Carbon-Confined Vanadium/Zinc-Based Oxide Derived From Coordination Polymer For Lithium Battery

With the rapid development of the economy and remarkable improvement of living standard,fossil energy consumption is continuously increasing.Human beings are facing increasingly serious energy shortages and environmental damage.Hence,the development of clean energy is of great significance for energy uses,environmental protection and reducing greenhouse gas emissions.Lithium-ion batteries as energy storage device have been extensively used in commercialization owning to their high energy density,long life and environmental friendliness.Commercial lithium-ion batteries usually use graphite as the negative electrode.However,due to the low theoretical capacity of graphite(only 372 mAh g^-1),it is difficult to meet the demand for commercial batteries with higher capacity and higher energy density.Transition metal oxides are considered as an alternative to carbon materials because of their high theoretical capacity,abundant resources and low cost.However,large volume expansion occurs in transition metal oxides during the process of lithium ion insertion and extraction,resulting in poor cycle performance.In addition,the poor conductivity of transition metal oxides also affects the rate performance of batteries,which greatly limits their application in practical production.Developing carbon-based metal oxide nanostructure composites is an efficient solution to solve these problem.Coordination polymer refers to a kind of polymer formed by coordination reaction between metal ions and organic ligands.This coordination polymer is a molecular-scale organic-inorganic hybrid structure with the advantages of simple synthesis,adjustable structure and rich components,including crystalline metal organic frameworks?MOFs?compounds and amorphous coordination polymers.Both of these two types of coordination polymers can be used as precursors for carbon materials or metal oxide carbon-based composites.The design and construction of carbon-based metal oxide composites from these two coordination polymers is the research focus of this thesis.The main research results are as follows:?1?We have developed a facile and efficient template-free synthesis of uniform nitrogen-doped carbon-confined V₂O₃ hollow spheres through a solvothermal process and subsequent heat treatment.During solvothermal process,the mismatched coordination reaction between vanadium ions and organic ligands occurs,resulting in the formation of amorphous organic-inorganic complex precursor.With the solvothermal time prolonging,the precursor morphology evolves from solid microsphere to yolk-shell microsphere and eventually hollow microsphere due to an inside-out Ostward-ripening.Then,after in situ carbonization,the as-prepared precursor hollow spheres are converted into uniform morphology-preserved V2O3@NC.As a proof-of-concept application,when evaluated as a LIB anode material,the resulting V₂O₃@NC hollow spheres manifest high capacity,excellent rate capability and long-term cycling stability.When the current density is 2000 mA g^-1,the V₂O₃@NC hollow microspheres have a capacity of 742 mAh g^-1 and a capacity retention rate of 100%after 700 cycles.?2?We have also for the first time developed a facile and general low temperature melting method to synthesize uniform metal-organic framework?MOF?shell on the surface of certain metal oxide by coordination reaction between melted 2-methylimidazole with metal ion.For example,at the low temperature,the melted dimethylimidazole reacted with Zn²⁺on the surface of Zn₂SiO₄ nanowires to form ZIF shells,and Zn₂SiO₄@ZIF nanowires were obtained.at high temperature,the ZIF shell will be further carbonized in situ to obtain Zn2SiO4@NC nanowires.This carbon confinement strategy using melted organic ligands can be easily applied to obtain other ternary metal oxide precursors with high bonding energy,including Zn₂WO₄ and Zn₂TiO₄.As a proof-of-concept application,the obtained carbon-confined Zn₂SiO₄nanowires,when tested as lithium ion battery anodes,exhibited enhanced lithium storage performance with a high reversible capacity of 685.2 mAh g^-1 after 100 cycles at 100 mA g^-1,compared to pure Zn₂SiO₄ nanowires.