The Preparation And Characteration Of Si/C Composite Anodes For Lithium Ion Batteries

Silicon is regarded as one of the most promising anodes for the next generation of high energy density Li-ion batteries due to its large theoretical specific capacity and low Li-insertion potential (vs. Li/Li+). In this thesis, the research process of anodes for lithium-ion batteries has been summarized in detail and three types of silicon/graphite/carbon composites have been synthesized by several methods. The three composites are separately silicon/spherical natural graphite/carbon composite, silicon/porous graphite/carbon composite and silicon/flake graphite/carbon spherical composite. And the morphologies, phase components and electrochemical properties of these materials also have been investigated.The effects of silicon content, carbon source, dispersion method, calcination temperature and pyrolytic carbon content on the structure, morphology and electrochemical properties of Si/C composites have been studied. And the optimum conditions have been obtained. Under the optimum conditions, silicon/spherical natural graphite/carbon composite (silicon:graphite:carbon=25:50:25) with the best electrochemical performance is synthesized via twice ultrasonic-mechanical mixing and subsequent pyrolysis (800℃,2h). The raw materials of the composite are nano-Si, spherical natural graphite and vinylidene chloride (CPVC), and the composite shows an initial reversible capacity of719.44mAh/g with an initial coulombic efficiency of63.22%, and a reversible capacity of446.80mAh/g with a capacity retention rate of62.10%after20cycles.Liquid phase oxidation of spherical natural graphite has been used to obtain porous graphite and silicon/porous graphite/carbon composite has been prepared. First of all, the effects of different oxidants and different amounts of oxidant on the structure, morphology and electrochemical properties of porous graphite have been studied. The results show that porous graphite oxidized by K2Cr2O7and H2SO4has a lot of micron pores on the surface, and presents higher discharge capacity (1206.50mAh/g) and charge capacity (445.47mAh/g) than spherical natural graphite without oxidation, but its initial coulombic efficiency is only36.92%and the reversible capacity for the20th cycle is only354.36mAh/g with a capacity retention rate of79.55%. Then, silicon/porous graphite/carbon composite is synthesized by twice carbon coating with nano-Si, porous graphite and CPVC, which exhibits an initial reversible capacity of728.52mAh/g with an initial coulombic efficiency of53.23%, and a reversible capacity of370.45mAh/g with a capacity retention rate of50.85%after20cycles.Using Nano-Si, flake graphite (<1μm) and glucose as raw materials, silicon/flake graphite/carbon composites are synthesized by a spray drying and subsequent pyrolysis approach, and the effects of different dispersants on the structure, morphology and electrochemical properties of the composites and their precursors have been investigated. Silicon/flake graphite/carbon composite prepared under the optimum conditions is spherical and porous particles with a diameter of about2-6μm, and delivers an initial discharge capacity of864.54mAh/g, an initial charge capacity of602.68mAh/g and an initial coulombic efficiency of69.71%. After20cycles, the reversible capacity of the composite keeps at551.95mAh/g with a capacity retention rate of91.58%.