The Synthesis Of Carbon-based Composite Fibers And Their Storage Lithium/sodium Properties

Li-ion battery,as an excellent energy storing device,has been widely used in mobile electronic devices and small portable appliances because of its many advantages,including high working voltage,high energy density,high power density,no memory effect,no pollution and long life cycle.As an ideal energy storage battery,Li-ion battery has become a hotspot in the international research field in the 21st century.However,lithium resources is scarce,high-cost and unevenly distributed,which greatly restricts the development of Li-ion battery.Sodium,which belongs to the main group of lithium,is similar to the chemical property of lithium.Therefore,Na-ion battery can be used as an effective substitute for Li-ion battery due to the abundant resources of sodium.The anode material,which is an important part of Li/Na-ion battery,has a significant impact on the performance of battery.Carbon materials are commonly applied as the anode materials of Li/Na-ion battery owe to its low platform,good stability and small volume expansion in the charge/discharge process,but its capacity is low.Transition metal compounds are one of the most promising electrode materials,which can be ascribed to their higher specific capacity compared to that of carbon materials.However,the large volume expansion of transition metal compounds will destroy the integrity of the electrode structure during the charge/discharge process,which may seriously affect the cyclic performance of Li/Na-ion battery.In this thesis,in order to improve the performance of Li/Na-ion battery anode material,we successfully fabricated high-performance carbon-based transition metal compounds of Li/Na-ion battery anode materials via electrospinning,hydrothermal and some other methods with rational design for obtaining the respective advantages of carbon materials and transition metal compounds.Moreover,we explored the effects of the material morphology and structure on the performance of Li/Na-ion battery with different characterization methods.And the main works are as follows:（1）In Chapter 2,the 1D MoO₂@NC nanofiber is successfully synthesized by electrospinning using nitrogen-containing polyacrylonitrile（PAN）as the carbon source.The MoO₂ nanocrystals,which are in-situ formed,can completely coat with carbon in the NC fibers.NC nanofibers not only accelerate the transfer of ions,but also act as a buffer layer to relieve the mechanical degradation of materials caused by volume changes during the charge-discharge cycle of MoO₂.When the prepared 1D MoO₂@NC fiber composite material is used as the anode material of Li/Na-ion battery,it exhibits an outstanding storage lithium/sodium performance.When used as the anode material of Li-ion battery,MoO₂@NC displays the discharge capacity of930mAhg^-11 at a current density of 200mAg^-11 after 100 cycles.At a current density of1Ag^-1,the discharge capacity can also maintain at 720mAhg^-1after 600 cycles.In addition,when it is used for Na-ion battery,the discharge capacity can be maintained at 350mAhg^-11 after 200 cycles at a current density of 100mAg^-1.（2）The transition metal selenides show the notable performance of anode material for Li/Na-ion battery.In Chapter 3,selenium powder can be uniformly suspended in the mix solution of polyacrylonitrile and DMF added with cobalt salt by stirring in the water bath.CoSe@N-doped carbon fibers（CoSe@NC）are successfully synthesized after annealing in the protective gas.Furthermore,the different heat-treated temperature experiments are developed to study the performance of Li/Na-ion batteries.The result reveals that CoSe@NC-550,which is annealed at550°C,has better storage capacity of Li⁺/Na⁺.When CoSe@NC-550 is used as anode material for Li-ion batteries,the discharge capacity is maintained at 796mAhg^-11 after100 cycles at a current density of 1Ag^-1.Moreover,when applied as anode material for Na-ion batteries,the discharge capacity of CoSe@NC-550 is stabilized at333mAhg^-11 after 100 cycles at the current density of 0.1Ag^-1.In addition,the pseudocapacitor effect of CoSe@NC material by the cyclic voltammetry（CV）calculate in Li-ion battery,especially the capacitance contribution of CoSe@NC-550reach 72.8%at a scan rate of 1mV/s.（3）The volume change of SnO₂ as anode material can lead to the pulverization of material in the charge/discharge process,affecting the cycle and rate performance of Li/Na-ion battery.In Chapter 4,the N-doped carbon nanofibers are obtained by electrospinning and to introduce SnO₂ materials via hydrothermal process.Finally,it forms the composite structure of SnO₂ nanoflowers grown in carbon fibers（NC@SnO₂）.NC@SnO₂ material is used as anode material for Li-ion batteries.The reversible capacity is as high as 750mAhg^-1after 100 cycles at a current density of1Ag^-1.In addition,the reversible capacity was stabilized at 270mAhg^-11 after 100cycles at current density of 100mAg^-11 for Na-ion batteries with NC@SnO₂ using as anode material.The high-performance storage lithium/sodium properties of the material may be attributed to that the introduction of N-doped carbon fibers during the growth of synthetic SnO₂ nanoflowers not only provides a support framework for the growth of nanoflowers,but also prevents its agglomeration during the formation of nanoflowers.Therefore,this method is developed to achieve thinner SnO₂ nanosheets with greater specific surface area and maintain stable structure during charge and discharge.In addition,the carbon fibers using as a conductive network can effectively accelerate Li⁺/Na⁺transmission.（4）MoS₂ with similar graphene structure consists of 3D atomic layers（S-Mo-S）superposed by weak van der Waals force to form 2D layered structure,with 0.62 nm large layer spacing,which is conducive to the insertion and extraction of Li⁺/Na⁺,and it is one of the promising anode material for Li/Na-ion battery.In Chapter 5,the composite material of MoS₂ nanoflower on the N-doped carbon fibers with S-vacancy is successfully prepared by a simple electrospun-hydrothermal synthesis-reduction annealing process（NC@MoS₂-VS）.When NC@MoS₂-VS is used as anode electrode material for Li-ion battery,the capacity is maintained at 928mAhg^-1after 100 cycles at a current density of 1Ag^-1.As the anode electrode material for Na-ion battery,the capacity was maintained at 495mAhg^-11 after 100 cycles at a current density of100mAg^-1for Na-ion batteries,and it is found that NC@MoS₂-VS has a remarkable capacitance characteristic during the storage of Na⁺by using CV.The significant performance of Li/Na-ion battery is attributed to that carbon fibers can provide channels for fast electron transport,preventing the agglomeration of layered for molybdenum sulfides,and the presence of sulfur vacancies,leading to more active sites for the adsorption of Li/Na⁺.The theoretical calculations display that the presence of sulfur vacancies in MoS₂ not only promotes the adsorption of Na⁺,but also enhances the electrical conductivity of MoS₂.