An electrochromic cell is provided having a Li.sup.+ doped oxymethylene-polyoxyethylene solid polymer electrolyte. The film of ionically doped oxymethylene-polyoxyethylene is sandwiched between two complementary electrochromic layers, one of which is tungsten oxide. The other electrochromic layer is a polyaniline or a Prussian Blue material. The complementary electrochromic layers are deposited onto electrically conductive transparent substrates, such as fluorine-doped tin oxide coated glass. The electrochromic cell colors and bleaches by the application of an appropriate electrical potential between the two electrochromic films. The Li.sup.+ doped oxymethylene-polyoxyethylene electrolyte is stable at elevated temperatures up to about 100.degree. C.