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Oxide/Carbon Composites as Anode Materials for Lithium Ion Batteries

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Tutor: YinLongWei
School: Shandong University
Course: Materials Science
Keywords: nanomaterials,carbon,metal oxide,electrochemistry,lithium ionbatteries
CLC: TM912
Type: Master's thesis
Year:  2012
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Lithium ion batteries (LIBs) with high voltage, big capacity and long cycling life, have been widely used in portable communication equipments such as mobile phones and laptop. The primary anode materials used for LIBs were graphite materials; however, the theoretical capacity of graphite was only372mAhg-1, which could not meet the need of new LIBs with high capacity. Compared with graphite materials, new carbon materials (such as carbon nanotubes and ordered mesoporous carbon) and transition metal oxides (such as manganese oxide, iron oxide, nickel oxide and copper oxide) have higher capacity. Nevertheless, transition metal oxides displayed capacity degradation due to the pulverization of electrode materials caused by volume expansion and contraction during charging and discharging progress. The large amounts of active sites due to the high specific surface of mesoporous carbon result in the high irreversible capacity. One effective way to gain the stabile performance is to fabricate composites of transition metal oxides and carbon materials, which can take full advantages of high capacity of oxides and good electrical conductivity of carbon materials, and avoid the structural deficiency of a single material. In this paper, the composite materials of transition metal oxides and carbon materials were synthesized. The composites can effectively reduce the capacity decay caused by volume change, and decrease the irreversible capacity of ordered mesoporous carbon. The main contents are summarized as follows:1. A facile one-step solvothermal reaction route to carbon homogeneously wrapped manganese oxide (Mn3O4@C) nanocomposites was developed using manganese acetate monohydrate and polyvinylpyrrolidone as precursors and reactants. The synthesized tetragonal structured Mn3O4(space group:I41/amd) samples display nanorod-like morphology with a width of about200-300nm and a thickness of about15-20nm. It is shown that the carbon layers with a thickness of5nm are homogeneously coated on the Mn3O4nenorods, and the weight mass of carbon was1.2%. Cyclic voltammetry (CV) and galvanostatically cycling were used to determine the electrochemical performance. It is indicated that even after50cycles at the current density of40mAg-1, the products remain stable capacity of473mAhg-1, which is as much3.05times as that of pure Mn3O4samples. Due to the low-cost, non-pollution and stable capacity, the carbon homogeneously coated Mn3O4@C nanocomposites are promising anode material for lithium ion batteries.2. Iron oxide and nickel oxide modified mesoporous carbon were synthesized by a two-step method. The FeOx/CMK-3and NiO/CMK-3were synthesized using OMC as a matrix, CTAB as a surfactant, FeCl3and Ni(CH3COO)2¡¤4H2O as metal oxides sources using hydrothermal and precipitation method, respectively. The product of FeOx/CMK-3was composed of Fe3O4with cubic structure (space group:Fd-3m) and hematite Fe2O3(¦Á-Fe2O3) with rhombohedral structure (space group:R-3c). The iron oxide particle size was50-100nm. The sample of NiO/CMK-3was composed of nickel oxide (NiO) with cubic structure with the particle size of50-80nm. Charge/discharge tests were used to determine the electrochemical performance. The initial coulombic efficiency, the reversible capacity and cycling performance were enhanced for the two metal oxide/CMK-3composite samples. Even at the current density of1600mAg"1, the capacity were still kept about304mAhg-1and230mAhg-1for FeOx/CMK-3and NiO/CMK-3electrodes, respectively, which were much better than that of pure OMC (58mAhg-1).3. Ordered mesoporous carbon composited with copper and cuprous oxide (CuxO/C) was synthesized through reduction method. The CuxO/C sample was prepared using OMC as a matrix, Cu(NO3)2¡¤3H2O as copper sources and ethylene glycol (EG) as reductant. There are two phases in the composites:copper and cuprous oxide. The partical size of copper oxides is nonuniform, from dozens of nanometers to hundreds of nanometers. With the different addition of copper sources, the amount of mess rate of copper is38%,44%,62%, respectively. The electrochemical performance was tested by electrochemical impedance spectroscopy (EIS) and charge/discharge progress. It was found that the sample of0.5CuxO/C(38wt.%) had the best electrochemical performance, even after100cycles it still kept the capacity of346mAhg"1at the high current density of372mAg-1. Similarly, the sample of 0.5CuxO/C had better lithium performance in the rate test.
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