Characterization of Ionic Liquid Electrolytes Based on Trifluoromethanesulfonyl-N-Cyanoamide for Li-Ion Battery Application
Authors: Hoffknecht Jan-Philipp , Paillard Elie 
Published: February 2020

Li-ion batteries are used in most portable electronic devices. However, as they are being developed for larger applications such as electric vehicles and stationary applications, the question of their safety is more crucial than ever. As conventional electrolytes are highly flammable and incorporate LiPF6 that generates hydrofluoric acid upon contact with atmosphere, alternatives must be found. In addition, operating batteries at higher temperature would facilitate their cooling as compared to batteries operating close to ambient temperature. One of the most promising approach for higher temperature applications is the use of ionic liquid (ILs)-based electrolytes. In particular, those based on the bis(trifluoromethanesulfonyl)imide (TFSI-) anion possess high thermal and electrochemical stability and are usually non-flammable and non-volatile. However, TFSI- high molecular weight induces high viscosities at room temperature, which limits the conductivities of the resulting ionic liquids. Thus, ILs incorporating smaller anions such as dicyanamide (DCA-)1 or bis(fluorosulfonly)imide (FSI-)2, have been proposed and DCA-based ionic liquids are among the most conductive ILs. Nevertheless, their electrochemical stability is limited as compared to that of TFSI-based ILs. In fact, the anion not only influence the electrochemical stability window of ILs, but also their liquid range. In particular, it is known that low symmetry anions allows obtaining ILs which do not crystallize 3–5. For instance, the the fluorosulfonyl-trifluoromethanesulfonyl imide anion (FTFSI-)6, is an asymmetrical hybrid between the FSI- and TFSI- anion and displays a higher thermal stability than FSI-. In addition, its ILs have viscosities and conductivities closer to those of their FSI- analogs with the main advantage of being amorphous, thus liquid and conductive on a wide temperature window5. Recently, a new asymmetrical anion has been reported: Trifluoromethanesulfonyl-N-cyanoamide (TFSAM-)6 shown in figure 1 which is a hybrid between DCA- and TFSI-.
We paired it with N-butyl-N-methyl-pyrrolidinium (PYR14
+), investigated the resulting IL in terms of physico-chemical properties for a use in Li-ion batteries, and compared it with ILs analogs incorporating other promising anions. The substitution of one nitrile group by a trifluoromethansulfonyl group increases the asymmetry of the resulting ionic liquid that remain liquid down to its Tg, but also allows reaching viscosities not only lower than that of PYR14FTFSI, but also, for most of the temperature range investigated, than that of PYR14FSI. Its thermal stability is also higher than both of these ILs. Moreover, reversible electrochemical (de)insertion of Li+ into graphite and LiNi0.33 Co0.33Mn0.33O2 using PYR14TFSAM-based electrolytes was demonstrated. Interestingly for a use in Li-ion battery, LiTFSAM, even in a conventional ethylene carbonate/dimethyl carbonate mixture, do not induce Al current collector corrosion, which is the main issue for LiTFSI-based electrolytes.

Acknowledgements

The research presented is partly funded by the ‘SPICY’ project funded by the European Union’s Horizon 2020 research and innovation program under grant agreement N° 653373.

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Figure 1