Application Research Of Transition Metal-Substituted Keggin Type Phosphotungstate In Li-S Batteries And Li-Se Batteries

The rapid growth in global energy demand is driving the development of the next generation rechargeable batteries with high energy density.Polyoxometalates（POMs）have a large amount of terminal oxygen,and transition metals in the corner are extremely prone to generating vacancies that are replaced by other ions or functional groups,and thus linking to other different ligands,which can maintain the stable state of the original structure and retain the respective characteristics of multiple atoms.POMs show many advantages compared with traditional inorganic acids,such as good redox properties,excellent thermal stability,and rich surface active groups.They exhibit excellent performance in the electrochemical field.However,the function of POMs is limited by their poor conductivity,uneven dispersion,and easy aggregation.Therefore,achieving uniform and firm loading of POMs to expose more active sites as well as simultaneously improving the conductivity and specific surface area of POMs are the key for the application of POMs in the electrochemical field and energy storage system.In this paper,the transition metal-substituted Keggin type phosphotungstate is used as the research object,and further combined with metal-organic frameworks（MOFs）,multi-walled carbon nanotubes（MWCNTs）or reduced graphene oxide（r GO）to prepare POMs-based composites with excellent properties.Furthermore,the application of these composites in Li-S batteries or Li-Se batteries is investigated.The specific research works are as follows:1.The designed Mo-substituted Keggin type phosphotungstate is uniformly decorated on MWCNTs through electrostatic attraction,which can be used as the modification material of separator to improve the performance of Li-S batteries.MWCNTs with good conductivity provide abundant attachment points for POMs,which can not only ensure the uniform dispersion of POMs but also accelerate the rapid ion transport.Especially,H₃PW₁₀Mo₂O₄₀(PW₁₀Mo₂)with more Mo atoms can be used as dual-function catalyst to improve redox reaction kinetics of Li-S batteries.As a result,the assembled cells with MWCNTs/PW₁₀Mo₂modified separator present high discharge capacity,excellent rate performance,and long-life cycle stability under high current density.The capacity can still maintain at 802 m Ah g^-1under 0.5 C up to 200 cycles as well as excellent long-life cycling stability with low recession rate of 0.051%under 3 C up to 1000 cycles.2.A series of V-substituted Keggin type phosphotungstate is explored to etch MIL-88A into hollow shell structure and uniformly disperse on r GO,which is used as the sulfur host material.The moderate V atom implantation in POMs ensures the integrity of the hollow structure as well as enhances the adsorption and catalysis abilities for polysulfides.r GO with the good electrical conductivity guarantees fast electron transfer and further accelerates the redox reaction between POMs and polysulfides.Thus,the S@PW₁₁V-M88A/r GO cathode exhibits a superior specific capacity of 520 m Ah g^-1and a low capacity recession rate of0.046%for each cycle upon 500 cycles under 3 C.3.Based on the work of the second part,we design a composite with hollow shell structure by carbonization process,which contains H₄PW₁₁VO₄₀(PW₁₁V),Co Fe₂O₄,and C components.PW₁₁V can effectively catalyze the conversion reaction of polyselenides and accelerate the redox reactions kinetics of Li-Se batteries.In addition,the V atom substitution in PW₁₁V induces the increase of Li-O bonding strength,and thus expressively enhances the chemical adsorption of polyselenides,which can further promote the catalytic efficiency of PW₁₁V.The carbonized hollow shell material allows the rapid diffusion of lithium ions and electrons,meanwhile,it also effectively inhibits the volume expansion of selenium.When using the composite as the selenium host material,Li-Se batteries demonstrate superior reversible capacity of 534 m Ah g^-1under 0.2 C after 300 cycles.At a higher rate of 1 C,it can still deliver 270 m Ah g^-1with low average capacity recession rate of 0.034%during 1200cycles of charge-discharge.