Effect of Supercritical Carbon Dioxide Processing on Ionic Association and Conduction in a Crystalline Poly(ethylene oxide)−LiCF<INF>3</INF>SO<INF>3</INF> Complex

To achieve fast ion transport in poly(ethylene oxide) (PEO)-based polymer electrolytes, we aimed to increase the ionic conductivity by using supercritical carbon dioxide (scCO<INF>2</INF>) as a processing solvent. In the crystalline PEO−LiCF<INF>3</INF>SO<INF>3</INF> complex system, a large difference was observed in the conductivities of the original and scCO<INF>2</INF>-treated samples. In particular, PEO<INF>7</INF><INF>-</INF>LiCF<INF>3</INF>SO<INF>3</INF> (oxyethylene unit:Li = 7:1) increased in conductivity approximately 100-fold at 40 °C with scCO<INF>2</INF> processing to a value of 1.8 × 10^-5 S/cm. Differential scanning calorimetry measurement showed that the processing reduces the glass transition temperature (T<INF>g</INF>), the melting point of the PEO crystalline phase (T<INF>m1</INF>), and the heat of fusion of the crystalline complex part (ΔH<INF>2</INF>). Raman scattering analysis clearly confirmed the decrease in the triple ion fraction and the increase in the ion pair fraction in scCO<INF>2</INF>-treated samples. Furthermore, subsequent time dependence of the ionic conductivity shows that scCO<INF>2</INF> processing maintains the conductivity more than 10-fold higher for at least one month. As a mechanism, we believe that the CO<INF>2</INF> molecules permeated into the sample play a crucial role in the dissociation of ions to increase the conductivity, according to the Lewis acid−base interaction.