Structural electrolytes were prepared using a fully formulated commercially available high performance epoxy resin (MTM57) and an ionic liquid based electrolyte: lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI). Through a systematic study, the composition of the formulations was found to have a greater effect than the curing temperature on the morphology and properties of the resulting structural electrolytes. The presence of lithium salt is essential to form a structurally homogeneous electrolyte. Bicontinuous morphologies containing continuous (coarse) epoxy networks surrounded by connected spherical epoxy nodules were obtained with different length scales upon varying the lithium salt concentration. Increasing the LiTFSI concentration improved the miscibility of MTM57 with the electrolyte and decreased the characteristic length scale of the resulting bicontinuous microstructure. The properties of the structural electrolytes correlated with the morphology, showing increased Youngs modulus and decreased ionic conductivity with increasing lithium salt concentration. The miscibility of the epoxy system with the electrolyte was also improved by substitution of EMIM-TFSI with an equal weight of an aprotic organic solvent, propylene carbonate (PC); however, the window of PC concentrations which resulted in structural electrolytes with bicontinuous microstructures was very narrow; at PC concentrations above 1 wt %, gel-like polymers with no permanent mesoporosity were obtained.
Polymer-based electrolytes based on commercially available epoxy resins were prepared through the addition of a liquid electrolyte, a solution of a lithium salt in an ionic liquid. The polymer monoliths were characterized using impedance spectroscopy, 3-point bending test, scanning electron microscopy (SEM) and nitrogen adsorption (BET). The balance of ionic conductivity and flexural modulus is crucially dependent on the relative proportions of epoxy resin to electrolyte. Also the effect of the liquid electrolyte on curing kinetics and processing was assessed by complex viscosity measurements and differential scanning calorimetry (DSC). Only one out of the three resins investigated exhibited a significant acceleration effect.