Application of electrochemical impedance spectroscopy to the study of ionic transport in polymer-based electrolytes

Lithium-ion batteries should benefit from new polymer-based electrolytes to answer both safety and performance issues for use in transportation applications. Rubber-like gel electrolytes are prepared by gelling of a ionic liquid based solution by a cross-linked epoxide-amine, as a simple and effective means to obtain safe electrolyte materials that exhibit minimal flammability and toxicity while possibly preserving high mechanical and ionic transport properties over a wide temperature window. The synthesis of gel polymer electrolytes based on epoxy-amine thermosetting polymers, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI TFSI) and the lithium salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is presented. Self-standing, transparent, optically homogeneous films are obtained after curing. Two gel polymer electrolytes differing by the nature of the epoxide precursor (aromatic versus aliphatic) are characterized in terms of thermo-mechanical properties with respect to their neat epoxy-amine counterparts. Electrochemical impedance spectroscopy (EIS) is used to investigate the electrical properties and measure the ionic conductivities over the temperature range of interest. The choice of a more flexible moiety in the thermoset precursors leads to an enhanced ionic conductivity. An electric model is presented to predict the gel conductivity on the basis of the conductivity values of neat compounds and to discuss the gel morphology at the molecular scale.