Preparation And Properties Of Nb₂O₅ Electrode Material For Aqueous Zinc-ion Hybrid Capacitor

The new aqueous zinc-ion hybrid supercapacitors（ZHSCs）combine high-capacity zinc-ion batteries with high-power supercapacitors to meet the energy and power needs of portable electronic products and electric vehicles.In practical application,it is a serious challenge to realize the aqueous zinc-ion hybrid capacitor with high energy density and high power density and to develop suitable battery-type electrode materials.Niobium pentaoxide（Nb₂O₅）has excellent rate performance and cycle performance in monovalent metal ion hybrid capacitors.However,the low conductivity of Nb₂O₅ limits the development of its electrochemical reaction kinetics and is not conducive to its high rate performance.Meanwhile,the specific capacitance of Nb₂O₅ needs to be further improved.In this thesis,a series of Nb₂O₅ electrode materials were prepared by nano-design and element doping modification to improve the transmission characteristics of zinc ions,in order to improve the electrochemical performance of Nb₂O₅ for the purpose.The research content is divided into the following three aspects:（1）The Nb₂O₅ microspheres were prepared via hydrothermal method and high temperature sintering process,and can be used as the battery-type electrode of aqueous zinc-ion hybrid capacitor,and the physical characterization and electrochemical performance were systematically analyzed.The results showed that the burr Nb₂O₅ microspheres were transformed into Nb₂O₅ microspheres with the sturcture like"sweet dumplings with exposed stuffing"（Nb₂O₅-2）via high temperature calcination.This special structure depended on uniform core-shell separation and showed abundant ion migration channels,which made Zn SO₄ electrolyte permeate into the whole Nb₂O₅ microspheres and showed excellent electrochemical performance.In the three-electrode system,the specific capacity of Nb₂O₅-2 reaches 324 F g^-1 at the current density of 0.1 A g^-1.After 5000 long cycles,its capacity retention rate was up to 85%.Meanwhile,AC//Nb₂O₅-2 ZHSCs were prepared with Nb₂O₅-2 electrode as negative electrode and activated carbon as positive electrode.The specific capacity of AC//Nb₂O₅-2 ZHSCs can reach 108 F g^-1 at the current density of 0.1 A g^-1.After 5000 cycles at the current density of 2 A g^-1,83%of the cycle capacity retention rate can be obtained,and the highest energy density can reach 60 Wh kg^-1,and the maximum power density is 8020 W kg^-1.（2）V doped Nb₂O₅ nanospheres were synthesized via hydrothermal method and high temperature sintering process.The results show that when the content of V is 20%,the dispersity of Nb₂O₅ electrode material（Nb₂O₅-V/20）is the best,which is attributed to the introduction of V element improves the conductivity and electrochemical reactivity of Nb₂O₅microspheres and optimizes the electrochemical performance.In the three-electrode system,the specific capacitance of Nb₂O₅-V/20 electrode reaches524 F g^-1 at the current density of 0.2 A g^-1.Meanwhile,the capacity retention rate reaches 86%after 8000 cycles at the current density of 2 A g^-1.In addition,AC//Nb₂O₅-V/20 ZHSC has a high specific capacity of138 F g^-1 at a current density of 0.2 A g^-1,showing a high power density of 9025 W kg^-1 and a high energy density of 70 Wh kg^-1.AC//Nb₂O₅-V/20ZHSC has a high specific capacity of 138 F g^-1 at a current density of 0.2A g^-1 and a high energy density of 70 Wh kg^-1.Excellent circulation capacity retention rate of 86%can be obtained at current density of 2 A g^-1after 5000 cycles.（3）With niobium oxalate as raw material,anhydrous ethanol as dispersant and ammonium metavanadate as dopant,the mixed material of niobium pentoxide and vanadate was prepared by ultrasonic spray and high temperature calcination.Through the combined action of ultrasonic spray and high temperature sintering,the addition of V element makes the surface of Nb₂O₅ rough and causes a little surface rupture,which provides more reaction sites and sufficient diffusion space for zinc ions,and improves the electrochemical properties of Nb₂O₅ materials.In the three-electrode system,the V-doped Nb₂O₅ electrode material（Nb₂O₅/V）exhibits a high specific capacitance of 650 F g^-1 at a current density of 0.2A g^-1,and its capacity retention rate reaches 91%after 8000 cycles at a current density of 2 A g^-1.In addition,the assembled AC//Nb₂O₅/V ZHSCs showed a power density of up to 8253 W kg^-1,an energy density of up to69 Wh kg^-1,and excellent cycle stability(capacity retention rate of 88%at current density of 2 A g^-1).