Investigation On The Properties Of Electrolyte Of Lithium (Fluorosulfonyl)(n-nonafluorobutanesulfonyl) Imide-Dimethyl Carbonate

In today's commercial Li-ion batteries?LIBs?,the electrolytes are mainly composed of lithium hexafluorophosphate?LiPF₆?as Li-ion conducting salt,blended solvents of cyclic and/or linear carbonate and a small amount of role-special additives.Ethylene carbonate?EC?has long been selected as an essential solvent in liquid electrolyte since the commercialization of LIBs.This is mainly attributable to its peculiarities of both its high dielectric constant??=89.8?for effectively dissociating lithium?Li?salt and excellent interfacial compatibility with graphite anode.However,the relatively high melting point?T_m=36.4°C?and viscosity??=1.9 cP?of EC has restricted it as a single solvent utilized in LIBs.Additionally,it was also reported that the use of EC as a co-solvent may have adverse effects in high-voltage LIBs.LiPF₆ universally utilized as the versatile conducting salt in commercial LIBs is largely due to its several crucial advantages over other Li salts,including its excellent ability of passivating Al current collector in the high potential region of 3⁵ V vs.Li/Li⁺,enough anodic stability towards oxidations of various kinds of 4 V class cathodes,and relatively high ionic conductivities(e.g.,10^-3 S cm^-1,25°C).However,it was widely demonstrated that LiPF₆-carbonate electrolytes have a poor chemical stability.Hence,it is desirable and urgent to seek for novel electrolyte recipes,which would be suitable for application in LIBs,particularly at elevated temperatures.Recently,we report a new lithium?fluorosulfonyl??n-nonafluorobutanesulfonyl?imide?Li[?FSO₂??n-C₄F₉SO₂?N],LiFNFSI?-EC electrolyte for graphite/LiCoO₂ cells at both room and elevated temperatures,which displays improved cycling and current rate performances in comparison with LiPF₆-based electrolyte.This has been essentially ascribed to the better compatibility of LiFNFSI with both Al current collector and cathode.Most importantly,it is found that LiFNFSI is an active SEI former and can form robust SEI films on graphite anode.Based on the above background and consideration,we herein continue to report a new EC-and additive-free base electrolyte,which is only composed of LiFNFSI and dimethylcarbonate?DMC?,forapplicationinLi/graphiteand graphite/LiNi_0.6Mn_0.2Co_0.2O₂?NMC622?cells.The main results could be summarized as below:?1?Fundamental properities:the EC-free LiFNFSI-DMC electrolyte exhibits a relatively low melting point of-16.9°C,low viscosity of 2.34 cP?25°C?,high ionic conductivities of 5.79 mS cm^-1?25°C?,high Li-ion transferences of 0.47?25°C?,and high oxidation potential of 5.45 V vs.Li/Li⁺?25°C?.?2?In the Li/graphite cells,the Coulombic efficiency?CE?at the first cycle is 92.2%for the LiFNFSI-DMC based cell,which is a bit higher than that of 92.0%for the LiPF₆-EC/DEC/EMC based one?with EC?,and is obviously higher than that for the LiPF₆-DMC based one?without EC?at 87.3%.These results suggest that the LiFNFSI-DMC electrolyte without SEI-forming EC has an excellent interfacial compatibility with graphite anode.?3?The EC-free LiFNFSI-DMC electrolyte can afford superior current rate and cycling performances for the graphite/LiNi_0.6Mn_0.2Co_0.2O₂ cells at both room and elevated temperatures,in comparison with the LiPF₆-based ones.?e.g.,capacity retention rate after300 cycles at 25°C:82.9%?LiFNFSI-DMC?vs.56.8%?LiPF₆-DMC?and 84.2%?LiPF₆-EC/DEC/EMC?;capacity retention rate after 200 cycles at 60°C:77.1%?LiFNFSI-DMC?vs.36.3%?LiPF₆-DMC?and 55.5%?LiPF₆-EC/DEC/EMC??.?4?X-ray photoelectron spectroscopy?XPS?and electrochemical impedance spectra?EIS?show that the EC-free LiFNFSI-DMC electrolyte has an extraordinary capability of forming a thin?<5 nm?,compact,and highly Li-ion conductive SEI films on the overall surface of graphite anode,whereas the EC-free LiPF₆-DMC electrolyte can only form a discrete SEI films on graphite anode.Therefore,the LiFNFSI would be considered as an active SEI former besides being a conducting salt,and be promising as an alternative salt of LiPF₆ for application in EC-and/or additive-free electrolyte for LIBs.