Poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) polymer electrolyte containing zirconium dioxide nanocrystals (ZrO2-NC) and lithium trifluoromethanesulfonate (LiCF3SO3) has been synthesized using the conventional solution casting method. The addition of ZrO2-NC into the polymeric substrate gave remarkable properties in terms of the electrolyte's ionic conductivity as well as its bulk mechanical strength. The enhanced amorphicity of the polymeric substrate due to ZrO2 and the nanofiller's high dielectric constant make an excellent combination to increase the ionic conductivity (above 10(-4)S cm(-1)). Increasing the nanofiller content raises the ionic conductivity of the electrolyte by two orders of magnitude of which the optimum is 2.65 x 10(-4)S cm(-1) at 13.04 wt% ZrO2-NC loading. Also, the Young's modulus, an indicator of electrolyte's mechanical stability, dramatically increased to 207 MPa upon loading 13.04 wt% ZrO2-NC. Using UV-vis spectroscopy, the electrolytes with 13.04% ZrO2-NC scanned from 200-800 nm wavelengths exhibited a maximum optical transmittance of 52.6% at 10 mu m film thickness. The enhanced conductivity, high mechanical strength and reasonable optical transmittance shown by our composite polymer electrolyte make an excellent electrolyte for future energy saving smart windows such as electrochromic devices. (C) 2014 Elsevier Ltd. All rights reserved.