Study Of Metal-Organic Frameworks-Conductive Polymer Based Sulfur Host For Lithium Sulfur Batteries

Metal-organic frameworks(MOFs),assembled by metal ions/clusters and polydentate organic ligands in infinite arrays,are new kind of porous crystalline materials.The topology and pore environment of MOFs could be precisely adjusted and varied through choices of specific secondery-building unit(SBU)of the metal part and ligand of the organic part.The permanent open pore structure of MOFs makes them ideally suitable for the interaction between guest molecules and the tatally exposed metal ions and organic lingker of the framework.Given these benefits,MOFs are widely explored for their potentials to be used in gas storage/separation,catalysis,drug delivery,sensors and optoelectronics.Comapered with the intense research in above applications,the study of MOFs in electrochemical energy storage is far behind.Though extremely high surface area,diverse pore environment and abduant topologies of MOFs provide perfect platforms for electrochemical reactions,but the poor electrical conducitivity of MOFs impedes the electron transfer process thus making them hardly suitable for electrochemical energy storage.We also notice the works using MOF derivated porous carbon for electrochemical energy storage through calcination at high temperature under inert atmosphere.Through this method,to some extent,could improve the electrochemcal performance,but the high energy input and the loss of crystallinity of MOFs make it hard to maximize the unique advantages of MOFs.In recent years,some conductive MOFs were also reported,but the application of these MOFs in electrochemical energy storage still remains uncovered.Besides,the conductive MOFs are only a thumbnail part of the huge MOF family and also preserved with high cost,which further impedes their practical applications.Based on this motivation,a universal method should be introduced to realize the promotion of electron transfer of MOFs for their applications in electrochemical energy storage devices.In this study,we reported a method to boost the electrical conductivity of MOFs for electrochemical application through combination of MOFs and conductive polymer(CP).In this stratge,the charge transfer process was facilitated by CP and the ion diffusion could be optimized by proper choice of the pore geometry of MOFs,thus providing fast-electrochemical reaction kinetics.Here,we used Li-S bateery as a prototype to illustrate the advantages of this method.Specifically,sulfur was firstly loaded in the pores of MOF through a melt diffusion method to yield the S-in-MOF.Then,we construct a concuctive polymer,here,polypyrrole(ppy,a low-cost conductive polymer)on the surface of MOF crystals to get the final ppy-S-in-MOF construct.Characterizations on crystallinity,morphology and conductivity revealed that the ppy-S-in-MOF constructs preserved well-maintained crystallinity and pososity,resembling that of the parental MOFs.Meanwhile,high electrical conductivity was achieved due to the integration of conductive ppy.We used these ppy-S-in-MOF constructs in the cathode of Li-S batteries to illustrate the advantages of the composites which preserved well-maintained polarity,high porosity and conductivity.Our research results are as follows:(1)MOFs with distinct pore geomoteries were shnthesized and composited with conductivie polymer in different configurations,such as inner growth,inner loading and outer surface wrapping of CP.We used tha CP wrapped MOF as host for proper containment of sulfur in Li-S battery by construction of ppy-S-in-MOF composites.(2)We evaluated the ppy-S-in-MOF constructs through a series of characterization and confirmed the well-maintained crystallinity and high conductivity of these composites.Electrochemical performance evaluated at low and high current rates proved the excellent polysulfides confinement of the ppy-MOF sulfur host which delived high capacity retention after long-term cyclying test.Compared with sulfur hosts from pristine MOF,ppy and porous carbon,these ppy-MOF sulfur hosts show obvious higher electrochemical performance.The signicant gain in capacity and stability of the ppy-MOF sulfur hosts benefited from the well-mentained polarity,porosity and conductivity of these materials.(3)The impact of MOF particle size and sulfur loading amount were charefully evaluated in ppy-S-in-MOF construct and the corresponding S-in-MOF.After ppy integration,the ppy-S-in-MOF composite show neglectable performance decay when the particle size and sulfur loading amount increased from 50%to 80%,ascribing to favoured ion and electron transfer in the ppy-MOF construct.High specific capacity retention of～910 mAh g-1 were achevied at 1.0 C in ppy-S-in-MIL-53 after 100 cycles,exceeding all MOF based Li-S batteries and equaling the best carbon-based Li-S batteries reported to date.(4)In situ PXRD unit was made to evaluate the phase change information of ppy-S-in-MOF in charge and discharge process.We clearly observed the reversible changes of certain peaks assingn to MOF in the charge-discharge process and transformation of crystalline sulfur to noncrystalline sulfur(molecules or oligomers).These in-situ PXRD results revealed that the crystalline framework could adjust to the electrochemical process for the charged and discharged products.And besides,the small sulfur molecular lay the foundation for the high rate performance.