Preparation And Sodium Storage Properties Of Anode Materials For Niobium-based Oxide Sodium Ion Capacitors

With the rapid development of portable electronic products and the popularization of electric vehicles and hybrid electric vehicles,the demand for electrochemical energy storage(EES)devices with high energy density and power density are more and more urgent.Lithium ion batteries exhibit high energy density,while their power density and cycle life are far from satisfactory.In contrast,although supercapacitors can deliver high power density and ultralong cycle life,the low energy density restricts their large-scale applications.Therefore,developing EES devices with high energy density,high power density and long cycle life is of extreme significance for their wide application.As a new type of EES devices,lithium ion capacitors composed of battery-type anode and capacitive-type cathode have sparked increasing attention in recent years based on the complementary features of high-energy batteries and high-power super capacitors.However,the uneven distribution and limited reserves of lithium resources in the earth limit their long-term development.In order to solve the disadvantages of insufficient lithium resources and high prices,the sodium ion capacitors have been considered as promising alternatives to the lithium ion capacitors because sodium resources are practically in exhaustible and evenly distributed around the world.In fact,the charge storage mechanisms between faradaic anodes and capacitive cathodes are differences,it causes kinetics imbalance because the sodium ion intercalation/deintercalation reaction in anode is much more sluggish than the anion adsorption/desorption process in cathode,and results in unsatisfied energy density and power density.Therefore,preparing high-rate faradaic anode materials is the key to construct high-performance sodium ion capacitors.Nb2O5 and Ti2Nb2O9 are usually considered as promising anode candidates for sodium ion capacitors because of their safety-voltages,stable crystal structure,large lattice spacing and typical pseudocapacitive characteristics.However,the intrinsic low electronic conductivity and sluggish sodium ion diffusion rate restrict their practical application in sodium ion capacitors.To address these issues,several strategies such as designing nanostructures,doping heteroatom,and introducing carbon matrix have been proved to be effective ways for improving the performance of sodium ion capacitors.Therefore,the preparation of Nb2O5-based and Ti2Nb2O9-based composite anode materials is a meaningful research topic.By optimizing the structure and component,Nb2O5-based and Ti2Nb2O9-based composite anode materials for high-performance sodium ion capacitors are expected to be prepared.The full thesis consists of seven chapters.The first chapter is an introduction,the composition,classification,working principle,development trend and challenges of the hybrid ion capacitor and sodium ion capacitors are reviewed,and the advantages and disadvantages of Nb2O5-based and Ti2Nb2O9-based composite anode materials are also analyzed.On the basis of these reviews,the significance,purpose,content and innovation for this research are proposed.Chapters 2 to 6 are experimental parts,mainly including the preparation of a series of Nb2O5-based and Ti2Nb2O9-based composite anode materials by solvothermal and followed by high temperature treating,electrospinning and followed by high temperature calcining,electrospinning and followed sulfidation treating and the flocculation followed by high temperature treating technology.The structures and morphologies of the as-synthesized materials have been analyzed,and the sodium storage performances of the prepared materials as anode materials for sodium ion batteries and the assembled sodium ion capacitors have been systematically studied.Chapter 7 is the conclusion of the full paper.The main research contents were as follows:(1)Preparation of Nb2O5/nitrogen doped graphene anode and its sodium storage property.Nb2O5/nitrogen-doped graphene anode material(Nb2Os/NG)has been prepared by calcining amorphous Nb2Os/NG at 700℃ for 2 h