Modification Of Vanadium-Based Electrode Materials And Research On Supercapacitor Performance

Supercapacitors（SCs）is a kind of efficient,stable,environmentally friendly,and potential energy storage devices.However,the energy density of current SCs is still not enough to independently complete the long-term energy-consuming system support task.Therefore,higher requirements are put forward for supercapacitor materials.VO₂（B）has a unique layered structure and a large interlayer spacing,which is conducive to the rapid transport of ions.However,the low electrical conductivity and dissolution problem of material has limited its use as electrode material in supercapacitors during the charge and discharge process.In this thesis,VO₂（B）is modified by metal cation doping and atomic layer deposition coating method.Material characterization and supercapacitor performance test are carried out as follows:VO₂ nanosheets with different nickel doping density are prepared by hydrothermal method.Furthermore,the effect of nickel doping on the electrochemical performance of electrode materials are investigated.All the Ni-doped samples show better electrochemical performance than the pure VO₂（B）.At a current density of 1 A g^-1,the Ni-VO₂-2 electrode exhibits a specific of 124.6 F g^-1,which is the best of the Ni-doped samples and superior to the pure VO₂(87.2 F g^-1).The results show that the introduction of nickel ions can change the VO2 lattice and electronic structure,generate more defect sites and improve the electrical conductivity of the material,thereby improving the specific capacity performance,rate performance and cycle performance of the electrode materials.VO2（B）will dissolve in aqueous electrolyte,resulting in poor electrode cycle performance.The Ni O coated Ni-VO₂ are further prepared by atomic layer deposition method,using the Ni-VO₂ as the substrate material（Ni-VO₂@Ni O）.Next,the effect of Ni O coating on the electrochemical performance of the material is investigated.Ni-VO₂@Ni O-200 of exhibits a specific capacity 218.5 F g^-1,at a current density of 1 A g^-1,which is the best of the Ni O coated samples.The Ni-VO₂@Ni O-200 has a capacity retention rate of 81.8%after 2500 charge and discharge cycles,which is significantly better than the pure VO₂ sample（38.6%）.Symmetric supercapacitor is designed and assembled,by using Ni-VO₂@Ni O-200 as electrode materials and PVA/Na₂SO₄ gel as electrolyte,with a sandwich structure.The Symmetric supercapacitor shows a maximum energy density of 4.03 W h kg^-1(with power density of 55 W kg^-1),with a peak power density is 385 W kg^-1（with energy density of 1.67 W h kg-1）.The capacity retention rate is 72.8%after 2500 cycles of charge and discharge,indicating that the material has certain practical application potential.The results show that the Ni O coating method can effectively improve the specific capacity and cycle stability of the electrode material.