Study On Microstructure Regulation And Sodium Storage Mechanism Of Layered Carbon Materials

The development of human society is closely related to the acquisition,storage and transformation of energy.Therefore,the research and development of new environment-friendly energy sources and large-scale energy storage devices has been developing continuously.Sodium-ion batteries are expected to complement lithium-ion batteries in the field of large-scale electrochemical energy storage and low-speed electric vehicles,due to the abundant reserves and low cost of sodium,as well as its similar physicochemical properties to lithium.The anode material is one of the important components of the electrode material required for the construction of sodium ion batteries.Carbon-based materials may become the anode materials of sodium-ion batteries commercially in the future due to their low cost,abundant resources,and environmental friendliness.In addition,with the strong demand for electric vehicles,portable electronic products,and wearable devices,the development of flexible carbon anode materials with high mechanical strength,high energy density,and excellent overall sodium storage performance has become one of the key research directions in academia and industry.Consequently,carbon fiber is used as a research clue in this thesis because of its high strength,high modulus,electrical conductivity,thermal conductivity,and corrosion resistance.The commercial carbon fiber as a starting point for the study to explore its energy storage properties in sodium ion batteries.Different microcrystalline graphite fiber and soft carbon fiber anode materials are prepared using a simple carbonization and graphitization process,while the microstructure of carbon fiber is controlled by adjusting the heat treatment temperature and pressure.The prepared materials are then applied to the sodium-ion battery system to systematically investigate their electrochemical performance and sodium storage mechanism.The details of research are as follows:(1)Commercial carbon fibers are investigated as anode material for sodium-ion batteries.The electrochemical test results display that carbon fiber exhibite a considerable capacity in DIGLYME electrolyte(148 mAh g-1),with a low plateau capacity accounting for 75%.This proves the feasibility and advantage of carbon fiber as anode material of sodium ion batteries.In-situ synchrotron small Angle X-ray scattering technique reveals that the active sites of sodium ion storage in commercial carbon fiber are unique closed pores.In particular,sodium deposition in carbon fiber samples after sodium storage is visually confirmed by spherical aberration correction transmission electron microscopy.Further combined with electrochemical tests and ex-situ X-ray diffraction,the sodium storage mechanism of carbon fiber can satisfy the process of "adsorption-metallic sodium deposition".(2)Microcrystalline graphite fiber(MCGF)with ordered grain boundary cavities and mesopores is prepared from cotton by carbonization and graphitalization.The electrode exhibits high initial coulombic efficiency(92.5%),fast-charging(within 4 min),and enhanced cycling stability(98%retention after 800 cycles).Herein,based on a variety of microstructure characterization techniques,in-situ synchrotron small-angle X-ray scattering and ex-situ synchrotron X-ray absorption spectra,a novel co-adsorption sodium storage mechanism of MCGF is discovered.Solvated Na+ can be stored reversibly in ordered grain boundaries and mesoporous pores of MCGF,which is different from the solvated Na+cointercalation mechanism reported in the literature.The ex-situ synchrotron absorption spectrum,spherical electron microscopy and DFT theoretical calculation further revealed the "co-adsorption"sodium storage behavior of microcrystalline graphite fiber electrode.This study finds a new mechanism for sodium storage behavior of crystalline carbon materials under ether electrolytes,develops a new high performance anode material for the fabrication of flexible sodium storage devices in the future,and develops a new test method for the study of metal ion storage in nanospace.(3)In order to further improve the integrated electrochemical performance of carbon-based sodium storage electrodes,soft carbon nanofiber(SCF)anode materials are successfully prepared by the hot-pressure coupling strategy.The electrode material is found that the capacity is up to 396 mAh g-1(with 222 mAh g-1 for low plateau capacity)and initial coulombic efficiency is 82%in DIGLYME electrolyte by optimizing the experimental process.Various microstructural test results demonstrate that the electrode has an anisotropic disordered turbine structure and expanded carbon layer spacing,which facilitates sodium ion storage.In-situ synchrotron small-angle X-ray scattering results confirm that the Na+pore filling and the structural stability of the electrode during charging and discharging.The results of synchrotron X-ray absorption fine structure spectra and extended X-ray absorption fine structure spectra further illustrate the electron transfer of Na-GIC.The microstructure and electrochemical test results demonstrate that the electrode material has an"adsorption-intercalation-filling" sodium storage mechanism.This study provides a new technical model for the construction of high-performance sodium storage anode materials in the future,and expands new ideas for exploring the sodium storage mechanism of new sodium storage electrodes.