an oxidatively coloring electrochromic layer of composition m y cro 2 + x where m = li , na or k with improved transmittance modulation , improved thermal and environmental stability , and improved resistance to degradation in organic liquid and polymeric electrolytes . the m y cro 2 + x provides complementary optical modulation to cathodically coloring materials in thin - film electrochromic glazings and electrochromic devices employing polymeric li + ion conductors .