Fabrication Of Polyoxometalate-based Metal Organic Frameworks With Azacrown Ethers For High-performance Anode Materials Of Lithium-Ion Batteries

Metal organic framework materials（MOFs）are organic-inorganic hybrid materials with intramolecular pores formed by self-assembly of organic ligands and metal ions or clusters through coordination bonds.Polyoxometalates（POMs）are a class of polymetallic oxygen cluster compounds constructed by the transition metal ions through oxygen.They are used in the fields of catalysis,conduction,adsorption,medicine,magnetochemistry,electrochemistry,and other fields with potential applications.Based on the unique functions of the above,a class of compounds based on polyoxometalates-based metal-organic frameworks（POMOFs）has also attracted the attention of researchers.Such compounds often have rich and unique structures and potential applications.In this paper,two kind of azacrown ether ligands L₁ and L₂ were synthesized,where L₁=11,23-dimethyl-3,7,15,19-tetraaza-tricyclo[19,13,1,19,13]-hexacosa-1（25）,9,11,13（26）,21,23-hexaene-25,26-diol;L₂=2,6,10,17,21,25-six-aza-[24,2,2,2]-1,12,13,15,26,27-six-azacrown ether.Further,five compounds[Co₃（L₁）_1.5(PMo₁₂O₄₀)·（H₂O）₅]·4H₂O（1）,[Ni₃（L₁）_1.5(PMo₁₂O₄₀)·（H₂O）₅]·4H₂O（2）,Zn₃（L₁）_1.5(PMo₁₂O₄₀)（3）,Fe₃（L₁）_1.5(PMo₁₂O₄₀)（4）and Cu₄（L₁）₂(Mo₈O₂₆)（5）,[(H₂Mo₈O₂₆)（H₄L₂）]·4H₂O（6）,[(H₁₂Mn₁Mo₁₀O₃₇)（L₂）]·2H₂O（7）was synthesized.The single crystal structures of the five compounds were determined by X-ray single crystal diffraction,X-ray powder diffraction（XRD）,thermogravimetry analysis（TGA）,infrared spectroscopy（IR）,scanning electron microscope（SEM）and X-ray Photoelectron spectroscopy（XPS）.Further,these compounds are used as electrode materials in lithium-ion batteries to study their electrochemical energy storage performance.X-ray single crystal diffraction result showed that compound 1 formed a 2D layered structure by Co（II）,phosphomolybdic acid and L₁ ligand.Compound 2 is isomorphic molecule with 1,and the 2D layered are formed by hydrogen bonding interactions between phosphomolybdic acid and L₁ ligand.The crystal structure of 3 is a discrete molecule,and is formed by a phosphomolybdic acid,Zn（II）and L₁ ligand.Moreover,the discrete molecules are extended to form a 2D layer by hydrogen bonds,where one phosphomolybdic acid molecule is sandwiched by two L₁ ligands.Compound 4 consists of a molybdenum-iron-crown ether structural of L₁-PMo₁₂O₄₀-L₁ chain through coordination bonds,finally builds a 2D layered structure.Compound 5 consists of[Mo₈O₂₆]^4-anion,Cu（II）and L₁ ligand connected through aπ-πbond to obtain a 1D structure.Compounds 6and 7 are POMOFs with known structures synthesized by L₂ ligand.In compound 6,the Mo₈O₂₆ and L₂ ligand are connected by Mo-N bond to form 1D L₂-Mo₈O₂₆-L₂chain,and further extended to a 3D layer through N-H···O and O-H···O bonds from the water clusters of adjacent chains.Compound 7 is a 2D supramolecular structure from Mo₁₀Mn₁O₃₇molecules and L₂ ligands connected byπ-πbonds.The results of infrared and XRD analyses showed that the high degree purities were shown in compounds 1-7.The compounds 1-7 exhibit thermally stable examined by thermogravimetric analysis.The lithium ion batteries performances of compound 1-5crystalline material shows that its specific capacity is not high,130 m A h g^-1,104 m A h g^-1,83.5 m A h g^-1,153 m A h g^-1,113 m A h g^-1,respectively.However,when the compounds1-5 have been recombined with polypyrrole and carbon nanotubes,the capacities increase greatly.The specific capacity of 5@Ppy/CNTs composite material can reach 1303 m A h g^-1,which is much higher than that of graphene material(372 m A h g^-1).For the L₂ ligand,the specific capacity of the compound 6 single crystal reached 600.2 m A h g^-1,and the specific capacity of the compound 7 single crystal reached 979 m A h g^-1.The subsequent electrochemistry of the two materials was also passed.The nature of behavior is characterized by XPS,SEM,XRD,IR and other means.