Cation Insertion Characteristics of Mesoporous Titania-Silica Composite Layers

Innovative approaches in materials and concepts will be key to address the energy transition challenge. A key area involves ion insertion materials which are not only interesting for energy storage but also for energy conversion and low power electronics and optoelectronics¹. Titanium dioxide (TiO₂) is well known ion insertion electrode capable of accommodating lithium ions. The ion storage capacity and ion insertion/extraction rates can be enhanced significantly by nano structuring and doping^2,3. For nanostructured TiO₂, insertion of other cations such as K⁺, Ca²⁺, Mg²⁺ and Al³⁺ has been shown⁴. The versatility and stability of titania chemistry lends itself to tuning of its morphology by simple fabrication methods. In this paper, mesoporous titania-silica composite thin films were fabricated by a facile sol gel approach. The thin film was spin-coated and cured onto a metal coated silicon substrate, which served as current collector and loaded in a three-electrode cell for electrochemical characterisation. A facile and reversible ion insertion and extraction of the titania-silica composites was found upon, respectively, reduction of Ti(IV) to Ti(III) and oxidation of Ti(III) to Ti(IV) from aqueous and non-aqueous solutions. Interestingly, the results show that next to Li⁺, also facile reversible ion insertion/deinsertion can be carried with other monovalent cations and even bivalent cations. The morphology and composition of the thin films were determined before and after the ion insertion process using scanning electron microscopy and transmission electron microscopy, combined with EDS and EELS. These measurements provided insights into the charge-discharge characteristics of the composite material and the charge capacity was determined. Proton intercalation/extraction was found a competing reaction during the charge/discharge process in aqueous solutions and depend on the pH. The total charge capacity of the material is then the sum of proton and cation insertion charges. The electrochemical characteristics of the material thus can be tuned according to the application.<br/><br/><br/>1. Sood, A., Poletayev, A.D., Cogswell, D.A. <i>et al.</i> Electrochemical ion insertion from the atomic to the device scale. <i>Nat Rev Mater</i> <b>6</b>, 847–867 (2021).<br/>2. Moitzheim, S., De Gendt, S. & Vereecken, P. M. Investigation of the Li-Ion Insertion Mechanism for Amorphous and Anatase TiO₂ Thin-Films. <i>J. Electrochem. Soc.</i> <b>166</b>, A1–A9 (2019).<br/>3. De Taeye, L. L., Mees, M. J. & Vereecken, P. M. Surpassing the 1 Li/Ti capacity limit in chlorine modified TiO_2−yCl_2y. <i>Energy Storage Mater.</i> <b>36</b>, 279–290 (2021).<br/>4. Koketsu, T., Ma, J., Morgan, B. et al. Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO₂. <i>Nature Mater</i> <b>16</b>, 1142–1148 (2017).