Preparation Of Novel Nitrogen Doped Multistage Porous Carbon Based Materials And Their Application As Cathode Host Materials For Lithium Sulfur Batteries

Lithium sulfur battery is expected to replace lithium-ion battery as the next generation of clean and efficient energy storage system owing to its high specific capacity and high energy density.However,the poor conductivity of sulfur,volume expansion of sulfur and the inevitable"shuttle effect"in the charge and discharge process seriously hinder the commercial application of lithium sulfur battery.Encapsulating sulfur in conductive host materials is an effective method to effectively solve the above problems of lithium sulfur batteries.Therefore,developing conductive host materials with excellent electrochemical performance has become the key to the preparation of high-performance lithium sulfur batteries.Among many host materials,nitrogen doped carbon mateirals with multistage pore have been proved to be excellent cathode host materials for lithium sulfur batteries.The rational channel design and heteroatom doping can effectively establish a good conductive network and effectively inhibit the shuttle effect,thus promote the conversion of polysulfides and improve the utilization of active substances.Based on the above ideas,we take nitrogen doped carbon mateirals with multistage pore as the research object of lithium sulfur battery cathode host materials and prepare high-performance lithium sulfur battery cathode host materials through the optimization of composition,morphology and structure.The research contents are as the follows:（1）Highly nitrogen doped hollow carbon spheres were prepared by coating silica with melamine resin.The resin is rich in nitrogen-containing functional groups,which is due to the nucleophilic addition reaction between the amino groups of melamine and formaldehyde to form carboxymethylation products.Therefore,in the subsequent carbonization process,a large number of nitrogen atoms are incorporated into the carbon lattice to form a large number of active sites.At the same time,nitrogen doped hollow carbon spheres with different nitrogen content were prepared by controlling the temperature and compare their electrochemical performance.The as repared highly nitrogen doped hollow carbon spheres with a ultra-high nitrogen content of 18.94 at.%exhibits a initial specific discharge capacity of 806.54 m Ah g^-1,and maintains a capacity retention of 85.71%.In addition,according to the result of DFT calculation and corresponding electrochemical test shows that a large number of active sites brought by high nitrogen doping not only strengthen the adsorption capacity of polysulfides,but also can act as electrocatalysis to promote the electrochemical conversion of polysulfides.Therefore,S₈ in NHCS-1/S is gradually and highly reversibly converted to Li₂S₆,Li₂S₄,Li₂S₂ and Li₂S in the electrochemical reaction process,thus the utilization rate of active substances is effectively improved in the process of host during the discharge/charge process.（2）By dissolving chitosan with green and environment-friendly solvent,porous carbon materials rich in micropores and mesopores were prepared for the cathode host material of lithium sulfur battery.The interaction between alkali and chitosan makes it produce enough micropores,mesopores and macropores in the process of high-temperature carbonization,thus the carrbon host reaches a ultra-high specific surface area of 2514.79 m²g^-1and giving it good conductivity.The as preapared carbon/sulfur electrode NMC-1/S exhibit a ultra-low decay rate of0.014%for 1000 cycles at 2C.Even when the sulfur load is 8 mg cm^-2,NMC-1/S still can maintain an area capacity of 5.10 m Ah cm^-2 after 50 cycles at 0.2C.Relevant tests show that a large number of micropores and mesopores contained in NMC-1 can serve as active sites for catalytic conversion of polysulfides,so that polysulfides can be fully transformed in electrochemical reaction,so as to inhibit the migration of long-chain polysulfides and make full use of active substances in the process of cycling process.（3）Nitrogen doped porous carbon supported oxide/carbide Mn O-Mo₂C-NPC were obtained by dissolving chitosan with alkali urea and adding compounds for heat treatment.The interaction of alkali urea with chitosan and compound precursors creates a porous carbon coated oxide/carbide structure.By comparing the Mn O-Mo₂C-NPC possesses the multiple advantages of dual phase structure and porous structure,the as synthesized S/Mn O-Mo₂C-NPC electrode exhibits a low decay rate of 0.019%after 1200 cycles at 2C.Even when the sulfur loading is as high as 8.0 mg cm^-2,S/Mn O-Mo₂C-NPC can still maintain a specific discharge capacity of 635.8 m Ah g^-1,correspongding to an area capacity of 5.07m Ah cm^-2 after 120 cycles at 0.2C,which shows excellent polysulfide inhibition and conversion ability.Relevant characterization and electrochemical tests show that the dual phase structure of Mn O-Mo₂C-NPC can realize the catalytic conversion of polysulfides during charge and discharge.The synergistic effect of Mn O-Mo₂C-NPC on the interaction of polysulfides is reflected in that Mn O improves the ability of the material to adsorb polysulfides,and Mo₂C can serve as a electrocatalysis to accelerate the catalytic conversion of polysulfides.