| The rapid development of high-power lithium ion battery has been requested to meet the growing demand for the miniaturization of portable electronic devices and electric vehicles.Moreover,the key technology of high energy density lithium ion battery is determined by the research and application of the electrode materials with high capacity and outstanding cycle performance.Due to low specific capacity(330 mAh g-1)for commercial carbon electrodes,cobalt oxide materials with high theory capacity has been intensively investigated as a high-energy store electrical material.Although Co3O4 has large theoretical lithium storage capacity(890 mAh g-1),it still suffers from low initial coulombic efficiency and poor cycling stability.Various kinds of methods,used for fabricating Co3O4 materials with different morphologies and sizes or Co3O4 composites,have been investigated to enhance their electrochemical performances.In this thesis,Co3O4 nanosheets and Cu/Co3O4 composites were prepared by a simple microwave-assisted and liquid-phase precipitation method.And their morphologies and Li-storage performances were evaluated by XRD,SEM,BET and electrochemical testing.The main research contents are summarized as following:?1?Nano sheet-like ?-Co?OH?2 precursors were synthesized by the route of liquid-phase precipitation under the condition of microwave radiation.The influence of microwave radiation reaction time and cobalt salt concentration on the morphology and structure of the obtained precursor was also investigated.The results shows that sheet-like?-Co?OH?2 can be formed under microwave radiation for one minute at 60°C.And the sheet-like morphologies got little change as reaction time prolonged.when the concentration of cobalt salt was more than 0.1 M,the ?-Co?OH?2 precursor can be confirmed to form sheet-like morphology.The morphology and electrochemical performance for both Co3O4 nanosheets formed by microwave-assisted method and Co3O4 nanoparticles formed by conventional heating method were compared and investigated.As for anode material,Co3O4 nanosheets prepared by microwave radiation method deliver an initial discharge capacity of1204.7 mAh g-1 at a current density of 100 mAh g-1,and they can remain a stable reversible capacity of 800 mAh g-1 after 30 cycles,which is far more than 400 mAh g-1 for Co3O4 nanoparticles prepared by conventional heating method.?2?The sheet-like morphology of ?-Co?OH?2 precursors can be controllably tailored through the addition of an appropriate amount of ammonia under the condition of microwaveradiation without using surfactants or templates.Under optimum conditions,Co3O4 nanosheets with appropriate size and morphology were successfully obtained to improve their lithium storage property.The optimized Co3O4 retains the original nano-porous plate-like structure with more homogeneous size,which could greatly improve its electrochemical performance.Electrochemical testing shows that the initial capacity of the electrode is about1153 mAh g-1,and the initial coulombic efficiency is 74.8%,which is higher than 73.7% for the unoptimized Co3O4.Moreover,the optimized Co3O4 sheets not only can maintain a stable reversible capacity of 922 mAh g-1 after 100 cycles at 0.1 C,but also have good rate capabilities.?3?The reaction conditions of preparation for Cu/Co3O4 composites were explored preliminarily.Copper nanoparticles were deposited on the surface of Co3O4 sheets by chemical reduction to form Cu/Co3O4 composites successfully.The Cu/Co3O4 composites electrode deliver an initial discharge capacity of 1208 mAh g-1 at 0.1 C.And the initial coulombic efficiency for Cu/Co3O4 composites is 82.24% higher than 74.8% for Co3O4 sheets.The results of ac impedance spectra demonstrated that electrical conductivity of Cu/Co3O4 composites were better than that of Co3O4 sheets,implying that addition of Cu would be an efficient method to improve the electronic properties of Co3O4 sheets by decreasing the resistance of the electrode materials. |
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