Synthesis And Lithium Storage Properties Of Ge/SnO₂ And Three-dimensional Graphene Composite Anode Materials For Lithium Ion Batteries

Rapid development of portable electronic devices,electrical vehicles,and renewable energy storage demands high-energy energy storage devices especially high-energy lithium ion batteries(LIBs).Ge and SnO2,which have high theoretical capacities of 1600 and 781 mA h g-1,respectively,attract increasing attention as anode materials for LIBs.However,drastic volume change during lithiation and delithition hinders their applications in LIBs.In this thesis,we prepared novel spider-like Ge@CNFs/3D graphene and SnO2@C/3D graphene electrodes and their electrochemical properties are investigated.Meanwhile,the morphology and structure dependent electrochemical properties were revealed.The main contents are as follows:(1)Three-dimensional graphene(3D graphene)foam is prepared by chemical vapor deposition(CVD)method using Ni foam as template,which is uses as substrate for the further growth of GeO2 nanoparticles through a facile solvothermal procedure.3D Ge@CNFs/3D graphene was produced by thermally treating the as-prepared GeO2/3D graphene foam under thermal reaction in the presence of benzene as carbon source.In this 3D nanocomposite,3D graphene can not only enhance the electricity of the composite electrode,but also facilitate diffusion of the electrolyte.At the same time,the carbon nanofibers(CNFs)grown in-situ on Ge nanoparticles(NPs)prevent Ge NPs detachment or breaking-down during cycling and avoid direct contact between Ge and the electrolyte leading to a stable Solid Electrolyte interface(SEI).The as-obtained spider-like Ge@CNFs/3D graphene composite shows a high capacity of 594 mAh g-1(based on total mass of the electrode)after 1200 cycles at a current density of 0.5 C(1 C=1600 mA h g-1).Furthermore,the Ge@CNFs/3D graphene nanocomposite delivers a reversible capacity of 474 mA h g-1 at 8 C,,corresponding to 74% capacity retention at 0.2C,showing good rate capability.The large capacity,long cycle stability and high rate capability of spider-like Ge@CNFs/3D graphene composite render promising application in high-performance LIBs.(2)The SnO2@C/3D graphene was prepared via hydrothermal growth of SnO nanosheet on 3D graphene,followed by glucose hydrothermal reaction and high temperature carbonization at 600 oC.3D graphene as the backbone enhances the electricity of the composite electrode and facilitates diffusion of the electrolyte.The carbon coating onto SnO2 nanosheet alleviates the volume change during alloy/de-alloy process and leads to the formation of a stable SEI.Meanwhile,the Sn O2 nanosheets connect together providing high stability of the electrode.The obtained SnO2@C/3D graphene composite electrode shows a capacity of 596 mA h g-1(based on total mass of the electrode)after 200 cycles at a current density of 0.25 C(1C=781 mA h g-1),decreasing only 0.2% for every cycle.When the current density increased from 0.2 to 6.4 C,66% capacity retention is obtained,showing high rate capability and good electrochemical performance.