Over the past few decades, rechargeable energy-storage devices have allowed for the development of portable devices and electric vehicles. Ideal characteristics of those devices are high capacity, fast charge/discharge capability and low cost while maintaining safe operating conditions. However, the drawback of increased capacity is that there is an increased safety risk. Runaway reactions are a common concern in such energy-storage devices including secondary batteries and supercapacitors. These uncontrolled reactions often result from in situ heat generation that increases the safety risks of the devices and further causes device failure [1]. To take advantage of safe energy-storage devices, durable thermally-activated safety components that do not interrupt normal operation are required.

Our work explores the potential of using switchable electrolytes that will provide a thermally-controlled safety mechanism in batteries. This switchable electrolyte system would minimize the need for any extra material or sub-devices that become necessary for thermal safety management in energy storage devices. The switchable electrolytes used are part of a larger class of switchable solvents that can switch between two states with the introduction of an external stimulus, such as carbon dioxide or heat[2]. Silylamine-based reversible ionic liquids (RevILs)[4, 5] are investigated here as the switchable electrolyte. In this case, the silylamine is in an ionic liquid-like state (RevIL state) that when dissolved in an organic solvent has shown to have substantial conductivity[3]. With heating, the RevIL will release CO₂ and drop in conductivity substantially (and be in what is termed a molecular liquid (ML) state). Upon cooling, the silylamine can re-react with the CO₂ and return back to its conductive state (RevIL state). This dramatic difference between two states may enable a device to reversibly shut off when heat is generated in the system. It is known that by altering the structure of the silylamine, the thermo-physical properties can be tuned[4, 5]. The impact of the structure of (3-aminopropyl)trialkylsilanes on the safety-switch relevant properties including ionic conductivity of both RevIL+solvent and ML+solvent, and reversal temperature of RevIL+solvent to ML+solvent will be reported in the presence of metallic salts and in the absence of metallic salts. The switch rates between the conductive RevIL and non-conductive ML states will also be reported.

References

[1] D.H. Doughty, Vehicle battery safety roadmap guidance, NREL, Golden, Colorado, 2012.

[2] P.G. Jessop, D.J. Heldebrant, X. Li, C.A. Eckert, C.L. Liotta, Reversible nonpolar-to-polar solvent, Nature 436 (2005) 1102.

[3] J.D. Jimenez, S. Jung, E.J. Biddinger, Ionicity analysis of silylamine-type reversible ionic liquids as a model switchable electrolyte, Journal of The Electrochemical Society 162 (2015) H460-H465.

[4] A.L. Rohan, J.R. Switzer, K.M. Flack, R.J. Hart, S. Sivaswamy, E.J. Biddinger, M. Talreja, M. Verma, S. Faltermeier, P.T. Nielsen, P. Pollet, G.F. Schuette, C.A. Eckert, C.L. Liotta, The synthesis and the chemical and physical properties of non-aqueous silylamine solvents for carbon dioxide capture, ChemSusChem 5 (2012) 2181-2187.

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