ELECTROCHEMICAL INSERTION IN SOL-GEL MADE CEO2-TIO2 FROM LITHIUM CONDUCTING POLYMER ELECTROLYTE - RELATION WITH THE MATERIAL STRUCTURE

Thin films of (CeO2)x-(TiO2)1-x prepared by a sol-gel process involving two alcoxides (Ce(OBu(s))4 and Ti(OBu(n))4 in BuOH) were studied by cyclic voltammetry in a lithium conducting polymer electrolyte. These films are made up of an amorphous matrix of titanium oxide including nanocrystallites of cerium oxide, less than 10A in size for x values lower than 0.5 and increasing to 50A for higher CeO2 contents in the coatings. The aim of this study was to examine the influence of the structure on the electrochemical insertion in such films, which are suitable counter electrode materials for lithium based electrochromic windows. It is shown that: (i) for films with a low CeO2 content, where the very small separate grains of CeO2 can be seen as pinpoint sites, the lithium ions pass through the TiO2 matrix and occupy these pinpoint sites very quickly according to a homogeneous diffusion process in very small particles; (ii) when the CeO2 grain size increases, the insertion rate in the grain bulk decrease and the diffusion of lithium ions tends to a semi-infinite mode; (iii) compared to pure CeO2 coatings, films of mixed oxides of compositions ranging from 25% to 50% CeO2 exhibit higher diffusion coefficients of lithium, their storage capacity remaining at an acceptable level.