POMs-based Nanocomposites As Anode Materials For Lithium-Ion Batteries

Polyoxometalates(POMs)are three-dimensional anionic molecular clusters consisting of earlier transition metals oxides(e.g.V,W,and Mo).Due to their remarkable structural and thermal stability,strong acidity,redox activity,and biological compatibility,POMs have captivated considerable interest in many research fields particularly in catalysis,energy conversion,biochemistry,and medicine,etc.In recent years,the redox and electrocatalytic activity of POMs have gained great consideration.In the earlier research,POMs have exhibited good redox activity and electrons transfer capability with an adaptive structure.Therefore,they seemed as good candidates for lithium-ion batteries(LIBs)and electrochemical supercapacitor's electrodes.POMs generally possess poor electronic conductivity that restricts their use in electronic devices.Because of the defective electronic properties,they performed poorly in batteries when used as electrode materials alone.During the past few years,POMs and nanocarbon hybrids have been proposed as electrode material for LIBs and have shown abundant opportunities to meet the challenges in electrochemistry.In this thesis,a series of ionic crystals and POMs/nanocarbon composites were successfully synthesized by ultra-sonication crystallization and covalent functionalization methods.The synthesized ionic crystals and nanocomposite were used as LIBs anode material,showing good electrochemical performance and good cycle stability.This thesis includes the following parts:1.In chapter one,we gave a brief introduction about POMs and POMs-based nanohybrids.Their applications in electrochemistry,especially in LIBs,were summarized and discussed here.2.In chapter two,Co(en)3PMo12(POMs-based ionic crystals)were prepared by using[Co(en)3]3+cation and[PMo12O40]3-anion via supramolecular interactions.The composition and morphology of these ionic crystals were characterized by several methods,such as FT-IR,powder XRD,scanning electron microscopy(SEM),TGA thermogravimetric analysis,energy dispersive X-ray spectroscopy(EDX)and X-ray photoelectron spectroscopy(XPS).The synthesized ionic crystals were used as the LIBs anode material for LIBs battery and showed good performance with a specific capacity of 977 mAh g-1 at 100 mA g-1 current density after 100 cycles.3.In chapter three,we designed and synthesized a new redox-active crystalline ionic solid(Co3PMo12O40)by complexation of trinuclear oxo-centered carboxylate cobalt-containing macrocation([Co3O(CH3CO2)6(C5H5N)3]+)with phosphododecamolybdate[(PMo12O40)3-]anion.A novel C03PMo12O40/SwNTs nanocomposite(SWNTs denotes single-walled carbon nanotubes)with necklace-like morphology was further obtained by using a facile one-pot strategy to mix the crystalline ionic solid and SWNTs.This nanocomposite was chosen to assemble the coin cell for LIBs anode test,and it showed a high specific capacity of 1012 mAh g-1 at current density 100 mA g-1 for 100 cycles,which is significantly better than the traditional graphite anode.This method can be applied to synthesize new kinds of LIBs anode materials.4.In chapter four,MWNTS-EDA/PMo12 nanocomposite(MWNTs are multi-walled carbon nanotubes)had been synthesized by covalent functionalization of MWNTs by EDA.Firstly,MWNTs were dispersed in water by ultra-sonication,then were treated with H2SO4 and HNO3 to introduce hydroxyl and carboxylic groups.The acid-treated MWNTs were further functionalized by using ethylenediamine via acylation reaction.After that,PMo12 was immobilized onto the surface of the MWNTs-EDA via strong electrostatic interaction.The MWNTs-EDA/PMo12 nanocomposite electrode exhibited good electrochemical properties with specific capacity 764 mAh g-1 under 100 mA g-1 current density after 80 cycles.More importantly,this method is convenient for the preparation of LIBs anode materials from an economic point of view.In summary,we fabricated new POMs-based anode materials by tuning the interaction between POMs and CNTs.As anode materials for LIBs,all these prepared nanocomposites demonstrated good electrochemical properties,such as high specific capacity,low electrochemical impedance,and long cycle stability.