Design And Optimization Of Viologen-Keggin Heteropolyoxometalate Composites And Their Application In Modification Separator Of Lithium-Sulfur Batteries

Lithium-sulfur（Li-S）batteries have many advantages such as high theoretical energy density,low cost and high safety,which are considered as one of the most promising next-generation high-performance electrochemical energy storage devices.However,slow reaction kinetics and the shuttle effect of lithium polysulfides（Li PSs）result in low capacity and poor cycling life of Li-S batteries,hindering their commercialization.The use of functionalized separator modified with catalytic materials in Li-S batteries can not only effectively prevent the shuttle effect of polysulfides between the cathode and anode,but also greatly promote the conversion reaction of Li PSs.Polyoxometalates（POMs）are a type of anionic metal cluster,which have good reversible redox activity,semiconductor-like properties and structural stability.Meanwhile,POMs as catalytic materials show the great potential in modifying separators of Li-S batteries.However,the poor conductivity and easy agglomeration of POMs limit their application in Li-S batteries.Therefore,this paper selects viologen with excellent redox property as the research object,which is combined with POMs through hydrogen bonding,aiming to design and synthesize viologen-POM based organic-inorganic hybrid materials as well as thoroughly investigate their working mechanism in adsorbing and catalyzing polysulfides.The specific work is as follows:1.Methyl viologen（MV）,ethyl viologen（EV）and aminopropyl viologen（AV）are selected for separator modification of Li-S batteries.Electrochemical tests and theoretical calculations demonstrate that AV has excellent lithium ion transport ability and high redox activity,meanwhile,it can effectively adsorb Li PSs as a cation adsorption site.Subsequently,viologen-POM hybrid materials are synthesized through hydrogen bonding using a solvothermal method.The experimental results confirm that AV can improve the redox kinetics of POM,and thus promote the application of POM-based materials in Li-S batteries.2.AV with the best electrochemical performance is selected to combine with K₃[PW₁₂O₄₀](PW₁₂)through hydrogen bonding to obtain AV-PW₁₂composites.PW₁₂can capture Li PSs by forming Li-O bonds,and AV transfers the obtained electrons to PW₁₂rapidly through intermolecular hydrogen bonding,thus ensuring higher electrical conductivity and catalytic activity of PW₁₂.When AV-PW₁₂modified separator is applied to Li-S batteries,a high initial discharge capacity of 1596 mAh g^-1is achieved at 0.5 C.Moreover,a low capacity decay rate of 0.041%is delivered after 1000 cycles at a high current density of 10 C.3.In order to achieve the application of POMs in high energy density Li-S batteries,an AV-PMo₁₂crystalline material with precise structure is designed and synthesized.AV-PMo₁₂with a three-dimensional pore structure accelerates the transfer of lithium ions,meanwhile,it can also effectively hinder the shuttle of polysulfides through physical confinement and chemical adsorption.In addition,AV and PMo₁₂are firmly combined through hydrogen bonding,which improves the structural stability of PMo₁₂in the electrolyte and reaction process,achieving strong adsorption and efficient catalysis of PMo₁₂for Li PSs.When AV-PMo₁₂modified separator is applied to Li-S batteries,a high initial discharge capacity of 1621mAh g^-1is achieved at 0.5 C,and the low capacity decay rates of 0.048%and 0.043%are obtained after 1000 cycles at high current densities of 2 C and 5 C,respectively.