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A reflective electrochromic device is fabricated on a 10 cm×10 cm flexible PI/Al substrate using magnetron sputtering. The device has a complementary all-thin-film structure and consists of seven layers. Indium tin oxide (ITO) acts as a transparent electrode deposited on the top, meanwhile, an aluminum (Al) film is adopted as an inter-counter bottom electrode and provides high reflectance. Tungsten oxide (WO 3 ) is used as the main electrochromic layer and nickel oxide (NiO) acts as the complementary electrochromic layer. Lithium niobate (LiNbO 3 ) is applied as a Li+ ion conductor layer. Especially, in the seven-layer structure, two tantalum oxides (Ta 2 O 5 ) are added as transition layers to prevent Li+ escaping from LiNbO 3 when the potential is not applied on the device. When the device is in an electrochromic process, both Ta 2 O 5 provide excellent conductivity for Li + ions and act as the dielectric of electrons. The complementary device with structure Al/NiO/Ta 2 O 5 /LiNbO 3 /Ta 2 O 5 /WO 3 /ITO exhibits good optical properties, and the reflectance modulation reaches up to 55% measured by a spectrophotometer in the range of 400–1600 nm. The cyclic stability of the electrochromic device is investigated. The results indicate that the charge density involved in the electrochromic process decreases and the electrochromic response time increases with the cycle number because of the Li + insertion in WO 3.