Preparation And Electrochemical Properties Of Cobalt-based Metal Organic Frameworks Composites

As the electrical vehicles(EVs)have grown rapidly in recent years,the demand of power batteries with both high energy densities and high security is ever-growing.However,low theoretical capacity(372 mA h g-1)of commercially available graphite anode has limited its large-scale applications in electrical vehicles(EVs).The development of new anode materials with higher capacity,security and low cost has been a hotspot in both theory and application.Metal organic framework(MOFs)is a kind of crystal material which is composed of metal ions and multifunctional organic ligands.MOFs can be considered as novel electrode material for lithium-ion batteries because of its large specific surface area and abundant pore structure,which may facilitate the insertion/extraction of Li ions.However,the poor electrical conductivity of MOFs materials limits their electrochemical performances.To increase the electrochemical performance of MOFs materials,three kinds of cobalt-base MOFs/conductive agent composites are successfully synthesized by in-situ compounding the MOFs materials with different kinds of conductive agents(conductive polymers,carbon fibers).The samples are characterized by X ray powder diffraction,infrared spectrum and scanning electron microscope.Then,the potential applications of cobalt-MOFs/conductive agent composites as anodes for lithium-ion batteries(LIB)are also studied.In this paper,polypyrrole nanofibers doped with methyl orange(MO)were prepared by using FeCl3·6H2O as the oxidant.Subsequently,Co-MOFs are grown on the as-obtained PPy nanofibers(PPy/Co-BDC)via solvothermal reaction technique.The nanofiber with small diameter can reduce the Li-ion and electron diffusion length when uesd as anode for LIB.Electrochemical tests revealed that the reversible capacity of the composite could be enhanced by introducing the PPy nanofibers.At a current density of 50 mA g-1,a reversible capacity of 363.5 mA h g-1 was obtained after 100 charge/discharge cycles,and revealed an excellent cycling stability and rate performance.Owing to the easy preparation,the PPy/Co-BDC composite may be promising anode materials for LIB.Using PPy nanofibers as substrate,polydopamine-functionalization polypyrrole nanofibers(PDA-PPy)were successfully prepared by introducing polydopamine.Subsequently,the PDA-PPy/Co-BDC composite were synthesized through solvothermal reaction,and the effect of different dopamine/pyrrole mole ratios on the structure and properties of composites were investigated.Results indicated that the best morphology and properties of the composites are obtained when the mole ratios of DA/Py is 0.064.The PDA-PPy/Co-BDC composites exhibit smaller charge transfer impedance,better rate capability and cycle stability for using as the anodic materials of lithium batteries.At a current density of 50 mA g-1,the highly reversible capacity of 558.8 mA h g-1 is obtained after 100 charge/discharge cycles,which is higher than that of pristine Co-BDC(314.1 mA h g-1).In addition,the mechanism of lithium storage was preliminarily discussed by analyzing the changes of the morphology and electronic structure of the materials before and after charging/discharging.With the development of flexible electronic devices,the research on flexible storage devices(especially flexible electrode)has received considerable attention.Herein,ZIF-L-Co nanosheets were in situ grown on the pretreated carbon cloth(ZIF-L-Co/CF self-supporting electrodes)through a facile liquid-phase deposition method.The results of XRD and SEM indicated that the ZIF-L-Co nanosheets with an average thickness of 100-200 nm are evenly anchored onto the surface of carbon cloth.Furthermore,ZIF-L-Co/CF can be used as flexible electrode for lithium-ion batteries without additional binder and conductive agent.When tested as binder-free anodes for lithium-ion batteries,the ZIF-L-Co/CF composite showed higher initial coulombic efficiency and better cycling performance compared with pure ZIF-L-Co.At a current density of 50 mA g-1,ZIF-L-Co/CF delivered a reversible capacity of 445.1 mA h g-1 after 100 charge/discharge cycles.The significantly improved electrochemical performance could be attributed to the synergetic effect between the conductive carbon cloth and the two-dimensional nanosheets of ZIF-L-Co.