Ionic conductivity of composite electrolytes based on oligo(ethylene oxide) and fumed oxides

The effects of fumed oxide fillers (SiO2, Al2O3, TiO2) and binary mixtures of oxide fillers (SiO2/Al2O3) on ionic conductivity of composite electrolytes based on poly(ethylene oxide) (PEO) oligomers (Mw=250, 500, 1000, and 2000)+lithium bis(trifluromethylsulfonyl)imide [LiN(CF3SO2)2] (LiTFSI) (Li/O=1:20) are studied using electrochemical impedance spectroscopy (EIS), differential scanning calorimeter (DSC), and Fourier transform infrared spectroscopy in the attenuated total reflectance mode (FTIR-ATR). Fillers show similar effect on conductivity in all systems: no distinguishable effect is found with filler type, and addition of filler decreases conductivity at temperatures above the melting point but increases conductivity at temperatures below. The addition of fillers stiffens polymer segments, as evidenced by enhancement in Li+–polymer interactions above the melting point seen in the IR spectra and an increase in Tg found from the DSC analysis. No reduction in ion-pairing upon addition of filler is observed from the IR spectra. The increase in conductivity at temperatures below the melting point is believed to be due to faster ion transport along the filler surface rather than through enhanced mobility of polymer segments. The insulating nature of fillers and stiffening of the polymer solvent in the presence of fillers cause a decrease in conductivity at temperatures above the melting point and is correlated solely with volume fraction of the filler.