CO3O4nanoplates are prepared by hydrothermal method. The process includes depositing Co(NO3)2at85℃, hydrothermal synthesis at200℃to acquire lamellar precursor, and obtain CO3O4nanoplates by calcination in air atmosphere. It was found that the lamellar precursor calcinated at600℃can form porous CO3O4nanoplate. The electrochemical tests show that the porous CO3O4nanoplates deliver initial discharge capacity of749mAh g-1at a current density of 100mA g-1. After40cycles, the remaining capacity is above700mAh g-1. Its good electrochemical performance is attributed to the porous nanoplate structure having advantages of large area contacting with electrolyte, fast charge transfer rate and effectively buffering for volume change during cycling process.Porous NiO nanosheets are prepared by hydrothermal method. The flaky precursor is obtained at200℃and transforms into NiO by calcinations. CO2and H2O are released during the transforming process, forming many irregular pores on the nanosheets. SEM images show that thickness of the as prepared porous NiO nanosheets is about100nm, and the length and width is a few micrometers. The porous NiO nanosheets deliver initial discharge capacity of689mAh g-1at a current density of100mA g-1, and remain644mAh g-1after20cycles. Compared with porous NiO nanosheets, bulk NiO materials delivers capacity of208mAh g-1after20cycles at the same current density, which means that sheet-like porous structure can effectively improve the electrochemical performance of NiO. At a current density of400mA g-1, porous NiO nanosheets deliver reversible capacity of about400mAh g-1, which show good electrochemical performance at a high rate condition. The good cycling performance of porous NiO nanosheets is attributed to its flaky two-dimensional structure and the wherein pores, which can increase contact area with electrolyte, shorten lithium ion diffusion distance, buffer volume expansion during cycling process.SiO2didn’t have good electrochemical performance because of its poor conductivity. In this thesis, carbon-coated SiO2nanobeads with different carbon content were prepared by using sucrose as carbon source. SEM and TEM pictures show that the SiO2nanobeads are embedded in amorphous carbon matrix. The amorphous carbon around silica beads form a network structure, which can improve the electronic conductivity between silica beads and buffer the volume expansion during cycling process, thereby improves the electrochemical properties. The a.c. impedance tests show that carbon layer coated on SiO2can effectively diminish interfacial impedance. The SiO2-C composite with50%carbon content delivers highest reversible capacity. It shows initial capacity of536mAh g-1and remains above500mAh g-1after50cycles, exhibiting good electrochemical performance. Charge and discharge tests show that the sample prepared by coating delivers higher specific capacity than those prepared by mixing with the same carbon content. Hard carbon has good compatibility with electrolyte and good safety, so it is also expected to replace graphite as anode material for lithium ion batteries. However, it has disadvantages of large irreversible capacity and low specific capacity. In this study, we focus on a hard carbon material of bamboo charcoal. The bamboo charcoal samples are prepared by milling and sulfuric acid treating. The effect of particle size and sulfuric acid treated time on the electrochemical performance of bamboo charcoal is investigated. The results show that initial Coulombic efficiency drops with the decresing of particle size. It is because that small size particle has big surface and lose more lithium during formation of SEI film. Sulfuric acid treatment can raise the specific capacity of bamboo charcoal by reducing the metal impurities, forming-HSO3and-HSO4groups which can reversibly react with lithium ion, taking channels on the surface for lithium ion migratting. The bamboo charcoal electrode prepared by sulfuric acid treatment for18h has a discharge capacity of328.2mAh g-1and it remains302.3mAh g-1after50cycles, exhibiting good electrochemical property. |
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