Preparation And Characterization Of Lithium Negative Graphene-based Composite Materials

With the rapid develop pment of the society, electronic information industry has developed as well, meanwhile more and more attention is paid on all kinds of portable electronic products. Compared to other batteries, the lithium ion battery (LIB), has the higher energy, higher working voltage and circulation performance, etc. Therefore, the research on secondary li-ion battery as a portable digital product has received much attention.The LIB is composed of the positive, the negative electrode and electrolyte etc. The lithium ion battery anode materials are the important factors which affect the comprehensive performance of the LIB. Among the materials, carbon materials are the first one been researched and applied in the lithium ion battery anode materials. But carbon anode materials have the following disadvantages:the low specific capacitance, the low first coulomb's efficiency and the bad discharge performance, etc. Therefore, the development of novel lithium ion battery anode material with high specific capacity, good circle performance and good safety performance has become the focus of research.In this thesis, high performance li-ion battery anode materials were prepared by combining graphene with molybdenum oxide or tin cobalt alloy, and their electrechemica performances were also tested. The structures of the materials were characterized by XRD, TEM, SEM and BET. The electrochemical performance of samples was evaluated by galvanostatic charge-discharge, electrochemical impedance spectra and cyclic voltammetry. The experimental results are listed follows.1. Graphene was obtained by reducing expanded graphite using hydrazine hydrate, which was prepared through high temperature expantion. Oxidation graphite was prepared by modified Hummers method. The graphene has a specific reversible capacity of1022.7mA h g"1in the first cycle and maintains a capacity of657.3mA h g-1after30cycles.2. MoO3-graphene composites were prepared through hydrothermal method using inexpensive graphite and (NH4)6Mo7O24-4H2O as the raw materials. The SEM images of MoO3-graphene composites show that dispersion degree is different with changing the hydrothermal temperature. When the ratio of MoO3-graphene is5:2, the hydrothermal temperature150℃and the hydrothermal time30h, the MoO3-graphene composite anode has the optimal electrochemical performance, the initial reversible capacity is977.9mA h g-1and the coulombic efficiencies70%. The MoO3-graphene composite anode maintained a capacity of856.4mA h g-1after80cycles.3. The Sn-Co-C-graphene composite material was prepared by the sol-gel method. The composite material was synthesized under the conditions:the ratio of stannic chloride to cobalt acetate was2.5:1and the amount of phenolic resin was0.25g with different content of graphene. It was found that when the content of graphene is9.5%(TCG-3) the composite anode had the best electrochemical performance. XRD showed that there are existed three crystalling phases including tin, CoSn2alloy and CoSn alloy in TCG-3. The homodispersion of metal alloy particles on graphene could be observed from the TEM. The initial reversible capacity of TCG-3electrode is837.2mA h g"1and it still remains a capacity796.6mA h g-1after80cycles, which is95.2%of the first charge capacity.