The Application Of Vanadium-based Pseudocapacitive Materials In Flexible Sodium Ion Capacitors

Sodium ion capacitors utilize two different charge-storage mechanisms:intercalation reaction storage and electrochemical double layer storage,which combine the advantages of high energy density of batteries with high power density of supercapacitors.It is of great significance to develop high performance flexible sodium ion capacitors.First of all,it is necessary to meet the flexible requirements of energy storage devices.On this basis,the key to construct high performance sodium ion capacitors is to combine appropriate anode and cathode materials to match their capacitance and dynamic reaction behavior at the same time.In this paper,we improve the capacitance contribution rate of the electrode capacity by choosing vanadium compounds as suitable pseudocapacitor materials,which have unique structure and properties.Vanadium-based compounds were designed to achieve the electrode flexibility as the basic requirement.The experiment focusing on the construction of large-area array structure and the method is to design different types of carbon and vanadium compounds.The goal is to promote the improvement of electrochemical performance.Three kinds of composite flexible electrodes,VO₂@mp-CNSs/CFC,VS₂@C and VS₂ NS@PCNFs,were synthesized by three ways:mesoporous carbon loaded nanoparticles,electrospinning carbon-coated fibers and two-dimensional nanosheet arrays grown on flexible carbonfibers.We tested the performance of vanadium-based compounds as anode electrodes of sodium ion capacitors.In Chapter 3,we fabricated VO₂@mp-CNSs/CFC composite electrodes.Nitrogen-doped mesoporous carbon nanosheets improved the conductivity of the electrodes and restrained the electrode materials from aggregation.Arrays structure not only enlarge the specific surface area of electrode material,promote the full contact between VO₂ and electrolyte,further improve the contribution of material pseudocapacitance,but also guarantee the stability of electrode structure in the cycle process.Under the design of these structures,sodium ion batteries have excellent electrochemical performance.At the current density of 0.1A g^-1,the specific capacity of 442 mAh g^-1 can be achieved.At 20 A g^-1,the specific capacity of 160 mAhg^-1 can still be obtained.The cathode electrode of NVP@mp-CNSs/CFC was prepared by the same method.The capacity retention is 87%after 10000 cycles at 100 C.A quasi-solid-state flexible NIC based on the NVP@mp-CNSs cathode and the VO₂@mp-CNSs anode is further assembled.This hybrid device delivers both high energy density and power density as well as a good cycle stability(78%retention after 2000 cycles at 1 A g^-1).The results demonstrate the powerfulness of MOF arrays as the reactor for fabricating electrode materials.In the fourth chapter,VS₂@C composites were prepared as the anode electrode of sodium ion batteries by the electrospinning technology.The active material was uniformly coated in the carbon fibers,which alleviated the volume expansion of the active material during charging and discharging.At the same time,nanofibers have higher specific surface area and structural stability than powdered materials.In the test of VS₂@C composites,the specific capacity of 432 mAh g^-1 can be released at current density of 0.1 A g^-1.At the current density of 20 A g^-1,it still has a specific capacity of 265 mAh g^-1 with a capacity retention rate of 53.2%.The composite electrode can stably circulate 1350 cycles at 2 A g^-1,and the specific capacity of the cycle reaches 248 mAh g^-1.It is proved that VS₂@C composites prepared by electrospinning have good sodium storage properties.At the same time,VS₂ NS@PCNFs was also designed.The electrode was based on highly conductive porous carbon fibers,which greatly increased the specific surface area and the proportion of active substances in the electrode.At the same time,VS2 bowl-shaped nanosheet arrays are uniformly grown on the fibers.The carbon fiber in the center ensures the conductivity of the electrodes,and the network formed by the interaction of ultra-thin layers reduces the ion diffusion distance.At the same time,the self-aggregation of active substances during charging and discharging process is alleviated by the array architecture,and the distribution of the structure improves the reaction efficiency.Using VS₂ NS@PCNFs as the anode material of sodium ion capacitor,the results show that the specific capacity can reach 659 mAh g^-1 at 0.1 A g^-1,which is close to the theoretical capacity of VS₂.The composite electrodes also show excellent cycling ability,with a capacity of 336.2 mAh g^-1remained after 6000 cycles at 2 A g^-1.Using graphene graphene as the positive electrode and VS₂ NS@PCNFs as the negative electrode,When the power density is 2kW kg^-1,the sodium ion capacitor device can get the high energy density of 132 wh kg^-1.