Thick CeO2-TiO2 sol-gel coatings for Li-ion storage electrode in electrochromic devices

CeO2-TiO2 sol-gel coatings are well known as Li-ion storage electrode in electrochromic (EC) devices of the form glass/TE/WO3/electrolyte/CeO2-TiO2/TE/glass (TE: transparent electrode, e.g. SnO2:F, FTO). The charge capacity of the CeO2-TiO2 coating is a limiting factor to get a high coloration intensity of such devices. In order to improve the charge capacity of these electrodes, new routes for the preparation of thick porous CeO2-TiO2 sol-gel layers were tested. One route was the preparation of thick porous TiO2 coatings on a conducting glass support (FTO) using a solution of colloidal TiO2 particles. After heat treatment at temperatures up to 550degreesC the coatings were soaked in a solution of a cerium-IV (Ce(NH4)(2)(NO3)(6)) or a cerium-III salt (Ce(NO3)(3) 6H(2)O) and heat treated again. Another route was the preparation of sols by mixing a solution of the cerium-IV or the cerium-III salt or a colloidal CeO2-sol with the colloidal solution of TiO2. After dip coating on FTO-glass the coatings were also heat treated at temperatures up to 500degreesC. All these coatings were studied electrochemically in I M LiClO4 in propylene carbonate electrolyte. Although thick porous single coatings could be obtained, typically 450 run for TiO2 and 600 nm for cerium-titanium oxide, the intercalated and deintercalated Li+ charges remain small and lie in the range of 2 mC/cm(2) to 3 mC/cm(2). The reasons for such low charge capacity is discussed.