"Solvate" Ionic Liquids, "Solvent-in-salt" Ionic Liquids, and "Deep Eutectic" Ionic Liquids. What are the Relationships - and the Ionicities?

Abstract

Recently, in the search for alkali metal-containing electrolytes with higher conductivities than available in common ionic liquid systems, some interesting systems have shown up. The most interesting ones are those that also have wide electrochemical windows and high alkali metal transport numbers, and when non-flammability is also included, the interest has indeed run high1 , 2. The aqueous systems ("water-in-salt" electrolytes) are the most recent on the scene, and are of special importance for obvious reasons, but they are closely related in principle to other also successful systems developed independently in different labs and so given different designations. All are characterized by being extremely concentrated in ions but modulated in properties by the presence of one or other type of molecular constituent in molar proportions up to 50% and in some cases greater. Some other examples, are the "solvate ionic liquids" of the Watanabe3 and Henderson4 laboratories in which Li+ ions exist 1:1 with polyglyme molecules, and the "deep eutectic" liquids used by Abbott and coworkers5 for the electrodeposition of higher valent metals, where again the autoionization of the multivalent ions is induced by equimolar proportions of a favorable (often oxygenic) ligand. Both are special cases of the more general "solvent-in-salt" class of electrolyte from the Chen-Armand collaboration6. In this talk we will review the relationship between these different types of ionic systems and examine their relative ionicities, ionic mobilities, and other key characteristic properties. 1 Suo, L. et al. “Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries. Science 350, 938-943 (2015).2 Suo, L. et al. Advanced High Voltage Aqueous Li-ion Battery Enabled by “Water-in-Bisalt” Electrolyte. Angewandte Chemie Int. Ed. (printed online) DOI (2016).3 Ueno, K. et al. Glyme−Lithium Salt Equimolar Molten Mixtures: Concentrated Solutions or Solvate Ionic Liquids? J. Phys. Chem. B 116, 11323-11331 dx.doi.org/11310.11021/jp307378j (2012).4 Henderson, W. A. Glyme-lithium salt phase behavior. . J. Phys. Chem. 110, 13177-13183 (2006).5 Abbott, A. P., Barron, J. C., Rydere, K. S. & Wilson, D. Chem.–Eur. J. 13, 6495. (2007).6 Suo, L., Hu, Y.-S., Armand, M. & Chen, L. A new class of Solvent-in-Salt electrolyte for high-energy rechargeable metallic lithium batteries. . Nat. Commun. 4, 1481 (2013).