Spatial extent of lithium intercalation in anatase TiO2

Thin smooth anatase TiO2 films are obtained by electron beam evaporation of reduced TiO2. These films show a preferential (004) orientation when deposited on electron beam evaporated amorphous titanium. Electrochemical lithium insertion into these films is studied with several optical and electrochemical techniques. A coloration efficiency of 13 cm(2) C-1 is found, which is twice as high as that reported for TiO2 films grown by chemical vapor deposition (CVD). potential-dependent capacitance measurements show that after the extraction of lithium ions has taken place a small region at the surface of the electrode has a much higher dielectric constant than that of the bulk of the electrode. This is explained by the presence of irreversibly trapped lithium ions in the region where a (reversible) phase transformation from anatase TiO2 to anatase Li0.5TiO2 has occurred. The extent of this region depends strongly on the intercalation potential; values of 7 and 17 nm are found after 2.5 h of intercalation at -1.0 and -1.2 V vs SCE, respectively. The dielectric constant of the modified surface region is found to range between 500 and 900. A scheme is proposed that describes the mechanism of lithium insertion in terms of a moving TiO2\Li0.5TiO2 phase front.