The Beneficial Effect of Protic Ionic Liquids on the Lithium Environment in Electrolytes for Battery Applications
Authors: Balducci Andrea , Menne Sebastian , Vogl Thomas , Kühnel Ruben-Simon 
Published: February 2020

Ionic liquids (ILs) are presently considered as one of the most promising candidates for the replacement of organic carbonates as electrolytes in lithium-ion batteries (LIBs). The main advantages of ILs towards organic carbonates are the high thermal, chemical and electrochemical stability as well as the negligible vapor pressure and the reduced flammability [1]. So far, several aprotic ionic liquids (AILs) have been investigated as electrolytes for LIBs. The results of these studies showed that AILs can be successfully introduced in LIBs, and their use has beneficial effects on the safety as well as on the temperature range of use of these devices [2,3]. Nevertheless, AILs are rather expensive and the performance of AIL-based LIBs still needs to be improved in order to be fully competitive with that of conventional electrolytes. Such improvement is particularly necessary for applications where high current densities are needed. In these applications the reduced lithium-ion transport of AIL-based electrolytes, compared to that of conventional electrolytes, might significantly affect the rate performance of LIBs [3]. For these reasons, the development of new IL-based electrolytes with improved lithium-ion mobility compared to the one of present AIL-based electrolytes appears nowadays of importance for the introduction of ILs in LIBs. Taking into account the high costs of AILs, it would also be very beneficial to develop cheaper ILs, as they could be easier introduced in commercial devices.
Protic ionic liquids (PILs) are a subset of ILs and they are typically synthesized by neutralization reactions of a Brønsted acid (proton donor) and a Brønsted base (proton acceptor) [1]. PILs display all favorable properties of ILs, but they have the advantage of being easier to synthesize and cheaper compared to AILs [1, 4]. Clearly, these properties make them interesting candidates for the use as electrolyte component for electrochemical devices.
In the past, the use of PILs as electrolyte for LIBs was not considered. The availability of an acidic proton and their strong reactivity towards lithium were seen as an obstacle for the introduction of PILs into these devices, and consequently all efforts were focused on AILs. Nevertheless, we recently showed that in dry PILs the labile proton of the cation is not “free” and these cations are not subject to reversible protonation-deprotonation [4]. We also proved that battery materials, e.g. lithium iron phosphate (LFP), can be used in combination with dry PILs without being subject to structural changes. Moreover, we showed that lithium-ion batteries containing PIL-based electrolytes can be realized and that they display promising performance in terms of capacity and cycling stability [4]. Taking into account these results, PILs can therefore be regarded as a new class of electrolytes for LIBs.
In this work we considered two pyrrolidinium-based PILs in view of their use as electrolyte for LIBs. We showed that (dry) PILs-based electrolytes display conductivity, viscosity and lithium-ion self-diffusion coefficient comparable to those of a pyrrolidinium-based AIL. However, they have the important advantage of displaying an improved performance when used in combination with battery materials, e.g. lithium vanadium phosphate (LVP), during tests at high C-rate. The lithium ions in PIL-based electrolytes do not move faster than in AIL-based electrolytes according to their self-diffusion coefficients [5]. However, fewer TFSI- anions form the solvation sphere of Li+ in the investigated PILs. We showed that the improved performance of LVP electrodes in the PIL-based electrolytes is related to a reduced charge-transfer resistance at the LVP-electrolyte interface. Taking into account that the limited performance at high rate of IL-based LIBs is presently considered as one of the main limitations of these devices, the use of PIL-based electrolytes can be regarded as a new and promising strategy to overcome this drawback. Additionally, since PILs are typically cheaper than AILs, the introduction of this innovative electrolyte could also be of importance for the development of safe and cheaper IL-based lithium-ion batteries.

References

[1] P. Wasserscheid, Handbook of Green Chemistry, Volume 6: Ionic Liquids Handbook of green chemistry, 2010.
[2] D.R. McFarlane, J. Sun, J. Golding, P. Meakin, M. Forsyth, Electrochim. Acta, 45 (2000) 1271-1278.
[3] S. Menne, R.S. Kühnel, A. Balducci, Electrochim. Acta, 90 (2013) 641-648.
[4] S. Menne, J. Pires, M. Anouti, A. Balducci, Electrochem. Commun., 31 (2013) 39-41.
[5]. T. Vogl, S. Menne, R.-S Kühnel, A. Balducci, J. Mat. Chem. A, DOI:10.1039/C3TA15224C