Preparation Of Vanadium Nitride Composites And Advanced Energy Storage Applications

The development and storage of solar,wind,and tidal energy have become a highly regarded subject nowadays in today’s deteriorating human environment.With the high energy-density of lithium-ion batteries and the high power-density of dual-layer capacitors,lithium-ion capacitors have great potential for development as an energy storage device for efficient energy transfer.Therefore,lithium-ion capacitors have been continuously explored to improve the storage capacity.In lithium-ion capacitors,the anode electrode undergoes a slow faraday reaction during the reaction,while the cathode electrode undergoes a fast physical adsorption/desorption reaction of PF₆^-on the material surface during the reaction,which results in the mismatch of reaction kinetics between the two electrodes,limiting the application of lithium-ion capacitors.Therefore,exploring a suitable anode electrode to improve the storage performance of lithium-ion capacitors is particularly important.Vanadium nitride（VN）with a theoretical specific capacity of 1200 m Ah g^-1 is an excellent pseudo-capacitor material,which can better match the cathode electrode of lithium-ion capacitors.However,VN can be accompanied by large volume expansion（～240%）during the reaction.In addition,VN nanoparticles tends to agglomerate together due to high surface energy of nanoscale VN particles,reducing the active sites of VN particles and the effective use of VN.Therefore,it is crucial to investigate and optimize the VN to solve the above problems.In this paper,the electrochemical performance of lithium-ion capacitors was explored and enhanced by using strategies such as double carbon confinement,spinning technology method of sacrificial silica templates combined with annealing treatment,and hydrothermal synthesis of barbed spherical special shaped VN to prepare the anode electrode of lithium-ion capacitors.The main work can be summarized as follows:（1）Based on a simple one-step heat treatment strategy,the double carbon layer constrained subminiature VN nanoparticles（VN/N-r GO-5）were prepared.Ammonium metavanadate（NH₄VO₃）was used as the vanadium source,dicyandiamide（C₂H₄N₄）was used as the ammonia gas,and graphene with good toughness was used as the template to inhibit VN agglomeration.In VN/N-r GO-5 composites,VN particles with an average particle size of 5 nm were tightly coated by nitrogen-doped dicyandiamide-derived carbon（DDC）and compounded on nitrogen-doped reduced graphene oxide（r GO）.Ultrafine VN particles can effectively reduce the diffusion distance of Li-ion and grow reactive active sites.The high specific surface area of r GO nanoflakes will provide many attachment sites,reduce the aggregation of VN particles,and improve the conductivity of the entire composite.Ultrafine VN particles were tightly wrapped by DDC with thickness less than 10 nm,which can mitigate the volume expansion of VN.The synergistic effect of VN nanoparticles and double carbon layer will effectively solve the core problem of dynamic imbalance between anode and cathode of LICs.Therefore,VN/N-r GO-5 composites as the anode in VN/N-r GO-5//AC LIC（activated carbon（AC）as the cathode）have excellent electrochemical properties,and the capacity retention rate was 78.5%at 40 m V s^-1 after 6000 cycles.Impressively,VN/N-r GO-5//AC LIC can achieve a high energy-density of 114.1 Wh kg^-1 with a power-density of 240 W kg^-1,and can still have a high power-density of 24 k W kg^-1 with an energy-density of 30.7 Wh kg^-1.VN/N-r GO-5 composites have synergistic structure and excellent electrochemical capabilities,which will introduce a fresh tactics for the optimal design of transition metal nitrides in the field of energy applications.（2）Porous VN carbon nanofiber flexible self-supporting composites were prepared by simple sacrificial silica（Si O₂）template-assisted electrospinning combined with annealing strategy（VN@N-PCFS-0.4）.VN@N-PCFS-0.4 composites do not need fluid collector,conductive agent,and adhesive,enhancing the energy-density of the batteries.The introduction of polyacrylonitrile（PAN）can inhibit the aggregation during the conversion of NH₄VO₃ to VN,increase the active sites involved in the reaction,and mitigate the volume change of VN.The porous structure of PAN-derived carbon fiber facilitates sufficient contact with the electrolyte and shortens the diffusion distance for lithium-ion/electron,which accelerates the reaction kinetics.In half-cells（lithium foil,reference electrode）,VN@N-PCFS-0.4 composites exhibit good electrochemical performance.VN@N-PCFS-0.4 composites still have a specific capacity of 357.1 m Ah g^-1 at 1 A g^-1 with～100%Coulomb efficiency（CE）after 500 cycles.Moreover,the specific capacity of 281.2 m Ah g^-1 is realized at 3 A g^-1 with～100%CE after 3500cycles.In addition,VN@N-PCFS-0.4 composites as the anode of VN@N-PCFS-0.4//AC LIC display a high energy density of 99.7 Wh kg^-1 and a high power density of 24k W kg^-1.（3）Using ammonium metavanadate（NH₄VO₃）and thioacetamide（C₂H₅NS）as raw materials,a hydrothermal reaction was carried out at 180℃under acidic conditions,supplemented by ammonia annealing technology to obtain sea urchin-like VN（SUK-VN）.The sea urchin-like morphology to a certain extent restrains the agglomeration phenomenon of VN nanoparticles owing to large specific surface energy on the surface,which increases the effective area in contact with electrolyte,increases the active sites of the reaction,and improves reaction kinetics.Therefore,the actual specific capacity of the electrode material is improved.SUK-VN has better rate performance compared to pure VN in half-cells.The specific capacities of SUK-VN at 0.1,0.2,0.3,0.5,0.7,1,2,3,and 5 A g^-1 are 299,218.7,183.0,158.3,141.3,127.0,94.9,77.5,and 58.5 m Ah g^-1,respectively.When the current density is 0.1 A g^-1 once more,the specific capacity of SUK-VN will achieve 361.5 m Ah g^-1.After 500 cycles at 1 A g^-1,SUK-VN can display the specific capacity of 164.5 m Ah g^-1 with CE of approximately 99.9%.In the SUK-VN//AC LIC assembled by matching SUK-VN with AC,a capacity retention rate of77.8%is obtained at 2 A g^-1 after 6000 cycles,demonstrating the good cycle life of the SUK-VN//AC LIC.