Improving the Electrochemistry of Anatase for Sodium Ion Batteries By Using Self-Organized TiO2 Nanotubes Prepared By Anodization Under Variable Voltage

The use of TiO2 as negative electrode in non-aqueous sodium ion batteries may show several drawbacks, such as the relatively low gravimetric capacity, low coulombic efficiency at low voltage and poor cyclability. To try to overcome these problems we have followed a new strategy: anatase-type TiO2 nanotubes (nt-TiO2) have been prepared by anodization of titanium under variable voltage. The titanium anodization process under variable voltage modifies de anodization curves (intensity vs. time) in comparison with the anodization under constant voltage at 100 V. As a result of the anodization process, self-organized TiO2 nanotubes with amorphous character are obtained. After annealing at 550°C in air atmosphere anatase-type nt-TiO2 are formed. The relative intensity of the anatase Bragg reflections are influenced by the anodization procedure. The resulting areal capacity values in sodium cells are in the order of 1.0-1.5 mAh/cm2 (as refereed to the fingerprint area of nt-TiO2). When the anodization voltage is linearly increased from 20 to 100 V during one hour, the obtained anatase nt-TiO2 electrode material exhibits improved electrochemical performance. After six hundred discharge-charge cycles the gravimetric capacity vs. Na is about 200 mAh/g. Consequently, the electrochemical behavior in sodium cell of the anatase nt-TiO2 electrode material that we have prepared can be competitive in comparison to lithium. On the other hand, the use of aqueous electrolyte batteries may be an alternative that is explored in this work. Using cyclic voltammetry, we have found that self-organized anatase nt-TiO2 vs. Pt electrode in aqueous NaCl solution exhibits reversible redox processes in the region between ca. -1.0 and -0.2 V.  In contrast to the true insertion of lithium ions into the framework of anatase, the reaction with sodium rather takes place in the surface of nt-TiO2.