Investigation On Preparation And Electrochemical Performance Of Lithium Sulfur Battery Cathode Materials By Electrospinning Technique

Energy storage devices,such as Li-ion batteries,are widely used in our daily life and advanced electronic devices.However,the current Li-ion batteries cannot meet the requirement of the fast development of portable electronic devices,electronic vehicles and space technology.Searching for advanced Li-ion batteries with high specific capacity,low cost and environmental friendliness are very important.The theoretical specific capacity of Li-S battery,which uses metal Li as anode and S or S based composite as cathode,is 1675 mAh g^-1 and the theoretical energy density of it is 2600 Wh kg^-1.In addition,the cost and toxicity of S is low,which makes Li-S battery a very promising energy storage system.However,the poor conductivity of sulfur,the volume changes during charge and discharge process,and the severe shuttle effect have led to extremely rapid capacity decay of lithium-sulfur batteries.Therefore,it is failed to meet the market demand for the long-life energy storage devices.The key to high-performance Li-S battery is improving the conductivity of S,accommodating the volume expansion of S and eliminating the shuttle effect of polysulfide.In order to solve these problems and obtain high-performance lithium-sulfur batteries,the thesis reports the preparation of functionalized carbon nanofibers by an electrospinning with simple processing steps.Based on the advantages of meso/macropores to alleviate volume expansion,three-dimensional conductive network to ensure electron/ion transport,and in situ polar substance modification for chemical adsorption,the free-standing cathode materials of lithium-sulfur battery with good electrochemical performance are designed and prepared.The main contents include:?1?Carbon nanofiber/sulfur composite flexible freestanding cathode material containing crumpled graphene:the CG@CNF/S film electrode introducing crumpled graphene is constructed by electrospinning technique and characterized for structure and electrochemical performance.The prepared freestanding CG@CNF/S cathode material exhibited good electrochemical performance,resulting from the large specific surface area of crumpled graphene,which is favorable for high S-loading,and excellent electron/ion transport capability and physical/chemical adsorption of CG and N-site.For example,the CG@CNF/S electrode still reach 100%Coulombic efficiency and maintain a discharge specific capacity of 382 mAh g^-1 after 200 cycles at 1.0 A g^-1,and it can also achieve a reversible capacity of 290 mAh g^-1 at 5.0 A g^-1.?2?Carbon nanofiber/sulfur composite flexible freestanding cathode material modified by copper particles:the sandwich-like CNF/S-Cu/CNF cathode in-situ dopping nano-copper is constructed by electrospinning and macrostructure design.And the materials are characterized by a series of test-ways.The synergistic effect of carbon nanofibers dopping the polar nano-copper and its own nitrogen active sites can effectively restrain the shuttle effect.Forthermore,the carbon nanofiber can alleviate the expansion/contraction stress and has excellent electron/ion transport path.Therefore,the CNF/S-Cu/CNF cathode shows high sulfur utilization and excellent electrochemistry performance.For example,the CNF/S-Cu/CNF cathode exhibits a high reversible capacity of 1295 mAh g^-1 at 0.1 A g^-1,capacities retention of around 85%(more than 530mAh g^-1)and?100%Coulombic efficiency after 300 cycles at 1.0 A g^-1.Especially,the CNF/S-Cu/CNF cathode exhibits a discharge capacity of 430 mAh g^-1 at 8.0 A g^-1.?3?Porous carbon nanofiber/sulfur composite cathode materials modified by g-C₃N₄catalyst:the g-C₃N₄@PCNF/S cathode with synergistic effect of anchoring/catalysis is prepared by electrospinning.The structure and electrochemical performance of the cathode materials are characterized and tested,and the catalysis and adsorption of g-C₃N₄are confirmed by the overpotential comparison at different current densities and the corrosion of cycled lithium anode.Due to the the anchoring and catalysis of g-C₃N₄ for sulfur species,effective electron/Li⁺transport and restrained shuttling effect,the g-C₃N₄@PCNF/S cathode shows excellent electrochemical performance.For example,the g-C₃N₄@PCNF/S cathode exhibits an initial discharge capacity of about 1250 mAh g^-1 at0.1 A g^-1.Moreover,the capacity decay of about 0.056%per cycle is demonstrated at 1.0A g^-1,indicating that the cathode materials with g-C₃N₄ can ensure good electrochemical reaction kinetics and long-term cycling stability.In conclusion,the cathode materials modified by crumpled graphene,nano-copper and g-C₃N₄ are designed and prepared by electrospinning technology,respectively,and the C/S composite materials exhibit good flexibility and excellent electrochemical performance.These modification and design strategies provide new ideas and are of great significance for the in-depth study of S based cathode materials by electrospinning.