Polar Porous Carbon Materials And Iron-Based Catalysts For Lithium-Sulfur Batteries

The new secondary lithium-sulfur batteries have high theoretical energy density(2600 kW kg^-1)and high theoretical specific capacity(1675 mAh g^-1),which is much higher than the energy density of commercial lithium-ion batteries.At the same time,sulfur has the advantages of being cheap and easy to obtain,abundant sources and environmentally friendly.Therefore,lithium-sulfur battery has become the most promising new secondary battery.However,the following defects seriously hinder the development of Li-S battery on the road of commercialization:?1?the active material sulfur and discharge product Li₂S₂/Li₂S are insulators,which seriously affect the reaction kinetics of the battery and reduce the utilization efficiency of the active material,resulting in low capacity of the battery;?2?in the process of battery charging and discharging,lithium insertion and lithium removal of sulfur will lead to a volume change of 80%of the cathode electrode,and even destroy the cathode electrode structure,resulting in a serious reduction in the battery cycle life and coulomb efficiency;?3?in the process of charging and discharging,long-chain polysulfide compounds are formed.Long-chain polysulfide compounds are easily soluble in the electrolyte,reach the lithium lithium metal electrode through the separation membrane,and have side reactions on the surface of lithium metal,resulting in a substantial attenuation of battery capacity and coulomb efficiency.In view of the above problems,especially the shuttle effect,this thesis mainly focuses on the design of the cathode structure and the separation membrane modification.The main contents are as follows.First,graphene oxide was used as the substrate,Ni²⁺and cyanamide were uniformly adsorbed,and nitrogen-doped carbon tubes were catalyzed in situ on the surface of graphene at high temperature.Ni was removed by acid pickling,and NCNTs-G was compounded with sulfur as the sulfur cathode electrode.Ni as a catalyst,cyanamide as a carbon source and nitrogen source,and cyanamide has a high nitrogen content,for carbon tube to provide a high nitrogen doping amount.On the one hand,NCNTs-G materials grow on the graphene surface in situ,and the two are connected as a whole,which greatly reduces the interface resistance between them and improves the electrical conductivity of the materials.On the other hand,due to the high polarity of carbon tubes,the doping of nitrogen greatly increases the polarity of the materials and enhances the inhibition of the shuttle effect of polysulfide compounds.Thanks to these advantages,the battery has achieved good electrochemical performance.At 1C,the first cycle discharge specific capacity is 835 mAh g^-1,and the attenuation rate of each cycle after 400 cycles of charging and discharging is only 0.06%,and the average coulomb efficiency is about 98%.On the basis of the research in the previous chapter,melamine foam was used as the framework,and phenolic resin was used to fill and coat it.Thiourea was added as the nitrogen source and sulfur source,and after carbonization,it was directly used as the cathode electrode material of N,S co-doped hierarchical pore structured carbon foam@porous carbon?CF@N-SPC?.This cathode material has a three-dimensional self-supporting structure with hierarchical pores.Without the introduction of binder,it can provide enough space to achieve a high sulfur load,promote rapid lithium ion and electron transport,and can well adapt to the volume change in the process of charging and discharging.At the same time,the addition of in-situ nitrogen/sulfur elements can improve the adsorption of polysulfide compounds,and can effectively inhibit the shuttle effect.The electrochemical performance was tested at CF@N-SPC/S electrode surface with sulfur load of 4 mg cm^-2,and at 0.5 C discharge specific capacity reached 923 mAh g^-1,reaching a high sulfur utilization rate.After 250 cycles of charge and discharge,the specific capacity remained at 765 mAh g^-1,the capacity retention rate reached 83%,and the average coulomb efficiency reached 98%.Graphene oxide was used as a base to load iron ions and glucose for hydrothermal reaction to generate graphene-supported carbon-coated Fe₂O₃ composites,which were then uniformly mixed with melamine and pyrolyzed under the protection of an inert gas to form nitrogen-doped reticulation-like porous carbon-supported Fe₃C/Fe-N_x materials?Fe₃C/Fe-N_x@NPCN?.The PP membrane was modified with the synthesized material,and polysulfide was adsorbed by physical resistance of porous carbon and polar nitrogen and iron based compounds.At the same time,the synthesized iron-based compounds have certain catalytic conversion effect on polysulfide compounds,improve the reaction kinetics of batteries,accelerate the conversion of long-chain soluble polysulfide compounds to insoluble sulfide?Li₂S₂/Li₂S?,and to a large extent,inhibit the shuttling effect.At the rate of 0.2 C,the initial specific capacity is up to 1268 mAh g^-1,and after 100 cycles of charge and discharge,the specific capacity remains at 1028 mAh g^-1,the capacity retention rate is up to 81%,and the average coulomb efficiency is about 98%,with high specific capacity and good cycle stability.