Imidazole Anions as Electrolyte Components: Ionic Liquids, Lithium Salts, Additives

Lithium-ion cells are still the fastest developing energy storage technology. Most of the research and development power is directed towards electrode materials evolution due to their main influence on cell capacity. Electrolyte, central component connecting the electrodes, needs bigger focus due to its limitations, which also define the limitations of the cell. These are: electrochemical stability (cathode material choice limitation), thermal stability (limiting applications of the cell or enforcing use of cooling devices) and conductivity/lithium cation transference number (limiting maximum current of the cell), to mention just the most important. As solvents and lithium cation are constant part of the electrolyte, the only variable is the anion. It affects all properties mentioned above, as well as plays the critical part in ionic liquids (ILs) synthesis.

As weakly-coordinating anions commercially available are known for their multiple disadvantages (notably PF₆^-anion), there is a substantial need for new ideas in the field. The concept we have chosen to obtain new anions led to synthesis of heterocyclic compounds with electron withdrawing groups. The most successful member of this family so far is the LiTDI salt [1,2,3].

As LiTDI is still optimized for better performance and to maximize its parameters [4], there is a room for new applications of such anions, like forming ILs [5] or use in salt form as functional additives. Increasing electrochemical stability or forming more stable SEI are among prospective use of such additives.

In this very presentation we will show newest results of investigation of imidazole anions used in lithium salts – both as main electrolyte and additives. We will also show novel application of this anions’ class in ILs.

As for LiTDI in liquid electrolyte, the most important achievement is the optimization of the electrolyte composition using combination of solvent ratio and salt concentration. As a result, the substantial saving of the salt use was obtained without sacrificing any electrochemical or stability parameters. Depending on solvent mixture, it can be as much as 66% savings of the material, contributing to lower overall cost of the cell.

TDI and PDI anions used in ILs also behave very well. Electrochemical and thermal stability for TDI anion-based ILs is up to 4.75 V vs Li and 260˚C, respectively. Also the typical ionic conductivity decrease after lithium salt addition to the pristine IL was substantially limited as it can be seen on the Figure 1. At 20˚C conductivity for PMImTDI and BMImTDI both pristine and after salt addition is in 4-5 mS cm^-1range.

Also use of LiTDI analog as SEI stabilizing additive resulted in positive outcome. Series of electrolyte were tested, similarly to the experiments mentioned above, for their ionic conductivity, lithium cation transference number, electrochemical stability window and SEI stability over time.

References:

[1] L. Niedzicki, G.Z. Żukowska, M. Bukowska, P. Szczeciński, S. Grugeon, S. Laruelle, M. Armand, S. Panero, B. Scrosati, M. Marcinek, W. Wieczorek, Electrochim. Acta 55 (2010) 1450.

[2] L. Niedzicki, M. Kasprzyk, K. Kuziak, G.Z. Żukowska, M. Marcinek, W. Wieczorek, M. Armand, J. Power Sources 196 (2011) 1386.

[3] D.W. McOwen, S.A. Delp, W.A. Henderson, Meeting Abstracts, Abstract #1182, 224th ECS Meeting.

[4] L. Niedzicki, E. Karpierz, A. Bitner, M. Kasprzyk, G.Z. Zukowska, M. Marcinek, W. Wieczorek, Electrochim. Acta 117C (2014) 224.

[5] L. Niedzicki, E. Karpierz, M. Zawadzki , M. Dranka, M. Kasprzyk, A. Zalewska, M. Marcinek, J. Zachara, U. Domańska, W. Wieczorek, Phys. Chem. Chem. Phys. (2014) Acepted Manuscript, DOI: 10.1039/c3cp55354j

Acknowledgements

This project has received funding from the European Union’s Seventh Programme for research, technological development and demonstration under grant agreement No 608502.