Preparation Of Cobalt Matrix Composites Derived From ZIF-67 And Its Application In Lithium-ion Batteries

With the rapid development of modern technologies such as portable electronic products and power tools,lithium-ion batteries(LIBs)are widely used due to their high capacity,safety and stability,small size,repeatable charging,and light weight.Graphite is the most commonly used anode for commercial lithium-ion batteries in the market,and its capacity is very close to its limited theoretical capacity(372 mA h g-1),which makes the further improvement of capacity face unprecedented challenges.Efforts to improve anode materials are undoubtedly important to improve the overall performance of lithium-ion batteries.Therefore,various MOFs materials have been extensively studied due to their high theoretical capacity.Zeolitic imidazolate framework ZIF-67 can be used as a template or precursor to synthesize various materials due to its large specific surface area,adjustable pore size and easy preparation.However,the conductivity of ZIF-67 is poor.Therefore,how to effectively improve the conductivity of ZIF-67 and increase the specific capacity of the material has become an urgent problem to be solved.Studies have shown that combining ZIF-67 with carbon materials,pyrolysis at high temperature to obtain nitrogen-doped porous carbon,carbon coating,or calcination of ZIF-67 precursor to prepare a variety of cobalt-based materials,such as cobalt phosphide,cobalt sulfide,cobalt selenide,etc.,can significantly improve the specific capacity of the material.In this paper,Co/NC derived from ZIF-67 high temperature pyrolysis is used as the main body,and new cobalt-based composites with better electrochemical performance can be prepared by reasonable design of phosphating,heteroatom doping and combination with carbon materials.The main research contents of this paper are as follows:(1)Using ZIF-67 as a precursor,CoP@NC-CNT nanocubes were further synthesized by two annealing treatments.The results show that this composite material can effectively alleviate the volume effect of the electrode material during the charge and discharge process,shorten the electron and ion transport path,accelerate the migration rate of lithium ions,promote the electrode reaction kinetics process,and improve the overall structural stability.The derived carbon nanotube structure can shorten the ion diffusion path,provide huge free space and facilitate the transport of electrolyte and charge.Therefore,it has excellent lithium storage performance.After 100 cycles at a current density of 100 mA g-1,the capacity is still maintained at 512 mA h g-1.After 1000 cycles at a high current density of 1.0 A g-1,the capacity is still maintained at 398 mA h g-1 discharge capacity and 402 mA h g-1 reversible charge capacity.The charge-discharge efficiency is as high as 99%.(2)Co/NC@MoS2/C composites were successfully synthesized by further growth of carbon-doped molybdenum disulfide nanosheets on the surface of hexahedral Co/NC by solvothermal method with appropriate glucose additives.In this special structure,the conductivity of molybdenum disulfide has been significantly improved,and the volume change caused by lithium insertion and extraction has been effectively alleviated.In particular,the outermost carbon produced by glucose leads to the expansion of the interlayer spacing of molybdenum disulfide,which makes it easier for Li+to shuttle between layers,thus accelerating the kinetic reaction and improving the electrochemical performance.According to the prepared materials,lithium ion batteries were assembled and electrochemically tested.After 100 cycles at a current density of 100 mA g-1,the capacity was still maintained at 750.2 mA h g-1,and the coulombic efficiency was as high as 98%.After 500 cycles at a high current density of 1.0 A g-1,the battery capacity still maintains a discharge capacity of 406 mA h g-1 and a reversible charge capacity of 405 mA h g-1,and the charge-discharge efficiency is as high as 99%.The Co/NC@MoS2/C composite exhibits good electrochemical performance as a lithium ion anode material.(3)Polypyrrole nanotubes were prepared by template-assisted method under ice bath conditions.Hollow nitrogen-doped carbon nanotubes(NCNTs)were obtained by high temperature calcination under the protection of inert gas nitrogen.NCNTs have a unique one-dimensional hollow nanotube structure with a diameter of 100-200 nm and a length of several microns.After calcination,the nitrogen-carbon-doped cobalt nanomaterials not only retain the morphology of the dodecahedron ZIF-67,but also can be uniformly attached to the surface of the nitrogen-doped carbon nanotubes.The surface is rough and dendritic.Its porous hollow structure and nitrogen-rich carbon have the ability to alleviate volume change and enhance electron transfer;the surface of N-doped carbon nanotubes will produce external defects and abundant active sites,which can improve the conductivity of carbon matrix and promote the improvement of specific capacity and rate performance.The capacity can reach 809.4 mA h g-1 after 100 cycles at a current density of 100 mA g-1,and the capacity remains at 674.7 mA h g-1 even after 500 cycles at a high current density.