Influence of high and low dielectric constant plasticizers on the ion transport properties of PEO: NH4HF2 polymer electrolytes

Abstract

Different composition ratio of polymer electrolytes based on poly(ethylene oxide) (PEO) as host polymer, ammonium bifluoride (NH4HF2) as salt, and propylene carbonate (PC), dimethyl acetamide (DMA), dimethyl chloride (DMC), and diethyl carbonate (DEC) as plasticizers has been prepared by solution casting technique. The influence of high dielectric constant plasticizers (PC and DMA) and low dielectric constant plasticizers (DMC and DEC) on the ion transport properties of PEO-NH4HF2 polymer electrolytes has been studied. The increase in ionic conductivity of polymer electrolytes containing PC and DMA is observed to be more as compared to those electrolytes containing DMC and DEC, which is due to an increase in both the amorphous phase and dielectric constant of PEO. X-ray diffraction study reveals the amorphous nature in case of plasticized polymer electrolyte. In the Fourier transform infrared study, the changes and shifting of the different characteristic peaks confirm the polymer–salt complex formation and the dissociation of ion aggregates present at higher concentration of salt with the addition of PC. Maximum ionic conductivity of 1.40 × 10−4 S cm−1 at room temperature has observed in case of plasticized polymer electrolytes containing optimum concentration of PC so that mechanical stability and flexibility be maintained. The variation of linewidth with temperature has also been studied by 1H and 19F nuclear magnetic resonance (NMR), which confirms that both cations and anions are mobile in these polymer electrolytes. Line narrowing associated with the glass transition temperature ( T g; low mobility region) and melting temperature ( T m; high mobility region) of PEO has also been observed for plasticized polymer electrolytes containing PC having optimum conductivity value. Conductivity versus temperature variation study reveals curved nature of plot in case of plasticized polymer electrolytes containing high dielectric constant plasticizers, which is significant for their amorphous nature. Smooth morphology observed in case of plasticized polymer electrolytes having optimum conductivity value is essential key factor for polymer electrolytes to be suitable for practical applications.