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.