Modification And Electrochemical Properties Of Graphitized Mesocarbon Microbeads For Lithium-Ion Battery

Since lithium ion batteries were introduced to the market by Sony Company in the early1990s, they have seen substantial growth in demand in recent ten years, and have been paid much attention by worldwide researchers and companies, due to their advanced electrochemical properties. Despite considerable efforts to find other substituents, carbon still remains the only commercial anode for the battery due to its steady structure and good circular properties.Carbon anodes for lithium ion batteries have such advantageous properties as good charge/discharge reversibility, low discharge platform, high specific capacity, high conductivity, and so on. But the main commercial carbon anodes have a rather lower specific mass capacity (theoretic specific mass capacity is lower than372mAh/g) compared with the metal anodes e.g. tin oxide and a rather high preparation cost. The structure and properties of carbon anode are affected by the origin and process condition of precursor remarkably. Based on the characteristics mentioned above, this thesis was focused on the study of different surface modifications of graphitized mesocarbon microbeads (MCMB). The preparation parameters, morphologies, structures and electrochemical behaviors of modified MCMB and their relations were investigated in order to develop a novel carbon anode with higher properties, and to prompt the research and commercialization of lithium ion batteries.In this thesis, a simple preparation method for spherical carbons is proposed. And the third anode SnO2/MCMB composites were prepared through homogeneous precipitation. The anode Si/MCMB composites were prepared through ball milling,The microstructure of these anode materials and their lithium insertion performance were studied by Scanning Electronic Microscopy (SEM), X-Ray Diffraction (XRD), Infrared (IR), Cyclic Voltammetry and charge-discharge measurements. The main research results of this dissertation are summarized as follows:1. The mesophase pitch and phenolic resin powders are employed as the precursor respectively. The irregular-shaped precursor powders are converted to spherica carbons by first coating with nano SiO2powder and carbonizing under nitrogen atmosphere. The conversion takes place near the softening point of the pitch and the glass transition temperature of the resin, respectively. The spherical carbon powders exhibit a good electrochemical performance,As a result, the initial coulombic efficiency of mesophase pitches-derived spherical carbon reached85%and its capacity maintained325mAh·g-1during20cycles.The initial coulombic efficiency of mesophase pitches-derived spherical carbon reached48%and the capacity of it maintained215mAh·g-1during20cycles.2. The SnO2/MCMB core-shell composite anode materials were synthesized via the homogeneous precipitation and their electrochemical performances were investigated. It was found that the electrochemical performance of the composite anode is critically dependent upon its composition, especially the tin oxide content. When the tin oxide content is50.0%, the experimental discharge and charge capacity are higher than that of others, and can reach623.2mAh/g and420.6mAh/g respectively, the initial coulombic efficiency of mesophase pitches-derived spherical carbon reached67.5%, As the tin oxide content increasing, the electrochemical behavior of the composite anode is much similar to that of the pure tin oxide.3. The MCMB and phenolic resin powders are employed as the precursor respectively, The Si/MCMB composite anode material are prepared by mixing with nano Si powder and mesophase pitch (phenolic resin) through ball milling. It was found that the structure and the electrochemical performance of the composite anode is critically dependent upon its composition. When the mass proportion of MCMB、 mesophase pitch、Si is5:2:1, the composite aonod material (MCMB/Si/Mesophase pitch)exhibit a good electrochemical performance. As a result, the initial discharge a capacity reachs754.9mAh/g. the initial coulombic efficiency of mesophase pitches-derived spherical carbon reached90.3%, and its capacity maintained600mAh·g-1during10cycles. Similarly,when mesophase pitch is taken place of phenolic resin, the initial discharge capacity of the composite aonod material (MCMB/Si/Phenolic resin) reachs731.3mAh/g. the initial coulombic efficiency of mesophase pitches-derived spherical carbon reached88.9%, and its capacity maintained640mAh·g-1during10cycles...