| Metal-organic frameworks(MOFs)are classical crystal materials which have periodic network structures strongly bonded by combing metal ions or clusters with organic ligands.MOFs present large surface areas and intrinsic abundant pores,which can provide diffusion pathways for Li+ and benefit for the filtration of electrolyte.Furthermore,the metal ions and the organic ligands can act as the active sites for additional Li+ storage,contributing to the high lithium storage capacity and excellent kinetics.Therefore,MOFs has been viewed as the promising electrode material for lithium storage.In this study,the materials of nickel-based MOFs have been prepared by using hexaiminobenzene(HIB)/2,3,6,7,10,11-hexaaminetriphenylene(HATP)as the organic ligands,and Ni2+(NiCl2·6H2O)as the metal center,respectively.And,the controlling of molecular pore sized and charge deloacation in MOFs have been studied.The electrochemical performance has been extensively studied,and the lithium storage mechanism in MOFs has been revealed.Moreover,Ni3(HITP)2/GO composite have been prepared by an in-situ growth of Ni3(HITP)2 MOFs on the surface of grapheme oxide,and the enhanced electrochemical performance have been explored according to the morphology evolution,electron and Li+ diffusion during the charge/discharge processes.The main research contents are as follows:(1)Ni3(HIB)2 MOFs has been synthesized by hexaiminobenzene(HIB)and NiCl2·6H2O via a solvothermal method.This MOFs is constructed by the small grains with the diameter of about 50 nm,and possess the molecular pore size of about 1.54 nm.At 50 mAg-1,and this MOFs exhibit the ability of storing Li+.Ni3(HIB)2 MOFs presents the initial lithium storage capacity of 405 mAhg-1 with the columbic efficiency of 61.9%.The low columbic efficiency is mainly due to the irreversible formation of SEI film.For 100 charge/discharge cycles,the capacity can keep at the value of 330.5 mAhg-1 with the capacity retention of 81.4%.The capacity fading is mainly due to the destroy of the MOFs' structure,which is related with low charge deloacation effect of the single benzene ring in the redox process of MOFs.(2)With the strengthen the charge deloacation ability,2,3,6,7,10,11-hexaaminetriphenylene(HATP)with long molecular chain is selected as the organic ligands to react with Ni2+ to synthesize Ni3(HITP)2 MOFs.This MOFs has the spindlelike morphology constructed by many nano fibers with the surface area of 347.2 m2 g-1.At 50 mAg-1,Ni3(HITP)2 MOFs presents the initial lithium storage capacity of 690 mAhg-1,and the capacity retention of 703.4 mAhg-1 after 100 cycles.At 1000 mAg-1,the capacity also can be 135.1 mAhg-1,and when the current density is reback to 50 mAg-1,the capacity can recover to the value of 736.7 mAhg-1?The excellent electrochemical performance of Ni3(HITP)2 MOFs is mainly due to the HATP ligand with the large aromatic structure,which is good for the charge deloacation of HATP ligand,improving the structure stability of MOFs and the electron transfer speed.And,the large molecular pore constructed by HATP and Ni2+ benefit to the diffusion of Li+ in the structure.XPS analysis and DFT calculation indicate that the metallic ions and ligands can take part in the redox process,which allows the intercalation of multi Li+ in MOFs structure.(3)Ni3(HITP)2/GO composite is synthesized by in-situ growing Ni3(HITP)2 MOFs on the surface of oxidized graphene(GO).The plate of Ni3(HITP)2 MOFs stick on the surface of GO,and the composite film has the thickness of 10 um.At 50 mAg-1 for 100 cycles,the capacity of the composite can be 740.0 mAhg-1,which is mainly contributed by pseudocapacitance from molecular pore and the Faraday capacity,and the contribution of pseudocapacitance is 52.6%.The excellent electrochemical performance of Ni3(HITP)2/GO is due to the plate of composite with large surface area and the good extension ability,and the GO film can supply the extra pseudocapacitance. |
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