Design Of Zinc Based MOFs And Their Derivates For Li Storage

Metal–organic frameworks?MOFs?,a new class of crystalline porous organic–inorganic hybridmaterials,have recently attracted increasing interest in the field of energy storage and conversion.MOFs materials and their derivatives have large specific surface area and unique morphological structure,and have a large number of metal active centers and hierarchical pore structures,which can provide a large amount of adsorption,reaction and energy storage active sites for the catalytic and energy storage processes.The active site is therefore well used in energy storage devices.Based on two kinds of zinc-based MOFs:MOF-5 and ZIF-8,a series of derivative composite materials were designed and prepared by means of hydrothermal conversion,graphene compounding and chelating agent modification,and the materials were studied.Lithium ion\lithium sulfur battery applications.The main research contents are as follows:?1?Using MOF-5 as a precursor,the Zn₄?O?O₁₂C₆ secondary building unit in the MOF-5 structure is opened by using HAc to more strongly coordinate the Zn²⁺than H₂BDC,thereby decomposing and forming MOF-5.ZnO self-assembled mesoporous microspheres?MSS?with uniform lattice orientation,ZnO MSS@N-C composites were obtained by carbon coating and applied to the negative electrode of lithium ion batteries.The cell demonstrates high reversible capacity of 1016.7 mAh/g at 100 mA/g after100 cycles,and excellent high rate stability which shows impressive reversible capacity of 698.4 mAh/g at1000 mA/g after 300 cycles.At current densities of 100,200,500,1000 and 2000 mA/g,the average discharge specific capacities were 1011.4,842.1,762.2,672.4 and 557.2 mAh/g,respectively,indicating that the electrode has good cycle performance and rate performance.?2?Co-ZIF-8 ultra-thin nanosheets were prepared by using ZIF-8 nanosheet as precursor and double-effect ion exchange and stripping method.Graphene oxide?GO?was added during the stripping process to obtain a Co-ZIF-8/GO composite aerogel structure with a surface stacking,and a Co@CoO@N-C/rGO composite with a special core-shell structure was formed after high temperature pyrolysis.The material was applied to a lithium-sulfur battery modified separator.After 200 cycles of charge and discharge cycles at 0.2 C,the reversible capacity of the device was maintained at 823.6 mAh/g and the capacity retention rate was 75.9%.In particular,after 500 cycles of 1C high current charge and discharge,the reversible capacity was maintained at 555.4 mAh/g,and the decay rate per revolution was only0.0651%.The mechanism analysis confirmed that Co@CoO metal nanoparticles with surface oxidation structure effectively inhibited the shuttle effect of polysulfides by forming chemical bonds with soluble polysulfides.Electrochemical kinetic analysis shows that the nitrogen-doped carbon coating of the shell not only helps to improve the conductivity of the material,but also alleviates the volume expansion of the CoO-Co metal nanoparticles during charge and discharge,greatly improving the rate performance of the device and cycle stability.?3?Based on ZIF-8 nanosheets,using the strong chelating ability of EDTA-2Na,by destroying the intrinsic metal coordination center of ZIF-8 nanosheets,a large number of 3D crosslinked mesopores and large pores are formed on the surface.After high temperature carbonization,a two-dimensional nitrogen-doped graded porous carbon material having a high specific surface area and a mesoporous structure is obtained.The microporous structure and nitrogen-doped sites of the material can inhibit the shuttle effect of polysulfide by limiting and bonding.The cross-linked mesoporous structure accelerates the diffusion and transport of the electrolyte and Li⁺,and synergistically alleviates the volume expansion of the active material during charge and discharge,thereby improving the cycle stability of the electrode material.The optimal device has a discharge specific capacity and a charge specific capacity of 722.4 mAh/g and737.5 mAh/g at a current density of 0.2 C.After 100 charge and discharge cycles,the reversible capacity is maintained at 581.5 mAh/g.The retention rate was 78.85%.