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Preparation and Properties of Carbon-coated Natural Graphite and Silicon-carbon Composite Anode Materials

Author: GanLei
Tutor: GuoHuaJun
School: Central South University
Course: Metallurgical Engineering
Keywords: natural graphite carbon-coating graphitization graphene silicon-carbon composite
CLC: TM912
Type: Master's thesis
Year: 2013
Downloads: 54
Quote: 0
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Abstract


Because natural graphite has shortcomings such as low coulombic efficiency, poor cycling performance and bad rate capability, a solution which uses carbon to coat on the surface of spherical natural graphite to form a core-shell structure is proposed.And silicon-based anode material with high capacity density is developed to break through the limitation of the theoretical capacity of graphite anode. In order to overcome the poor cycling stability of silicon-based anode, graphene is used as a carbon skeleton to improve the electronic conductivity and ease the volume effect of silicon particles caused by lithium insertion and extraction process.Asphalt and spherical natural graphite are directly mixed and carbonized to coat carbon on the surface of the graphite.The carbon-coated natural graphite composites are prepared by selecting the proper asphalt source,controlling the route of carbonization treatment and the amount of coated carbon.Composite material C-5%which is prepared by two stage carbonization treatment(first heating to150℃from room temperature and holding at150℃for1h,then heating to750℃and carbonizing at750℃for2h) and with a carbon coating amount of5%shows the best charge-discharge performance.Its initial charge capacity is361.8mAh/g with the coulombic efficiency of90.2%and capacity retention ratio after100cycles is93.0%. Carbon-coated composites are graphitized to prepare asphalt-based artificial graphite/spherical natural graphite composite materials. The graphitized composite CGI5which is composed by asphalt-based artificial graphite and spherical natural graphite with a mass ratio of15%has excellent performance in both initial coulombic efficiency (85.4%) and rate capability. Its reversible specific capacity is approximately343.7mAh/g,286mAh/g,230mAh/g and193mAh/g at the rate of0.1C (1C=300mA/g),1C,5C and10C, respectively.Flake graphite and nano silicon with a certain proportion by mass are dispersed in graphene oxide, then the suspension is used to prepare flake graphite/nano silicon/graphene composites by spray drying-heat treatment method.The influence of three-phase mass ratio of graphite, silicon and graphene on the electrochemical properties of composites is investigated in this paper. The optimal ratio of flake graphite, silicon and graphene is that the mass ratio of flake graphite to silicon is9:1and the graphene content in the composite is5.3%. Under this condition, the first charge specific capacity of the composite is575.1mAh/g with the initial coulombic efficiency of74.5%and its capacity retention ratio after50cycles is73.1%at a current density of50mA/g. It also shows a good rate performance.The reversible specific capacity is540mAh/g,480mAh/g and400mAh/g at a current density of200mA/g,500mA/g and1000mA/g, respectively.

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