Metal/Covalent-Organic Framework Based Composites For Lithium/Potassium Ion Batteries

Lithium-ion batteries（LIBs）are widely used in portable electronics products and electric vehicles owing to their long lifespan and high energy density.However,the increasing price of Li due to the scarcity and uneven geographical distribution of Li resources may limit its sustainable application in the near future.Therefore,looking for a promising alternative to LIBs in future energy storage systems is currently the hot research topic.Thereinto,potassium-ion batteries（PIBs）are one of the most competitive candidates owing to their abundant earth resources,low cost,eco-friendliness and high energy density.Moreover,the rapid development of the electronic market raises higher requirements for the energy density and long cycle lifespan of LIBs and PIBs.As a component part of battery,the anode material is the key to improve the performance of batteries.Commercial graphite anode usually shows limited capacity and energy density,which cannot meet the growing demand of various devices.Therefore,the rational design of new anode materials with high capacity and good cycle stability is crucial for constructing high-performance LIBs and PIBs.Metal-organic framework（MOF）and covalent-organic framework（COFs）,as a new class of porous electrode materials,have attracted extensive attention in recent years because of its large surface area,adjustable pore structure,and abundant redox sites.Yet,most of MOFs and COFs suffer from poor electroconductivity and low adsorption energy（ΔEa）,hindering their application in practice.According to these two problems,the paper proposes constructing the heterostructure of MOFs and COFs and unlocking MOFs nodes to optimize the electronic structure,and investigates the electrochemical properties.The specific research contents and conclusions are mainly divided into three parts:（1）A heterostructure（NF-MOF@MXene）is constructed by preparing a conductive ferrocene based MOF and introducing a highly conductive Ti₃C₂Tx MXene,which shows a high conductivity and improved ΔEa of Li⁺.Charge density difference and planar average potential charge density show substantial redistribution of charges at the interfaces,transferring from MXene to MOF layers,which optimize the band alignments and electronic structures of MOFs.Moreover,density functional theory（DFT）calculations reveal that the interaction between MXene and MOF significantly increases the Δ Ea.The NF-MOF@MXene heterostructure as the LIBs anode shows the high capacity and excellent cycle stability.Furthermore,the heterostructure anode is built into a full cell with a commercial LiNi_0.5Co_0.2Mn_0.3O₂（NCM 532）cathode,delivering a high energy density and power density.（2）Two-dimensional（2D）MOFs nanosheets（NMD-MOF）are prepared using 2D conductive V2CTx MXene and highly π-conjugated porphyrin（H2TCPP）as metal precursor and ligand source,respectively.Thermogravimetric analysis coupled with mass spectroscopy（TGA-MS）is applied to understand the conditions needed for unlocking the MOF nodes.After partically unlocking the MOF nodes,the original coordination saturated site is in the unsaturated state.DFT calculations reveal the ΔEa of K⁺at the unlocked node sites is greatly improved compared to that of the porphyrinic centers and intact node sites,which elucidates the ultimate cause of the increasing capacity.The NMD-MOF as PIBs anodes exhibits good rate performance and excellent cycle stability the specific capacity,which is much higher than that of the locked MOF.Furthermore,homologous V2CTX MXene-derived K⁺-intercalated vanadium oxide（MD-KVO）is used as the cathode to assemble the PIB full cell,which show good compatibility with the NMD-MOF anode.The PIB full cell shows a high energy density and power density.（3）Highly crystalline COF is prepared by Schiff base reaction using 1,3,5-triformylphloroglucinol（Tp）and 4,4’-diaminobiphenyl（DAB）as ligands.Functionalized graphene（FG）with substantial functional groups（e.g.carboxyl and hydroxyl）is used as the active sites to anchor DAB ligands,which can induce the vertical growth of COF nanosheets on the surface of FG,thus constructing the heterostructure of COF@FG.After pyrolysis,COF is converted into porous carbon nanosheets perpendicular to the surface of reduced FG,constructing a 2D carbon matrix（NCM）.The NCM shows the massive N-doping and a high surface area,which provides extra reactive sites for fast insertion/extraction kinetics of Li ions.The as-obtained NCM as an anode in LIBs exhibits high capacity,good rate performance,and cycling stability.