Fabrication Of Iron-Based Oxysulfide Composite Nanofibers And Their Lithium Storage Properties

Lithium ion battery is considered to be the optimal electrochemical energy storage devices due to its high energy density,fast charge and discharge speed,no memory effect,and good safety performance.Its electrochemical performance mainly depends on the electrode material,iron-based oxysulfide is considered as one of the most practical ele ctrode materials because of its high theoretical capacity,abundant resources,low cost,and environmental friendliness.However,there will still be some problems along with the use of iron-based oxysulfide as electrode materials,which restrict the improvement of their electrochemical properties and their commercial application such as:①The inherent low conductivity severely limits the conduction of electrons and the exertion of electrochemical properties in the electrode materials;② The iron-based oxysulfide has a serious volume expansion effect in the charging and discharging process,which may be due to the pulverization or exfoliation of material from the electrode surface.In view of the above problems,this dissertation uses electrospinning technology to construct one-dimensional iron-based oxysulfide composite nanofibers with a variety of controllable morphologies.Its one-dimensional structure can limit the electron transport to one dimension,and provide short diffusion channels for the electrons and ions;Compounding with carbon can increase the conductivity of the material to solve the problem of low conductivity of iron-based oxysulfide.The structure of nanofibers with internal voids and nanotubes(tube in tube)can provide sufficient buffer space for the volume expansion during charge-discharge cycles,thereby improving the cycling stability of the electrode material.This provides theoretical basis for the practical application of iron-based oxysulfide in the field of energy storage.The main research content in this dissertation are divided into the following five parts:1.Synthesis of flexible Fe3O4/C electrospun nanofibers with internal voids and their application in lithium-ion batteriesα-FeOOH/PAN precursor nanofibers have been obtained by electrospinning α-FeOOH and PAN.The Fe3O4/C flexible nanofibers with internal voids were prepared by the dehydration reaction of α-FeOOH at high temperature,accompanied by in situ volume contractions.Fe3O4/C composite nanofibers have good electrical conductivity and excellent flexibility,which can be directly sliced as electrode slice to assemble button batteries without any need for current collector,binder and conductive additives.Compared with Fe3O4 nanoparticles and solid Fe3O4/C composite nanofibers,the Fe3O4/C flexible nanofibers with internal voids have higher reversible capacity and better cycle stability.At a current density of 1Ag-1,the specific discharge capacity can still be maintained at 611 mA h g-1 after 300 cycles,this is attributed to the fact that the internal voids of the Fe3O4/C flexible nanofibers can effectively buffer the volume expansion of Fe3O4 during the lithium ion insertion/extraction process,prevent the nanofibers from breaking,and maintain the structural integrity,thereby improving the cycle stability of the electrode material.The volumeshrinkage effect of FeOOH during high temperature calcination was further demonstrated by the preparation of Fe3O4@C nanocapsules with yolk-shell structure.2.Fabrication of electrospun Fe7S8/C flexible nanofibers with nano-buffered space and their application in lithium-ion batteriesCompared with iron oxides,iron sulfides are more advantageous lithium-ion battery materials.In order to further verify the feasibility of nano-buffer space in the lithium storage process of other iron-based derivatives,the chapter makes use of the volume shrinkage reaction of FeOOH at high temperature in chapter 1,the pre-oxidized FeOOH/PAN precursor nanofibers were vulcanized at high temperature to obtain the Fe7Ss/C flexible nanofibers with buffering volume.The Fe7S8 nanoparticles are uniformly attached to the carbon nanofibers,and there are internal voids between Fe7Ss nanoparticles and the carbon matrix that can buffer the volume expansion effect.Experimental results show that as a negative electrode material for lithium ion batteries,Fe7Ss/C flexible nanofibers can still achieve a reversible specific capacity of 675 mA h g-1 after 400 cycles at a current density of 1 A g-1.Its excellent electrochemical performance is mainly attributed to the fact that the carbon nanofiber provides good electrical conductivity to the material,and the voids between Fe7S8 nanoparticles and carbon nanofiber provides sufficient buffer space for volume expansion of Fe7S8 during charge and discharge cycles.3.The construction of scaly Fe7S8/C composite nanotubes and their electrochemical propertiesThe nano-voids generated by FeOOH self-shrinkage at high-temperature can effectively alleviate the volume