Ultraviolet Assisted Ionic Liquid Electrodeposition And Electrochemical Properties Of Germanium Based Anode Materials

Germanium deserves to be considered as a promising anode candidate for LIBs owing to high theoretical specific capacity and low working potential.Nevertheless,Ge suffers from electrochemical pulverization problem due to the large volume expansion/shrink during the repeated charge/discharge processes.Additionally,the repeated volume changes hinder the charge transfer and reduce the electrical contact between the active material and the collector,resulting in serious capacity fading and battery performance degradation.In this thesis,in order to buffer the volume expansion and improve the electrochemical performance of germanium based materials,one-dimensional germanium nanomaterials were grown in situ on different substrates by UV assisted ionic liquid electrodeposition without catalyst and binder.The electrochemical property of germanium nanowire clusters deposited on foamed nickel substrate were tested,and the effects of Ge nanowire cluster materials in improving the cycling stability of materials were studied.Ge/graphene was prepared in situ on nickel foam by UV assisted ionic liquid electrodeposition.The electrochemical propertie of Ge/graphene materials prepared by UV assisted ionic liquid electrodeposition and chemical reduction method was compared,and the advantages and disadvantages of the two methods were determined to provide some references for the preparation of germanium/graphene composite materials.The deposition behavior of germanium was studied by ultraviolet light(UV)assisted ionic liquid electrodeposition.The effects of irradiation wavelength,deposition potential,deposition time,irradiation height and substrate on the deposition morphology were investigated.UV light affected the morphology of the deposit obviously.The Ge film composed of nanoparticles was obtained without UV.In comparison,one dimensional structure Ge deposits were obtained by UV-assisted electrodeposition.The deposition potential showed an important influence on the morphology of Ge deposits under 365 nm UV irradiation.The coral-like Ge nanowire clusters were obtained at-1.3 V.With the negative shift of the deposition potential,compact nanorod clusters could be obtained.The influence of UV irradiation on the deposition process and ionic liquid was investigated,so as to explore the mechanism of electrodeposition of Ge nanorods by UV assisted ionic liquid electrodeposition.The effect of UV light on the ionic liquid and deposition process was investigated,and the differential capacitance of the assembled ITO electrode was analyzed by electrochemical impedance spectroscopy(EIS).The influence of UV irradiation on the arrangement of anions and cations was deduced,and the mechanism of UV assisted electrodeposition of germanium nanorods was determined:UV irradiation can retard the decomposition of [EMIm]+ cations and promote the reduction of germanium.During the deposition process,the imidazole ring of [EMIm]+adsorbed is equivalent to forming a solution template on the electrode surface,which hinders the lateral growth of Ge nuclei and promots the formation of Ge nanorod structure.Ge nanowires cluster arrays(Ge NWCA)were obtained on nickel foam via an ultraviolet light assisted,template-free and catalyst-free electrodeposition process.The deposition mechanism was studied by the cyclic voltammograms and chronoamperometry experiments.The reduction process of germanium in the deposition system was controlled by diffusion,and the nucleation process of germanium was consistant with the characteristics of three-dimensional nucleation.As a binder-free anode material,the Ge NWCA electrode delivered a specific capacity of 1612 m Ah·g-1 and 740 m Ah·g-1 after 200 cycles at a current rate of 0.2 C.The Ge NWCA electrode exhibited excellent rate capacity at 0.1 C-2 C and retains 959 m Ah·g-1at a current density of 2 C.Furthermore,the specific capacity well recovered to 998 m Ah·g-1 when the rate decreased to 0.1 C.The Ge NWCA can achieved excellent cycling stability and rate capacity compared with the Ge film electrode used as the anodes of LIBs.The agglomeration of Ge nanowires with their neighbors led to the emergence of interconnected,porous network after repeated Li discharges/charges.This porous network structure of the Ge ligaments provided flexible space for the material to expand or self-adjust,while also allowing fast diffusion of Li ion.Thus,this structure is highly stable and capable of withstanding the volume expansion.Germanium/graphene/nickel foam(Ge/RGO/Ni foam)composite anode materials were obtained by UV assisted ionic liquid electrodeposition after loading graphene oxide(GO)onto Ni foam by dipping method.The reduction of GO and the deposition of germanium were completed at the same experimental step.The Ge/RGO/Ni foam anode exhibited a specific capacity of 1636 m Ah·g-1with an initial Coulombic efficiency of 72.9%.The specific capacity of Ge/RGO/Ni foam was higher than that of Ge NWCA-2,indicating that the introduction of RGO substrate was beneficial to the capacity retention of Ge electrode.The Ge/RGO/Ni foam anode retains capacities of 895 m Ah·g-1 after 200 cycles at 0.2 C,which was higher than that of the chemically reduced Ge/graphene of 676.6 m Ah·g-1.Compared with the chemical method,the Ge/RGO/Ni foam composite electrode obtained by electrodeposition possessed a larger lithium ion diffusion coefficient,thus showing better rate performance.The UV-assisted ionic liquid deposition of german-based/graphene composites,which does not require binder or conductive agent,can improve the energy density of batteries,simplify the preparation process,and can be extended to the preparation of other element and alloys of Group IV.