| Presently,the lithium ion battery have been acclaimed as advanced powersources,slowly replacing several versions of conventional systems such as the leadacid and nickel cadmium batteries.Because of its two features,efficient andenvironmentally friendly,can contribute to the solution of energy crisis andenvironmental pollution,research and development of lithium-ion battery havebecome a strategic imperative of our nation. LiVPO4F cathode material is a fluorideapplication in lithium-ion battery phosphate compounds, because it has a good cycleperformance,155mAh/g theory capacity and about4.2V operating voltage platform,and much of attention.In spite of the enormous commercial success of LiCoO2-based lithium-ionbatteries, there is still a continual requirement for new electrode materials withimproved electrochemical and safety characteristics. In recent years there has beenconsiderable interest in the application of phosphate-based active phases such asLiFePO4（Olivine）and Li3V（2PO4）3（Nasicon）. Recently, BARKER et al extendedtheir investigations to include the electrochemical evaluation of a series offluorophosphate materials represented by the general formula LiMPO4F, where Mrepresents a3d transition metal. Among these materials, LiVPO4F is the mostpromising and attractive one because of its relatively high capacity with a plateau ataround4.2V.Firstly, we successfully prepared LiVPO4F using a Super P assisted sol-gelmethod. The decomposition process of the precursor was discussed via TG analysis,based on which we determined the proper synthesis temperature of LiVPO4F.We studied the structure properties of the material using varies techniques including XRD,FTIR,SEM,and XPS.XRD patterns of LiVPO4F with different temperatures （500℃；550℃;600℃；650℃） show that crystalline phase of Li3V2（PO4）3is more and moreobvious with the heat treatment temperature increased.The morphologies of LiVPO4F materials at various temperatures were observedusing scanning electron microscopy （SEM）,and indicate that the actual grain sizes ofmaterials are minished with the increase of sintering temperatures. In the infraredspectral maps of the material we have not found the vibration peak of crystallinecarbon, show that the residual carbon exists in amorphous.Observing550℃prepared sample morphology using transmission electron microscopy analysis, wefound that the LiVPO4F material is completely coated by the residual carbon, andthickness of the carbon layer is about3nm.The LiVPO4F particles are embedded inthe network of carbon. The carbon network structure can provide good electricalcontact between the LiVPO4F particles.X-ray photoelectron spectroscopy （XPS） has been extensively used to study theelectronic structure and chemical composition of the materials.Here,LiVPO4Fprepared at550℃was characterized by XPS spectra.It can be found that the V2p3/2and V2p1/2binding energies of the LiVPO4F XPS spectra are located at517.5eV and524.8eV，very closed to that of V3+.We studied the electrochemical properties of LiVPO4F through galvanostaticcharge-discharge and cyclic voltammetry experiments. Charge-discharge cyclingexperiment was carried out at0.1C rate in the potential window on2.04.5V.Theinitial discharge capacity of the LiVPO4F material was189.0mAh/g,which decreasedto174.0mAh/g after50cycles.However, the charge process was characterized by asingle sloping profile in the present LiVPO4F sample prepared by sol-gel method,which is quite different from the results of the samples prepared through conventionalsolid-state reaction.The kinetic properties of Li+ion is one of the most important factors for theelectrochemical performance of cathode materials.In this study,potentiostaticintermittent titration technique was employed to evaluate the Li+ion diffusioncoefficients.The lithium diffusion coefficient of the material between2.0and2.6Vwere much higher than those between4.0and4.5V. The diffusion coefficient between2.02.6V voltage showing a V-shaped trend. Materials of Li+ion diffusioncoefficient DLi+=3.51×10-15cm2s-1at2.0V. When from2.0V gradually reduced to2.49V minimum DLi+=3.39×10-16cm2s-1, the diffusion coefficient of the materialbegan to larger DLi+=2.63×10-13cm2s-1. After the diffusion coefficient of thematerial have a small amplitude of the fluctuations. When the voltage is3.97V, thediffusion coefficient of the material rapidly dropped to DLi+=5.87×10-17cm2s-1,andwhen the voltage is4.02V,diffusion coefficient is the minimum DLi+=3.36×10-17cm2s-1, followed by an upward trend.On all accounts, this work gives us a comprehensive understanding on thepreparation of LiVPO4F polyanion cathode materials, as well as their structural andelectrochemical properties. |
