Study On Preparation And Electrochemical Performance Of Silicon/Carbon Anode Materials For Lithium Ion Batteries

Silicon（Si） is a very promising candidate anode material for next-generation lithium ion batteries,as it can alloy with lithium and possesses the high theoretical energy storage capacity of 4200 mA h g^-1, more than 10 times higher than that of graphitic carbon(372mA h g^-1). However, the large volume change of Si（>300%） during repeated lithium insertion and extraction often causes electrode pulverization and gradual loss of electric contact between Si and the current collector, leading to the decrease of the electrical conductivity and inefficient utilization of Si. In general, the above phenomena can cause serious irreversible capacity and poor cyclability.In recent years, significant progress has been made in improving the electrochemical performance of Si-based anodes through preparing nanostructure materials,composites, and porous mat;erials devoted to alleviating the volume change during cycling and to improving the conductivhy. Based on these methods, in this thesis, porous and nano structure silicon/carbon composites anode materials for lithium ion batteries were prepared, characterized by physic and chemical technique, and carried out systematic research on their electrochemical performance.Firstly, in order to develop the carbon anodes from the waste the carbon powders, we employed the spray drying method to make the porous carbon microspheres from the representative fine needle coke and graphitized needle coke powders using sucrose as the binder. The prepared porous carbon microspheres had not only the preferred spherical morphology, but also the well-developed inner porous structure and hard carbon network,very obvious structural characteristics conducive to the high performance carbon anodes. It was found that the capacity of graphhized microspheres was comparable to that of graphke microspheres but with much better rate performance. The work demonstrated the hightechnical feasibility of making microspherical carbon-based anode materials in lithium ion batteries from graphitized and non-graphitized fine carbon powders, which had guiding significance for recycling the waste fine carbon powders in industrial production.Secondly, the tap density of porous carbon microspheres （0.47-0.56 g mL-1） was less than that of graphite microspheres （1.18 g mL-1） because of the existence of inner porosity, leading to relatively low volume energy density of porous carbon microspheres. Therefore, in this section, we prepared porous Si/C microspheres by the successive ball milling and spray drying procedures using waste fine graphitized needle coke as the primary carbon resource, and Si nanoparticles as the Si source, as well as sucrose as the binder. The obtained spheres were carbonized in high temperature calcination in N2 and further deposited with carbon by the chemical vapor deposition method. It was found that the reversible capacity of carbon-coated porous Si/C microspheres was significantly more than graphite microspheres, as well as excellent cycling performance and rate performance. This work demonstrated the high technical feasibility of making high-performance micro-spherical Si/C anode materials for lithium ion batteries from graphitized fine carbon powders and Si nanoparticles.Thirdly, for further improving the reversible capacity of Si-based anode materials, we designed and developed a simpler and greener approach for the preparation of porous Si by directly reacting metallurgical-grade Si powders with ethanol in the presence of Cu-based catalysts in an autoclave. The Si/C composites were obtained after chemical vapor deposition of carbon. Differing from the established synthetic routes, this new route effectively avoided the use of any expensive equipment and template, highly toxic reactants （e.g., SH4 and HF） and complicated processing steps （e.g., template removal and chemical etching） needed to produce Si anodes with comparable cost and scalability to graphite anodes. Furthermore, the pore size, shape, depth and wall thickness, particle size, and the yield of porous Si materials could be readily tuned by varying the synthesis conditions. The synthesized Si/C composites were used as anode materials for lithium ion batteries which showed both high reversible capacity and good cycle life. This low-cost, easy-handling and scalable fabrication process of the high performance porous Si/C anode materials should contribute to the next generation lithium ion batteries.Fourthly, based on the section 3. we prepared Si nanoparticles by directly reacting metallurgical-grade Si powders with more ethanol and more reaction time in the presence of Cu-based catalysts in an autoclave, followed by ball milling. The size of Si nanoparticles could be controlled by adjusting the reaction time. The Si/C nanocomposites could be obtained after chemical vapor deposition of carbon on the surface of Si nanoparticles. When used as anode materials of lithium ion batteries, the Si/C nanocomposites exhibited high reversible capacity and excellent cycling stability. More importantly, the use of Cu composite and metallurgical-grade bulk Si, which was much cheaper than the noble metals and could be recycled, was expected to significantly lower the cost of Si nanostructure materials, making it possible for facile and low-cost production of Si nanostructure materials for applications in lithium ion batteries.At last, based on our previous research, we prepared Si/C nano branches via Rochow reaction by reacting metallurgical-grade Si powders with CH₃Cl. Si/C nano branches were fabricated by directly reacting metallurgical-grade Si powders with CH₃Cl in the presence of Cu-based catalysts in a fixed bed, followed by acid etching of Cu compounds. The obtained Si/C nano branches as anode materials of lithium ion batteries showed good electrochemical performance. The overall method was simple, energy-efficient and easy to scale-up for fabrication of Si/C nano-composites for lithium ion batteries.