The Control And Mechanism Study On Polysulfides Transformation Of Carbon-Supported Metal-Based Materials

Lithium-sulfur battery(Li-S batteries)is attractive for its advantageous such as high theoretical capacity,low cost and environmentally friendly,which is considered as one of the most promising next-generation rechargeable battery systems.However,the volume expansion of sulfur result in the detachment of active materials,polysulfides generated at the cathodes shuttle towards the anodes,leading to the rapid capacity degradation.The other hand,low conductivity of sulfur and Li2S is difficult to transform Li and S,result in the sluggish reaction kinetics and poor cycle life.To address the above problems,this paper designs multifunction electrodes/interlayer materials with adsorption and electrocatalytic activity by controlling the pore structures and catalytic activity,restraining the shuttle effect as well as buffering the volume expansion effect,and further improves the conductivity and enhances the reaction kinetics of polysulfides catalytic conversion to obtain a high performance and practical Li-S batteries,and fundamentally explores the mechanism of the polysulfide conversion.To resolve the shuttle effect of polysulfides and buffer the volume expansion effect of sulfur,sandwich-like hierarchical reduced graphene oxide doping core-shell Fe2O3 microspheres(RGO/Fe2O3)was prepared as electrodes.Core-shell Fe2O3 with stable structure and large surface area(121.8 m2 g-1)suppresses the shuttling of LiPSs by chemical bonding instead of merely physical adsorption,the meantime exhibits the electrocatalytic effect on the transformation of intermediate polysulfides to lower order polysulfides.promoting the conversion in both discharging and charging period;highly conductive the RGO with multi-lamellar structure promotes the rapid electron/ion transfer;RGO and Fe2O3 support each other,reducing the volume expansion,enhancing the cycling stability with the enhanced discharge capacity from 476.7 mAh g-1 to 607.7 mA h g-1 at 1 C after 100 cycles in Li-S batteries,with the 0.32%decay per cycle.To further improve the catalytic activity of materails,accelerate the conversion of Li2S to form Li and S,the paper introduces the carbon membranes as interlayer with catalytic activity,and expands the sulfur storage space.Pt@C is used as catalyst,which is dispersed in PAN casting solution amd obtain the Pt/CM interlayer by phase inversion.Pt nanoparticles dispersed in membranes catalyst Li2S to form Li and S,increasing the electrochemical reaction kinetics.Pt@C slowes down the accumulation of PAN during phase inversion and increase the porosity of carbon membranes from 44.7%to 88.5%.The abundant pore structures buffer the volume expansion effect and provide the space for polysulfides reservation;interconnected networks enhance the electron transport.The N in PAN form pyridine N,pyrrole N and graphene N in carbon membranes,the binding energy of pyridine N,pyrrole N and graphene N is-1.541 eV,-1.605 eV and-1.459 eV respectively,providing the efficient adsorption for polysulfides and suppressing the shuttle effect.The electrochemical performance was improved with the capacity of 691.7 mAh g-1 at 1 C after 100 cycles significantly,decay per cycle is 0.22%.To improve catalytic activity of Pt material,a Pt@Ni core-shell bimetallic catalyst is prepared in porous carbon spheres,and the mechanism of synergic catalysis is first proposed based on DFT:The interface of Pt and Ni core-shell catalysts can facilitates the charge transfer between the two metals,the bimetallic catalysts offer a synergic effect on catalyzing reactions for catalytic oxidation of insoluble polysulfides to Li and S.The electronic migration from Ni to Pt becomes a driving force for Ni activating Li2S2/Li2S molecules(decomposition energy barrier of Li2S on Pt and PtNi is 0.486 eV and 0.101 eV)by largely reduce the reaction energy barrier(Pt:5.19 eV,PtNi:4.34 eV),enhance the reaction kinetics and cycle stability of Li-S batteries.At the same time,bimetallic catalysts show the strong adsorption of LiPSs,suppress the shuttle effect.The capacity of batteries is 654.1 mAh g-1 at 1 C after 100 cycles,and maintains 600.0 mAh g-1 after 300 cycles with 0.18%decay per cycle?To avoid the use of current collector inactive materials,further improve the active material mass and enhance the electronic transfer in the electrode,a flexible asymmetric porous carbon membrane doped with iron carbide(FexC)nanoparticles(partical size is 30 nm)is designed by a facile phase-inversion method,realizing the integrated electrode.The dense conductive lower layer can replace A1 foil as a current collector,which improves active material mass of S,and enhances electronic transfer in the electrode and overall conductivity of the electrode.The unique triple-layer-structured pores of multifunctional membrane(pore size is 3.9 nm,porosity is 81.2%)relieve the volume expansion and improve the S loading(4.6 mg cm-2 high sulfur loading).The high adsorption of FexC promote their gradual diffusion into the porous carbon membrane on the opposite side of the electrolyte,suppressing the shuttle effect,the high electrical conductivity effectively promote entrapped polysulfide redox kinetics and fast electron transfer.The areal capacity of batteries is 2.6 mAh cm-2 and the capacity is 879.8 mAh g-1 at 1 C after 100 cycles with 0.11%decay per cycle.The flexible preparation method offers a new strategy to achieve smooth trapping-diffusion-conversion of polysulfides for large-scale processes toward high-performance Li-S batteries.