Preparation And Electrochemical Performance Of Biomass Carbon Cathode Materials For Lithium-sulfur Batteries

Lithium-sulfur batteries with high theoretical specific capacity(1675m Ah·g^-1)and low-cost raw materials have become a promising candidate for energy storage devices in the field of energy storage.However,monolithic sulfur and its discharge products（such as Li₂S₂ and Li₂S）have inherently low electron/ion conductivity,and the shuttling of intermediates of lithium polysulfides in the electrolyte is a major obstacle to commercialization.Encapsulating sulfur within a conductive porous carbon material that contains a polar surface is considered one of the most promising strategies to overcome these issues.The conventional method for preparing carbon materials is complex and expensive,making it unsuitable for large-scale production.Therefore,biomass-based biocarbon materials obtained from abundant sources with a simple preparation process and low cost have attracted widespread attention.The introduction of biomass carbon into lithium-sulfur battery energy storage devices is increasingly becoming one of the hot spots in current lithium-sulfur battery research.This thesis focuses on improving the electrical conductivity and active material utilization of sulfur composite cathode material by utilizing optimal biomass carbon raw material,together with various pretreatment methods,elemental doping,and introduction of polar compounds.These approaches aim to reduce the negative impact caused by the shuttle effect occurring in the intermediate product lithium polysulfide,and ultimately achieve a substantial performance improvement.Details of the study are as follows:（1）Three types of biomass raw materials—water hyacinth,loofah flesh,and sycamore wood—were subjected to pyrolysis reactions at high temperature to prepare corresponding biomass carbon materials named WHC,LC,and FSC.The unique growth structures of these biomass materials served as templates for the successful conversion into carbon materials.Next,three sulfur-carbon composites（S/WHC,S/LC,and S/FSC）were prepared by melt compounding sulfur with these biocarbon materials.They were compared using physical and electrochemical testing techniques.The results showed that the biocarbon material LC,obtained by using loofah flesh as raw material,formed micro and mesoporous structures,resulting in a large surface area(661.5 m²·g^-1)and pore volume(0.4228 cm³·g^-1).Finally,the sulfur/biocarbon material（S/LC）was used as the positive electrode for a lithium-sulfur battery,which exhibited good electrochemical performance at 0.2C with a specific capacity of 910.3 m Ah·g^-1in the first week of discharge.Even after 200 weeks of charge and discharge,the specific capacity of the battery could still maintain at 554.1 m Ah·g^-1,with a capacity retention rate of60.87%.（2）To enhance the electrochemical properties of biomass materials and eliminate potential impurities,utilization loofah flesh as a source of biomass and subjects it to two simple pretreatment methods-acid washing and hydrothermal reaction.The study aims to investigate the impact of such treatments on the physical and electrochemical properties of loofah-derived biomass carbon materials.The study found that the surface area of biocarbon materials increased significantly after undergoing acid-washing（LC-1）and hydrothermal（LC-2）pretreatments,resulting in an improvement in the electrochemical properties of the composites.Specifically,S/LC-1,which was prepared using loofah carbon material derived from acid-washed pretreatment,demonstrated a specific capacity of 1180.8 m Ah·g^-1 during the first discharge.After 200 cycles,it exhibited a reversible capacity of 814 m Ah·g^-1 and a capacity retention rate of 68.98%.（3）Using P-doped loofah-based biomass carbon materials were prepared by carbonization and activation at different temperatures using loofah flesh as a biomass feedstock and H₃PO₄as a pore-forming agent and a source of elemental phosphorus.The physical structure and properties of these materials were characterized.After sulfur melt compounding,this biomass carbon material was tested in lithium-sulfur batteries for its application.Our experimental results showed that the optimal loofah-based biomass carbon material was fabricated at a carbonization temperature of 550°C,with a specific surface area of 2724.2 m²·g^-1 and a pore capacity of 2.359 cm³·g^-1.The first-week discharge specific capacity reached 1271.3 m Ah·g^-1at 0.2 C,and the reversible capacity was still 870.5 m Ah·g^-1 after 200 cycles.These results demonstrate the excellent performance of the P-doped loofah-based biocarbon material in lithium-sulfur batteries,highlighting its potential and effectiveness as a green energy storage device.（4）GO-HPC-550 was synthesized by using P-doped loofah-fleshed carbon material as a substrate.The substrate was further doped and coated with purified graphene oxide（GO）in an aqueous solution under ultrasonic assistance.Finally,it was compounded with sulfur to form the GO-S/HPC-550 electrode material.Structural and compositional analyses were carried out and revealed that the layered structure of GO can act as a physical barrier to sulfur and polysulfides.Furthermore,the oxygen-containing functional groups in the system,in combination with P atoms,play a role in chemisorption of polysulfides.In the electrochemical performance test,GO-S/HPC-550 was used as the sulfur cathode and demonstrated excellent performance.It exhibited a high initial capacity of 1445.8 m Ah·g^-1 at 0.2C and maintained a reversible capacity of 1046.9 m Ah·g^-1 even after 200 cycles.It also showed outstanding capacity performance even under high current density（2C）and high sulfur loading(～4 mg·cm^-2).Overall,this study successfully developed a novel GO-S/HPC-550 electrode material with exceptional electrochemical performance and polysulfide adsorption capabilities.The material overcomes challenges posed by complex environments such as high current density and sulfur loading,and it provides new insights and research avenues for further development in the field of lithium-sulfur batteries.