The Synthesis And Modification Of The Hollow Carbon Materials And Their Cathode Performance In Lithium Sulfur Batteries

Hollow carbon-based materials are crucial for lithium-sulfur battery cathode material research due to their high specific surface area,adjustable pore and pore size distribution,high conductivity,and robust chemical and mechanical stability.However,when directly applied to lithium-sulfur battery systems,hollow carbon materials exhibit suboptimal capacity and cycle performance.Thus,modifications to the hollow carbon material are necessary to fulfill the requirements of lithium-sulfur battery cathode materials.Addressing common scientific challenges in lithium-sulfur batteries,such as poor electrical conductivity of active substances,volume expansion,and the shuttle effect,this paper optimizes hollow carbon materials through heteroatom doping,metal nanoparticle modification,and sulfide nanoparticle embedding.It outlines the design and preparation of three types of enhanced hollow carbon-based materials,significantly improving the performance of hollow carbon materials as lithium-sulfur battery cathode materials,detailed in the following sections:（1）Addressing the problem of polysulfide shuttling in lithium-sulfur batteries,we designed and synthesized a nitrogen/sulfur co-doped multistage porous hollow carbon material（N/S-HCS-CNT）from the angle of heteroatom co-doping.This co-doping modification significantly enhances the hollow carbon material’s polysulfide adsorption capacity,thereby improving the lithium-sulfur battery’s cycle life.As a cathode material for lithium-sulfur batteries,the specific capacity is maintained at 550 m Ah·g^-1 even after 1000 cycles at 5 C.The material’s excellent properties and cycle stability were confirmed by CV,EIS,and GCD tests.XPS test results reveal that N/S co-doping dramatically improves the carbon material’s polysulfide adsorption capacity.（2）To address the volume expansion and shuttle effect issues in lithium-sulfur batteries,an in situ dopamine-etched metal-organic framework method was devised to create hollow materials.Following high-temperature heat treatment,nitrogen-doped graphitized hollow carbon materials（Co-NHGC）were obtained.TEM analysis confirmed the successful injection of the active substance,sulfur,into the hollow carbon cavity.The hollow carbon shell effectively inhibits volume expansion and physically confines polysulfides.Building on the previous chapter’s discussion of heteroatom-mediated polysulfide adsorption,cobalt nanoparticles were introduced.These nanoparticles possess high catalytic activity and effectively suppress the shuttle effect dynamically.This results in a remarkable specific capacity of up to 1,600 m Ah·g^-1 at a0.1 C current density in lithium-sulfur batteries.A series of CV and EIS tests showed that cobalt nanoparticles significantly enhance the lithium-ion diffusion coefficient in Li-S batteries.Electrochemical tests confirmed that cobalt nanoparticle incorporation substantially catalyzes the REDOX reactions in lithium-sulfur batteries.Moreover,the presence of cobalt nanoparticles not only dynamically improves the REDOX reactions in Li-S cells but also thermodynamically increases the diffusion current during the reaction.（3）To address the significant polarization and low capacity issues under high load and rate conditions,a nitrogen/sulfur co-doped graphitized hollow carbon material modified with cobalt disulfide nanoparticles（Co S₂-NHGC）was developed.This significantly enhanced the material’s catalytic activity and ion transport.For example,even at a 5 C current density,the specific capacity reaches 350 m Ah·g^-1 after 800 cycles,and at a 10 C current density,a specific capacity of 340 m Ah·g^-1 is maintained after600 cycles,demonstrating robust specific capacity and cycle stability.Additionally,even at a high load of 4 mg·cm^-2,an area-specific capacity of 4.3 m Ah·cm^-2 is sustained after 200 cycles.Electrochemical tests,such as CV and EIS,confirmed that Co S₂nanoparticles significantly enhance the lithium-ion diffusion coefficient in hollow carbon materials.Other electrochemical tests,like GCD and PCD,coupled with TEM and XRD analyses,revealed that Co S₂ nanoparticles exhibit higher catalytic activity and longer catalytic lifespan than Co nanoparticles.In conclusion,this study addresses existing scientific challenges in lithium-sulfur batteries by designing and preparing three types of hollow carbon-based lithium-sulfur cathode materials,incorporating heteroatom co-doping,metallic cobalt nanoparticle modification,and cobalt disulfide nanoparticle modification.The shuttle effect is progressively mitigated through heteroatom chemisorption,physical confinement by hollow carbon shells,and kinetic catalysis.The catalytic capacity of the hollow carbon materials is incrementally enhanced from non-metallic heteroatoms to metal nanoparticles,and ultimately to sulfide nanoparticles.Key synthesis methods employed include the wet chemical method,sintering method,and low-temperature hot melt method,all of which are straightforward and highly reproducible.The cobalt disulfide nanoparticle-modified hollow carbon material,developed based on the findings of the previous chapters,demonstrates exceptional rate performance and cycle stability.This work holds significant implications for future research and practical applications of sulfur cathodes.