Hydrophobic and hydrophilic interactions of ionic liquids and polymers in solid polymer gel electrolytes

Poly(ethylene oxide), PEO, or poly(vinylidenefluoride-co-hexafluoropropene), PVdF-HFP, and the ionic liquids 1-n-propyl-2,3-dimethylimidazolium tetrafluoroborate (MMPIBF4) and 1-n-propyl-2,3-dimethylimidazolium hexafluorophosphate (MMPIPF6) with and without 0.5 M Li salt were used to prepare solid polymer gel electrolytes. Experimentation indicates that for the PEO films, the 20 and 30 wt % polymer gels exhibited the highest ionic conductivity, although the 20 wt % film is fragile and exhibits poor mechanical properties. Therefore, the composition of all gels studied is 30 wt % polymer and 70 wt % ionic liquid. For all systems, addition of the lithium salts results in approximately a 33% drop in ionic conductivity. Results indicate that for the formation of polymer gel electrolytes, selection of a nonpolar polymer paired with a hydrophobic electrolyte results in greater ionic conductivity and reversibility in the intercalation of Li ion into graphite. These results are interpreted in terms of the type of solid solution formed between the hydrophobic and hydrophilic polymers and the ionic liquids. (c) 2007 The Electrochemical Society.