Free volume structure at interphase region of poly (ethylene oxide)-Al2O3 nanorods composites based solid polymer electrolyte and its direct correlation with Li ion conductivity

Abstract

Poly (ethylene oxide), PEO, based solid polymer electrolytes (SPEs) filled with nanosize passive fillers are shown to have higher ionic conductivity compared to pure PEO based SPEs. The enhancement in ionic conductivity has been attributed to an alternate path of ion transport through larger size free volumes/voids available for ionic conduction in the vicinity of nanofillers i.e. at the interphase region without experimental evidence. In order to investigate the exclusive role of molecular packing/free volume at interphase region on ionic conductivity, PEO based SPEs filled with different amounts (0–10 wt%) of Al2O3 nanorods have been prepared using solvent casting method. The variation in semicrystalline morphology has been investigated using differential scanning calorimetry and X-ray diffraction. Fourier transform infrared spectroscopy and scanning electron microscopy have been used to investigate the interfacial interaction, fracture morphology and nanofiller dispersion in SPEs. Free volume size distribution determined using positron annihilation lifetime spectroscopy becomes broader with the loading of nanorods confirming the creation of an interphase region with larger size free volumes. In case of 5 wt% Al2O3 nanorods, a bimodal free volume size distribution attributed to bulk and interphase region is observed to be directly correlated with the enhancement in ionic conductivity. The results have been compared with PEO-Al2O3 nanoparticles based SPEs. The lower ionic conductivity in case of PEO-Al2O3 nanoparticles based polymer electrolytes was observed to be consistent with the limited modifications observed in the free volume structure at the interphase region. A direct correlation between the free volume structures at interphase region with Li ion conductivity is obtained. The present study provides an experimental evidence of an alternate ion-transport path through the interphase region in polymer composite based SPEs.