Synthesis And Properties Of Vanadoborate And Vanadate

Polyoxometalates（POMs）are ionic oxygen clusters formed by the condensation of several identical or different transition metal ions.Due to the tunability of the size,charge density,and redox potential of POMs,they have excellent physical and chemical properties and have been widely used in energy storage,catalysis,optoelectronics,and biomedical applications.Vanadoborates and vanadinoferrates have received much attention as members of the growing family of polymetallic oxygenates.Since vanadium elements can reveal abundant ionic valence states（+3,+4,+5）,they chemically bond with oxygen ions to synthesize different vanadium-oxygen polyhedra,while B atoms can form boron-oxygen tetrahedra/triangles（BO₄ and BO₃）and Fe atoms can form iron-oxygen tetrahedra/octahedra（Fe O₄ and Fe O₆）.They build different novel structures by combining them with boron/iron-oxygen polyhedra.Among them,the introduction of templating agents such as transition metal complexes,protonated amines,and alkali metal cations interconnected with polyanion clusters to form a complex network structure,and these features enable vanadium borates to exhibit excellent proton conductivity under high humidity conditions.The combination of vanadium-oxygen polyhedra with iron-oxygen polyhedra further enriches the structural valence states and brings into play the synergistic effect of both,enabling them to exhibit excellent electrochemical properties.The diversity of valence states and the rich structural features of vanadium have motivated researchers to further synthesize novel structures and exploit the potential value of such metal oxides.The main research work of this thesis is to introduce boron and iron into the vanadate system to synthesize novel and high-performance vanadium-based polymetallic oxides and to investigate their structure,proton conduction properties,and lithium-ion anode performance.（1）We have successfully synthesized a novel vanadoborate[Ni（en）₃]₃·[1,3-dap]₂·H₆[V₁₀B₂₆O₆₀（OH）₁₀]（1）by a solvothermal method.In the structure of 1,the{B₁₃O₃₀（OH）₄}cluster is formed by BO₃ planar triangles,BO₄ tetrahedra interconnected by oxygen atoms.VO₅ square pyramids are interconnected by shared oxygen atoms on the bottom edge to form a ring of{V₁₀O₃₀}units,and one{V₁₀O₃₀}unit and two{B₁₃O₃₀（OH）₅}units are interconnected byμ₃-O atoms to grow out of the[V₁₀B₂₆O₆₀（OH）₁₀]^12-cluster.The organic complexes Ni（en）₃²⁺and 1,3-dap molecules are interconnected with the polyanionic clusters by hydrogen bonds to form a three-dimensional open framework.The crystal structure of this compound was resolved and characterized by PXRD,TG,FT-IR,ICP,CHN,and XPS techniques.The proton conductivity was tested using an electrochemical impedance meter,and the results showed that compound 1 exhibited a significant proton conductivity of1.78 10^-3 S cm^-1 at 50°C and 100%relative humidity with an activation energy of0.61 e V.（2）We prepared hydrated ferric vanadate Fe₂（H₂O）[V₂O₇·VO_3.5]·0.5H₂O single crystals（2）by hydrothermal method.In the structure of compound 2,the Fe₂（H₂O）[V₂O₇·VO_3.5]·0.5H₂O skeleton consists of VO₄ tetrahedral moiety and Fe O₆octahedral moiety.The VO₄ tetrahedral Fe O₆ octahedra are interconnected by symmetric operations to form the V₂O₇ vanadate anion and the Fe₂O₁₀ diferric anion.Three V₂O₇ units and two Fe₂O₁₀ units are interconnected by shared oxygen atoms to form the three-dimensional supramolecular structure.The crystal structure of compound 2 was resolved and characterized using PXRD,TG,FT-IR,ICP,CHN,and XPS.After variable temperature XRD tests,the structure could be maintained stable at 250°C.The electrochemical properties of the samples before and after calcination（250°C）were investigated.The test results showed that the final product as a negative electrode exhibited excellent performance in electrochemical tests,such as higher capacity,coulomb efficiency,multiplicative performance,and cycling stability.It is worth mentioning that under high multiplicity test conditions(700 m A g^-1),the specific capacity was still 262 m Ah g^-1 after 100 cycles,which indicates that Fe₂（H₂O）[V₂O₇·VO_3.5]·0.5H₂O has a promising application in high energy density Li-ion secondary batteries.