The role of surface films during lithiation of amorphous and anatase TiO2 nanotubes

X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy are used to investigate the electrochemical lithiation/delithiation behaviour of self-organized amorphous TiO2 and anatase TiO2-x nanotubes. With proceeding bulk lithiation of the active material, surface films form on both nanotube arrays. Carbonates start forming with the beginning of the phase transitions and are reversibly desorbed upon delithiation, on both materials. LiF is found to mainly influence the lithiation/delithiation properties on amorphous nanotubes due to an important decrease of the surface film resistance and an increase of the surface film capacitance at potentials <= 2.0 V, where large amounts of LiF adsorb at the surface. Surface film formation is found to be completely reversible on amorphous nanotubes, while C-O, C=O and fluoride containing adsorbates partially stay adsorbed on anatase nanotubes. The higher reversibility of the surface chemistry on amorphous TiO2 nanotubes is therefore assumed to be essential for the higher rate capability, faster kinetics, and the higher capacity retention observed for amorphous TiO2 materials. This demonstrates that lithiation properties can be substantially altered through changes of the surface chemistry, which can be used to improve kinetics and cycling stability of titania based anode materials in general.