Preparation And Energy Storage Properties Of Carbon Nanofiber Interlayer

The rechargeable lithium-sulfur battery is one of great promise for new-generation energy storage systems due to the large theoretical energy density(2600 Wh kg^-1)and the high theoretical specific capacity(1675 mAh g^-1)of sulfur.Additionally,sulfur also exhibits of advantages of low cost,non-toxicity and abundant.However,lithium-sulfur batteries have a series of problems,such as the poor electron conductivity of sulfur,shuttle effect and 80%of the volume expansion during charge/discharge.The above problems lead to a short cycle life and low Coulombic efficiency,which restricts the commercialization of lithium-sulfur batteries.In the present thesis,based on an overview of the research and development of lithium-sulfur batteries at home and abroad,aim to problems of the poor electron conductivity and“shuttle effect”,development of new-type carbon nanofiber interlayer that increase conductivity of cathode and suppress“shuttle effect”,by optimizing batteries configuration and electrospinning technology,which obtain high performance lithium-sulfur battery.This paper mainly studies the preparation of excimer UV-irradiated carbon nanofibers?EUV-CNFs?,iron carbide composite carbon nanofibers?Fe₃C-CNFs?and silk carbon nanofibers?SCNFs?and its applications of the lithium-sulfur battery.The main research contents are as follows:?1?Using PAN as a carbon source,EUV-CNFs were prepared by electrospinning,carbonization,and following excimer UV lamp irradiation.After 20 min irradiation,the pores appeared on the surface of the CNFs and the oxygen content increased obviously,and the specific surface area increased from 599 m² g^-1 to 621 m² g^-1.Electrochemical performance tests showed that the batteries have a high initial discharge capacity of 1356 mAh g^-1 and retention capacity of917 mAh g^-1 with high reversibility?per cycle 0.16%capacity decay?and Coulombic efficiency?98.8%?at a current density of 0.2 C after 200 cycles.The oxidized functional group on the surface of the EUV-CNFs adsorbed polysulfides through Li-O interaction and provided a fast electron transfer path by the conductive carbon to achieve high utilization of the active material.?2?FeSO₄ and PAN are mixed and dissolved in DMF,following electrospunning and pyrocarbonized to form Fe₃C-CNFs.The effect of FeSO₄ on the morphology of CNFs was investigated,the results showed that FeSO₄ is converted into Fe₃C nanoparticles and uniformly dispersed on the surface/inner of the fiber,and the fiber integrity is deteriorated.XPS analysis and SEM images results showed that Fe₃C nanoparticles could adsorb polysulfides.Lithium-sulfur battery with Fe₃C-CNFs interlayer presents a first discharge capacity of 1077 mAh g^-1 and the reversible retention capacity of 914 mAhg^-1 with an average coulomb efficiency of 98.0%at a current density of 0.2 C after 100 cycles.It is found that the Fe₃C does not influence the lithium ions diffusion rate of the lithium-sulfur batteries by calculating the Randles-Sevick equation.?3?Carbon sources of silkworm cocoons are degumming,dissolving,freeze-dried,electrospun and carbonized to form silk fibroin nanofibers.At 320°C,the fiber mass loss was severe;at 360°C,the?-sheet crystallites of the silk fibroin nanofiber gradually lose the N element and transform into a conjugated carbon structure.After carbonization at 800°C,SCNFs were obtained with a fiber diameter of about 400 nm,a specific surface area of 445 m² g^-1,and a pore volume of 0.24 cm³ g^-1.Four different structural configurations are designed to test the electrochemical performance of lithium-sulfur batteries that include cathode interlayer,anode interlayer,double interlayers,and no interlayer.The results show that the lithium-sulfur battery with double interlayers presents an initial discharge capacity of 1164 mAh g^-1 at 0.2 C and the discharge retention capacity of 799 mAh g^-1 after 200 cycles.