| Graphene possesses extensive applications in different fields such as micro-nano fabrication,sensors,and energy owing to its superior optical,thermal,electrical,and electrochemical properties.However,agglomeration easily occurs between two-dimensional graphene layers,which is unfavorable for fully exploiting its superior properties.The development of three-dimensional graphene is an important strategy for solving this problem.Currently,China has made it a key task in the research and development of new materials to realise the large-scale preparation of graphene materials and develop energy-storage devices that are modified by graphene.Silicon is one of the most promising energy-storage materials,due to the advantages of high lithium-storage capacity,abundant resources,and low cost.However,the poor conductivity and high volume expansion hinder its application as the anode material for lithium ion batteries.One of the effective strategy of improving the performance is employing graphene.In this thesis,the preparation and structure controlling of three-dimensional graphene were mainly studied.The as-prepared three-dimensional graphene was utilized to improve the lithium-storage performance of silicon-based anodes.Concretely,thermal chemical vapor deposition was used to grow vertically oriented three-dimensional graphene on silicon particle substrate.Methane and hydrogen were employed as the carbon source and etching gas,respectively.Meanwhile,effect of the following factors on the structure of three-dimensional graphene was studied,including the concentration of methane,temperature,average particle size of the silicon substrate,growth time,and surface carbon coating.The scanning electron microscopy results show that the optimized concentration of methane for growing three-dimensional graphene is dependent on the temperature.The higher the temperature,the lower the concentration required.The optimized concentrations of methane at 1050 ℃,1100 ℃,1150 ℃,and 1200 ℃ were proved to be 20%,14.3%,11.1%,and 4.8%,respectively.Although the average particle size of the silicon substrate has no significant effect on the morphology of the three-dimensional graphene,the results of X-ray diffraction and Raman spectroscopy indicate that silicon carbide by-products would form during CVD process when the average particle size of silicon substrate is 50 nm.Differently,when the average particle diameter of silicon substrate is 100 nm or larger,no silicon carbide is formed.By gradually optimizing the preparation conditions,the three-dimensional graphene/silicon composites and three-dimensional graphene/carbon-coated silicon composites with good dispersibility were obtained.Vertically oriented graphene nanosheets are uniformly distributed on the surface of silicon particles and carbon-coated silicon particles,which connect with each other,forming a well-developed three-dimensional conductive network.The optimum samples were verified to be prepared at the following condition: temperature,1050 ℃;silicon particle diameter,100 nm;growth time,3 h;methane concentration,20%.In order to study the effect of carbon coating and three-dimensional graphene on the lithium-storage performance of silicon-based anodes,electrochemical analysis was performed on three-dimensional graphene/silicon composites,carbon-coated silicon composites,and three-dimensional graphene/carbon coated silicon composites.The constant current charge and discharge cycling curves show that the three-dimensional graphene/carbon-coated silicon composites possess the best lithium-storage performance.After further optimizing the carbon content and silicon mass loading of the three-dimensional graphene/carbon-coated silicon composites,an anode material with excellent lithium-storage performance was obtained,which has an initial specific capacity of 3000 mAh/g at the current rate of 0.1 C.The specific capacity still remained 3000 mAh/g even after 120 cycles,and was about 2400 mAh/g after 240 cycles with a capacity retention of 80%. |
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