Understanding the Ionic Behavior of Silylamine-Type Reversible Ionic Liquids for Use As a Battery Safety Switch
Authors: Chen Josephine , Jung Sungyup , Messinger Robert J. , Biddinger Elizabeth J 
Published: February 2020

The commercial prevalence of energy-storage devices, including secondary batteries, has necessitated development of higher capacity and more energy dense systems.1 However, even as the battery components are modified to accommodate these needs, the flammable electrolytes found in certain batteries pose an inherent safety risk that has threatened their continued use. Furthermore, current safety mechanisms often irreversibly shut down the battery should thermal runaway occur. Reversible ionic liquids (RevILs) are a viable alternative to the electrolyte solvents currently used. These RevILs switch between an ionically-insulating molecular liquid (ML) state to an ionically-conducting state (IL) via external stimuli such as carbon dioxide (CO2) or temperature.2 Thus, the battery would operate under the premise that the electrolyte would be in its IL form during normal battery operation and in its ML form when a thermal safety limit is triggered. This would address the underlying safety concerns. More interestingly, with the application of an external stimulus, the battery can be revived simply by switching from the ML form back to the IL form and battery cycling may resume as normal.

From our previous study, silylamine-based RevILs have demonstrated promise as potential electrolytes based on their ionicity.3
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Additionally, the work was expanded to determine the effect of different organic co-solvents on the conductivity. We found that the conductivity of trialkylsilylamines, in particular, increased up to 40 times in their RevIL state (~0.5 mS/cm) compared to their ML state (~0.01 mS/cm), thereby indicating that the silylamine could act as a safety switch via a conductivity change. With relevance towards formulating a silylamine-based electrolyte, we found that a tripropylsilylamine-based solution had an enhanced conductivity of 2.16 mS/cm upon the addition of a metallic salt (LiPF6). However, there are current difficulties associated with their use, including (1) ion aggregation, (2) low solubility of metallic salts, and (3) high viscosity. Towards this end, structure functionalization is one avenue of exploration that has been shown to simultaneously maintain the switchability while enabling other properties of the silylamine system to be tuned.4 However, these experiments should be guided by fundamental understanding of the ion interactions that underlie the observed phenomena. This study focuses on the elucidation of the ion-ion and ion-solvent interactions that dictate ion aggregation behavior and low salt solubility. These interactions were probed via two-dimensional nuclear magnetic resonance (NMR) spectroscopy experiments and electrochemical methods. By isolating the structural effects that result in the aforementioned difficulties, the silylamine structures can be tuned accordingly to address these challenges, resulting in improved RevIL-based electrolytes for battery applications.

References:

(1) Scrosati, B.; Garche, J. Lithium batteries: Status, prospects and future. J. Power Sources
2010, 195, 2419-2430.

(2) Blasucci, V.; Dilek, C.; Huttenhower, H.; John, E.; Llopis-Mestre, V.; Pollet, P.; Eckert, C. A.; Liotta, C. L. One-component, switchable ionic liquids derived from siloxylated amines. Chemical Communications
2009, 116-118.

(3) Jimenez, J. D.; Jung, S.; Biddinger, E. J. Ionicity analysis of silylamine-type reversible ionic liquids as a model switchable electrolyte. J. Electrochem. Soc.
2015, 162, H460-H465.

(4) Switzer, J. R.; Ethier, A. L.; Hart, E. C.; Flack, K. M.; Rumple, A. C.; Donaldson, J. C.; Bembry, A. T.; Scott, O. M.; Biddinger, E. J.; Talreja, M.; Song, M.-G.; Pollet, P.; Eckert, C. A.; Liotta, C. L. Design, synthesis, and evaluation of nonaqueous silylamines for efficient CO2 capture. ChemSusChem
2014, 7, 299-307.