Synthesis And Modifications Of High-performance Metal-Organic Frameworks As Electrode Materials

Along with the growing demand for high-performance lithium-ion batteries?LIBs?,there is a need for the development of novel anode materials to improve the capacity,cycle life,and rate performance of LIBs.Some metals that can reversibly form alloys with lithium are promising alternative anode materials for LIBs because of their high theoretical capacities,such as silicon(forming the Si Li_4.4 alloy,4200 m Ah g^-1),tin(forming the Sn Li4.4 alloy,993 m Ah g^-1),antimony(forming the Sb Li₃ alloy,660 m Ah g^-1)and germanium(forming the Li₂₂Ge₅ alloy,1600 m Ah g^-1).However,these metals undergo very large volume changes during the alloying and dealloying processes,resulting in fast capacity degradation.As a novel class of porous materials,metal organic frameworks?MOFs?have attracted much attention for energy storage and conversion applications such as LIBs,fuel cells,and supercapacitors due to their porous architecture that allows rapid insertion of species and offers improved electrochemical properties.However,some intrinsic drawbacks,such as the poor electrical conductivity and low lithiation activity,limit their practical application,and the purposeful preparation of highly electrically conductive MOFs is still a big challenge.Herein,we developed three novel post-synthetic methods to prepare modified MOFs.By using these facile novel methods,we successfully synthesized thin-film electrode based on zeolitic imidazolate frameworks?ZIF-8 and ZIF-67?,Sn O_x/UiO-66 composites and Zn Mn₂O₄ nanograins.The crastal structure and morphology of the as-obtained materials was confirmed by X-ray powder diffraction,field emission scanning electronmicroscopy and transmission electron microscopy.The electrochemical performance was also investigated by cyclic voltammetry?CV?,charge-discharge measurements and electrochemical impedance spectroscopy?EIS?using LIR 2032 coin type half cells.The main keypoints of this dissertation can be summarized as follows:?1?Thin-film electrodes based on zeolitic imidazolate frameworks?ZIF-8 and ZIF-67?were prepared using a one-step drop-casting strategy without polymer binder and carbon black.The electrodes exhibit a thin-layer structure,which is an intriguing architecture for lithium-ion battery applications.A high reversible capacity(335.3 and 311.6 m Ah g^-1 for ZIF-8 and ZIF-67,respectively),good rate performance,and exceptionally cycling stability were observed in this study.Such an excellent electrochemical performance of the film electrodes is attributed to the thin-layer architecture,which provides easy access for Li ions to permeate the whole electrodes because of the shortened diffusion paths,and also to the unique property of the MOFs without the collapse of its structure.?2?A novel method that combines the dehydration of inorganic clusters in metal-organic frameworks?MOFs?with nonaqueous sol-gel chemistry and pyrolysis processes is developed to synthesize Sn O_x quantumdots/Zr-MOFs?UiO-66?composites.Beneting from the advantages of the smaller size of Sn O_x nanoparticles and the synergistic effect between Sn O_x nanoparticles and the Zr-MOFs,the Sn O_x/Zr-MOF composite exhibits enhanced electrochemical performance.Specically,the discharge specific capacity of the Sn O_x/Zr-MOF electrode can still remain at 994 m Ah g^-1 at 50 m A g^-1 after 100 cycles.?3?New functional groups were introduced into the organic ligands of ZIF-8 through a post-synthetic modification approach?-CH₃?-COOH?with the formation of ultral fine Mn O₂ on the surface of MOFs.After a subsequent cacination process,nanophase Zn Mn₂O₄compounds was successfully prepared.When used as an anode material for LIBs,Zn Mn₂O₄ electrodes exhibited an excellent electrochemical property with a high reversible capacity(444 m Ah g^-1),good rate performance,and long-term cycling stability.Such a facile novel synthetic route will broaden our horizons in the preparation of mixed metal oxides.