Study On The Silicon-based Composites As Anode Materials For The Lithium Ion Batteries

In this thesis, the recent research development on the related materials for lithium-ion batteries (LIB) at home and abroad, especially Si-based anode materials, are detailed reviewed. Based on the advantages and disadvantages of Si as anode material, two Si-based nanocomposites (Si/Li4Ti5O12 and Si/C) with a core-shell structure were synthesized through Sol-Gel method and thermal vapor decomposition method, respectively. The microstructures of the synthesized Si-based nanocomposites were investigated by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) as well as element analysis, etc. The electrochemical properties of the composites were studied by galvanostatic charge-discharge, Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) methods. The effects of the fabrication parameters on the microstructures and electrochemical properties of the synthesized Si-based nanocomposites were systematically studied.The Si/Li4TisO12 composites with a thin shell of Li4Ti5O12 coating on the surface of silicon nanoparticle core were synthesized via a Sol-Gel method by using nano-Si powders, tetrabutyl titanate and lithium acetate as the starting materials. Sintering temperatures ranging 500-1000℃were used. The agglomeration of the synthesized Si/Li4Ti5O12 composites was reduced by the pre-treatment of the dry-Gel by ball milling with conductive agent, and hence, the electrochemical properties were improved effectively. The purity of the synthesized Li4Ti5O12 was high in the sintering conditions of 600-800℃, a few impurity phases would exist when the sintering temperature was too high or too low. The presence of Li4Ti5O12 coating-layer bated the formation of SEI film as well as weakened the silicon volume variation during Si alloying/dealloying with lithium, so that the cycle performance of composite materials was effectively improved. The reversible capacity of the synthesized composite with the sintering temperature of 700℃is 2075 mAh/g for the first cycle which is the largest of the synthesized samples, and 490 mAh/g after fiftieth cycle, which is two times of 245 mAh/g for the bare Si. The synthesized composite with the sintering temperature of 1000℃has a high initial Coulomb efficiency (80%) and better cycling stability, though its reversible capacity is relatively lower. The causation may be that there are more impurities which are inactive or weaker active.The Si/C nanocomposites with a uniform layer of carbon-coated were successfully prepared through thermal vapor decomposition method. Acetylene decomposed into amorphous carbon which through the temperature of 700-900℃for different time (15-90 min). The thickness of carbon coating was a few nanometers, and the carbon content increased from 1 wt%to 40 wt%along with the extention of the decomposition temperature and time. The cycle stability of nano-Si was significantly improved through the carbon uniformly coated on the surface of the Si nanoparticles. For example, the Si/C nanocomposite synthesized at 800℃for 30 min has a capacity of 705 mAh/g after 50 cycles, which is 3 times of the bare Si sample.