Ion transport properties of ionic liquid based gel electrolytes

Ion transport properties of two types of gel electrolytes based on the same ionic liquid (IL) 2,3-dimethyl-1-hexyl imidazolium bis (trifluoromethanesulfonyl) imide (DMHxImTFSI), namely polymer-in-IL, containing DMHxImTFSI and polyethylene oxide (PEO) and polymer gel electrolytes containing DMHxImTFSI, PEO and propylene carbonate (PC) have been studied over a wide temperature range. The ionic conductivity of polymer-in-IL gels is of the order of 10−3 S cm−1 at 25 °C, which increases to 10−2 S cm−1 at 127 °C and these gels are thermally stable up to 400 °C. Polymer gel electrolytes containing PC show relatively higher ionic conductivity than polymer-in-IL gels but have lower thermal stability. Line narrowing observed in the variation of line width of 1H and 19F NMR lines with temperature shows the simultaneous presence of high and low conducting phases in polymer-in-IL gels whereas high conducting amorphous phase is mainly present in polymer gel electrolytes containing PC. The onset of long range translational motion in these gel electrolytes has been observed to take place at the glass transition (Tg) and melting (Tm) temperature and is closely related to the ionic conductivity of the gel electrolytes. The range of temperature over which the gel electrolytes possess higher ionic conductivity has also been identified. The polymer has been observed to play an active role in polymer gel electrolytes whereas it mainly acts as a stiffener in polymer-in-IL gels. The polymer gel electrolytes containing PC can be used in practical devices at temperatures below 100 °C whereas polymer-in-IL gels can be used up to 400 °C.