Synthesis And Electrochemical Application Of Hollow Materials Based On MOFs

With the shortage of fossil energy and environmental degradation,the interest in renewable energy storage systems is rapidly increasing.Lithium-ion batteries（LIBs）,as the main conventional energy storage technology,are widely used in various electronic products.However,LIBs have problems such as limited energy density and high price.Therefore,there is a need to develop a new generation of batteries with high energy and low cost.Transition metal oxides（TMOs）and transition metal sulfides（TMSs）have been widely used in LIBs in recent years due to their high specific capacity,but their own severe volume changes have seriously affected their further development.The hollow structure of the material can be a good solution to the above volume expansion problem.Metal-organic frameworks（MOFs）materials have superb tunability and can provide a variety of derivatives after pyrolysis,including carbon nanostructures,metal oxides,metal composites,etc.Therefore,MOFs are often used as precursors or sacrifices to synthesize high-performance electrode materials,and this direction has become a research hotspot in recent years.In thesis paper,Co₃O₄@Fe O_x dodecahedra with three-shell hollow structure and Zn–Co–Fe–S@N–C dodecahedra with three-metal core-shell structure were prepared mainly by using MOFs as precursors combined with high-temperature calcination process,and the electrochemical properties of the two materials were investigated as the anode of LIBs.（1）Using ZIF–67 as precursor,Co₃O₄ dodecahedron with three shell hollow structure was obtained by controlling the heating rate in the process of high temperature calcination.Specifically,Co₃O₄ of the single–shell hollow structure was obtained at the heating rate of 20℃min^–1.The Co₃O₄ of the three–shell hollow structure was obtained at the heating rate of 0.5℃min^–1.After modified with PVP,the Co₃O₄ was coated with Fe Cl₃·6H₂O,and the Co₃O₄@Fe O_x dodecahedron of the three–shell hollow structure was obtained after calcination.Among them,Co₃O₄@Fe O_x has an obvious three-shell hollow structure inside,and the unique three-shell hollow not only provides a very large electrochemically active surface area,but also can effectively alleviate the volume expansion brought by Li ion insertion/deinsertion process and enhance the structural stability of the material.The C and N in the composite contribute to stability and electrical conductivity.When applied to LIBs anode materials,multishell dodecahedra exhibit excellent electrochemical properties with high reversible capacity(1141.67 m A h g^–1 at a current density of 100 m A g^–1),excellent multiplicative performance and stable cycling stability(The specific capacity can reach 1442.8 m A h g^–1 after 100cycles at a current density of 100 m A g^–1).（2）ZnCo Fe–ZIF was synthesized by a one–pot method under an inert gas environment to prepare core–shell structured Zn–Co–Fe–Fe trimetallic sulfide@nitrogen–doped carbon composites（Zn–Co–Fe–S@N–C）.The precursor Zn Co Fe–ZIF was first synthesized under inert gas protection,and then a layer of dopamine（PDA）was coated on its surface by a surface polymerization process,and the core–shell structure of Zn–Co–Fe–S@N–C dodecahedra was obtained by sulfidation during high–temperature carbonization.The core–shell structure can effectively buffer the volume expansion and shorten the diffusion path of Li ions.When it is used as a anode for LIBs,the specific capacity reaches 966.6 m A h g^–1 after 100 cycles at a current density of 100 m A g^–1.Even at a high current density of 2000 m A g^–1,the specific capacity of the trimetallic core–shell Zn–Co–Fe–S@N–C dodecahedron can still reach 497.72 m A h g^–1 after 200 cycles.The good electrochemical performance is mainly due to the ability of the core–shell structure to provide sufficient void space to accommodate the volume change during lithium ion insertion/exit and shorten the ion/electron diffusion path.The presence of nitrogen–doped carbon matrix not only enhances the electron transfer kinetics,but also improves the structural stability.