A mixture of the ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIBTI) and 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIPF6) was formulated. The thermal properties of the 50/50 binary mixture were determined by differential scanning calorimetry and cloud point measurement. No evidence of crystallization or phase separation during cooling at 1 degrees C/min to -60 degrees C was found. The devices constructed in this study were light weight, flexible, and hermetically sealed. These sealed devices proved effective against electrolyte leakage and performance degradation during temperature cycling. The performance of poly(propylenedioxy) thiophene (PProDOT)-based type I supercapacitors using a 50/50 wt % EMIBTI/EMIPF6 electrolyte mixture was compared to the performance using neat EMIBTI. It was difficult to deposit PProDOT films directly from the ionic liquid mixtures. However, it was possible to deposit PProDOT films from EMIBTI and then assemble and test the devices using the 50/50 mixture. The low temperature performance of the EMIBTI/EMIPF6 supercapacitors was superior to that of the EMIBTI-based supercapacitors, evidenced by the retention of capacitive behavior below -30 degrees C. The high temperature performance of the EMIBTI/EMIPF6 supercapacitors was nearly as good as the EMIBTI-based supercapacitors with 10% loss after 10,000 cycles at 70% depth of discharge at 60 degrees C.