Development of a new plasticizer for poly(ethylene oxide)-based polymer electrolyte and the investigation of their ion-pair dissociation effect

One approach to increasing the ionic conductivity of polymer electrolyte is to add plasticizers to the polymer-salt complexes. Recently, we have synthesized a new plasticizer, modified carbonate (MC3), by attaching three ethylene oxide units to the 4-position of ethylene carbonate. A.c. impedance studies showed that an ionic conductivity of 5 × 10−5 S cm−1 could be achieved at room temperature, by adding 50 wt.% of MC3 into poly(ethylene oxide) (PEO)-LiCF3SO3 complex. This is two orders of magnitude higher than that found in PEO-LiCF3SO3 electrolyte without a plasticizer, and one order of magnitude higher than that found when using same amount of propylene carbonate (PC) as plasticizer. Temperature-dependent conductivity measurement and thermal analysis show that this new plasticizer increased the ionic conductivity throughout the entire complex system, while the conventional plasticizers only create a high conductivity pathway through the plasticizer itself. The samples are free-standing films with good mechanical properties. When MC3 is used as a plasticizer, the ionic conductivity increase is much higher than using PC as a plasticizer at high temperature (65 °C), implying an increase in the number of charge carriers. Therefore, we believe that MC3 has a stronger ion-pair dissociation effect than PC, when used as a plasticizer. The ion-pair dissociation effect was studied by Raman, Fourier-transform infrared spectroscopy, and near-edge X-ray absorption fine structure spectroscopy.