Study On Inhibition Of Shuttle Effect And Cycling Stability Of Lithium-sulfur Batteries

Lithium sulfur(Li-S)battery shows high development prospect due to its high theoretical energy density which far exceeds the main lithium-ion battery and the abundant,nontoxic and low cost of sulfur.However,there are some challenges which to their commercial application,such as the insulation of sulfur and lithium sulfide,the shuttle effect of lithium polysulfides(LiPSs)and the growth of lithium dendrites.Among them,the shuttle effect leads to the loss of active materials and the corrosion of lithium anode,which seriously shortens the cycle life of the battery.Herein,the dissolution and shuttle effect of Li PSs are inhibited by designing sulfur host structure and component,separator modification layer and tuning electrolyte structure.The conversion process of Li PSs on polar materials and the mechanism are studied.The effect of solvate structure on the discharging mechanism is also investigated.The spherical hollow structure manganese dioxide(MnO₂)is synthesized by template method and serves as the sulfur host.Polymeric multilayers coating S@MnO₂ material(S@MnO₂@PM)is further achieved by the hydrogen bonding between polymer to improve the cycle stability and rate performance of the cell.Statical adsorption experiment and the X-ray photoelectron spectroscopy(XPS)test indicate that MnO₂ can react with Li PSs via redox reaction to suppress the irreversible consumption and shuttle effect of Li PSs.The polymeric multilayers film can further anchor the Li PSs and promote the transmission of Li⁺,improving the rate performance and cycle stability.After 1000 cycles,the S@MnO₂@PM cathode can still display a capacity of 481mAhg^-1 at 0.2C,corresponding to 74.2% of the initial capacity and an average decay of 0.0258% per cycle.Ketjen black/molybdenum carbide(KB/Mo₂C)composite is prepared by high-temperature carbonization process.The static adsorption experiment,XPS test and electrochemical testing method are carried out to study the effect of Mo₂C on Li PSs.The Mo₂C shows strong chemisorption for Li PSs and can accelerate the conversion of Li PSs,inhibiting the shuttle effect.Li-S battery,with KB/Mo₂C modified separator and the sulfur loading to 6.5mg cm^-2,delivers a high capacity of 5.2mAh cm^-2 at 0.1C after 60 cycles.XAS test shows that the electron transfer between Mo and S occurs during the charge/discharge process,and the complex S-Mo-C compound forms on the Mo₂C surface.The localized high concentration electrolyte(LHCE)is achieved by adding an inert diluent 1,1,2,2-Tetrafluoroethyl 2,2,3,3-Tetrafluoropropyl Ether(TTE)into the high concentration electrolyte(HCE).Its characteristic and the influence on the discharge mechanism of sulfur active materials is studied.It is indicated that the activity of the free solvent in the electrolyte decreases with the increasing lithium salt concentration.After being diluted by the inert diluent TTE,the ion solvate structure remains its original state.The LHCE exhibits high ionic conductivity,low solubility of Li PSs and good nonflammability.The conversion kinetics of the sulfur in this electrolyte becomes slow,reducing the formation of Li PSs.In addition,the electrolyte supports stable SEI film on the Li anode,thereby effectively inhibiting the shuttle effect.At 60?and the rate of 1C,the Li-S battery using this electrolyte could still deliver specific capacity of 598mAhg^-1 after 200 cycles.A carbon fibers-sewed high sulfur loading cathode is explored.It is indicated that pulverization of active materials could be prevented by adding 10%carbon fibers.The electrode with sulfur loading of more than 10mg cm^-2 could be successfully achieved.The interwoven carbon fibers in the high loading electrode ensure rich electron transport network.This method shows good feasibility for different cathode components and binders.The lithium anode is an important factor restricting the cycle life of the high loading Li-S batteries.By building stable Li F film on Li anode surface,the Li-S battery with sulfur loading of 10mg cm^-2 could still demonstrate a capacity of more than 7mAh cm^-2 after 96 cycles.