Evidence of the Electrochemical Ca2+ Intercalation in Anatase Nanotubes

Here, we demonstrate the electrochemical intercalation of Ca2+ ions within the lattice of anatase nanotubes (a-NTs) synthesized by hydrothermal treatment of TiO2/NaOH precursors followed by Na+/H+ ion exchange and H2O-loss at high temperature in air. Scanning electron microscopy, X-ray diffraction, and Raman spectroscopy confirm the formation of nanosized anatase, whereas transmission electron microscopy highlights the formation of nanotubular morphologies with an average diameter of 10 nm. TiO2 electrodes are able to deliver reversible specific capacities in aprotic batteries vs. calcium metal or in hybrid configurations vs. capacitive activated carbon using aprotic electrolytes (i. e., Ca(BH4)(2) in tetrahydrofuran or Ca(TFSI)(2) in dimethoxyethane, respectively). The electrochemical intercalation of Ca2+ ions into the anatase lattice is confirmed by X-ray absorption spectroscopy in close comparison with Na+ and Li+ intercalations. Ca2+ incorporation leads to the partial amorphization of the TiO2 lattice despite the limited Ca/Ti ratio (i. e., 0.09) obtained in discharge. The analysis of the extended X-ray absorption fine structure region allows the determination of the local structure of the incorporated Ca2+ ions and confirms that a disordered environment is obtained after the electrochemical reaction.