Ionicity Analysis of Silylamine-Type Reversible Ionic Liquids as a Model Switchable Electrolyte

Abstract

Separation of the electrochemical product from an electrolyte can add challenges to the overall process. Use of a switchable electrolyte offers the possibility to reduce or eliminate these separation challenges while maintaining high conductivity. Reversible ionic liquids (RevILs) are a type of switchable solvent that could be used as a switchable electrolyte. Switchable solvents are solvents that can have dramatic step-changes in their properties with the introduction of an external stimulus, in this case CO 2 . In order to determine the feasibility of using TEtoxySA-RevIL, which behaves similarly to protic ionic liquids, as an electrolyte an ionicity analysis utilizing binary mixtures of the RevIL and organic solvents were studied to determine the ionic behavior of the system. Both protic and aprotic solvents with a broad range of dielectric constants were used in order to elucidate the ionicity of the RevIL mixtures. When in the pure RevIL state, the conductivity is prohibitively low due to a heightened viscosity and poor ion mobility due to hydrogen bonding and coulometric attraction. We found that in the presence of protic solvents with high dielectric constants the conductivity of the RevIL increased by up to four orders of magnitude and achieved ideal ionicity.

Structure-Property Relationships of Silylamine-Type Reversible Ionic Liquids for Use as a Switchable Electrolyte

“…5,[9][10][11][12][13] We previously reported on the feasibility of using silylaminetype RevILs as an electrolyte by studying the ionicity and electrochemical window of commercially-available (3-aminopropyl) triethoxysilane (TEtoxySA) neat and when mixed with solvents. 8 We found that both the neat TEtoxySA RevIL and ML states had negligible conductivity. When the TEtoxySA-RevIL state was combined with high dielectric constant solvents (polar protic or polar aprotic), conductivities up to three orders of magnitude higher than the TEtoxySA-ML in solvent were observed, illustrating the possibility of TEtoxySA being used as a switchable electrolyte.…”

“…Transferability of TEtoxySA findings to TEtSA.-While a baseline understanding of TEtoxySA-RevIL/solvent as an electrolyte has previously been established, 8 there was a need to determine if the more stable (3-aminopropyl)trialkylsilanes behaved similarly as an electrolyte. TEtSA, an analog to the TEtoxySA with the ether linker removed, was used to make the preliminary comparisons.…”

“…Also, with TEtoxySA-RevIL we observed higher conductivities with methanol, DMSO, and acetonitrile than with THF. 8 We have yet to fully elucidate the reasoning between the differences in conductivity with solvent type; however, the dielectric constant, polarity, and protic nature of the solvent appear to have an impact. These properties likely all influence the ion dissociation ability of the RevIL; solvents that can better separate the anion and the cation lead to increased mobile charge carriers in the solution and thus higher conductivities.…”

“…When comparing the impact of silylamine structure, TEtSA RevIL had the highest conductivities and TPSA RevIL had the lowest conductivities, regardless of the RevIL-DMSO ratio, following the observations that were made at 5 mol% RevIL discussed above. We previously observed a marked drop in conductivity at TEtoxySA RevIL concentrations above 8 mol%, 8 when viscosity and anioncation interactions increase significantly, so we did not study the higher concentrations here. Temperature effects on RevIL solutions.-Temperature has a significant influence on the properties of the silylamines, including the switch from RevIL to ML state, conductivity, and viscosity.…”

“…Reversible ionic liquids (RevILs) take some of the desirable properties of ILs and combine them with a switch that can turn the IL-like solvent into a traditional molecular liquid (ML). [5][6][7] Electrochemically, these can be used as switchable electrolytes where both conductivity and solubility can be altered on-demand, 8, including for battery safety purposes and product separation from electro-organic syntheses. The switchable behavior could be utilized as a battery safety switch (Fig.…”