Synthesis Of Three Dimensional VO_x,VS₂ Nanomaterials And Their Supercapacitive Performance

Supercapacitors are promising energy storage devices with high power density,long cyclic life and high charge/discharge speed compared to conventional capacitors and batteries,which have been widely researched worldwide.The most important part of supercapacitors is electrode materials that determine the electrochemical performance of supercapacitors.Vanadium based materials are ideal electrode materials for supercapacitors,owning to its multiple valences,high theoretical capacitance,abundant resources and low price.However,the common vanadium-based materials have poor electrical conductivity,unstable structure,and poor cycling stability,which limit their application in supercapacitors.In this thesis,the common vanadium-based materials are constructed into three-dimensional nano structures and/or incorporated with highly conductive materials,in order to improve their electrochemical performances.The main contents are as follows:Commercial V2O5 powder was used to prepare three-dimensional(3D)nanocorals by hydrothermal method.The nanocorals were assembled by short nanorods arranged in radial pattern.According to XRD and FT-IR results,the composition of nanocorals was pure V2O5 crystal.N2 adsorption/desorption analysis showed that the material of this structure has a high specific surface area of 42.6 m2/g,with an average pore diameter of 18 nm.The electrochemical performance tests showed that the specific capacitance of 3D nanocorals were as high as 414 F/g at a current density of 0.3 A/g and about 70% initial specific capacitance had been remained after 3000 charge/discharge cycles.Novel hierarchical VOx microspheres forming from hyperbranched growth of nanoribbons have been successfully synthesized by solvothermal method.The hierarchical VOx microspheres were composed of V6O13 with metal conductivity at room temperature and VO2,the mass fraction of V6O13 is 86.2 wt%,and that of VO2 is 13.8 wt%,the products had excellent conductivity due to the high content of V6O13.This VOx microspheres with a diameter of ~5 μm had a porous interior structure,and the specific surface area is as high as 51 m2/g,the average pore size is about 16 nm,belonging to mesopore.The hierarchical VOx microspheres exhibit a specific capacitance as remarkably high as 456 F/g,with a corresponding volumetric specific capacitance of 3.09 F/cm3,at a current density of 0.6 A/g.The maximal energy density and power density achieved are up to 22.8 Wh/kg(0.16 mWh/cm3)and 1.2 kW/kg(8.14 mW/cm3).65% initial specific capacitance had been remained after 2000 charge/discharge cycles.VS2 nanoflowers were synthesized by hydrothermal method using sodium orthovanadate dodecahydrate(Na3VO4·12H2O)and thioacetamide(C2H5NS)as raw materials.By studying the influence of mass ratio and concentration of raw materials,reaction temperature on the morphology of products,the optimum synthesis conditions were obtained: the mass ratio of Na3VO4·12H2O:C2H5NS is 1:3,the concentration of raw materials is 0.12 g/mL,and reaction temperature is 140 ℃.The VS2 nanoflowers were constructed by the crimped VS2 nanosheets.The specific capacitance of VS2 nanoflowers was 211 F/g at the current density of 0.3 A/g.It can be maintained at 74% of initial specific capacitance when the current density was increased to 1.0 A/g,indicating the good rate performance of VS2 nanoflowers.However,the specific capacitance of VS2 nanoflowers dropped to 50.6% after 500 charge/discharge cycles,showing their poor cycling performance..