Research For The Preparation Of Graphene Based Hybrid Fiber And Its Application In Flexible And High Line-capacity Lithium Ion Batteries

With the rapid development of wearable electronic devices and artificial intelligence devices,the market demand for flexible energy storage devices is also increasing year by year.LIBs have been applied in lots of fields and achieved enough attention as mainstream energy storage devices due to high energy density and low self-discharge.Accordingly,it is necessary that develop fiber shaped electrode for assembling flexible lithium ion batteries in order to meet the application scenarios and application requirement of flexible electronics market.The electrode for traditional LIBs could be prepared by mixing active materials,conductive material and binder following coated on metal foil.Owning to the high rigidity,easy deformation after bending and unmatched macroscopic shape,traditional electrode is difficult to apply in fiber shaped LIBs directly.Graphene fiber,as a novel carbon based materials,could be constructed via layer-by-layer stacking between graphene sheets by?-?interaction.On the one hand,graphene fiber inherits exccellent properties coming from graphene sheets.Based on the various process methods such as wet spinning,dry spinning,dry-jet wet spinning and limited hydrothermal and combined with post processing methods,graphene fiber could express a part of properties belonging to the graphene nanosheets,including mechanical,electrical and electrochemical performance.In the other hands,the layer size and diatance,stacking types and pore structure could be accurately controlled by in situ hybridization in wet spinning process and fiber surface modification,which contributed to the the active materials loading precisely.Therefore,it is possible that graphene fiber applied in fiber shaped energy storage devices for high flexibility and functionalization.The targets for lithium ion battery in different application scenarios show an obvious difference.Different from the traditional lithium ion batteries which pay attention to mass capacity and volume capacity,the line capacity become a more important indicator for evaluating electrochemical performance for fiber shaped battery for one-dimensional structure.However,there are several problems for applying graphene fiber in flexible LIBs.(1)Compared with metal oxide and silican active materials,graphene show low capacity and poor adaptation in flexible LIBs as electrode;(2)The fiber LIBs usually presented poor cycling performance owning to introducing high capacity active material which express huge volume expansion in cycling;(3)Because of strong interaction between two sheets leading to tight array of fiber structure,electrolyte usually presented low diffusion rate in graphene fiber.Therefore,how to achieve remarkable increase in line capacity for fiber shaped batteries by the control design of fiber structure and the hybridization of multi-component has been the main problem.This dissertation focuses on the preparation of porous graphene fiber by wet spinning.The physical feature and electrochemical performance was measured systematacially and the possibility of application in fiber shaped lithium ion batteries was discussed.The main results are as follows:(1)The preparation of graphene hybrid carbon nanotubes fiber for the application in flexible lithium ion battery.The uniform spinning solution could be obtained by the ultrasound process for GO and CNT in deionized water depending on the strong?-?interaction between two carbon based materials.Then,the continuous GO/CNT fiber was prepared by wet spinning that chitosan solution regard as coagulating bath.Finally,the r GO-CNT fiber was received after reduced at 900^oC under the N₂ atmosphere A network with high alignment,optimal porosity and low junction contact resistance is formed.Because of the strong shear force at wet spinning,the r GO and r GO-CNT fiber presented high orientation and lay-by-layer stacking structure.However,r GO-CNT fiber showed loosen situation compared with r GO fiber owning to existence of CNT as insert,leading to the larger distance for adjacent graphene sheets and more number for continuous pores.As a result,the specific surface area of r GO-CNT increased from127.5 m~2 g^-1 to 204.5 m~2g^-1 after adding CNT.The half-cell fiber shaped lithium ion battery was assembly that r GO-CNT fiber regarded as anode materials for measuring the electrochemical performance.At the 5%content of CNT,the r GO-CNT fiber presented maximum line capacity reached to 522 m Ah g^-1(0.287 m Ah m^-1)at current density of 0.05 A g^-1.What's more,after 600 cycles the retaining capacity was 374 m A h g^-1(0.206 m Ah m^-1)at the current density of 500 m A g^-1.Finally,to demonstrate the principle of one dimensional material as insert in graphene fiber system,the graphene/silver nanowire(r GO-Ag)fiber was fabricated for anode as control group,revealing that carbon based material played a more significant role in capacity growth.