Preparation And Sodium/potassium Storage Of Ti And Fe Based Carbon Composite Nanofibers

Portable energy storage equipment is an important energy supplier for mobile electronic devices,electric vehicles and other products.Research and development of low cost,high stability,high capacity secondary battery is an important way to ensure energy storage.Among them,lithium-ion batteries（LIBs）have been highly praised since their commercialization and are widely used in electric vehicles and various electronic products.But there are limited reserves of lithium on earth and it is difficult to meet the increasing demand for LIBs.Sodium ion batteries（SIBs）and potassium ion batteries（KIBs）work in a similar way to LIBs.What’s more,sodium and potassium are abundant and cheap on earth,giving them a huge cost advantage.Therefore,they are expected to replace LIBs as portable power sources.However,because the mass and radius of Na⁺/K⁺are larger than that of Li⁺,it is easy for the material structure to collapse when they are embedded/released between the positive and negative SIBs/KIBs,thus limiting the long cycle performance of the battery.Therefore,many LIBs anode materials are not suitable for high-performance storage applications of Na⁺/K⁺.Based on the above considerations,focusing on the problems of low conductivity,poor stability and low capacity of anode materials,the electrochemical performance and energy storage mechanism of Ti and Fe composite nanofibers as anode materials were studied from the aspects of composite material preparation,crystal structure control and micro structure control.The specific research contents are as follows:（1）Because TiSe2 is an attractive two-dimensional（2D）material,it has good electrical conductivity and layered structure.Therefore,using various strategies to synthesize TiSe2 and Ti O2 composite materials can improve the electrochemical performance of Ti O2 composite materials for sodium/lithium storage.In Chapter 2,TiSe2-Ti O2-CNF nanocomposites were prepared by simple electrospinning,carbonization and selenization.It is found that anatase（Ti O₂-A）can be partially converted into TiSe₂ at 600℃.However,TiSe₂ formation is not favorable when the temperature is lower than 400℃or higher than 700℃.TiSe₂-Ti O₂-CNF as SIBs and LIBs anode material shows excellent stability and good storage performance.Therefore,this experiment not only achieves the purpose of improving the storage capacity of composite materials by regulating the crystal structure,but also finds that anatase titanium dioxide（Ti O2）is more easily converted into titanium diselide（TiSe2）than rutile Ti O2.The mechanism of high performance energy storage of TiSe2-Ti O2-carbon nanofiber（TiSe2-Ti O2-CNF）composites was revealed.This provides new research ideas for the synthesis and application of TiSe2 and Ti O2 in the future.（2）As a typical metal sulfide,ferric thiovanadate（FeV2S4）always faces the problems of volume attenuation and poor cyclic stability caused by volume expansion effect.In view of this,on the basis of the previous research,Chapter 3 describes the use of GO to control the size and distribution of FeV2S4 nanoparticles on the basis of electrostatic spinning technology,so that it can be better coated in the interior of carbon nanofibers,and then prepare FeV2S4@GO@CNF composite material and apply it to SIBs system.The results show that the FeV2S4@GO@CNF anode material has better electrochemical Na storage performance than the FeV2S4@CNF composite without GO regulation,which has more particles on the fiber surface.On the one hand,the excellent cyclic stability and storage capacity of FeV2S4@GO@CNF is due to the fact that GO reduces the size of FeV2S4 nanoparticles,changes their distribution in CNF,and slows down the volume expansion effect of FeV2S4,resulting in the excellent structural stability of FeV2S4@GO@CNF.On the other hand,because GO can also improve the conductivity of the material,FeV2S4@GO@CNF shows better rate performance and cycling performance.In this study,a strategy to improve the energy storage stability of CNF composite electrode was proposed and realized by regulating the microstructure of CNF composite electrode with GO.It provides a new strategy for the synthesis of nano-particle-CNF composites in catalyst and battery fields.（3）Ideas inspired,in the first two chapters experiment for metal chalcogenide volume expansion is serious,in the process of energy storage materials in the process of preparation of complex operation and so on,based on the electrostatic spinning technology in Chapter 4,through the way of internal selenide and carbonization,simply synthesized carbon fiber coated Fe3Se4（Fe3Se4@CNF）composite fibers.Compared with the external selenization method,the internal selenization synthesis process is not only simple and safe,but also saves raw materials and does not need too high temperature conditions.At the same time,Fe3Se4nanoparticles were better distributed in CNF by this preparation method,and the aggregation of particles on the fiber surface was reduced.Therefore,Fe3Se4@CNF showed enhanced electrochemical K⁺storage performance compared with Fe@CNF-Se0.1-400,which was more external selenide products of fiber surface particles.In addition,the storage mechanism of K⁺in Fe3Se4@CNF electrode material was further revealed by electrochemical kinetic analysis and ex-situ XRD characterization test,which laid a certain foundation for the study of Fe3Se4as potassium storage anode material.Therefore,this study proposed and realized the purpose of using internal selenization method to synthesize high-performance carbon fiber-selenide composite fiber easily and efficiently,and provided a new idea for the synthesis of selenide in the field of energy storage.（4）In the previous chapter,we found that PAN fibers carbonized at 400℃basically had no capacity contribution.Further study showed that It was Fe that promoted PAN to have excellent potassium storage after carbonization at 400℃.In view of the above research basis,in Chapter 5,PAN fiber（Fe-CNF-400）containing Fe element carbonization at 400℃and PAN fiber（CNF-400）without Fe element carbonization at 400℃were prepared based on electrostatic spinning technology.It is found that Fe promotes CNF-400 to have stable morphology and structure,and can reduce the impedance of CNF-400 and increase its conductivity,which makes CNF-400 show excellent cyclic stability and rate performance in the process of potassium storage.In addition,the storage mechanism of K⁺in Fe-CNF-400electrode material was further revealed by electrochemical kinetic analysis and ex-situ XRD characterization.This study found that Fe promoted PAN carbonization at low temperature,revealed the mechanism of high performance potassium storage of carbonization products,and laid a certain foundation for the subsequent application of Fe-carbon composite materials in the field of energy storage and catalysis.