Design,Synthesis And Electrochemical Performance Of Carbon-based Nanostructured Materials For Lithium Stroage

As the best power sources for electric vehicles（EVs）,the performance of lithium-ion batteries（LIBs）is directly related to that of EVs.For example,the energy density determines the cruising distance of a single charge.Lithium-ion battery technologies have been developing for nearly 30 years since its first commercialization in 1991.However,the current performance still below people’s expectations,especially in the aspects of energy density and safety,towards which new electrode materials with high capacity are needed.As far as anodes,it is necessary to explore novel electrode materials with a higher capacity than that of the currently commercialized graphite(372 mA h g^-1).Among the available materials,transition metal compounds have attracted considerable attention as their ability for multiple electron transfer based on conversion reaction,manifesting high theoretical specific capacities and high value in fundermental reserch.But they still suffer from unsatisfied capacity retention and short life span,which are derived from their drastic volume expansion during the Li⁺insertion-extraction processes,electrode structural failures in continuous cycles and their intrinsic poor electronic conductivity.This thesis will devote to exploring the improvement towards the performance of such materials by constructing active/inactive composites with nanosized transition metal compounds particles being evenly encapsulated into conductive matrices.Firstly,we design the synthesis of N-doped carbon nanosheets decorated with hollow copper oxide nanoparticles（CuO@NCSs）by chemical vapor and solid deposition methods based on the template effect of Cu foil and the carbothermal reduction.The well-designed hollow CuO@NCS composites inherit the advantages of hollow nanostructures,carbon nanosheets and N-doping which can effectively mitigate the volume expansion during the repeated cycles and improve the electrical conductivity of the active material.Thus,the CuO@NCS electrode delivers an extremely long cycle life(688 mA h g^-1 after 1000 cycles at 2 A g^-1)for lithium storage.Furthermore,we expand on the basis of the above novel method.We directly spin coat nickel nitrate on the reduced copper foil to prepare the final product by the virtue that metal nitrate is easily decomposed to form oxides.The in situ formed graphene-like carbon coated NiO nanoparticles evenly embed in the nitrogen-doped carbon sheets（C@TMOs@NCSs）composites are fabricated by the high catalytic activity of metallic nickel.As lithium storage materials,such synthesized composites demonstrate excellent long-term cycle stability(651 mA h g^-1 after 1500 cycles at 1 A g^-1)and high-rate capability(400 mAh g^-1 at 4 A g^-1).The morphology of the material will significantly influence the intercalation/de-intercalation performance of lithium ions.Metal–organic frameworks（MOFs）possess specific morphology as well as tunable size and composition.We synthesis highly uniform 1D nanorods structure by triggering the reconstruction of bimetal Ni/Co-MOFs in a hydrothermal process.The obtained MOF template can be easily transformed into monodispersed graphene-coated bimetal oxide NiCo₂O₄（NiCo₂O₄@graphene）nanorods composite material by subsequently annealing treatment.The synergetic effects of the bimetallic compounds are investigated.The synthesized NiCo₂O₄@graphene nanorods as the anode material exhibit extraordinary stability(610 mA h g^-1 after 1200 cycles at 1 A g^-1)by virtue of the advantages of unique graphene coating,bimetallic elements and monodispersed nanostructure.Metal sulfides have higher electrical conductivity and lower voltage plateau than the corresponding metal oxides.We demonstrate the synthesis of a hierarchical carbon based bimetal sulfide CoNi₂S₄ porous nanocomposites（CoNi₂S₄@PCS/CNTs）from one-step sulfurizing the controllable designed Co/Ni bimetal-organic frameworks（Co/Ni-MOF-74）precursors.In this composite,the CoNi₂S₄ nanoparticles are evenly embedded in the carbon nanosheets which are self-assembled into porous carbon spheres with short carbon nanotubes（CNTs）deeply rooted in the surface.As anode materials,the as-synthesized composites demonstrate greatly improved electrochemical performance relative to the single metal counterparts.