Preparation And Study Of Carbon Nanofiber-based Functional Interlayers For Lithium-sulfur Batteries

Due to the continuous development of society,the energy demand is constantly increasing,and optimizing energy utilization is an important means to solve energy problems.The role of energy storage technology in optimizing energy utilization has attracted widespread attention.Lithium-ion batteries have been widely used in the field of energy storage,but the current energy density of lithium-ion battery electrode materials is close to the theoretical limit,so it is difficult to meet the increasing energy demand.Lithium-ion batteries cathode materials are composed of heavy metal compounds,which are expensive and easy to harm the environment.Therefore,there is an urgent need to find suitable alternative materials.Sulfur elemental resources are abundant,non-toxic,and non-polluting.Lithium-sulfur（Li-S）batteries with sulfur elemental as the cathode have a high theoretical capacity and high energy density,which are considered to be the next-generation energy storage systems with the most potential applications.However,the current commercialization of Li-S batteries still faces many problems,such as the insulation of sulfur cathodes and the volume expansion during charging and discharging,the shuttle effect of polysulfides,and the dendrites of lithium anode.Among,the shuttle effect of polysulfides is one of the key factors restricting the development of Li-S batteries.Studies have shown that the shuttling of polysulfides can be suppressed and the electrochemical performance of Li-S batteries can be improved by introducing functional interlayers.This dissertation starts from the limitation of the shuttle of polysulfides,using electrospinning technology to design and prepare four kinds of carbon nanofiber-based multifunctional interlayer materials.To explore the effect of functional interlayers on the shuttle of polysulfides and the electrochemical performance of Li-S batteries.The main findings are as follows:1.Using electrospinning technology and NH₃ activation technology,a flexible porous carbon nanofiber membrane（D-Ti O₂@NPCNF）containing dual active sites（N and defective Ti O₂）was successfully prepared.NH₃ activation increases the specific surface area of the composite fiber,the nitrogen content increases,and Ti O₂ produces oxygen vacancy defects.N and D-Ti O₂ enhance the interfacial conversion and chemisorption capacity for polysulfides by forming"Li-N"and"Ti-S"bonds.The Li-S batteries using D-Ti O₂@NPCNF interlayer has a discharge specific capacity of up to609 m Ah g^-1 at 3 C,the capacity decay rate per cycle is only 0.062%after 500 cycles at 1 C.2.TiO₂ and SiO₂ double oxide doped carbon nanofiber membrane（Ti O₂@Si O₂/CNF）with the necklace-like structure was designed and fabricated by electrospinning technology.The Si O₂ nanospheres exposed more active surfaces of the necklace-like fibers,increased the contact area with the electrolyte,and made the Ti O₂nanoparticles more uniformly distributed on the fiber surface.The co-doping of Ti O₂and Si O₂ provides more adsorption active sites,accelerates the lithium-ion diffusion rate,and enhances the adsorption and catalytic conversion of polysulfides.The Li-S batteries based on Ti O₂@Si O₂/CNF interlayer exhibits a high discharge specific capacity of 658 m Ah g^-1 at 3 C and a reversible specific capacity of 696 m Ah g^-1 after500 cycles at 1C.3.A flexible hollow carbon nanofiber（HCNF）was prepared by coaxial electrospinning technology as a matrix,and then through a solvothermal reaction,the Sn S₂@HCNF membrane with excellent flexibility was prepared.Compared with carbon nanofiber,HCNF has a larger specific surface area and a higher degree of disorder,and the hollow structure provides a larger Sn S₂ growth surface.Sn S₂ promotes the nucleation of Li₂S,which not only accelerates the transport of lithium-ion but also accelerates the reaction kinetics of polysulfides.The Li-S batteries assembled by the Sn S₂@HCNF interlayer has a discharge specific capacity of up to 694 m Ah g^-1 at 3 C;the capacity decay rate per cycle is only 0.056%after 500 cycles at 1 C.4.Based on the research of Sn S₂@HCNF work,using bimetallic sulfides with higher catalytic activity to replace monometallic sulfides.Using HCNF as a matrix,a flexible bimetallic sulfide Ni Co₂S₄@HCNF membrane was prepared by hydrothermal technology.The hollow structure of the fiber shortens the diffusion distance of ions.The abundant space between Ni Co₂S₄ nanowires can increase the contact area with the electrolyte,effectively immobilization and catalyzing the redox reaction of polysulfides;the synergistic effect of HCNF and Ni Co₂S₄can significantly inhibit the shuttle effect of polysulfides.The Li-S batteries assembled based on the Ni Co₂S₄@HCNF interlayer exhibits an initial discharge capacity of 1213 m Ah g^-1 at 0.2 C and a specific discharge capacity of 755 m Ah g^-1 at 3C.