Preparation And Electrochemical Properties Of Intrinsic Doped Carbon-based Cathode Materials For Lithium Sulfur Batteries

In order to achieve the goal of building a beautiful China,China proposes to achieve the peak of carbon dioxide emissions by 2030,and strive to achieve carbon neutrality by 2060.Therefore,the use of fossil energy must be reduced,so the development of new clean and renewable energy is imminent.However,such as solar energy,wind energy,tidal energy and other green energy has the characteristics of large amount but not concentrated distribution.Therefore,it is necessary to develop efficient and clean energy conversion and storage devices.Lithium sulfur（Li–S）batteries have a very high theoretical specific capacity（1672 m Ah/g）,theoretical mass energy density（2600 Wh/kg）,low cost and nontoxic.Therefore,Li–S batteries are considered as one of the most promising next-generation high energy-density chemical energy systems.However,the application of Li–S batteries is restricted by the problems of poor conductivity of elemental sulfur and discharge products（Li₂S₂ and Li₂S）,the shuttle effect of soluble polysulfides during the charge discharge process,and the 80%volume expansion of cathode material from elemental sulfur to Li₂S during the discharge process.In order to solve the problems of the cathode side of lithium sulfur battery,carbon nanofiber materials with intrinsic doping heteroatoms and pore structure and metal organic framework（MOF）-derived-carbon materials with intrinsic doping Oxygen were prepared in this paper to explore the influence of MOF-derived-carbon material-based integrated electrode on the electrochemical performance of lithium sulfur battery,and to optimize the performance of modified lithium sulfur battery.The specific research contents are as follows（1）By using the poly（phthalazione ether nitril ketone）（PPENK）and polyvinylpyrr-olidone（PVP）as carbon source and adding silica aerogel（Si O₂）through the electrospinning process,intrinsic nitrogen/oxygen dual-doping carbon nanofiber interlayer with meso-/micropores for Li–S batteries（Porous carbon nanofibers referred to PCNFs）PCNFs-2 was prepared.The results of X-ray photoelectron spectroscopy（XPS）,X-ray energy dispersive spectroscopy（EDS）and nitrogen adsorption desorption show that the PCNFs-2 interlayer has a uniform distribution of nitrogen and oxygen heteroatoms of 3.93 at.%and 4.21 at.%,and a micro mesoporous structure with specific surface area of 844 m²/g.The PCNFs-2 interlayer can effectively improve the utilization of cathode materials,and have a high discharge specific capacity of 1259.3 m Ah/g at 0.1 C,a specific capacity of 632.8 m Ah/g at 2 C,a specific capacity of 862.5 m Ah/g maintained after 200 cycles at 0.2 C with the average capacity decay per cycle of only 0.08%.Even at a high sulfur area loading of 7.5 mg/cm²,it still has an area specific capacity of 5.22 m Ah/cm² after 100 cycles at a current density of 0.1 C for the Li–S batteries with the PCNFs-2 interlayer.All above indicates good initial discharge specific capacity,rate performance and cycle stability of the introduction of PCNFs-2.（2）Two kinds of MOF-derived carbon materials were prepared by MIL-101-Fe.O-doped sulfur host material（MIL900-H）carbon nanosheets with many mesoporous and iron microparticles composite carbon nanofiber network MIL1000 interlayer materials were prepared by carbonization and chemical etching.MIL1000 was used as the interlayer and MIL900-H was composited with sulfur（S@MIL900-H）as the cathode,which were coated on the polypropylene（PP）separator by layer by layer to construct the metal current collector free flexible all MOF-derived-carbon materials based-integrated cathode（S@MIL900-H/MIL1000/PP）.The nitrogen adsorption and XPS characterization shows MIL900-H contains a large number of mesopores with pore volume of 1.706 cm³/g and doped oxygen atoms with6.44 at.%,which can provide sufficient space for high sulfur loading and chemical adsorption of soluble polysulfides.X-ray diffraction（XRD）shows that MIL1000 has Fe particle composite.The test results show that the integrated cathode has good initial discharge capacity of 1316.7m Ah/g at 0.1 C,and good rate performance with the rate capacity at 2 C current density of809.3 m Ah/g,and high cycle stability after 500 cycles at 1 C with the capacity retention rate of 69.3%.