Study On The Design And Mechanism Of Sulfur Host Materials For High Performance Lithium-Sulfur Batteries

Lithium-sulfur?Li-S?batteries have great competition among many new battery energy storage systems.In order to meet the requirements of commercialization,however,many challenges need to be conquered,such as conductivity,shuttle effect of intermediate active species and safety.Therefore,the reasonable design of sulfur host materials is significant,but challenging.Only by deeply studying the mechanism in the charge and discharge process as well as the effect of intrinsic properties of materials on battery performance,and revealing the properties of host materials,can we provide better theoretical support and ideas for the design of materials,so that the high-performance Li-S battery is obtained.This paper aims to study the mechanism of non-porous and porous sulfur host materials in the battery system,while non-porous materials mainly discuss the structural change of non-active and active hosts as well as their effects on battery performance,including the following three aspects:?1?Molybdenum,for the first time,was used as a new type of sulfur host for lithium-sulfur batteries.Further analysis using in situ X-ray diffraction,DFT calculations and electrochemical methods,it demonstrated that the structure of Mo did not change in charge and discharge process.In addition,the surface of metal Mo showed high catalytic activity of reducing lithium polysulfide and decomposing Li₂S.A Mo-S composite,with 80 wt%sulfur loading,delivered reversible capacities of 1003 mAh g^-1 after 130 cycles at 0.1 C,and 786 mAh g^-1 after 100 cycles at 5.0 C.The results show that the host materials without pore structure and lithium storage still possess the good electrochemical performance.?2?At present,most work aim to design rational materials for sulfur host and have research on the interaction between host materials and intermediate polysulfides.There is one critical,but often neglected,phenomenon that some hosts themselves can be lithiated-delithiated during the voltage range of Li-S batteries,and their structures change in the charge-discharge process.It is the lithiated or partial lithiated host materials that interact with polysulfides.Here,we chose the transition metal dechalcogenides which can be lithiated by themselves and has no pore structure,TiS₂as sulfur host.The results of in situ X-ray diffraction revealed that in the discharge process,lithium ions first react with TiS₂ to form Li_xTiS₂?0<x<1?.Then the lithiated process of TiS₂ and formation of polysulfides occur at the same time.The discharge products are LiTiS₂ and Li₂S.While the host,LiTiS₂,is converted to Li_xTiS₂?0<x<1?,not TiS₂ during charge process.It results from the partial retention of lithium ions in the bulk of TiS₂.These results illustrated that the host material is dynamically changed during charge-discharge process.Hence,it is Li_xTiS₂?0<x?1?,rather than TiS₂,that effectively traps polysulfides and catalytically decomposes Li₂S.Electrochemical performance shows that the TiS₂-S composite electrode exhibited excellent electrochemical performance,delivering the reversible capacities of 886 and613 mAh g^-1 at 1.0 C up to 200 and 1000 cycles,respectively.Here,for the first time,we explore the dynamic structural change of sulfur host materials and their influence on battery performance is revealed by DFT calculations and electrochemical analysis,which provides a new understanding for the intrinsic properties of host materials.?3?Porous materials are widely studied in the development of Li-S batteries.Many work has studied the effect of pore size and pore volume on battery performance.While the influence of pore geometry is still unknown.Therefore,metal-organic framework?MOF?with designable and controllable pore structure was chosen as sulfur host materials.In virtue of electrostatic interaction,transition metal dechalcogenides?TMDs?will encapsulate on the surface of MOF to form TMDs-MOF composites.The coating effect of NbS₂ and PCN-224 is ideal,and NbS₂-70S-PCN-224 electrode exhibits the outstanding battery performance.It remained 330 mAh g^-1 after 550 cycles at 8.0C and capacity retention is 92%.The capacity decay rate is 0.014%.The results show that the electrochemical performance of MOF-S composites can be improved by constructing a polar and conductive interface layer in MOF.Inspired by NbS₂-PCN-224 composite,the conductive polypyrrole was chosen to encapsulate MOF,which constructed a polar and conductive interface layer on the surface of each MOF particle.And the conductivity of composites improved dramatically,increasing from 1.2×10^-8 S m^-1 to6.7 S m^-1.At 10.0C,the initial capacity of ppy-S-in-PCN-224 is 680 mAh g^-1 and the capacity retention is 99%,94%,88%,74%and 65%after 200,400,600,800 and 1000,respectively.By the design of MOF-based composites,the conductivity,polarity and porosity of sulfur host materials will influence the battery performance.It can be found that PCN-224 based composites electrode is more suitable for high-rate conditions,which owes to the fact that the mesoporous PCN-224 with cross-linked pore and tunnel is more favorable for ion diffusion.For the first time,this work discussed the effect of pore geometry on the electrochemical performance.It is a new benchmark in the field of MOF-based composites for sulfur host materials.The innovation of this paper is as follows:?1?the metal molybdenum powder is used as sulfur host material of Li-S battery for the first time,which expands the research direction of Li-S battery;?2?By constructing the catalytic interface between host materials and sulfur as well as lithium sulfide,the dynamic structural change of host materials and their influence on the performance of Li-S battery was systematically studied by in situ characterization technique;?3?the effect of pore structure on the performance of Li-S battery is studied by constructing a conductive reaction interface on the surface of MOF.The relationship between the pore geometry and rate performance is revealed for the first time.