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The Study of Si/Graphene Nanocomposite as Anode Materials for Lithium-ion Batteries

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Tutor: PanQinMin
School: Harbin Institute of Technology
Course: Chemical Engineering and Technology
Keywords: Lithium-ion battery,Nano-Si,Graphene,Electrostatic self-assembly,Three-dimension
CLC: TM912
Type: Master's thesis
Year:  2012
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Owing to silicon exhibits the highest known theoretical specific capacity(4200mAh¡¤g-1), which is expected to replace the traditional graphite anode and becomenewly anode material for lithium-ion battery. Unfortunately, the commercial applicationof silicon as an andoe material is hindered by the huge volume changes £¨>300%£© andlow electrical contact between silicon paticles during repeated charging/discharging,leading to dramatic pulverization of silicon particles and rapid capacity fading. In thispaper, we improved the cycling performance of silicon anode by using graphene thathas high specific surface area, excellent electrical conductivity and strong mechanicaltoughness. Specifically, we used electrostatic self-assembly to obtain Si/G composite,also fabricated three-dimensional£¨3D£© porous Si/G composite via VC reduction andfreeing-drying. Finally, we studied their structure and composition also with theelectrochemical performance.The Si/G composite was prepared by electrostatic self-assembly. XRD, SEM, TEMtests proved that effective mixture between the nano-Si particles and graphene happened,and the result of TG text showed the content of graphene in the Si/G composite was25%. The Si/G composite had better electrochemical performance than Si anode. Whenthe current density is200mA¡¤g-1, the first delithiation capacity was1061mAh¡¤g-1, and itsreversible capacity could remain at707mAh¡¤g-1after50cycles, while Si anode only had62mAh¡¤g-1. The reason was that graphene connected Si nanoparticles to each other,which acted as a conductive network, prevented the aggregation of the Si particles andenhanced the electrical contact between the Si particles.We prepared the3D porous structure of Si/G composite via VC reduction andfreeing-drying. SEM and TEM texts proved that Si nanopaticles uniformly dispersedinto graphene sheets, and graphene sheets stacked into each other to form athree-dimensional porous structure. TG text showed the mass fraction of graphene in thecomposite was40.72%. This material exhibited good cycling performance, its reversiblecapacity kept at756mAh¡¤g-1after50cycles. Pores in the3D Si/G composite wasformed via self-assembly of graphene sheets, which could effectively buffer the volumeexpansion of silicon in the process of lithiation and delithiation, promoted thepenetration of electrolyte, also the fast transport of lithium-ion and electron.
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