The present invention relates to electrochromic devices and methods for functionalizing cellulose-based materials, in production and post-production stages, in order to obtain solid-state electrochromic devices. The invention is in the field of electrochemistry. These functionalized cellulose-based materials have typical electrochromic characteristics, specifically the capacity to change the oxidation state, leading to a modification of the physical properties, shown by a color change when exposed to an electric potential difference, being this color change reversible. The color remains in the absence of any electric stimulus, demonstrating a memory effect. An example of an electrochromic device according to the present invention comprises a cellulose-based material soaked with a dispersion of electrochromic inorganic material nanoparticles or with a solution of electrochromic organic molecules, wherein the solution/dispersion contains at least one salt, and is finished by deposition of an electrode in each side of the cellulose-based material, wherein at least one of the electrodes is transparent.