Study On The Preparation And Electrochemical Properties Of CO₃O₄ And Composites Derived From ZIF-67

With the rapid development of science and technology,the demand of human society for energy is increasing continuously.However,the over-exploitation of fossil energy and low utilization rate lead to a series of environmental problems such as global warming,haze,air pollution and so on.Therefore,we need a new energy source which is efficient and clean.At the same time,we need new technology about energy conversion and storage urgently.Lithium-ion batteries have many advantages,such as good rate performance,high energy density,long cycle life and no memory effect.Metal-organic frameworks（MOFs）are a new type of crystalline materials formed by coordination of metal ions and organic ligands.They are porous materials with periodic topological structure.It has many advantages,such as large porous and specific surface area,diversity of structure and function,adjustable pore size,etc.Therefore,it has broad application prospects in many fields such as lithium ion batteries,supercapacitors,catalysts,adsorbents and so on.Based on the above points,we combined the carbon-based materials and pyrolyzed MOFs materials,and the products and reaction mechanism were further explored.In this paper,dodecahedron ZIF-67 was synthesized by using cobalt nitrate hexahydrate（Co（NO₃）₂·6H₂O）as metal cobalt source and 2-methylimidazole（C₆H₆N₂）as organic ligand.N-doped carbon-coated core-shell structure of Co@Co₃O₄nanoparticles was synthesized by two-step pyrolysis and hydrothermal oxidation.The effect of concentration of NaOH on Co/Co₃O₄@C was investigated,and the junction was obtained by adjusting hydrothermal time.Co/Co₃O₄@C-8h as the final product has complete structure,regular morphology and uniform size.Co/Co₃O₄ nanoparticles are uniformly distributed in the regular dodecahedron structure of amorphous carbon.Cobalt metal and nitrogen-doped carbon form conductive network,which effectively shorten the lithium ion transmission path and make the composite present higher conductivity.Moreover,amorphous carbon has more porous pores,which fully contacts with electrolyte during charge-discharge reaction,and alleviates the generation during cycling process.Volume expansion retards the capacity attenuation caused by structural collapse.Under constant current of 0.1 A g^-1,the discharge capacity after 50 cycles is stable at 620mAh g^-1（carbon content 56.8 wt%）.Under constant current of 2 A g^-1,the capacity can reach 572 mAh g^-1 after 250 cycles,the coulomb efficiency is about 99.8%.The experimental results show that the Co/Co₃O₄@C composite electrode material has excellent rate performance and has a promising application prospect in electrochemical energy storage.Co₃O₄ prepared by annealing ZIF-67 was combined with reduced graphene oxide by electrostatic self-assembly by hydrothermal method.The hollow dodecahedron of Co₃O₄was formed by calcining ZIF-67 in air.The effect of carbonization temperature on the morphology and electrochemical properties of Co₃O₄ derived from ZIF-67 was investigated.The electrochemical test results show that the composite material has better performance than single Co₃O₄.It has a high reversible capacity of 1343 mAh g^-1 at 0.1 A g^-1 current density.After 250 cycles at 2 A g^-1 current density,the reversible capacity gradually stabilizes to 872 mAh g^-1,and the Coulomb efficiency is about 99.8%.It indicates that the synergistic effect of rGO and Co₃O₄ improves the stability and capacity of the material.The composite rGO/Co₃O₄ has excellent electrochemical properties and is a potential material in the field of electrochemical energy storage.