in Ar atmosphere,in which amorphous Nb2O5/NG has been synthesized via solvothermal method using graphene oxide as raw material,niobium pentachloride as niobium source and urea as nitrogen source.The effects of nitrogen doping on the structure,morphology and electrochemical performance of Nb2O5/NG anode materials have been systematically studied.Experimental results show that Nb2O5 nanoparticles with average size of 17 nm are uniformly anchored on the surface of the nitrogen-doped graphene because their aggregation and growth are effectively hindered due to the introduction of nitrogen-doped graphene.The Nb2O5/NG electrode shows superior rate capability and good cycling performance,it exhibits the specific capacity of 68 mAh g-1 at 2 A g-1 and the capacity retention of 85%after 200 cycles at 0.2 A g-1.A sodium ion capacitor is constructed by using Nb2O5/NG as anode and AC as cathode,it delivers an energy density of 40.5 Wh kg-1 at a power density of 100 W kg-1.Moreover,its capacity retention is 63%with a high Coulombic efficiency near 100%after 5000 cycles at 1 A g-1.This research provides a new strategy for developing anode materials for sodium ion capacitor with high energy and power density.(2)Preparation of Nb2O5 nanorod/nitrogen-doped microporous carbon fiber thin film anode material and its sodium storage performance.The precurcor H2Nb2O6·H2O nanorods are firstly prepared by grinding H2Nb2O6·H2O nanowires for 3 h,then PAN/PMMA/H2Nb2O6·H2O nanofiber film is fabricated by electrospinning using polyacrylonitrile as carbon and nitrogen sources,polymethylmethacrylate as pore forming agent,H2Nb2O6·H2O nanorods as niobium source.PAN/PMMA/H2Nb2O6-H2O nanofiber film is calcinated at 700℃ in Ar for 2 h,flexible and free-standing Nb2O5 nanorods/nitrogen-doped microporous carbon nanofiber film anode materials(Nb2O5 NRs/NMCNF)are finally prepared.The effects of the doped H2Nb2O6·H2O nanorods amounts on the crystal structure,morphology,and the electrochemical property of the as-prepared film anode materials are systematically studied.Experimental results indicate that Nb2O5 nanorods are uniformly embedded in nitrogen-doped microporous carbon nanofiber when the amount of H2Nb2O6·H2O nanorods is 0.7 g and its agglomeration is effectively inhibited.Benifiting from the synergistic effect between three-dimensional nitrogen doped microporous fiber network structure and Nb2O5 nanorods,0.7-Nb2O5 NRs/NMMCN film electrode delivers excellent rate capability and cycle stability,its specific capacity is 101 mAh g-1 at a large current density of 4 A g-1.A high capacity retention of 91%can be achieved even after 10000 cycles at 2 A g-1 and the Coulombic effciency keeps near 100%.A sodium ion capacitor based on the 0.7-Nb2O5 NRs/NMMCN anode and AC cathode is assembled and its operating voltage window can be widened to 4 V.The energy density of the as-assembled device is as high as 91 Wh kg-1 and its maximum power density is 7499 W kg-1.Furthermore,the capacitor displays relatively stable cycling ability,the capacity retention is still as high as 77%after 10000 cycles at 1 A g-1.(3)Preparation of sulfur-doped Nb2O5/nitrogen-sulfur co-doped microporous carbon fiber film anode material and its sodium storage performance.At firstly,PAN/PMMA/NbCls nanofiber films are prepared by electrospinning using polyacrylonitrile as carbon and nitrogen sources,polymethylmethacrylate as pore forming agent,niobium pentachloride as niobium source and sublimated sulfur powder as sulfur source.The as-prepared PAN/PMMA/NbCl5 nanofiber films precursor are stabilized in air at 250℃ for 3 h,and then the preoxidized films and sublimed sulfur powder with different mass ratios are heated in Ar at 800℃ for 3 h,flexible and free-standing sulfur doped Nb2O5 quantum dots/nitrogen and sulfur co-doped microporous carbon nanofiber film anode materials(S-Nb2O5@NS-PCNF)are finally prepared.The structure and morphology of the obtained film anode materials are investigated,and the effects of sulfur doping on the electrochemical performance of the prepared film electrodes are systematically studied.XPS results show that sulfur atom is doped into Nb2O5 lattice,inducing