Preparation Of Covalent Organic Framework-based Separator Modification Materials And Their Application In Lithium-Sulfur Batteries

The combustion of fossil fuels leads to greenhouse gas emissions and environmental issues such as global warming.The use of renewable energy has become popular in recent decades.It is necessary to develop reliable,low-cost,durable and environment-friendly rechargeable energy storage systems to store energy from renewable energy on a large scale.Lithium-sulfur（Li-S）batteries are ideal for next-generation energy storage devices due to their extremely high theoretical energy density(2600 Wh kg^-1)and natural abundance and cost-effectiveness of sulfur.Nevertheless,the commercialization of Li-S batteries is still limited by battery components.Sulfur cathodes are challenged by the dissolution,diffusion,and side reactions of polysulfide intermediates（Li₂S_x,4≤x≤8）produced during charge and discharge.Lithium anodes are limited by uncontrolled dendrite growth,low coulombic efficiency（CE）,and lithium metal inactivation during cycling.Aiming at above problems,this paper takes covalent organic frameworks（COFs）as precursors to improve the rate performance and cycle stability of lithium-sulfur batteries,and obtains COFs post-modification materials,COFs composite materials and COFs-derived carbon materials,respectively,so as to systematically study the modification of lithium-sulfur battery separators.The main contents includes the following aspects:（1）Using the post-modification strategy of COFs,COF-FS with flexible sulfonic acid has been prepared for separators modification of lithium-sulfur battery.COF-FS exhibits dynamic adaptive behavior for electrostatic repulsion of polysulfide anions.At the same time,the post-modified flexible sulfonic acid chain was conducive to Li⁺migration and inhibited the formation of Li dendrites.When compared with pure COF and COF with rigid sulfonic acid（COF-S）,COF-FS exhibits higher discharge specific capacity and better cycle stability,with a discharge specific capacity of up to 1265.1 mA h g^-1at 0.1 C,and can stably cycle 2000 cycles at a rate of 2 C,maintaining a high reversible discharge specific capacity of 556.7 mA h g^-1,and the capacity attenuation rate of the average cycle is 0.014%.（2）Using the composite strategy of COFs,ionic covalent organic nanosheets（i CONs）and Ti₃C₂heterostructure（Ti₃C₂@i CON）were prepared as the separator modification materials,it has been proposed that i CONs could adsorb polysulfides by electrostatic action,and Ti₃C₂could accelerate polysulfide redox through catalysis.ionic covalent organic nanosheets are grown on Ti₃C₂nanosheets,which can be effectively adsorbed by the electrostatic interaction of cationic main frame and negatively charged polysulfide anions,and then make them difficult to diffuse and dissolve into the electrolyte,and use Ti₃C₂to catalyze the conversion of adsorbed polysulfides,thereby effectively inhibiting the polysulfide shuttle effect.Therefore,when the Ti₃C₂@i CON is used as a modification material for the PP separator of lithium-sulfur batteries,the electrochemical performance of carbon nanotubes/sulfur cathodes is effectively improved.The average capacity attenuation over 2000 cycles is as low as 0.006%at 2 C.Even with a sulfur content of 90 wt%and a sulfur load of 7.6 mg cm^-2,the reversible capacity,areal capacity,and volumetric capacity of the Ti₃C₂@i CON modified separator at 0.1 C were still as high as 1186 mA h g^-1,9.01 mA h cm^-2,and 1201 mA h cm^-3,respectively.（3）Using the derivative strategy of COFs,nitrogen-doped carbon materials doped with iron and nickel biatomic sites were prepared by high-temperature roasting technology,using the bimetallic porphyrin covalent organic framework as the precursor.The modified separator can not only effectively intercept the passage of polysulfide through physical/chemical action,but also play a step-by-step electrocatalytic role,using Fe-N₄active sites to accelerate long-chain polysulfide conversion,while Ni-N₄active sites are conducive to promoting Li₂S deposition kinetics.The use of Fe₄/Ni₁₆-N₄-NC modified separators allows carbon nanotubes/sulfur cathodes to exhibit a high initial specific capacity of 1562.7 mA h g^-1at 0.05 C,and almost maintain a high discharge specific capacity of 774.4 mA h g^-1after 1000 cycles at 2 C.