A study of pyrrolidinium bis(fluorosulfonyl)imide based ionic liquids for lithium metal batteries
thesisposted on 28.02.2017 by Yoon, Hyun Gook (Martin)
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The use of ionic liquids (ILs) as electrolytes for lithium metal secondary batteries was investigated in this thesis. An electrolyte based on bis(fluorosulfonyl)imide, or FSI, which has relatively lower viscosity and higher conductivity than the well-known bis(trifluoromethylsulfonyl)imide ionic liquids (NTf2 or TFSI), was investigated for its application in a lithium metal battery. An electrolyte based on dicyanamide, or DCA, which does not have any fluorinated functional group, was also investigated. The FSI based electrolyte was characterised using various electrochemical, physical and spectroscopic techniques as well as for its performance in Li metal batteries containing a lithium cobalt oxide (LiCoO2) cathode. Unlike the commercial organic liquid based electrolyte, FSI showed good lithium cycle-ability when the lithium salt concentration was exceptionally high, up to 3.2 mol.kg-1 or 1 : 1 Li to cation ratio - in spite of its high viscosity. High rate charge and discharge (up to 5C) was achieved with this high salt concentration (and highly viscous) electrolyte in Li | LiCoO2 cells; which is superior to the performance with the standard organic electrolyte. The nature of this unusual behaviour was rationalised with respect to its transference number and the conformational structure of the FSI anion. It was suggested that the change from trans-conformer of FSI to cis-conformer in the presence of Li+ alters the lithium ion transportation mechanism. A non-fluorinated dicyanamide (DCA) based ionic liquid was investigated as an alternative electrolyte for its exceptionally low viscosity, high conductivity and lower cost compared to fluorinated electrolytes. It was shown that this electrolyte exhibited good capacity retention for Li | LiFePO4 batteries, achieving 130 mAh.g-1 at 50 oC. The moisture content of the IL was shown to be extremely important in determining performance and approximately 200 ppm of moisture produced the optimum cycling ability. The physical properties of this DCA ionic liquid containing several different fluorinated lithium salts (lithium bis(fluorosulfonyl)imide(LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiNTf2 or LiTFSI)and lithium tetrafluoroborate (LiBF4)) was compared with when LiDCA was added. The solution had the lowest viscosity, highest ionic mobility and highest degree of ionization when Li DCA was added. LiBF4 addition produced the highest viscosity and lowest ionic mobility with the lowest degree of ionization. Unlike the other added salt anion species, BF4- was shown to form larger clusters rather than simple ion pairs. The difference of qualitative description of the ‘Walden plot’ result and ‘ionicity’ result was compared and this suggested that ‘ionicity’ (the ratio of the molar ionic conductivity versus the conductivity calculated from Nernst-Einstein equation using NMR self-diffusion coefficients) better represents the behaviour of a salt mixed system than the Walden plot.