Synthesis And Lithium Storage Properties Of Tetrathiafulvalene-based Metal-Organic Frameworks

Lithium-ion batteries(LIBs)have been widely used in portable electronic devices,electric vehicles,large energy storage systems and so on because of their high energy density,excellent reversible capacities along with long cycle life and environmental friendliness.The anode is the key component of LIBs.Commercial graphite-based anodes cannot meet the needs of new electronic devices due to their low theoretical specific capacity and poor rate performance.It is urgent to develop new anode materials.At present,some inorganic materials,such as Si-based materials and transition metal oxides,and organic materials have been widely studied.However,their disadvantages limit their further applications.Organicinorganic hybrid materials possess the characteristics of both organic and inorganic materials,and can achieve the complementary optimization of properties.Metal-organic frameworks(MOFs)are a unique kind of organic-inorganic hybrid materials.Their orderly pore structures allow the rapid storage and transmission of Li+.The stable frameworks enable them to maintain the structural integrity during charging and discharging.In particular,MOFs with redox active ligands can provide a large number of active sites for the insertion of Li+.The focus of this dissertation is to construct MOFs with redox-active tetrathiafulvalene(TTF)-based ligands,develop new anode materials for LIBs,and study their electrochemical properties and potential applications by using their reversible redox activity and lithium affinity of sulfur rich structure.The main contents of the dissertation are as follows:(1)In introduction,the development and working principle of LIBs were firstly summarized.The development status of anode materials was further introduced.Then the difficulties faced by commercial graphite-based anodes and the advantages/disadvantages of some new kinds of anodes were discussed.The research progress of MOFs-based anodes was elaborated.The recent research development of TTF-based MOFs in the field of energy storage was summarized.Finally,the idea and research content of this topic were expounded.(2)In chapter two,two three-dimensional TTF-Zn-MOFs were synthesized,formulated as[Zn2(py-TTF-py)2(BDC)2]·2DMF.H2O(1)and[Zn2(py-TTF-py)(BDC)2]·DMF.2H2O(2)(py-TTF-py=2,6-bis(4’-pyridyl)tetrathiafulvalene and H2BDC=terephthalic acid).1 can work as a high-performance anode material for rechargeable LIBs.1 electrode displayed a high discharge specific capacity of 1117.4 mA h g-1 at a current density of 200 mA g-1 after 150 cycles along with good reversibility.Based on the X-ray photoelectron spectroscopy and structural analysis of 1 and 2,the TTF moiety and the twofold py-TTF-py pillar play a key role in the excellent electrochemical performance.(3)In order to study the effect of different metal centers and porosity of TTF-MOFs on lithium storage,in chapter three,the electric active Co(Ⅱ)ion was selected to coordinate with redox-active py-TTF-py to create two TTF-Co-MOFs,formulated as[Co2(py-TTFpy)2(BDC)2]·2DMF.H2O(3)and[Co2(py-TTF-py)2(BPDC)2]·3DMF·3H2O(4)(H2BPDC=biphenyl-4,4’-dicarboxylic acid).The two MOFs possess similar 2-fold-interpenetrating 3D frameworks but with two different pore sizes.And 3 is isostructural with 1.Compared with 1 and 4,3 presents the highest reversible specific capacity of 1186.6 mAh g-1 at 200 mA g-1 after 287 cycles and the best rate performance.On the basis of the series analysis of theoretical calculations,X-ray photoelectron spectroscopy,and crystal structures,the competitive performances of 3 are attributed to the synergistic effect of the Co(Ⅱ)metal centers and S-rich py-TTF-py ligand as well as suitable porosity.(4)Hybrid lithium-ion capacitors(HLICs)combine the advantages of LIBs and ECs with both high energy density and high power density.Considering that the great majority of MOFs are electrical insulators,the poor intrinsic electric conductivity of the MOFs blocks the charge transfer and limits the fast kinetics of the MOF-based electrode,which hinders their applications in energy storage devices.In chapter four,the 3 and 4 anodes with low conductivity were optimized by adjusting the content of the conductive agent in the electrode.Each of the optimized anodes presented a higher reversible specific capacity,better rate performance and a longer cycle life.Furthermore,the optimized 3 electrodes were selected as the pseudocapacitive anodes of HLICs.The HLICs with optimized 3‖AC configuration presented a high energy density as well as a high power density,and stable cycling performance with 88%capacity retention after 8000 cycles at 2000 mA g-1.The results help to extend the application of pristine TTF-MOFs with low conductivity as remarkable anodes for advanced energy storage devices.(5)Mixed metal MOFs have become a research hotspot in recent years.The synergistic effect between different metals may make them have better electrochemical performance than single metal MOFs.In addition,the improved conductivity of electrode materials can optimize the lithium storage properties of TTF-MOFs.In chapter five,we prepared three isostructural MOFs with high intrinsic conductivity formulated as Co2TTFTB(5),Mn2TTFTB(6)and MnCoTTFTB(7)(H4TTFTB=tetrathiafulvalene-tetrabenzoate).The performance of them as anode materials for LIBs was studied.All three MOFs showed good lithium storage capacity,among which bimetallic 7 presents the highest reversible specific capacity and the best rate performance.The results show that mixed-metal MOFs may have advantages over traditional single-metal MOFs as energy storage materials.(6)At last,the overall work is summarized,and the future research is prospected.