Study On Modification Of Nitrogen-doped Carbon Micro-nanofibers And Their Application In Lithium Sulfur Batteries

The energy crisis and environmental issues have promoted the development and application of new energy.Energy storage and efficient conversion and utilization have become one of the current research focuses.As a traditional energy storage device,secondary lithium-ion batteries cannot meet the needs of future energy storage devices due to their low specific capacity of electrode materials and expensive raw materials.In recent years,lithium-sulfur（Li-S）batteries based on sulfur as the active material and lithium as the negative electrode have gradually attracted attention due to their high theoretical specific capacity(1675 mAh g^-1)and high theoretical specific energy density(2600 Wh kg^-1),which have been considered as the promising energy storage device.Meanwhile,the active material sulfur reserves are abundant,the cost is low,and the environment is non-polluting.However,the battery system has several problems that limit its commercial application,such as the active material sulfur and the electrochemical final reaction products（Li₂S₂/Li₂S）being non-conductive,thereby reducing the sulfur utilization and cycle stability;the"shuttle effect"of a series of polysulfides（Li₂S_x,4≤x≤8）,which are the intermediate products of electrochemical reactions,causes the lithium metal anode to corrode and destroy the electrode structure.This thesis focuses on the above issues.Using polyacrylonitrile（PAN）and N,N dimethylformamide（DMF）as raw materials,the fibrous of PAN membrance precursor membrane precursors were prepared based on electrospinning technology,and a three-dimensional conductive network structure nitrogen-doped carbon Micro nano fibers（N-CNFs）membrane.The influence of heat treatment temperature on the physical and chemical properties of N-CNFs was explored.Using Li₂S₆ solution as the active material,N-CNFs/Li₂S₆ composite electrodes were prepared,and the electrochemical performance of high Sulfur loading（4.74 mg）electrodes was studied.The results show that the N-CNFs（900-N-CNFs）prepared at 900℃have high pyridine nitrogen content,which can effectively chemisorb polysulfides,inhibit their shuttle effect,and exhibit good electrochemical cycling performance.Based on the optimized process for preparing N-CNFs,the metal-organic framework material（ZIF-67）was mixed in the electrospinning raw material,and the cobalt and nitrogen co-doped carbon micro-nano fiber membranes（Co,N-CNFs）were prepared in situ by heat treatment.Electrochemical tests investigated the mechanism of metallic cobalt in electrochemical reactions.The results show that metallic cobalt can induce the nucleation of Li₂S,catalyze the conversion of polysulfides to Li₂S,accelerate the electrochemical reaction kinetics,and then inhibit the dissolution and diffusion of polysulfides and the corrosion of lithium metal negative electrodes.At the same time,compared with N-CNFs,Co,N-CNFs can effectively reduce electrode polarization and improve lithium ion migration rate.When the sulfur loading was 7.11 mg,the Co,N-CNFs/Li₂S₆ composite electrode showed excellent electrochemical performance.The first specific discharge capacity at 0.2 C is 1018 mAh g^-1,and the capacity after 100 cycles is 859 mAh g^-1（retention rate 84.4%）.There is still 817 mAh g^-1 at 1 C rate.Based on the Co,N-CNFs prepared by the above-mentioned optimized process,the CoS₂ supported Co,N-CNFs composite micro nano fibers（CoS₂@Co,N-CNFs）were prepared in situ in combination with the hydrothermal method,and the synergistic catalysis of CoS₂ and metallic Co was used to further improve the electrochemical performance of high sulfur composite electrodes.The results show that,compared with Co,N-CNFs,the CoS₂@N-CNFs/Li₂S₆ composite electrode has a sulfur content of 7.11 mg,and the first specific discharge capacity at 0.2 C is 1127mAh g^-1,and the capacity is 877 mAh g^-1 after 200 cycles.When the sulfur load is14.22 mg,0.1 C has a capacity of 12.7 mAh,and the capacity retention rate after 100cycles is 93.7%.