Lithium Ion Batteries, The Synthesis And Performance Characterization Of Carbon Anode Materials

Recently, graphite and modified graphite are mostly used as anode material in commercial lithium-ion batteries. The graphite electrode has many advantages such as flat potentials as low as Li metal. small volume expansion, and high reversibility of lithium-ion intercalation or de-intercalation. However, at high rates, the graphite electrode can be polarized to such an extent that lithium dendrites/moss form on the surface of the negative electrode surface, potentially rendering thermal runaway because of internal shorts triggered. Exploring new generation anode materials with higher capacity has become the focus of the research on lithium-ion batteries. It has been demonstrated that hard carbon, present higher capacity, lithium intercalation potential, higher rate ability and compatible with propylene carbonate. However, the low coulombic efficiency at first cycle and large hysteresis hinder the application of hard carbon. How to circumvent the limitations of hard carbon is the key point to develop high performance anode. Moreover, Exploring new generation anode materials with higher capacity and high rate has become the focus of the research on lithium-ion batteries.This dissertation is on the foundation of existing research, and creatively introduces a facile method to prepare A series of soft carbon modified hard carbon materials with different amount of pitch at different heat-treatment temperatures. The initial coulombic efficiency and rate capability of the sample were improved. In addition, this dissertation explores the nano-porous graphitized carbons anode, and the electrochemical performance of this material has also been investigated. The ordered three-dimension structure anode is also included in this dissertation. The graphene/Si material anode is also included in this dissertation.In Chapter3. A series of soft carbon modified hard carbon materials with different amount of pitch at different heat-treatment temperatures was prepared. The effects of heat-treated temperature and the amount of soft carbon from the pitch on the internal structure, the charge/discharge profile, the irreversible capacity, the rate capability and the cycling stability were detailedly studied. The sample obtained at1200℃with30wt%soft carbon exhibits a reversible capacity of about290mAh/g with a high initial coulombic efficiency of81%. Furthermore, it also displays high power performance and excellent cyclicity.The three-dimensional structure materials have some unique advantages and show great promising for enhancing the performance of rechargeable lithium-ion batteries. In Chapter4, nano-porous graphitized carbons were successfully prepared by a CVD process using SBA-15as a template and toluene as a carbon source with final heat treatment temperature of2500℃. The rate capability of lithium-ion intercalation/de-intercalation of the nano-porous graphitized carbons is outstanding than the artifical graphite. The excellent electrochemical performance could be attributed to the presence of the porosity and platelet graphitized pore walls.In Chapter5, the graphene/nanosized silicon composites was prepared by using assisted by ultrasonic wave. The electrochemical properties of the composite were investigated. The graphene/nanosized silicon composites shows the high initial coulombic efficiency and excellent cycling stability. The significant enhancement on cycling stability could be ascribed to the better electrical conductivity of the graphene materials and absorption of volume changes of silicon by graphene sheets during the lithiation/delithiation process.