(55c) Thermomechanical and Electrical Properties of Ionic Liquid Imbibed Poly(ethylene glycol) Gels Designed for Use in Lithium Batteries

Lithium ion batteries (LIBs) are attractive as energy storage devices because of their higher volumetric and gravimetric energy densities than the nickel metal hydride batteries that are currently used in hybrid vehicles. However, conventional LIB electrolytes are volatile and flammable liquids, and therefore, limit the durability and temperature range of operation of the device. Ionic liquids (ILs) have been found to be promising alternatives because of their low volatility, satisfactory ionic conductivity, and good electrochemical stability [1], but to overcome the problems of hermetic sealing and lithium dendrite formation associated with liquids, there is a significant interest in solid polymer electrolytes for lithium conduction. Completely dry solid polymer electrolytes have rather low room-temperature ionic conductivities, often below 10^–5 S/cm. Liquid electrolytes (e.g., those based on blends of ethylene carbonate and diethyl carbonate), on the other hand, have at least three orders of magnitude higher conductivity [1].

With the goal of combining the high conductivity of IL electrolytes with the desirable mechanical properties solid polymer electrolytes, we investigated structure–property correlations in lithium-ion conducting PEGylated gels that were imbibed with ILs. In this work, the lithium ion conductivity and mechanical properties of membranes prepared by crosslinking poly(ethylene glycol) (PEG) diacrylate with a T_g-modifying comonomer, such as ethyl acrylate, were investigated using electrochemical impedance spectroscopy and dynamic mechanical analysis. Three different comonomers and six different ILs (all with the bistriflamide anion) were screened for incorporation in these gels. Consistent with our previous study [2], the extent of charge delocalization in the cation was found to greatly affect transport properties such as viscosity and conductivity of the IL. An optimal IL/comonomer combination was identified based on gel ionic conductivity measurements at different temperatures (in the range of 0 to 100 °C). The resulting composite gel electrolyte had good thermal, mechanical, and electrochemical properties. In this talk, the synthesis and characterization of these PEGylated gels will be discussed.

[1] Rock, S.; Lin, W.; Crain, D.; Krishnan, S.; Roy, D. Interfacial Characteristics of a PEGylated Imidazolium Bistriflamide Ionic Liquid Electrolyte at a Lithium Ion Battery Cathode of LiMn₂O₄, ACS Appl. Mater. Interfaces 2013, 5, 2075–2084.

[2] Lebga-Nebane, J. L.; Rock, S.; Franclemont, J.; Roy, D.; Krishnan, S. Thermophysical Properties and Proton Transport Mechanisms of Protic Ionic Liquids, Ind. Eng. Chem. Res. 2012, 51, 14084–14098.