Research On The Crucial Role Of Lithium Polysulfides In Lithium Sulfur Battries

Human beings have been lived in a great historic era of mobile Internetsince stride into the 21 st century.There is a pressing demand forhigher energy density of battery than before with the energy consumption of mobile IT equipment?cell phones,computers,digital cameras,wearable devices,etc.?is increasing;as well as the global extension of new energy vehicles andthe large-scale application of energy storage system.Lithium sulfur batteries are considered a very promising system for the next generation of energy storage because of their high theoretical energy density(2600 Wh kg^-1),which is expected to play an important role on solve theshortage problem of energy shortage.Lithium sulfur batteries use sulfur elemental as the cathode and the anode using lithium metal,employing ethers as the electrolyte.The battery system has been proposed since the 1970 s and caused the attention of the academic height.At present most of the research work mainly focus on the following three aspects: firstly,how to scientifically design the electrical conductivity of the composite material to improve the positive and inherit the volume change of sulfur because the sulfur elemental intrinsic properties as electrical insulation,andprone to volume expansion;secondly,while the battery is goingon the process of charging and discharging,sulfur elemental?Li₂S_n,1?n?8?will undergo the transformation process of the solidphase to liquid phase and to solid phase again when it changes to polysulfide.There will be a part polysulfide dissolve into the electrolyte,leading the irreversible damage of the battery capacity.Moreover,higher-order polysulfide?Li₂S_n,4?n?8?can cause side effects in the anode with lithium metal,along with the electrolyte are transported back to the anode,resulting in serious of "shuttle effects ".Therefore,how to inhibit the dissolution of polysulfide and "shuttle effect" are key research.Thirdly,dendrite growthwill accord on the lithium metal anode with the cycle of a cell,part of the dendrites can apart from the rootto form "dead lithium ",and some dendritesmay even piercethe separatorscausing short circuit of the batteries.Therefore,how to give a scientific protection of lithium anode correctly,inhibit lithium dendrites have been gradually paidenough attention and research.Polysulfide plays an important role in lithium sulfur batteries,is also the key factor realizethe industrialization and commercialization oflithium sulfur batteries.Thus giving a correct information and better understanding of the role played byploysulfide in lithium sulfur batteries own the positive significance.Research emphasis of this paper is to explore the key role of polysulfide in the lithium sulfur battery,the research work mainly focuses on three aspects including the design of composite porous structure to host sulfur,the design of flexible compositei nterlayer materials to adsorption polysulfide and the stability of the solid-liquid interface layer between lithium metal and polysulfide,aiming to understand the characteristics of polysulfide.On the aspect about design of porous composite materials,lotus leaf biomass materials were used as carbon source,the hierarchical structure of nano carbon skeletonwas prepared with high specific surface area(572.1 m2 g^-1)and the average pore size distribution in 3.31 nm by employing the method of multistage temperature carbonization under argon atmosphere;The skeleton carbon materials with sulfur elementalwill improve the poor conductivity of the cathode materials by taking advantage of the high electronic conductivity characteristics,and the hierarchical porous materials can provide good reaction sites for sulfur transformation,inhibitthe secession of polysulfide from the cathode skeleton,reduce the irreversible lossof the battery capacity.Under the composite material system,the initial capacity can reach 1246 mAh g^-1 at 0.1 C and discharge specific capacity can retain 600 mAh g^-1 in battery capacity even after 100 cycles,XRD and EDS characterization results show that sulphur and polysulfide produced by charging and discharging process can be host on the carbon skeleton.On the aspect of the flexible composite interlayer materials,which were prepared with carbonization lotus leaf,60 wt% of polytetrafluoroethylene?PTFE?emulsion andketjen black byhigh energy ball milling method.The flexible interlayer was placed between cathode and separator.The composite materialsown multi-stage pore structure and high electron conductivity,high lithium ion conductivity connectivity features at the same time,can provide the further three-phase buffer reaction sites about reduction of high order polysufide,making use of chemical adsorption to preventpolysulfide toward the anode electrode.The high rate performance and coulomb efficiency were improved in lithium sulfur batteries with the interlayer materials,the initial discharge capacity is 1350 mAh g^-1at 1 C and still maintaina capacity of 960 mAh g^-1after100 cycles,cell average coulomb efficiency can reach 95% on the absence of lithium nitrate additives,showing the good cycle stability.On the aspect of exploring the connection and stability about polysulfide and the solid electrolyte interface layer on lithium metal,we useddual-salt additives electrolyte made of different concentrations of polysulfide and lithium nitrate.The result shows found that 0.02 M Li2S5-5.0 wt % LiNO3 system can form a more stable SEI filmthan other system in the solid liquid interface?SEI?membrane stability tests,the cycle number in Li-Cu half cell can up to 233 times,neither too high nor too low concentration of polysuifide on the anode surface can't form a stable SEI.This provides a quantitative reference for us in the design of the cathode material structure,also offers a new insight of designing materials scientificly to control the suitable concentration of polysulfide contact with lithium metal anode.