Application Of FeN_x Biomimetic Catalytic System For Sulfur Conversion In Lithium-sulfur Batteries

The shuttle effect of lithium polysulfides（LiPSs）limits the commercialization of lithium-sulfur（Li-S）batteries.Introducing efficient catalysts to accelerate the reaction kinetics of the LiPSs conversion is proved to be effective in inhibiting the migration of LiPSs.In this thesis,the biomimetic catalysts were introduced into the cathode to accelerate the conversion of LiPSs.The main contents of this thesis are as follows:（1）An iron phthalocyanine（FePc）and octafluoronaphthalene（OFN）functionalized graphene（Gh/FePc+OFN）was designed and developed,and introduced into the cathode of Li-S battery as a biomimetic catalyst.The Li–S battery using the Gh/FePc+OFN catalyst based cathode exhibited the optimal initial specific capacity(1604 m Ah g^-1)and cycle durability.In addition,a series of in situ spectroscopy,X-ray photoelectron spectroscopy（XPS）combined with density functional theory（DFT）were conducted to investigate the internal sulfur conversion mechanism in Li-S batteries.The results show that the FePc acts as an efficient anchor and scissor for LiPSs through Fe-S coordination,mainly facilitate the liquid–liquid conversion of LiPSs,whereas OFN enables Li-bond interaction with LiPSs and accelerates the kinetics of the liquid–solid nucleation and growth of Li₂S.（2）A hemin and melamine（MM）functionalized carbon nanotubes（[CNTs-MM-Hemin]）were prepared by condensation reaction and introduced into the cathode of Li-S battery as a biomimetic catalyst.The Li–S batteries integrated with[CNTs-MM-Hemin]catalysts exhibit extremely excellent rate performance and cycle durability.Further investigation of the catalytic mechanism of[CNTs-MM-Hemin]indicated that the Fe N₅ moieties in Hemin are able to anchor LiPSs through Lewis acid-base interaction（Li???N bond）and coordination bond（Fe-S bond）,and significantly promot the conversion of long-chain LiPSs,whereas the MM enriches the Li⁺（or LiPSs）via multiple Li-bonds and accelerates the kinetics of the conversion of short-chain LiPSs.The stepwise catalysis and synergistic effect of Hemin and MM in[CNTs-MM-Hemin]can realize the rapid conversion/nucleation of LiPSs in an ideal manner at the cathode interface,which effectively inhibits the LiPS shuttling and significantly improves the sulfur utilization.In summry,to address the critical interface science issue,i.e.,serious shuttle effect of LiPSs,which leads to the rapid battery performance decay,this thesis proposed to design and develop biomimetic catalysts（Gh/FePc+OFN and[CNTs-MM-Hemin]）to efficiently convert LiPSs,and suppress the shuttle effect.Herein,the biomimetic enzyme（FePc and Hemin）containing Fe N_xmoietie is endowed with excellent anchoring ability and catalytic activity for long-chain LiPSs;whereas the F/N-rich organic molecules（OFN and MM）can concentrate Li⁺（or LiPSs）through Li-bonds（Li???F/N）,thereby promoting the conversion of short-chain LiPSs.On the basis,various in situ characterition techniques were adopted to comprehensively investigate the micro-structures at the cathode surface during charge and discharge processes,and a sulfur conversion reaction mechanism on the biomimetic catalyst surfaces was revealed at molecular level.The implemention of this theis is helpful to deepen the understanding of the correlations between electrode structure,battery performance and reaction mechanism,which will provide a theoretical guidance for designing efficient catalysts for LiPSs conversion and developing high-performance Li-S batteries in future.