Investigation On The Microstructure Of Polyacrylonitrile Fiber And Its Application In Lithium Batteries

Carbon fiber material is a new type of high-tech fiber material that has become a hot research in recent years.It plays an increasingly important role in our life and practical production.By virtue of its high strength,high modulus,high temperature resistance,corrosion resistance,creep resistance and other excellent properties,it is widely used in highly sophisticated aerospace fields,fast developing transportation fields,demanding construction industry fields,and diverse sports supplies.With the diversification of carbon fiber properties and the reduction of its production costs,its application fields are more and more extensive.Although the performance of carbon fiber is excellent,it is far from its theoretical performance.Many scholars have carried out a lot of scientific research and have not achieved significant results.The actual tensile performance of the best carbon fiber is only about 5%of the theoretical value.The structure of the material determines its performance,so it is urgent to study the changes in the microstructure of the fiber and its effect on the performance during the production of carbon fiber.The carbon fiber preparation process mainly includes the preparation of carbon fiber precursor,preoxidation process,low-temperature and high-temperature carbonization.Existing studies have shown that the properties of polyacrylonitrile?PAN?carbon fiber precursor determine the properties of carbon fibers.In this thesis,we systematically studied PAN fibers and preoxidized fibers from the perspective of microstructure.Detecting the microstructure changes of samples obtained from the important process steps in the spinning process of PAN fibers,the quantitative analysis method of micropore parameter was improved,and a new way was opened for exploring the performance mechanism of PAN carbon fiber precursor.The in-situ crystal structure detection equipment was designed,which provided guarantee for fiber manufacturers to control product quality and improve production efficiency.Taking the research results of PAN fiber microstructure as the theoretical guidance,through the combination of production,teaching and research,the production process of PAN fibers is optimized and production cost is reduced,and the performance of PAN fibers is improved.By studying the microstructure of preoxidized fibers at various stages,the mechanism of crystal transition of preoxidized fibers was explored,the in-situ preoxidation diffraction attachments of fibers and the related experiments was researched,the preoxidation process and crystal structure changes of the fiber were simulated.Based on the excellent properties of PAN carbon fibers precursor,a new type of lithium ion battery anode material was developed through the method of composite metal oxides,and the electrochemical performance of composite materials was systematically studied.The following innovative research results have been achieved:1.X-ray diffraction?XRD?was used to characterize fiber samples at key process nodes during the spinning prcocess.The change of crystal structure during the spinning process of PAN fiber was fitted by the method of full spectrum fitting,and the quantitative analysis method of generalized crystallinity and orientation of PAN fiber was established.The results showed that the crystallinity of PAN fibers varies from 38.70%increase to 74.34%in the wet spinning process.After the hot-densification process,the structure of PAN fiber was basically shaped.With the increase of the draft force,the degree of orientation of PAN fiber gradually increased from 59.12%to 83.51%.The crystallographic parameters of PAN fibers can be precisely adjusted by changing the spinning conditions of PAN fibers.A set of in-situ on-line two-dimensional X-ray diffraction detection device has been design,which can detect the generalized crystallinity,generalized orientation,and grain size of the fiber in-situ on-line.The device can feedback the crystal structure information of key process nodes in time,and provide equipment support for manufacturers to monitor the product quality of fiber production lines and develop high-performance products.2.Based on the small-angle X-ray scattering?SAXS?principle,the quantitative analysis of the micropore parameters of PAN carbon fiber precursor was improved.It is the first time to put forward a method to analyze the parameters of micropores in PAN fibers by stitching SAXS/USAXS data,and to systematically explore the change rule of micropores structure obtained by each key process in the preparation process of PAN fiber.The results showed that the micropores were formed in the double diffusion process.As the process progresses,the size,absolute number and porosity of the micropores gradually decrease.The micropores defect was reduced during the hot-densification process.The fractal dimension of the micropores gradually decrease,which indicated that the inner surface of the micropores tended to be smooth.In this process,the tensile strength and modulus were gradually increased,which showed that the performance of PAN fibers is getting better and better.The scanning electron microscope?SEM?was used to study the morphology of the fiber samples obtained from the key process nodes during the spinning process.The groove structure on the surface of the fibers has been retained in the PAN carbon fiber precursor,and the fiber diameter decreased from 61.9 to 10.3?m.Throughing a systematic discussion on the correlation among the production process,micropore parameter and tensile strength,it was found that the optimization of spinning process can reduce the structural defects of PAN fiber and improve the quality and performance of PAN carbon fiber precursor,that is to improve the quality and performance of carbon fiber,which provides theoretical guidance for carbon fiber enterprises to reduce production costs and improve product performance.3.The two-dimensional XRD was used to study the microstructure of the sample from each stage of the preoxidized fiber.The crystal structure parameters of the sample were calculated.The results showed that the preoxidation process was very slow when the temperature was below 180°C.The preoxidation reaction starts in the amorphous region,and then gradually expands to the crystalline region.The oxidation and cyclization reactions were intensified when the temperature was above 230°C,and the crystallization peak of the graphite-like phase can be detected when the preoxidation temperature reaches 240°C.The SEM image showed that applying a certain drafting force can effectively prevent the preoxidized fiber from shrinking and improve the performance of the proxidized fiber.The fiber in-situ preoxidation diffraction accessory was successfully developed,which can realize continuous temperature heat treatment from room temperature to 300°C,and the temperature control accuracy is±2°C,it can apply a draft force from 0 to 15 kg.The accessory is delicate and practical.It can simulate the process of preoxidation well by combining two-dimensional XRD with this accessory.The effect of process conditions on the structure of the preoxidized fiber is studied by a single factor.The results showed that due to the longer preoxidation time in the in-situ experiment,the transformation of the fiber crystalline structure was accelerated;applying the draft force of 1.1 times can improve the mechanical properties of the preoxidized fiber.4.Based on the groove structure on the surface and internal microporou structure of PAN carbon fiber precursor,a new type of anode material?CNF/SnO₂/Ni?of lithium ion battery with a multi-stage structure was prepared by a method of composite tin dioxide?SnO₂?and elemental nickel?Ni?.The structure of the composite was characterized by X-ray photoelectron spectroscopy?XPS?,SEM,and Transmission electron microscope?TEM?.The results showed that SnO₂ and Ni nanoparticles were uniformly distributed on carbon nanofibers?CNF?Surface,through the electrochemical performance test of CNF/SnO₂/Ni composite,it was found that the material has better cycle stability than tin oxide,which may be that the combination of SnO₂ and Ni nanoparticles with CNF can provide a specific framework and shortened the transmission path of lithium ions?Li⁺?.After multiple cycles,it can suppress the fragmentation of the SnO₂ lattice and improve the electrochemical performance of lithium ion batteries.It is a promising lithium ion battery anode material and can expand the application fields of carbon fibers precursor.