<title>Structure of sol-gel-made CeO<formula><inf><roman>2</roman></inf></formula>-TiO<formula><inf><roman>2</roman></inf></formula> and relation with electrochemical insertion of lithium</title>

Abstract

Thin films of CeO2-TiO2 have been prepared by a sol-gel process involving 2 alcoxides (Ce(OBu2)4 and Ti(OBun)4 in BuOH). The films are formed in an amorphous matrix of titanium oxide including nanocrystallites of cerium oxide whose size increases considerably (10 to 50 angstroms) for compositions with more than 50% CeO2 in the film. The voltammetric data are in perfect concordance with the structural model previously described and deduced from X-ray Scattering: (i) the TiO2 amorphous matrix accepts only a small amount of lithium up to the ITO reduction limit. The lithium ion mores in this matrix with a mobility similar to that of lithium in TiO2 on its own. (ii) in the presence of very small separate grains of CeO2 equivalent to pinpoint sites, the lithium passes through the TiO2 matrix and occupies these pinpoint sites very quickly according to a homogeneous diffusion process in very small particles. This behavior appears for low values of CeO2 contents. The global mobility should be close to that of Li+ in TiO2. (iii) when the grain size increases, CeO2 occupation of sites in the grain bulk is not instantaneous: the behavior of the intercalation in CeO2 tends to the semi-infinite diffusion mode. On the other hand, TiO2 content decreases, reducing the possible paths necessary to reach the deepest CeO2 grains. The global mobility of Li+ will decrease for these 2 reasons and reaches that of Li+ in pure CeO2.