| Polyoxometalates-based metal-organic frameworks?POMOFs?,as a kind of stable hybrid material,is formed by introducing POMs into MOFs.POMOFs can combine the advantages of POMs and MOFs,which provide a stable and fast channel for the transport and storage of electrons and charges.For example,the oxygen-rich surface,strong coordination ability and multi-electron transfer of POMs,and the long-range ordered framework and stable channels of MOFs.POMOFs is one of the excellent candidates for the anode materials of the new generation of rechargeable batteries.However,the electrical conductivity of POMOFs is quite low,especially when POMs exists as molecular form.Carbon nanotubes,such as multi-walled carbon nanotubes?MWCNT?,single-walled carbon nanotubes?SWCNT?and graphene,have been widely used in the design and synthesis of POMs composite electrode materials because of their excellent electrical conductivity,large specific surface area and easily functionalized surface.In order to enhance the electrochemical performance of POMOFs,it is the most direct and effective way to form composites with carbon materials.In this thesis,we aiming at the problems of low electronic conductivity and soluble in electrolyte of POMs,a series of polyacid quaternary ammonium salts and derivatives,zero-to three-dimensional POMOFs materials and carbon-coated POMOFs crystal composites were designed and synthesized based on the application function of materials.The effects of material morphology,type of unit?source unit?,structure dimension and carbon coating on the cycle life and specific capacity of the battery were deeply studied and explored.1.Tetrabutylammonium bromide was used to aggregate different POMs into insoluble polyacid quaternary ammonium salt complexes,namely?NBu4?3[PMo12O40]?1?,?NBu4?3[PW12O40]?2?,?NBu4?4[SiW12O40]?3?and?NBu4?6[P2W18O62]?4?.And then the quaternary ammonium carbon coating materials were obtained by polypyrrole and carbon nanotubes,namely?NBu4?3[PMo12O40]@PPy@CNT?5?,?NBu4?3[PW12O40]@PPy@CNT?6?,?NBu4?4[SiW12O40]@PPy@CNT?7?and?NBu4?6[P2W18O62]@PPy@CNT?8?.Finally,1@PPy@CNT?9?,2@PPy@CNT?10?,3@PPy@CNT?11?and 4@PPy@CNT?12?were synthesized by carbonized the carbon mixed with polypyrrole coating polyacid quaternary ammonium salt materials at high temperature.The POMs was thermally decomposed into source units.The electrochemical performance studies show that the discharge specific capacity and cyclic performance of polyacid quaternary ammonium salt complexes and their derived materials are not ideal,and the simple solidification and multiple compound strategies of POMs molecules can not improve the performance of electrode materials.The strong coordination characteristics of POMs should be immobilized to improve the multi-electron redox ability of POMs in the lattice space.2.Three low-dimensional POMOFs materials were obtained by immobilizing POMs molecules into MOFs.H3[C12H8N4S]2[PMo12O40]·4H2O?BPTD-POM??13?[Ag2(C11H7N6O2)2(SiW12O40)][Ag2(C11H7N6O2)2]?Ag-BTBA-POM??14?[Cu4(C12H8N4S)4?OH?5][PMo12O40]?Cu-BPTD-POM??15?Metal ions as stabilizers can improve the bonding degree between POMs and organic ligands,so that the electrochemical properties of one-dimensional chain complexes can be promoted.In addition,although POMs was used as template agent for simple and complex one-dimensional chain structures,the discharge specific capacity is different.It is proved that the dimension and complexity of the structure are important factors affecting the performance of lithium-ion batteries.3.Two high-dimensional POMOFs materials were obtained by adjusting the type of building blocks and reaction conditions.H[Ag(C12H8N4S)2]2[PMo12O40]?Ag-BPTD-POM??16?[Cu2(C12H6N6)2?OH?][PMo12O40]?Cu-BTDB-POM??17?The specific capacity of Ag-BPTD-POM with two-dimensional interpenetrating structure is slightly poor,which indicates that the interpenetrating structure affects effective immersion of electrolyte and can not get POMs to play with the multi-electron transfer capability.However,the specific capacity of Cu-BTDB-POM with three-dimensional structure is significantly increased,the specific capacity was 834 mA h g-1after attenuation of initial capacity and the specific capacity was stable at 788 mA h g-1after 300 cycles.This shows that the Cu-BTDB-POM provides a stable channel for the insertion/ejection of lithium ions,and the reasonable structure type enabled POMs to maximize the performance of high specific capacity.Generally speaking,the complexity of structure is an important factor affecting the electrochemical performance.4.In order to increase the conductivity and cycle stability of POMOFs,the carbon nanotubes coating POMOFs materials were obtained.CNT@BPTD-POM?18?CNT@Ag-BTBA-POM?19?CNT@Cu-BPTD-POM?20?CNT@Ag-BPTD-POM?21?CNT@Cu-BTDB-POM?22?The outer layer of POMOFs were coated with carbon nanotubes by ultrasonic wave,On the one hand,the carbon nanotubes coating method can effectively increase the electronic conductivity of POMOFs,On the other hand,it can buffer the shock caused by a large number of electrolytes immersed in the lattice during charge and discharge since carbon nanotubes can store electrolytes in their internals.It can not only protect the crystal structure of POMOFs from collapse and improve the cyclic stability,but also prevent the aggregation of lithium ions on the surface of POMOFs to generate dendrites and maximize the advantage of high specific capacity of POMs.The specific capacity of CNT@Ag-BPTD-POM and CNT@Cu-BTDB-POM were basically stable at the level of 850 mA h g-1and 1000 mA h g-1after cycle of initial activation. |
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