Design And Preparation Of Silicon And Germanium Based Anode Materials With High Performance

Rechargeable batteries with a high specific energy are vital for a wide variety of ap-plications in portable,transportable,and stationary energy storage applications.Among various energy storage technologies,lithium ion batteries(LIBs)are most attractive en-ergy storage system due to high energy density,wide working temperature ranges and long cycle life.Conventional graphite anode have been widely used as anode mate-rials for LIBs,but the relative lower theoretical capacity(372 mAh g-1)and safety issues prevents its application.Recently,many high theoretical capacity anode mate-rials based on alloy reaction mechanism such as Si,Ge,and metal oxides have been considered as the alternative high-capacity anode materials for traditional carbon-based materials.However,the limited and unevenly distributed availability of lithium in earth prevent its wide application in large scale application.As LIBs alternatives,sodium-ion batteries(SIBs)have attracted increasing attention because of the abundant resources and low cost of sodium resource.As for SIBs,most usually applied electrode materi-als for LIBs are not suitable for Na+ insertion due to the radius of Na+ is 55%larger than that of Li+.Many efforts have been made to develop superior cathode materials with large interstitial space to achieve rapid ion insertion and extraction.In this thesis,we focus on the investigation of silicon and germanium based materials,designing and constructing several nanostructure materials with high performance.We also study ad-vanced cathode materials Na3V2(PO4)3 with excellent electrochemical perforinance for SIBs.In chapter 1,we focus on the recent progress of anode and cathode materials for LIBs and SIBs.The reaction mechanism and crystal structures of electrode materials are discussed.Based on the review,we provide the research background and approaches.In chapter 2,we provided the information of precursor materials,characterization methods and research equipment in the thesis.In chapter3,we gave a general strategy for the fabrication of freestanding sandwich-like graphene-based hybrid films by electrostatic adsorption and following reduction reaction.We demonstrate that by rational control of pH value in precursors,GO sheets can form 3D sandwich frameworks with nanoparticles decorated between the layers of graphene.In our proof-of-concept study,we prepared the graphene/Si/graphene(G@Si@G)sandwich-like films.When used as negative electrode materials for LIBs,it exhibited superior lithium-ion storage performance(1800 mA hg-1 after 40 cycles at 100 mA g-1.Importantly,with this simple and general method,we also success-fully synthesized graphene/Fe2O3/graphene and graphene/TiO2/graphene hybrid films,showing improved electrochemical performance.In chapter 4,we provided a simple method to synthesize Ge-reduced graphene oxide nanocomposites using organic germanium as a precursor.The nanocomposites exhibit improved electrochemical performance with a reversible specific capacity of 814mAhg-1 after 50 cycles at a current density of 0.1 Ag-1.When cycled at a high current density of 2 A g-1,it still delivers a reversible specific capacity of 690 mA hg-1 after 150 cycles.In chapter 5,we provided a general strategy for the synthesis of one-dimensional(ID)organic-inorganic hybrid Ge-based nanowires in the present of transition metal via a simple one-step solvothermal method.Through this strategy,various transition metal/Ge oxides-ethylendiamie(EDA)hybrid materials,including Fe,Mn,Co,Zn and bimetal Fe-Co,have been successfully synthesized.We demonstrated that more transi-tion metal/Ge oxides-EDA nanowires could be synthesized,which can greatly broaden the application of organic-inorganic hybrid materials.What is more,1D Ge-based car-bon nanowires prepared by carbonization of transition metal/Ge oxides-EDA nanowires are employed as anodes for LIBs,exhibiting excellent lithium storage performance.In chapter 6,we designed porous Ge-Fe bimetal oxide nanowires(Ge-Fe-Ox-700 NWs)synthesized by a large-scale and facile solvothermal reaction.When used as anode materials for LIBs,the porous Ge-Fe-Ox-700 NWs exhibited superior electro-chemical performance(1120 mAh g-1 at a current density of 100 mA g-1and good cycling performance(750 mAh g-1 after 50 cycles at a current density of 100 mAg-1).In chapter 6,we provided carbon coated Na3V2(PO4)3 anchored on freestanding graphite foam(denoted as NVP@C-GF)as cathode for SIBs.The resulting NVP@C-GF possesses the advantages of 3D free-standing graphite that offers high electrical conductivity and porous structure for electrolyte to soak in.Furthermore,carbon coated NVP particles anchored on the surface of GF not only accommodates the volume change of NVP during charge/discharge,but alsso reduces the diffusion distance of Na+ ion.The NVP@C-GF exhibits superior sodium-ion storage performance,including rate ca-pability(56 mAhg-1 at 200C)and long cycle life(54 mAhg-1 at 100 C after 20000 cycles).In chapter 8,we provided a Li+/Mg2+ hybrid-ion batteries using freestanding MoS2 nanoflower anchored graphene foam as cathode and Mg metal as anode.The battery exhibited excellent cycle performance(104 mAh g-1 after 50 cycles at 0.02 A g-1)and superior rate capability(65 mAh g-1 at 1 A g-1).In chapter 9,we summarized the work during the doctoral period and looked for-ward to the future research plan.