Lithium Coordination in Protic Ionic Liquid-Based Electrolytes
Authors: Vogl Thomas , Menne Sebastian , Balducci Andrea 
Published: February 2020

The commercially available lithium-ion batteries (LIBs) contain electrolytes based on organic carbonates (e.g. ethylene carbonate (EC) and diethyl carbonate (DEC)). These electrolytes feature good conductivities, but since they are easily flammable and volatile, their use strongly reduces the safety as well as the temperature range of use of LIBs. Ionic liquids (ILs) are presently considered as one of the most promising candidates for the replacement of organic carbonates. [1-3]
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] 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.
Recently, we showed that protic ionic liquids (PILs) represent a new and interesting class of electrolytes for LIBs. [4] As a matter of fact, PILs display all favorable properties of ILs, but they have the advantage of being easier to synthesize and cheaper compared to AILs. [5] Moreover, their use makes possible the realization of LIBs with higher performance at high current densities compared to AIL. [6]
The higher performance of PIL-based electrolytes compared to the AIL-based ones can be explained with the different lithium coordination in these electrolytes. Using Raman spectroscopy we investigated several TFSI-based ILs (protic and aprotic) and we showed that lithium ions are coordinated by a significantly lower number of TFSI--anions in PILs than in AILs. [7] Furthermore, we also showed that the charge-transfer resistance of composite electrodes is lower in PIL-based electrolytes compared to AIL-based ones, resulting in higher charge-discharge capacities. [6]
In this work we investigated the lithium coordination in mixtures containing propylene carbonate (PC) and a PIL (Fig. 1). Furthermore, with the aim to further improve the performance of PIL-based LIBs, we also investigated the influence of the cation size on the lithium coordination of PIL-based electrolytes.

References

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[8] T. Vogl, S. Menne, A. Balducci, Physical Chemistry Chemical Physics, 16 (2014) 25014-25023.

Figure 1