Study On The Influences Of Fluorinated-Ether Based Electrolyte On Electrochemical Performance Of Li-S Battery

At present,the energy density of lithium-ion batteries has been difficult to meet the growing needs of electric vehicles,hybrid vehicles and next-generation mobile electronic devices.Lithium-sulfur batteries are considered as one of the most promising next generation battery technologies,since elemental sulfur has very high theoretical capacity of 1672 mAh g-1.However,the "shuttle phenomenon" caused by the dissolution of polysulfides(Li2Sn,n=4?8)in the lithium-sulfur battery and the growth of lithium dendrites in the negative electrode constrain the development of lithium-sulfur batteries.In recent years,various efforts have been undertaken to address the above challenges,including confining sulfur within porous carbon to trap lithium polysulfides within the cathode,applying new binders,adding functional interlayer,coating the lithium anode with protective layer and modifying the electrolyte to inhibit the dissolution and shuttle of lithium polysulfides.In this paper,a fluorinated electrolyte consisting of fluorinated ether 1,1,2,2-Tetrafluoroethyl-2,2,3,3-Tetrafluoropropylether(F-EPE)and 1,2-Dimethoxyethane(DME),and high-concentration electrolyte(HCE)are evaluated as new electrolytes for Li-S batteries.F-EPE/DME based electrolyte and high-concentration electrolyte can effectively suppress the dissolution of polysulfides in the electrolyte and thus mitigate the shuttle effect.The coulombic efficiency and capacity retention of Li-S batteries are significantly improved with the use of F-EPE/DME based electrolyte and high-concentration electrolyte combing with PEO as binder.An electrolyte containing fluorinated ether F-EPE as cosolvent combining with PEO as binder results in high cycling efficiencies and outstanding cycling stability in Li-S battery even without the addition of widely applied LiNO3 additive,as compared to widely used 1.0 M LiTFSI DOL/DME electrolyte with LiNO3 additive.Compared with conventional 1.0 M LiTFSI DOL/DME electrolyte,the capacity retentions of the Li-S cells with 1.0 M LiTFSI F-EPE/DME electrolyte are significantly increased from 67%to 87%after 50 cycles.In addition,higher coulombic efficiency of?96%was obtained for the cells with 1.0 M LiTFSI F-EPE/DME electrolyte,compared with the low efficiency of?86%for the cells with 1.0 M LiTFSI DOL/DME electrolyte.The function mechanism of F-EPE as cosolvent was investigated by self discharge test,polysulfide dissolution experiments,SEM and UV-vis spectra.The results indicate that the less solubility of polysulfides in F-EPE/DME based electrolytes suppresses the lithium polysulfide shuttle effect during cycling.In the meantime,F-EPE involves in the formation of a dense SEI layer on the lithium metal anode,which prevents the corrosion of lithium metal with polysulfides during cycling.Based on the above research works,the effect of high concentration electrolyte on the electrochemical performance of lithium-sulfur battery is further investigated.The function mechanisms of HFE are preliminarily studied.The results show that,based on the F-EPE/DME electrolyte,increasing the lithium salt concentration to 3 M can further reduce the solubility of polysulfides,inhibit the "shuttle effect",and improve the electrochemical performance of lithium-sulfur batteries.Lithium-sulfur batteries with 3 M LiTFSI F-EPE/DME electrolyte exhibit excellent cycling stability and high coulombic efficiency.After 56 cycles at 0.1 C current density,the discharge capacity of Li-S battery is still as high as 1028 mAh g-1,and the coulombic efficiency is maintained above 97%for all the cycles.Solubility tests and UV-vis spectra results show that the solubility of lithium polysulfides in 3 M LiTFSI F-EPE/DME electrolyte is lower than 1 M LiTFSI F-EPE/DME electrolyte.The results of SEM and EDS show that a dense SEI layer is formed on the surface of the lithium metal electrode of the battery with high concentration electrolyte,and the SEI layer shows higher fluorine content.