(2)The preparation of tin oxide in situ deposition on graphene hybrid fiber for the application in flexible lithium ion battery.Inserting CNT between adjacent graphene sheets for r GO fiber capacity growth was achieved mainly by preventing the stacking of graphene sheets and exposing more active site for lithium reaction.To improve the theoretical capacity of graphene hybrid fiber,a feasible method is adding enough active material such as metal oxide.The uniform spinning solution could be obtained by the ultrasound process for GO in deionized water.Then,the continuous GO fiber was prepared by wet spinning that chitosan solution regard as coagulating bath.Subsequently,GO fibers were immersed into Sn Cl₂ concentrated hydrochloric acid/ammonium hydroxide solution for in situ GO reduction and Sn O₂ deposition.After that,GO fibers with precipitates were washed by distilled water,dried,and annealing successively,resulting in the final r GO-Sn O₂ fibers.With the increase amount of ammonium hydroxide,the distance of neighbor graphene sheets and the content of Sn O₂ were enhanced simultaneously,while the specific surface area and fiber strength were decreased.These results attributed to the p H influence of ammonium hydroxide leading to the different swelling degree.In half-cell battery,the specific capacity of r GO-Sn O₂ fiber reached a peak at 0.6 m L ammonium hydroxide added.The proposed device acquire a remarkable line-capacity reaching to 1.35 m Ah m^-1 and still maintaining 1.16 m A h m^-1 after 100 cycles at 0.1 A g^-1.When the current density enhanced to 5 A g^-1,the line capacity still retain to 0.312 m A h m^-1,which indicated that active materials could promote the line capacity obviously for fiber shaped electrode.In addition,matching with Li Fe PO₄/carbon fiber(Li Fe PO₄/CF),the full cell shows volume capacity at 0.51 m A h m^-1 after 50cycles at the current density of 50 m A g^-1 and holds a stable charge discharge cycle performance in different bending angle.This chapter developed a method for controlled loading metal oxide nanoparticles between graphene sheets,a principle about the hybridization of graphene and fuctional components and a fiber with high line capacity and stable cycle performance.(3)The preparation of porous graphene/carbon nanotube/tin oxide hybrid fiber for the application in flexible lithium and sodium ion battery.Sn O₂ as active materials could improve the line capacity proved in part two.To further improve the line capacity,enlarging fiber diameter is another method.However,the diffusion rate of electrolyte in fiber will become slower along with the increase of diameter leading to a poor line capacity.Therefore,how to construct rational structure could provide enough channels for electrolyte as diameter large enough is the main point that we should solved.GO powder,CNT and Sn O₂ nanoparticles were suspended into deionized water by sonicating.The profiled r GO-CNT-Sn O₂ aerogel hybrid(PP-GCS)fiber could be assembled via wet-spinning following freeze-dried and mechanically compressed.The graphene hybrid aerogel fiber was fabricated by wet spinning following freeze dried for building porous structure assuring the faster and thicker diffusion of electrolyte,and the fiber flexibility was promoted by the mechanical power derived rearrangement of graphene sheets.Compared with traditional r GO-CNT-Sn O₂(C-GCS)fiber and porous r GO-CNT(PP-GC),r GO-Sn O₂(PP-GS),the assembled fiber shaped battery demonstrated a stable cycle performance in that the discharge capacity maintained 17.43 m A h m^-1 after 100 cycles in 0.1 A g^-1 and 9.12 m Ah m^-1 after 500cycles in 1 A g^-1.Besides,with the increase of PP-GCS fiber diameter,the mass-capacity still keeps a high level.The highest line capacity of porous fiber,which prepared at inject rate at 900?L min^-1,reached to 47.78 m A h m^-1 at current density of 0.1 A g^-1.PP-GCS hybrid fiber,assembled to fiber shaped SIBs as anode,possessed high line capacity,reached to 15.74 m Ah m^-1at 0.05 A g^-1.Therefore,this work provided a booming future in the preparation of high energy device in next-generation wearable electronics.This chapter developed a method for porous graphene fiber by cold compress drived,achieved relationship between line density and line capacity and prepared a fiber with higher line capacity,stable cycle performance and flexibility.