The Study Of Fluorine Additive In Electrolyte For Lithium Sulfur Batteries

Lithium-sulfur(Li-S)batteries have been considered as the most promising candidates for the next generation of batteries,due to sulfur’s high specific energy density(2600 Wh kg-1),high theoretical capacity(1675 mAh g-1),affordably low cost,effectiveness and non-toxicity.However,progress of Li-S batteries have been limited due to unsolvable issues,that is,the low electrical conductivity of sulfur and the products from its discharge process,the diffusion of polysulfide species in organic electrolytes and its high solubility,and thus side effect reactions named "shuttle effect" and the lithium metal interface issues,not to mention cell’s volumetric expansion caused by sulfur during lithium intercalation.Therefore,improving the conductivity of insulating sulfur,preparation for volume expansion,limiting polysulfide diffusion,and confining the soluble long chained polysulfide within the cathode side of the battery are the key points to be able to industrialize Li-S batteries.Among the issues,the shuttle effect and the interface problem attract the most attentions.Functional additives in electrolyte play a critical role for improving the performance of LI-S BATTERIESLi-S batteries through the alleviation of the shuttle effect.Herein,we study the functions of perfluorooctanoic acid(PFOA)additives in 1.0 M lithium bis-(trifluoromethane sulfonyl imide)in DME:DOL=1:1 Vol%(LiTFSI)electrolyte to Li-S batteriesLI-S BATTERIES.Compared different content of PFOA in LiTFSI electrolyte,the PFOA additive enhances the specific capacity and cycle stability of Li-S batteriesLI-S BATTERIES,specifically,2 wt%of PFOA/LiTFSI,which display the best performance in Li-S batteriesLI-S BATTERIES In addition,we also compare PFOA/LiTFSI with other additives,such as C11H5F15O2,C8H3F15O,LiF and LiNO3.The EIS test and SEM morphology analysis of a charged lithium anode from a cell tested with 2 wt%of PFOA/LiTFSI showed PFOA has definitely created passivating layer on top of the lithium metal anode.The result indicates 2 wt%of PFOA/LiTFSI have the best effect on the specific capacity and cycle stability of Li-S batteriesLI-S.According to Fan L et al.[96]PFOA creates passivating layer with C=C bond along with LiF on the lithium anode surface and with its carboxylic acid bondage,contributes the superb polysulfide absorption ability.The result displays in its galvanostatic charge and discharge in Li-S battery which has an excellent cycle ability and its stability.A capacity as high as 788.9 mAh g-1 when its 149th cycle,is retained with till 660.8 mAh g-1 after 500 cycles at 0.2 C charge-discharge rate,which is comparably higher than that of commercialized LIBs with LiNO3 electrolyte additive,presenting an important progress to practical application of high energy Li-S battery.By contrast,PFOA with LiNO3 showed little improvement in its initial efficiency.