Preparation Of Covalent Organic Frameworks (COFs) And Their Heterostructure Materials And Study Of Their Potassium Storage Propertie

The development of new energy vehicles,3C digital products,and all kinds of clean energy（solar energy,wind energy,etc.）,can’t do without efficient energy storage equipment.Lithium-ion battery（LIBs）,which has been studied for decades,is the most widely used energy storage equipment at present.Researchers gradually dedicate to search the next generation of batteries to replace LIBs due to the scarcity of lithium resources and high cost.Abundant potassium resources make potassium ion batteries（PIBs）become the first choice.However,the traditional LIBs electrode materials are not suitable for efficient PIBs due to because of the large ionic radius of K⁺.Two-dimensional layered covalent organic frameworks（2D-COFs）are identified as the ideal anode materials for high-performance PIBs because of their merits,including low density,low cost,stable and ordered crystal framework structure,and adjustable aperture.The graphene-like structures of 2D-COFs are owing toπ-πinteraction densely packed,which can result in deep burial of active sites and low specific capacity.In this paper,via introducing more chemical bonds with energy storage activity（C=O,C=N）,changing the conjugate main chain structure（benzene ring,triazine ring）,composite high-conductivity materials（MXene,carbon nanotubes,graphene oxide,etc.）,to provide material with rich active sites,excellent conductivity,and strong stability,the anode material for PIBs with excellent electrochemical performance can be obtained.In the first part,two-dimensional multilayer structural COF connected by double functional groups,including imine and amidogent through a simple one-step solvothermal process,have been successfully synthesized.The multilayer structure of COF can provide fast charge transfer and combine the merits of imine（the restraint of irreversible dissolution）and amidogent（the supply of more active sites）.It presents superior potassium storage performance,including the high reversible capacity of 229.5 m Ah g^-1at 0.2 A g^-1and outstanding cycling stability of 106.1 m Ah g^-1at the high current density of 5.0 A g^-1after2000 cycles,which is superior to the individual COF.In the second part,covalent organic framework composite carbon nanotubes with plenty of carbanyl group（COF@CNT）were synthesized by solvothermal process,after pyromellitic dianhydride,melamine and carbon nanotubes were mixed at room temperature for a while.The addition of CNT not only can improve the poor conductivity of COFs,but also CNT can provide COFs with growth sites.It can weaken theπ-πstacked structure interaction force and expose the active sites which were deeply buried,which can increase the theoretical specific capacity.The pyromellitic dianhydride has rich active functional groups（C=O）,which can also improve the specific capacity of the material.Therefore,the above merits make COF@CNT possess superior rate performance and cycling stability.Specifically,the COF@CNT electrode presented a superhigh capacity of about 396.6 m Ah g^-1at the current density of 0.2 A g^-1.It also delivered long-term cycling stability(116.7 m Ah g^-1after 1400cycles at the high current of 2 A g^-1).In the third part,a few layers of MXene were synthesized by the non-HF etching process.Then,the imine group covalent organic framework composite MXene（COF@MXene）were successfully synthesized by stirring 1,4-phthalaldehyde,melamine and MXene at room temperature for 5 days continuously.The own characteristic of MXene is strong electrical conductivity,so the addition of MXene can greatly improve the conductivity of COFs.At the same time,MXene can provide COFs with growth sites and guide COFs to grow on MXene,which can weaken theπ-πstacked interaction force and expose deeply active sites,thus it can achieve the purpose that improving the utilization of active sites.Therefore,COF@MXene has excellent potassium storage performance.Specifically,COF@MXene anode presented a high initial capacity of 399.2 m Ah g^-1at a current density of 0.2 A g^-1and long-term cycling stability(126.1 m Ah g^-1at a high current density of 2.0 A g^-1after 1500 cycles).