We developed a new concept for improving the ionic conductivity of polyether electrolytes. It was applied supercritical carbon dioxide (scCO 2 ) to the poly(ethylene oxide) (PEO)-lithium trifluoromethane sulfonate (LiCF 3 SO 3 ) mixture in order to improve the chain mobility in the amorphous phase. The ionic conductivity at room temperature of the mixed sample was more than 10 times improved by the scCO 2 treatment. Moreover, we found a phenomenon that the dissociation of LiCF 3 SO 3 in PEO is promoted by the scCO 2 treatment.