DOI:10.1021/cm102263s PEGylated Imidazolium Ionic Liquid Electrolytes: Thermophysical and Electrochemical Properties

We report the synthesis and characterization of a series of imidazolium iodide ionic liquids (ILs) containing monomethoxy-terminated poly(ethylene glycol) (mPEG), and n-alkyl groups. These PEGy-lated ILs contain 7, 12, or 16 ethylene glycol units in the side chains, and are designed as potential electrolytes for energy conversion and storage devices such as dye-sensitized solar cells, supercapacitors, and Li ion batteries. The thermophysical (density, viscosity, and the temperatures of glass-transition, crystallization and melting) and electrochemical (nonfaradaic window, and capacitor leakage resistance) propertiesof the ILs, that are critical to these targetedapplications, are studiedusinganarrayof techniques. 1-Alkyl-3-methylimidazolium iodide ILs are synthesized and characterized in parallel with the PEGylated ILs, to compare how the electrolyte properties of the two systems are affected by their detailed molecular structures, and especially by the ether oxygen atoms. The mPEG side chains show strong intramolecular interactionswith the imidazoliumring,weakeningCoulombic interactionsbetween the imidazoliumcation and the iodide anion. The PEGylated ILs, therefore, exhibit conductivities of the order of 0.1 mS cm-1 (25 C) despite their relatively high viscosities, and support a temperature-independent electrochemical window of about 2 V, demonstrating their suitability for extended temperature applications. 1