Design Of Fe/Co-based Oxygen Compound Heterojunction And Its Application In Lithium-sulfur Batteries Separator Modification

Vigorously promoting new energy is an effective strategy to achieve the goal of carbon neutrality.However,there are many kinds of new energy,and the production of this type of energy is limited by time and space.This makes it difficult for the existing regulation system based on traditional energy to match the energy regulation mode of new energy,and the use of electrochemical reaction to regulate electric energy is considered to be a very effective way.Among the existing electrochemical energy storage batteries,lithium-sulfur batteries(LSBs)are considered to be one of the most promising energy storage methods with its high theoretical specific capacity(1675 mAh g-1)and rich sulfur resources.However,the low conductivity of charge and discharge products S8 and Li2S,the shuttle effect of lithium polysulfides(LiPSs,Li2Sn,4≤n≤8)and the slow reaction kinetics seriously hinder the development of LSBs.In view of the scientific problems above,this paper uses Fe-Co metal oxide group compound to construct multiple heterojunction structure,and composite carbon material modified polypropylene separator(PP)for LSBs to achieve the "captureadsorption-catalysis" synergistic effect.The above three-in-one functional separator modification layer can effectively inhibit the shuttle effect of LiPSs and accelerate the transformation of LiPSs,so as to fully release the capacity of LSBs.The main contents of the paper are as follows:(1)In order to improve the long-cycle stability of LSBs,a three-in-one heterojunction is prepared based on the different ways of suppressing the LiPSs shuttle effect in LSBs by Fe7S8,CoS2 and rGO respectively.Fe7S8/CoS2 heterostructure particles are constructed on the surface of N-doped graphene(NG)by hydrothermal method and then calcined at high temperature,and then modified on the LSBs separator(Fe7S8/CoS2/NG-PP).Firstly,the lamellate structure of NG enhances the domain limiting ability of LiPSs and improves the electrolyte infiltration on the membrane surface and provides strong non-polar and weak polar adsorption(N-doped).Secondly,Fe7S8 showes strong polar adsorption,which could effectively bind LiPSs.Thirdly,CoS2 promotes the rapid transformation of LiPSs.Fourthly,the constructed Fe7S8/CoS2/NG heterojunction exhibits strong "capture-adsorption-catalysis" function.Therefore,the LSBs based on the heterojunction modified separator has a high capacity retention rate of 71.7%(500 cycles)at the current density of 1 A g-1,and a low attenuation rate of 0.057%per cycle.(2)In addition,Fe-Co metal oxides have excellent adsorption capacity,while FeCo metal sulfide has strong catalytic effect.Therefore,in this study,by controlling the ratio of CH4N2O and CH4N2S,a heterojunction(FeCo(OH)x/FeCo(SH)y/CNT)of FeCo hydroxide complex hydrogen sulfide growing on CNT is constructed by hydrothermal method.A heterojunction of Fe-Co oxide sulfide composite carbon nanotubes(FeCoOxSy/CNT)is prepared by carbonization.With the introduction of S element,the original ordered atomic arrangement is broken,and the microstructure of the heterojunction gradually changed from lamellar structure to granular structure.The results show that the intermediate FeCoO1.3S1.2/CNT heterojunction material has better"capture-adsorption-catalysis" characteristics.The capacity retention rate of LSBs based on FeCoO1.3S1.2/CNT-PP separator is 70.4%after 100 cycles at the current density of 0.2 A g-1.(3)In this study,isoelectronic heterojunction is constructed by controlling the degree of sulfuration,thus maximizing the adsorption-catalytic effect on LiPSs.Firstly,FeCo-LDH/CNT is grown on the surface of CNT by co-precipitation method,and then CH4N2S is used as sulfur source.2D/0D-1D FeCoOxSy/NC multiple isoelectronic heterojunction is constructed by gas phase method.The results show that a 3D LiPSs trap can be constructed by combining 2D/0D layer and particle FeCoOxSy with 1D CNT.Among them,interconnected CNTs have good electrical conductivity and can promote the transformation of LiPSs.However,2D/0D FeCoOxSy heterojunction lamellar layer has higher specific surface area and stronger polar adsorption,which can enhance the capture and adsorption effect of LiPSs.Therefore,the capacity retention rate of LSBs based on FeCoO1.5S1.0/NC-PP is 81.4%after 100 cycles at the current density of 0.2 A g-1.In-situ UV,in-situ Raman and in-situ XRD are also used to further verify the adsorption catalytic effect of isoelectronic heterojunction.