Controllable Construction Of Electrospun Vanadium-based Nanofiber Composites And Their High Performance For Alkali Ion Batteries

The limited and uneven distribution of Li resources in the earth's crust make the commercial success application of lithium-ion batteries(LIBs)difficult to meet the diversified storage demand in the future.It is gratifying that the promising sodium-ion batteries(SIBs)and potassium-ion batteries(PIBs)have gotten enormous attention over the past few years to supplement LIBs on account of the abundance resources of Na and K resources as well as similar charge/discharge mechanisms with LIBs.However the development of SIBs and PIBs has been hindered by the lack of suitable anode materials for hosting the relatively large ion radius of Na⁺(1.02A)and K⁺(1.38A).It is unfavorable to maintain high stable cycle stability due to the problems of sluggish redox reaction kinetics and high volume expansion during charge/discharge process.Vanadium-based material is considered as one of the most promising sodium/potassium Anode materials because of its rich vanadium valence,high theoretical capacity,abundant resources and low cost.However,there still are some problems such as the poor electrical conductivity and large volume expansion in the process of ion insertion/deinsertion process,causing the poor cycle stability and low specific capacity.Numerous efforts have been devoted by scholars to solve the above problems,such as obtaining the nanometer scale particle size,compositing with conductive carbon matrix,introducting the defect structure,heteroatom doping,constructing the heterogeneous interface and so on.In this dissertation,electrospun nanofibers are used as the substrates to confine with vanadium-based materials,and meanwhile,controllable construction of vanadium based/nanofiber composites by adjusting their components and morphologies according to the above effective strategies.The main research contents were as follows:(1)The polyacrylonitrile(PAN)was employed as polymer template,and the vanadium/carbon composite(VN/VSe_1.5)was fabricat by electrospinning followed by calcination treatment technology.The active components such as vanadium based materials were confined in carbon fiber.The aim is to enhance the electrochemical performance of vanadium-based materials by improving the electrode conductivity,buffer volume variation and increase cycling stability.As expected,the experimental results show that the carbonfibers really improve the cycling stability of the electrodes.(2)The rich S defect D-V₅S₈/CNF composite was prepared by electrospinning method,followed by a simple one-step sulfurization treatment.The experimental results and DFT calculations all show that the S defect can provide additional storage sites for ion storage,and also plays an important role in improving electrode conductivity,promoting ion migration by charge rearrangement and reducing the migration barrier.(3)The vanadium source and PAN were electrospun to form A fiber film,and then selenized synchronously.In the selenization process,the Se was embedded in the polymer chain at the molecular level via the carbon-selenium bond between selenium And polyacrylonitrile.The vanadium selenide was confined in the Se PAN fiber finally.The dissolution of polyselenides was almost completely inhibited,thus obtaining high performance electrode materials.(4)Using Chlorella as the heteroatom source,the vanadium selenide embedded in nitrogen/phosphorus co-doped carbon nanofibers(V₃Se₄/NPCNF)was obtained by electrospinning,and employed as Si Bs and PIBs anode materials with high capacity and long cycle life.The heteroatom in the composite material is directly derived from the rich nitrogen/phosphorus components,chlorella.The experimental results and DFT calculations show that the introduction of P and N into carbon composites is beneficial to improve their electrochemical performance.(5)The heterostructure can provide a stable storage interface.Simultaneously,it can adjust its electronic properties and improve its activity by mixing the vanadium-based materials with other oxides or sulfides.Herein,Chlorella was used as adsorbent and reactor.The amino groups on the surface of Chlorella and the hydroxy groups in phosphotungstic acid could form hydrogen bonds.Eventually,the phosphotungstic acid can be adsorbed on the Chlorella,and the two metal sources of W and V with different dissolution properties can be co-spun to synthesize WVO₄/V₃Se₄/CNFS electrode materials with the assistance of Chlorella.As anode materials,the composite structure reveals high reversible capacity,excellent rate characteristics and long-term cycling stability.