Ionic liquids (ILs) are more and more investigated as substitutes of volatile and flammable organic electrolyte solvents in rechargeable lithium-ion battery systems [1] to enhance safety of the electrochemical device. So far, single ILs do not generally match or fully satisfy the requirements and/or operative conditions of practical devices, even if the ionic liquid properties could be finely tuned by suitably modifying their architecture [1]. A promising approach is represented by properly combining different ionic liquids, leading to beneficial synergic effects on the physicochemical properties of the resulting IL mixture [1].

In the present work, we have developed ternary mixtures by incorporating different fractions of the LiTFSI salt into the proper PYR₁₃TFSI-PYR₁₃FSI blend. Pyrrolidinium-based ILs were successfully proposed for lithium batteries [1]; the TFSI anion has proved to be stable toward oxidation and thermally robust [1] whereas FSI exhibits high conductivity even at low temperature coupled with protective film-forming capability onto electrodes [1].

Results and Discussions

An eco-friendly procedure route [2] allowed to synthesize high purity, anhydrous, clear, colorless and odorless ionic liquid samples with yields ranging from 85 to 90 mol.% and water content below 2 ppm.

Proper PYR₁₃TFSI-PYR₁₃FSI formulations have shown ion conduction approaching 10^-3 S cm^-1 at -20 °C whereas the pure IL materials, still in solid state, exhibit conductivity values four orders of magnitude lower. This behavior is likely due to different hindrance of the anions, resulting in worse ion packing. On the basis of the obtained results, the TFSI:FSI mole ratio was selected equal to 2:3.

The LiTFSI-PYR₁₃TFSI-PYR₁₃FSI mixtures (Figure 1) have exhibited high conductivity (about 10^-3 S cm^-1), even at low temperatures (-20 °C), in good agreement with the thermal and rheological properties. Cyclic voltammetries (Figure 2) have indicated electrochemical stability up to 5 V in conjunction with the possibility to reversibility intercalate Li⁺ ions into carbonaceous anodes, this also suggesting good mobility of lithium at the interface with electrodes.   

Acknowledgements

The authors wish to thank the financial support of the European Commission for the GREENLION project within the 7^th Framework Program (Grant agreement n°: 28526).

References

[1] G.B. Appetecchi, M. Montanino, S. Passerini, Ionic Liquid-based Electrolytes for High-Energy Lithium Batteries, in Ionic Liquids Science and Applications, ACS Symposium Series1117, A.E. Visser, N.J. Bridges and R.D. Rogers editors, Oxford University Press, Inc., American Chemical Society, Washington, DC, USA.

[2] M. Montanino, F. Alessandrini, S. Passerini, G.B. Appetecchi, Electrochim. Acta 96 (2013) 124