Investigations on the Chemical and Electrochemical Stability of Imidazolium Containing Ionic Liquids

The rapid depletion of fossil fuels, as well as the increasing environmental concerns resulting from their combustion, makes the demand for the development of electrical energy storage systems. Those systems have to fulfill several requirements like high energy density and long-term cycle stability to replace fossil energy sources.

Due to the fact that commercially available Lithium ion batteries cannot provide the needed specific energies and are still expensive, alternative technologies are of great interest. One approach is the use of lithium air batteries with their high theoretical energy density (11.680 Wh•kg^-1).^[1] The typical cell design consists of a lithium metal anode and a porous carbon cathode, which are separated by an electrolyte. During the discharge process the lithium metal electrode is oxidized, while oxygen from the air is reduced at the porous carbon cathode.

2 Li + O₂ → Li₂O₂

Lithium metal is the negative electrode material of choice due to various advantages like its low atomic weight (6.94 g•mol^‑1), a low redox potential (-3.04 V versus standard hydrogen electrode) and a high specific capacity (3862 mAh•g^‑1).^[2] Nevertheless, there are still challenges which have to be faced. One is the formation of high surface area lithium (HSAL), which can be observed in three different morphologies: mossy, needle-like and granular.^[3]

It has been known that HSAL formation especially occurs while using organic carbonate based electrolytes, so the replacement of this electrolyte type is considered. Intensively researched candidates are solid polymer- as well as ionic liquid based electrolytes,^[4] since it has been proven that dendrite growth is hindered when ionic liquids are used as electrolyte components.^[5] Further benefits of this category of substances are their wide electrochemical stability window, non-volatility and non-flammability.^[6]

Figure 1 shows a cyclic voltammogram of a lithium metal full cell (Lithium vs. Lithium iron phosphate (LFP)) with an 1‑Ethyl‑3‑methyl-imidazolium bis(trifluoromethanesulfonyl)imide ([C2MIm]TFSI) based electrolyte. In this work we present investigations on the chemical and electrochemical stability of imidazolium containing ionic liquids, especially [C2MIm]TFSI, which is stated as a suitable electrolyte solvent for lithium air batteries.^[7]

The chosen ionic liquids will be investigated concerning their chemical (infra-red spectroscopy) and electrochemical stability (cyclic voltammetry, galvanostatic cycling) at different temperatures. Furthermore the effect of an additive (fluoroethylene carbonate (FEC)) on the cycling performance of the ionic liquid based electrolytes is researched.

[1] G. Girishkumar, B. McCloskey, A.C. Luntz, S. Swanson, W. Wilcke, J. Phys. Chem. Lett., 1, (2010), 2193-2203.

[2] M.-H. Ryou, Y. M. Lee, Y. Lee, M. Winter, P. Bieker, Adv. Funct. Mater., 25 (6), (2015), 834-841.

[3] Z. Li, J. Huang, B. Y. Liaw, V. Metzler, J. Zhang, J. Power Sources, 245, (2014), 168-182.

[4] R. Jakelski, M. Winter, P. Bieker, Conference Poster No. 508, 224th ECS, (2013).

[5] G. B. Appetechi, G.-T. Kim, M. Montanino, M. Carewska, R. Marcilla, D. Mecerreyes, I. De Meatza, J. Power Sources, 195, (2010), 3668-3675.

[6] N. Schweikert, A. Hofmann, M. Schulz, M. Scheuermann, S. T. Boles, T. Hanemann, H. Hahn, S. Indris, J. Power Sources, 228, (2013), 237-243.

[7] C. J. Allen, S. Mukerjee, E. J. Plichta, M. A. Hendrickson, K. M. Abraham, J. Phys. Chem. Lett., 2, (2011), 2420-2424