Microstructure of the epitaxial film of anatase nanotubes obtained at high voltage and the mechanism of its electrochemical reaction with sodium

Titania nanotubes (nt-TiO2) with high aspect ratio and amorphous character were prepared by Ti anodization at high voltage (100 V). After annealing, the nanotubes crystallized to anatase phase and the XRD patterns and TEM images showed that self-organized anatase nt-TiO2 are composed of crystallographically oriented nanocrystalline domains on the Ti substrate. The contribution of faradic and pseudocapacitive processes was studied for anatase nt-TiO2 in Na cells obtaining the b-parameter values from cyclic voltammetry experiments. The reaction between anatase nt-TiO2 and Na is dominated by redox surface processes (pseudocapacitance) in the region over ca. 0.5 V and by irreversible faradic processes below ca. 0.3 V. The cycling performance of anatase nt-TiO2 in sodium cell was very strongly affected by the imposed electrochemical conditions. Reversible capacity values of about 150-200 mA h g(-1) and good cycling behaviour were found for 2.6-0.5 V of potential limits. Impedance spectra and Na-23 MAS NMR spectra were also used to study the reaction mechanisms. In addition, it was found that by using variable voltage during the anodization of titanium, the resulting electrode exhibits excellent cycling behaviour (ca. 190 mA h g(-1) after ca. 800 cycles). These results are the best cycling behaviour reported to date for anatase in the form of self-organized nanotubes for sodium ion microbatteries.