anionic oxidation of Nb2O5(O2-→O-)and forming numerous oxygen vacancies,they can improve the conductivity of Nb2O5 and promote the diffusion of sodium ions.When the mass ratio of the sublimated sulfur powder and the preoxidized film is 3,3S-Nb2O5@NS-PCNF film electrode exhibits the best rate capability,it gives a specific capacity of 267 mAh g-1 at 0.02 A g-1.After charging-discharging 10000 cycles at 2 A g-1,it still delivers a specific capacity of 173 mAh g-1 with a high Coulombic efficiency near 100%.A sodium ion capacitor is assembled by integrating 3S-Nb2O5@NS-PCNF anode with AC cathode,and its operating voltage can reach 4.3 V.The assembled device delivers the maximum energy density of 112 Wh kg-1 at 80 W kg-1.Even at 7949 W kg-1,this capacitor still achieves an energy density of 55 Wh kg-1The device exhibits a good cycle stability with a capacity retention of 81%after 10000 cycles at 1 A g-1.This sulfidation treatment strategy provides a new idea for prepareing anode materials for sodium ion capacitors with excellent electrochemical performances.(4)Preparation of Ti2Nb2O9 nanosheet/graphene anode material and its sodium storage performance.HTiNbO5/GO composite precursor is firstly fabricated via flocculation method using delaminated HTiNbOs nanosheets and GO nanosheets as assembly units,1 mol dm-3 HCl as flocculating agent.By using a high temperature solid-state method,layered precursor KTiNbO5 is obtained.It is treated in HCl solution,layered HTiNbO5 is prepared via an ion-exchange reaction and and followed by exfoliating in TBAOH solution into the delaminated HTiNbO5 nanosheets.Ti2Nb2O9/RGO composite anode material is finally prepared by annealing HTiNbO5/GO composite at 500℃ in Ar for 1 h.The graphene effectively restricts the stacking of Ti2Nb2O9 nanosheets and promotes fast electron transport between Ti2Nb2O9 nanosheets and current collector.Experimental results indicate that Ti2Nb2O9/RGO electrode can obtain 95%capacity retention after 1000 cycles at 1 A g-1.Meanwhile,Ti2Nb2O9/RGO electrode displays superior rate capability,it gives a specific capacity of 283 and 206 mAh g-1 at 0.1 and 4 A g-1,respectively.The sodium ion capacitor comprising Ti2Nb2O9/RGO anode and an AC cathode delivers an energy density of 96 Wh kg-1 at 75 W kg-1,and its energy density can still maintain 39 Wh kg-1 even at a high power density of 10.03 kW kg-1.In addition,the assembled device exhibits remarkable cyclic stability with 79%capacity retention even after 10000 cycles at 1 A g-1.This nanosheet flocculation assembly technology provides a new way for the design and preparation of Ti-Nb oxides graphene based composite anode materials.(5)Preparation of Ti2Nb2O9 nanosheet/nitrogen-doped carbon fiber thin film anode material and its sodium storage performance.By using polyacrylonitrile as carbon and nitrogen sources,delaminated HTiNbO5 nanosheets as niobium and titanium sources,HTiNbO5 nanosheets/PAN nanofiber films are firstly prepared by electrospinning method.The as-prepared HTiNbO5 nanosheets/PAN nanofiber films are calcined in air at 250℃ for 3 h and then calcined in Ar at 500℃ for 1 h,flexible and free-standing Ti2Nb2O9 nanosheets/nitrogen-doped carbon nanofiber film anode materials are prepared(Ti2Nb2O9/NCNF).The carbon nanofiber network can not only inhibit the restacking of Ti2Nb2O9 nanosheets,but also improves the conductivity and structural stability of the film electrode.The electrochemical test results show that the Ti2Nb2O9/NCNF film electrode exhibits excellent electrochemical performance,its specific capacity is 324 mAh g-1 at a current density of 0.1 A g-1,and the capacity retention is 63%when the current density increases from 0.1 to 4 A g-1.After charging-discharging 2000 cycles at 1 A g-1,it still delivers a specific capacity of 250 mAh g-1 with a high capacity retention of 97%.A sodium ion capacitor is devised based on as-obtained Ti2Nb2O9/NCNF anode and AC cathode,it achieves the high energy density of 129 Wh kg-1 at 75 W kg-1,and still obtains 63 Wh kg-1 even at a high power density of 7.56 kW kg-1.After 10000 cycles at 1 A g-1,the capacity retention of the assembled device is as high as 85%.