Preparation Of Multifunctional Composite Separators And Their Application In Lithium-sulfur Batteries

The widespread use of portable electronic devices,electric vehicles,and the rapid growth of renewable energy storage have raised higher demands for the energy density,manufacturing cost,and safety of secondary batteries.The development of new energy storage systems is imminent.Lithium-sulfur batteries,with their high energy density and low cost,are considered the most promising candidates.However,the practical application of lithium-sulfur batteries still faces many challenges,especially the shuttle effect caused by the diffusion of soluble intermediate polysulfides（Li PSs）and slow reaction kinetics,resulting in a severe decay of battery capacity,which has become the bottleneck of lithium-sulfur batteries.This paper aims to improve the cycle stability of lithium-sulfur batteries by designing and preparing nanocomposite materials with special structures to construct multifunctional separators,focusing on limiting the diffusion of Li PSs,accelerating the redox reaction of Li PSs,and thus inhibiting the shuttle effect of Li PSs.First,physical blocking and chemical adsorption anchor Li PSs on the cathode side,limiting the migration of Li PSs;second,by catalytic conversion,promoting the rapid completion of the redox reaction of Li PSs on the cathode side,reducing the accumulation of Li PSs;finally,by taking full advantage of the synergistic effect of composite materials,combining blocking,adsorption,and chemical catalysis organically,further improving the electrochemical performance of lithium-sulfur batteries.Based on a series of tests including in-situ tests,the prepared separator’s blocking,adsorption,and catalysis processes for Li PSs are analyzed from a mechanistic perspective.The main research content and results are as follows:1.Mesoporous boron carbonitride/graphene composite separator as an effective barrier for Li PSs.A composite separator consisting of mesoporous boron carbonitride（BCN）microribbons and conductive graphene sheets was prepared on a polypropylene（PP）separator using a vacuum filtration method,forming a BCN/G functional separator.The separator has strong physical blocking and chemical adsorption for Li PSs,confining them to the sulfur cathode side and inhibiting their shuttle effect.Electrochemical tests show that the battery displays a high specific capacity and good cycle stability.After 100 cycles at a current density of 0.2 A g^-1,its capacity remains at837.7 m Ah g^-1.After 400 cycles at 0.5 A g^-1,the reversible specific capacity is 701m Ah g^-1.The good electrochemical performance is mainly due to the strong chemical interaction between Li PSs and conductive BCN/G.2.Ion-selective covalent organic framework/graphene composite separator as a catcher for Li PSs.A novel TAPP-ETTB COF@G/PP separator with high lithium ion permeability was developed based on the covalent organic framework COF and graphene composite separator,and the mechanism of selective adsorption and catalytic conversion of Li PSs by the separator was comprehensively analyzed.The crystalline TAPP-ETTB COF’s nitrogen-rich scaffold has a regular pore geometry,which strongly adsorbs Li PSs and provides sufficient lithium-friendly sites for the catalytic conversion of Li PSs.The high electron mobility of graphene accelerates the redox kinetics of sulfur.Electrochemical tests show that the TAPP-ETTB COF@G/PP separator lithium-sulfur battery has a high reversible capacity and good cycle stability.At a current density of 0.2 A g^-1,the initial capacity is 1489.8 m Ah g^-1,and it remains at 920 m Ah g^-1after 400 cycles.When the current density increases to 2.0 A g^-1,the specific capacity is as high as 827.7 m Ah g^-1.3.Metal-coordinated covalent organic framework/graphene composite separator for synergistic adsorption and catalysis of Li PSs conversion.A new multifunctional material,Salen-COF@Ni,was synthesized by coordinating metal nickel ions on the porous material Salen-COF,and combined with graphene to prepare a high-performance lithium-sulfur battery separator.Salen-COF has strong chemical adsorption ability for Li PSs,and the loaded metal ions are efficient electrochemical catalysts for Li PSs.The high specific surface area of graphene and the rich and tunable microporous structure of the COFs framework effectively restrict Li PSs on the cathode side.Thanks to these excellent features,the Salen-COF@Ni modified separator battery shows good electrochemical performance.The specific capacity is 1173.1 m Ah g^-1after 100 cycles at a current density of 0.3 A g^-1.The specific capacity is 675.8m Ah g^-1after 300 cycles at 1.7 A g^-1.4.Au-Pt nanoclusters/graphene composite separator for efficient catalysis of Li PSs conversion.A composite material of Au-Pt nanoclusters and graphene,Au₂₄Pt（PET）₁₈@G,was designed and prepared.It was found that on the one hand,the Au₂₄Pt（PET）₁₈nanometal clusters can form a chemical bond with Li PSs to fix them,and on the other hand,Au₂₄Pt（PET）₁₈has a catalytic effect on Li PSs,accelerating the redox reaction of Li PSs.Meanwhile,the rich sulfur-friendly active sites on the surface of the Au₂₄Pt（PET）₁₈@G composite material effectively promote the formation of lithium sulfide.The prepared Au₂₄Pt（PET）₁₈@G/PP separator LSBs show excellent rate performance and cycle stability.The high reversible specific capacity of 1535.4 m Ah g^-1was obtained in the first cycle at a current density of 0.2 A g^-1,and the reversible specific capacity is 887 m Ah g^-1at 5 A g^-1.After 1000 cycles at 5 A g^-1,the capacity is558.5 m Ah g^-1,with an average capacity decay rate of only 0.041%per cycle.