High Concentration Electrolytes, or Almost Ionic Liquids, for Advanced Batteries
Authors: Blomgren George E 
Published: May 2020

For most of the commercial life of lithium ion batteries, the electrolyte of choice has been LiPF6 salt in mixtures of organic carbonates, such as ethylene carbonate, propylene carbonate, ethyl carbonate, methyl carbonate and ethyl methyl carbonate1 . In the last several years, however, the electrolyte phase for lithium ion batteries has been given increased scrutiny. One productive area of research has been in the general area of high salt concentration in coordinating organic solvents2. In particular, when the mole ratio of electrolyte salt to coordinating solvent becomes less than 1 to 3 or 4, many changes occur in the solution structure. The amount of free (uncoordinated) solvent drops to undetectable levels, as measured, for example by Raman spectroscopy. This means that the "solvent" is completely coordinated to the cation and the solution now resembles an ionic liquid rather than a mixture of coordinated compact ion pairs and free ions in a medium of the excess free solvent molecules like the conventional 1 molar LiPF6 lithium ion battery electrolyte. The transition to the disappearance of free solvent usually occurs at around 3 to 5 molar salt concentration depending on the salt-solvent combination. It has been known for many years that such transitions occur at high concentrations and models have shown that the degree of association of anion and cation into ion pairs may drop many orders of magnitude3 . Thus, we view such solutions as ionic liquids where a quasi-lattice exists of cations surrounded by anions and anions by cations.
Several important effects occur with these solutions. Since the solvent molecules are no longer freely available at the negative electrode interface, the composition of the solid electrolyte interphase (SEI) is found to be quite different from that of the conventional electrolyte. The passivation of the aluminum positive electrode, which in the conventional electrolyte depends upon the unique characteristics of LiPF6 is no longer restricted to the use of this salt. It has been found, for example, that lithium bis-(flurosulfonyl) imide salt - Li(FSO2)2N - in various solvents creates a viable SEI on the negative electrode as well as passivates the Al carrier at the positive electrode. It has also been found that coordinating solvents other than organic carbonates may be used and adequate SEI and Al passivation are observed depending on the salt-solvent combination.
There are a couple of negative attributes of these nearly ionic liquid electrolytes, however. The solution viscosity is found to increase dramatically with salt concentration after the disappearance of free solvent. The conductivity is also found to decrease substantially after the transition as well. The use of solvents different from the carbonates, such as small molecule nitriles, gives sufficiently lower viscosity and higher conductivity that these solutions may be of use for lithium ion batteries even at high salt concentration.
These aspects will be discussed in detail and some promising results for lithium ion batteries will be presented. In addition, the application of these concepts will be considered for lithium and other metal anode batteries.
References:
1. G. E. Blomgren, J. Power Sources 81-82, 112 (1999)
2. For a recent review of this work, see: Y. Yamada, J. Wang, S. Ko, w. Watanabe and A. Yamada, Nature Energy, 4, 427 (2019)
3. G. E. Blomgren, J. Power Sources, 14, 39 (1985)