effect of the material during lithium ion insertion and extraction,but the low content of active components in the composite nanofibers limits the specific capacity of the material.The nanotube structure is also an excellent structure capable of buffering the volume expansion effect,and the nanotube has a larger buffer space and specific surface area,which is a better electrode material structure.In this chapter,the Fe7S8/C composite nanotube with tubular structure is prepared by changing the spinning solution of polymer and using the reaction of partial decomposition and swelling outward of polymer PVA when calcined under inert atmosphere.The Fe(NO)3·9H2O/PVA nanofibers are prepared by blending Fe(NO)3 · 9H2O,low molecular weight and high molecular weight PVA.Then,using TAA as a sulfur source,vulcanize Fe(NO)3·9H2O to Fe7S8 by solvothermal reaction to obtain scaled Fe7S8/PVA nanofibers.After calcining under the protection of argon,PVA decomposed and spread outwards,and finally obtain scaly Fe7S8/C composite nanotubes.The scaled nanotubes have a large specific surface area,can provide large activation space,more lithium storage sites and the buffer space for the volume expansion effect of Fe7S8.Moreover,the combination of Fe7S8 and carbon effectively improves the conductivity of the material,which is conducive to the conduction of electrons in the material.As the anode material of lithium ion battery,the scaled Fe7S8/C composite nanotubes exhibit high specific capacity and excellent cycle stability After 400 cycles at a current density of 1 A g-1,the reversible specific capacity can still maintain 688 mA h g-1,which is very excellent electrochemical performance.4.Investigation on controllable synthesis and vulcanization of Fe3O4@C composite nanotubes with various morphologies and their energy storage applicationsBased on the research in the previous chapter,it can be seen that the nanotube structure can alleviate the volume expansion effect of iron sulfide and has a large specific surface area.However,carbon coated nanotubes can inhibit the crushing and shedding of active materials in the cycling process and enhance the conductivity of materials,which is a more advantageous electrode material structure.In this chapter,FeOx/polymer or Fe3O4@C tube in tube are used as precursors to prepare Fe7S8@C tube in tube composite nanofibers using the high-temperature vulcanization strategy in the second part.In this chapter,three kinds of tubular Fe3O4@C composite nanofibers are designed and synthesized,based on the swelling phenomenon of PVA at high temperature and the difference between the diffusion rate of polymer(PVA)and iron ions caused by the heating rate,by adjusting the heating rate,the morphology of the composite nanofiber can be adjusted,and their electrochemical properties are studied.The experimental results show that compared with the hollow nanotube and pea-like nanotube,the Fe3O4@C tube in tube exhibit the largest discharge specific capacity and the best cycle stability.This is attribute to the Fe3O4@C tube in tube with the best carbon content,the largest specific surface area,and structural advantages that can alleviate the effect of volume expansion.Although iron sulfide tube in tube cannot be obtained by high-temperature vulcanization,however,Fe3O4@C tube in tube composite nanofibers have excellent electrochemical properties,indicating that the carbon-coated tube in tube structure is an outstanding electrode material structure5.Construction of transition metal oxide tube in tube and their electrochemical propertiesThe research in previous chapter shows that the Fe3O4@C tube in tube has the best electrochemical performance.In order to demonstrate the universality of the synthesis method of carbon coated transition metal oxide tube in tube,we synthesized 12 kinds of carbon-coated tube in tube through by single needle gradient electrospinning and controlled pyrolysis by changing the type of inorganic salts.The NiO@C tube in tube is used as electrode material to assemble the three-electrode system supercapacitor and asymmetric supercapacitor,and it also exhibits excellent electrochemical performance.Compared with single metal oxides,bimetallic oxides will exert the synergistic effect of two metal oxides during the electrochemical reaction,thereby improving the electrochemical performance.The experimental results show that the carbon-coated bimetallic oxides tube in tube can be obtained by adding the two inorganic metal salts together into the spinning solution.Among them,when iron-based binary metal oxide composite tube in tube(NiFe2O4@C and CoFe2O4@C)are used as anode materials for lithium ion batteries,the synergistic effect of bimetals can be fully exerted,and showing higher discharge specific capacity and better cycle stability than Fe3O4@C tube in tube,this further proves that the carbon-coated bimetal oxide tube is advantageous electrode material and has great development potential.