Study On Ionic Liquids/Fluorinated Ether Functional Electrolytes For Lithium-Sulfur Batteries

High energy density is one of the most significant advantages of lithium-sulfur batteries compared with other commercial secondary batteries.At the same time,lithium-sulfur batteries have the advantages of low cost of raw materials and environmental friendliness,which have great development and application prospects.However,at present,there are some problems that greatly restrict its commercial application,such as low utilization of active materials,poor cycle stability,low coulomb effi,ciency and serious self-discharge.The main reason is the "shuttle effect" formed by lithium polysulfides during the charge/discharge processes.As the dissolution,diffusion and shuttle of polysulfides mainly occur in the electrolyte,optimizing the composition of the electrolyte naturally becomes an effective way to improve the performance of lithium-sulfur batteries.In this paper,a series of new functional electrolytes for lithium-sulfur batteries were prepared by using Lithium bis(trifluoromethanesulphonyl)imide(LiTFSI)as lithium salt and using ionic liquids&organic ethers co-solvent.The physical and chemical properties of various electrolyte systems and the electrochemical performance of batteries were systematically studied.The dissolution and shuttle inhibition mechanism of polysulfides and the action mechanism of different solvents were preliminarily explored.The following phases of results and progress have been achieved:(1)The effects of different proportion of fluorinated ether TTE on the physical and chemical properties of imidazole-type ionic liquids(EMITFSI)based electrolytes and the electrochemical properties of batteries:the addition of TTE increases the ionic conductivity and has a positive impact on the ionic migration in the electrolyte.EMITFSI/TTE system shows good flame retardancy,wide potential window and good electrochemical stability.Batteries containing TTE electrolyte have higher reversible capacity,better cycle and rate performance than batteries without TTE electrolyte.Insufficient or excessive fluorinated ether is not conducive to battery performance.When TTE is added at 50 wt%(ET50),the performance of the battery is the best.The specific discharge capacity of the first cycle is 902 mAh g-1,and there is still a high specific capacity(505 mAh-1)after 50 cycles.Fluorinated ether can help to form a dense passivation protective film on the surface of lithium and prevent the continuous corrosion of lithium by polysulfides.(2)Introducing etherified ionic liquids(DEMETFSI)with ether groups,the effects of different solvent composition on the physical and chemical properties of etherified ionic liquids-based electrolytes and the electrochemical performance of batteries are studied.When TTE and DOL are used as co-solvents of ionic liquids at the same time,they are more conducive to the formation of a relatively stable and low-resistant interface layer for Li ion migration on lithium metal surfaces.The cycle and rate performance also reach the best level.For DDT system,the specific discharge capacity of the first cycle is 1164 mAh g-1,and the capacity retention rate is 78%after 100 cycles.The rate test further shows that the specific capacity of DDT is always higher than that of other electrolytes at each rate.After several charge-discharge cycles,the SEI on the surface of lithium anode is complete with lower surface roughness.(3)The dissolution and shuttle inhibition mechanism of polysulfides and the action mechanism of different solvents were preliminarily explored are studied.The results of UV-Vis spectroscopy indicate that the overall order of solubility of polysulfide in etherified ionic liquid-based electrolyte is DD>DDT>PD>DT.Analysis of XPS further confirms that fluorinated ether participates in the formation of film on the surface of lithium negative electrode,and forms a stable,low resistance,LiF-rich SEI film on the surface of lithium.It can effectively prevent further decomposition of electrolyte and continuous corrosion of lithium anode.Benefiting from the synergistic effect of co-solvents,a benign balance is established in dissolution,diffusion and contact reaction,which effectively prevents the shuttle effect.