Synthesis Of Carbon Composite Materials And Their Applications As Anode Materials For Lithium Ion Batteries

Lithium-ion battery is a new generation environment-friendly green batterywhich entered the market in1990s. They have been considered as the mostpromising power sources because they hold a series of considerable specificadvantages, such as high potential, high energy density, long cycle life, no memoryeffect and so on. The anode material of Li-ion battery is a key factor to control itsoverall performance. Most of metal, metal oxide and metal sulfide have highcapacities, but poor cycling stabilities. Carbon materials have good cyclingstabilities. Therefore, the preparation of carbon supported metal, metal oxide andmetal sulfide composite materials may improve the Li-ion storage properties andcycle stabilities of the present lithium battery system, which has great value ofpotential application.In this article, carbon composite materials, such as graphene nanosheet （GNS）,carbon nanotubes （CNTs） and carbon nanofibers （CNFs）, were the research targets.Graphene wrapped CoS nanoparticles and graphene-In₂S₃composites weresynthesized by the solvothermal method and microwave-assisted fast hydrothermalapproach, respectively. Carbon nanotube encapsulated CoO nanoparticles and carbonnanofibers （CoO-CNF@CNT） composite was prepared by a templated chemicalvapor deposition（CVD） method. The good electrochemical properties were foundwhen the materials were used as anode materials for Li-ion batteries.Graphene wrapped CoS nanoparticles were synthesized by a solvothermalapproach. The product was significantly different from porous CoS microspheresprepared in the absence of graphene under similar preparation conditions. Thenanocomposite exhibited an unprecedented high reversible capacity of1056mAh/gamong all cobalt sulfide-based anode materials, which still maintained71.3%after40cycles. The CoS/graphene composite was found to be better suitable as an anodematerial in terms of higher capacity and better cycling performances, which was attributed to the the synergistic action of graphene and cobalt sulfide. Good cyclingperformances area also observed at both small and high current rates.Flower-like In₂S₃was synthesized by the microwave-assisted fast hydrothermalapproach. The influence of differnent reaction temperature and time were also studied.The crystal form of In₂S₃was not changed with the change of the reaction temperatureand time. But the size of In₂S₃was increased with the rise of temperature. Thenanoflower nanostructure was formed by the self-assembly of the folded intermediateproduct of nanoparticles and nanosheets. When reaction time was increased further to30min, nanosheets were heavily folded and a heavy agglomeration of many flowerswas formed. In₂S₃-graphene nanoparticle-on-sheet and flower-on-sheet compositeswere also prepared by the same method. When fabricated as anode materials,In₂S₃-graphene composites showed extraordinary large reversible capacities and goodcycling performances and high rate capabilities at both small and high current rates.The reversible initial lithium-extraction capacities of1249mAh/g and1056mAh/gwere observed for flower-on-sheet nanostructure and nanoparticle-on-sheet at70mA/g, which larger than their theoretical values and the capacities of In₂S₃andgraphene. The charge capacities were657mAh/g and614mAh/g after40cycles,respectively. They also showed good capacities and cycling performances at largecurrents of1C，2C and5C. In comparison, the In₂S₃-graphene nanoparticle-on-sheetcomposite showed slightly lower reversible capacities but more stable cyclingperformances at both small and high currents. The presence of graphene can improvethe electrical conductivity of In₂S₃and reduce the resistance between electrode andelectrolyte interface. Graphene nanosheet also has strong toughness and mechanicalstability, improve the structural stability of In₂S₃during cycling.The presence ofIn₂S₃can prevent the agglomeration of graphene, thus maintaining their promisingproperties relative to their thin-layer structure during cycling.Carbon nanotube encapsulated CoO nanoparticles and carbon nanofibers（CoO-CNF@CNT） composite was synthesised by a templated chemical vapor deposition（CVD） method. CoO-CNF@CNT possessed good cycling performancesat both small and high current rates. The initial reversible capacity ofCoO-CNF@CNT composite was920mAh/g and the capacity reached630mAh/g upto40cycles. It was suggested that the high reversible capacity could be ascribed tothe fiber-in-tube structure and the confined volume change in the nanotube cavities.CNT can increase the electrical conductivity and mechanical strength of materials,and prevent the agglomeration of the CoO nanoparticles.