Fabrication And Electrochemical Performance Study Of Hollow Structure Materials

Lithium-sulfur（Li-S）batteries have garnered widespread attention due to high energy density,low cost,and environmental friendliness.However,significant challenges remain on the path to commercialization of Li-S batteries.Firstly,the low conductivity of S and its discharge product（Li₂S）reduces the utilization efficiency of the active material.Secondly,the dissolution of lithium polysulfides（Li₂S_n,4≤n≤8,Li PSs）leads to continuous loss of active material,severely impairing the cycling performance of the battery.Additionally,the sluggish reaction kinetics between sulfur and Li₂S results in the poor rate performance of Li-S batteries,further diminishing their practical application potential.Lastly,the growth of lithium dendrites can penetrate the separator,posing safety hazards.To address these issues,the introduction of highly active catalytic materials is considered one of the most promising approaches.These materials not only adsorb Li PSs,but also facilitate the conversion between Li PSs and Li₂S,thereby improving the cell reaction kinetics.Among various catalysts,alloys（metals）have garnered widespread attention due to their excellent conductivity and high catalytic activity.In addition to composition,the structure of catalysts also plays a crucial role.Hollow structure materials generally possess a large specific surface area,which can accommodate abundant catalytic active sites,thereby enabling sufficient interaction with Li PSs.Moreover,hollow structure facilitates the diffusion of lithium ions,thereby enhancing the catalytic activity of the material.Based on these considerations,this dissertation focuses on separator modification and designs various types of alloy（metal）/carbon composite hollow materials to improve the electrochemical performance of Li-S batteries.The main content is as follows:Firstly,employing ZIF-8@ZIF-67@PDA core-shell architecture as precursor,highly catalytically active and conductive CoNC@NC hollow catalyst material was synthesized through high-temperature annealing.The high adsorption and catalytic properties of Co metal not only effectively suppress the shuttle effect of Li PSs but also promote the conversion of Li PSs to Li₂S,thereby accelerating the reaction kinetics.Additionally,the high conductivity of carbon materials facilitates efficient electron transfer,enhancing the utilization efficiency of active materials.Finally,the hollow material with thin shell structure promotes sufficient contact between catalytic sites and electrolyte,facilitating lithium ion diffusion.After coating the separator with CoNC@NC material,the Li-S battery exhibited excellent electrochemical performance.At 0.2 C,the battery achieved a specific capacity of 1153.3 mAh g^-1.Even at 4 C,the battery can also maintain a high specific capacity of 601 mAh g^-1,outperforming the batteries utilizing CoNC and NC materials,demonstrating the superior catalytic characteristics of hollow CoNC@NC material.Secondly,using ZIF-8/ZIF-67@PDA/Fe as the precursor,CoFeNC@NC hollow material,characterized by abundant surface cracks,was synthesized via high-temperature annealing.Compared to single-component Co metal materials,the Co Fe alloy demonstrates a superior capability in effectively reducing the reaction barrier for the conversion of Li PSs to Li₂S,facilitating bidirectional conversion between them and optimizing reaction kinetics.Moreover,the hollow structures with numerous surface cracks can promote electrolyte infiltration,ensuring adequate contact between Li PSs and the Co Fe alloy.Harnessing its composition and structural advantages,the Li-S batteries with CoFeNC@NC as catalyst achieved a specific capacity of 1250.9 mAh g^-1 at a rate of 0.2 C.Furthermore,even under high sulfur loading conditions of 5.3 mg cm^-2,the area-specific capacity remained at 4.05 mAh cm^-2 after 100 cycles at 0.1 C,highlighting the outstanding practical performance of CoFeNC@NC.Thirdly,utilizing ZIF-8 as template,the NC/NiCo double-shell structured hollow polyhedrons（NC/NiCo DSHPs）catalyst was obtained through dopamine coating and high-temperature annealing.Compared with single-shell materials,the dual-shell architecture significantly increases the catalytic active sites of the NiCo alloy,effectively suppressing the shuttle effect of Li PSs and accelerating battery reaction kinetics.Moreover,the dual-shell carbon material along with abundant carbon nanotubes greatly enhance the electrochemical conductivity of the battery,thereby improving the utilization efficiency of active materials.With NC/NiCo DSHPs coated on the separator,Li-S batteries exhibited an initial specific capacity of 1310 mAh g^-1 at 0.2 C and excellent rate performance of 621 mAh g^-1 at 4 C.Moreover,even with a sulfur loading of 6 mg cm^-2,Li-S batteries maintained a high area-specific capacity of 4.5 mAh cm^-2 after 100 cycles at 0.1 C.