A high contrast, self-standing, and thermoplastic solid polymer electrolyte (PE) with electrochromic property is fabricated by utilizing N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), heptyl viologen (HV(BF4)(2)), succinonitrile (SN), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP). Various amounts of the ionic liquid (IL, BMIMBF4) are added into the PE, and the amount of IL is found to play an important role in affecting the ionic conductivity, morphology, and thus electrochemical property of the obtained PE. The IL content in the PE is optimized in order to achieve a better electrochemical and electrochromic properties of the obtained electrochromic device (ECD). The effect of the IL content in PE on the electrochemical and electrochromic properties of the ECD is discussed in terms of the ionic conductivity and morphology of the PE. The optimized ECD, which uses the PE containing 5 wt% IL, exhibits a reversible transmittance change of 60.1% at 615 nm during the switching between 0 and 0.9 V with a promising long-term stability; both the transmittance change and the long-term stability of the ECD using the PE containing 5 wt% IL are much better than that using the PE without IL (C) 2015 Elsevier B.V. All rights reserved.