The Preparation And Application Of Ultrathin Two-dimensional Metal-organic Framework(MOF) Nanosheets

As a new type of nanomaterials,two-dimensional metal-organic frameworks（2D MOFs）nanosheets have broad application prospects in different fields such as catalysis,gas separation and energy conservsion due to their many disadvantages such as large specific surface area,controllable nanometer-level thickness and abundant accessible active sites.However,at the present preparation of 2D MOFs nanosheets has some problems such as uneven thickness and uncontrollable morphology,especially the fabrication of single-layer 2D MOFs nanosheets is particularly difficult.Therefore,the precise design and synthesis of ultrathin 2D MOFs nanosheets still remain a significant challenge.Here,we developed a novel"top-down"chemical exfoliation to prepare one-layer 2D MOFs nanosheets with controllable thickness and surface functionalization.This method also shows good results when applied to the preparation of monolayer nanosheets similar to MOFs isoforms,indicating that the developed method has universal applicability.Foremore,in order to further investigate the application of 2D MOFs nanosheets in the lithium-sulfur（Li-S）battery,we employed them as modified materials of the separator for Li-S batteries to suppress“shuttle effect”of polysulfide in view of the characteristics of the 2D MOFs nanosheets,which obviously improve the overall cycle stability and rate performance of the battery.The specific research contents and results are as follows:（1）Preparation of the 2D MOFs nanosheets through"Top-Down"chemical exfoliation method.Using the pillar-layered structure three-dimensional MOFs（3D MOFs）as the precursor,the O₃ molecular is utilized as a cutting tool to break the olefin groups of 1,2-bis（4-pyridine）in the MOF precursor,which results in selective fragmentation and cleavage of pillar ligands.So we obtained single-layer 2D MOFs nanosheets through striking the interlayer interaction of the 3D MOFs precursor.The microstructure of the material was characterized by transmission electron microscopy（TEM）,atomic force microscopy（AFM）,which showed that the lateral size of the prepared nanosheets can reach the micron level,and the thickness is only 1-2 nm;Meanwhile,the treatment of O₃ molecules can introduce a large amount of-COOH on the surface of single-layer 2D MOFs nanosheets to achieve surface regulation of nanosheets.Other isomorphic MOFs were successfully exfoliated into single-layer 2D MOFs nanosheets with via this simple and low-cost strategy,which shows that the approach is universal.（2）Application research of 2D MOFs nanosheets for modified separator in Li-S batteries.Based on the above method,the prepared 2D MOFs are applied to the modification of the separator of Li-S batteries.Firstly,the 2D Zn-MOF nanosheets were loaded on the commercial Celgard-2400（PP）porous separator by vacuum filtration method to form a densely stacked two-dimensional structure modified layer with an optimized loading amount of 0.25 mg cm^-2.Moreover,assembling the modified separators into the cells for charging and discharging test,which shows that the Li-S cells with 2D Zn-MOF@PP delivered an initial discharge specific capacity of 1111 m A h g^-1 at 0.2 C,and 785 m A h g^-1 can still be retained after 100 cycles;After200 cycles,a low decay rate of merely 0.156%per cycle at 0.5 C was achieved.The results show that the 2D Zn-MOF@PP separators can effectively inhibit the"shuttle effect"of polysulfides and improve the electrochemical performance of the Li-S battery,which speculated that its excellent barrier effect originated from the the synergistic effect of size sieving and charge repulsion...