The Ordered Construction And Properties Of Vanadium Oxide @C Composite Electrode Materials

One-dimensional?1D?vanadium oxides exhibit excellent performances in acoustics,optics and electricity owing to their unique characteristics on the nano-scale.Especially in electrochemical energy storage,the 1D vanadium oxides show widely application foreground in electrode materials of lithium ion batteries because of the characteristics such as high specific surface area,short ion transmission path,high specific capacity and low cost.In this thesis,1D vanadium oxides with nano-structure were prepared by a facile-hydrothermal method.Subsequently,the growth mechanism of 1D vanadium oxides were studied using X-ray Diffraction?XRD?,Scanning Electron Microscopy?SEM?and X-ray Photoelectron Spectroscopy?XPS?analysis.What's more,while used as the cathodes of lithium ion batteries,the cyclic voltammetric performance,galvanostatic charging-discharging performance and cyclic stability were studied by analyzing the electrochemical properties of synthesized vanadium oxides.?1?The 1D vanadium oxides with nanoscales were synthesized using VOSO₄·2H₂O and distilled water as raw material and solvent at the temperature of 190?.The analysis of XRD and XPS indicated that the product was V₃O₇·H₂O crystal with orthogonal structure.Combining the morphology and structures of V₃O₇·H₂O nanoribbons,the growth process of samples were concluded as Dissolution-Oxidation-Exfoliation-Flaking through analyzing the phase of products prepared at different hydrothermal reaction time.Phase characterization,cyclic voltammetry?CV?,galvanostatic charging-discharging performance and electrochemical impedance spectroscopy?EIS?were performed on the prepared samples.The results demonstrated both crystallinity and electrochemical performances of samples were improved by increasing the time of hydrothermal reaction.The initial discharge specific capacity of the product,of which the hydrothermal reaction time were 48 hours,reached 245.7 mAh g^-1 and the capacity retention rate wasstill maintained at 87.99%after 50 cycles of charging and discharging.?2?The nanobelts of V₃O₇·H₂O prepared in experiment one were chosen as precursors and annealed in air at 400?.The samples after annealing were analyzed by XRD,and the results showed that the products were V₂O₅ with oblique structure.What's more,the SEM pictures showed that V₂O₅continued to hold the structure characteristics of V₃O₇·H₂O,which indicated the strong structural stability of products.Furthermore,the electrochemical test displayed that the electrochemical properties of the samples was improved with the increasment of the hydrothermal time of the precursors.For example,the V₂O₅,obtained from calcining the V₃O₇·H₂O precursor prepared with 12 hours during the hydrothermal process,showed initial specific discharge and charge capacity of 232.2 mAh g^-1 and 202.5 mAh g^-1 separately,performing the coulomb efficiency of 87.2%,which was much higher than the samples obtained with the hydrothermal time of 48 hours.This phenomenon was related to the crystal structure of V₂O₅.The higher degree of amorphousness of samples,the smaller particle size of the electrode active material was obtained,which facilitated the contact between the electrode material and the electrolyte,shortened the lithium ion transport path,promoted ion diffusion thus improving the electrochemical properties of batteries.?3?However,vanadium oxides generally suffered the poor electronic conductivity and low lithium ion diffusion coefficient,accompaniedwith structural changes during charging and discharging.While used as electrodes of lithium ion batteries,the cycle stability and rate performance performed poor.Glucose was selected as the carbon source to coat the V₂O₅,and the effects of different mass fractions of carbon adding were invested.The results showed that the structure of vanadium oxides were not changed and the electrochemical performances of products after carbon-coating were modified owing to that surface carbon layer increased the electronic conductivity of active electrode material and relieved the stress caused by the volume change of vanadium oxides during charging and discharging process and improved the cycle stability of batteries.Especially,the optimum properties of the productswere obtained when the carbon content was 3%.