A new series of flexible nanocomposite-solid polymer electrolyte (SPEs) in the form of poly(vinyl alcohol) (PVA) impregnated by core-shells polyaniline (PANI)-La2CuO4 (20:2 wt%) of nanofillers, 10wt% LiClO4 as electrolyte and various amount 2, 4, 6, and 8 wt% of propylene carbonate, PC as plasticizer via solvent intercalation method. The obtained plasticized PVA-SPEs films were evaluated for their microstructural and morphological behaviors via X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy, respectively. The surface morphology of plasticized PVA-SPEs films illustrated the finer dispersion of inclusion fillers in the PVA matrix with increasing the dosage of PC content. FTIR spectra showed a reduction in the characteristic peaks of PVA in composite films and that denoted the interaction between PVA-OH and fillers. Optical findings exhibited higher absorbance of PVA-SPE in the visible region compared with pure PVA; in addition, the band-gap energy was reduced to 2.68 eV for PVA-SPE containing 8wt% PC. The current-voltage characteristics showed a slight deviation for all PVA-SPE films denoting to non-ohmic behavior. Besides, the maximum ac-conductivity was found at 40.3 × 10−5 S/cm for PVA-SPE film containing 8wt% PC with enhanced their specific capacitance by two folds compared with pure PVA. Mechanical testing showed that elongation at break has been increased attributed to increasing flexibility of polymeric segments with the increase in PC content.