Electrochemical and Chemical Reactivities of Titanium Oxide-Based Materials with a Chloroaluminate Ionic Liquid Electrolyte for Aluminum Batteries

By selecting three different types of electrode materials, we intended to better understand the Al3+ intercalation chemistry of titanium oxide-based frameworks with an acidic chloroaluminate electrolyte. In agreement with previous reports, we confirmed that the native interstitial sites of anatase TiO2 are less prone to accommodate Al3+ than Li+ or Na+ ions, while introducing cationic vacancies largely increases the electrochemical storage capacity. Upon the first cycle, the highest reversible capacity, up to 277 mAh/g, was obtained for a hydrated layered structure featuring cationic vacancies. Total scattering data showed that the insertion of Al3+ ions induced a strong distortion of the framework. In addition, combined 27Al MAS NMR and DFT calculations revealed that in oxy-hydroxylated vacant sites, the coordination mode of Al3+ ions depends on the arrangement of anions around vacancies inducing the occurrence of 4-, 5-, and 6-fold coordination modes. Further cycling experiments revealed a progressive capacity fading for all electrode materials. Using cyclic voltammetry on the used electrolyte, we evidenced that a partial dissolution has occurred, which is more pronounced for the layered hydrate compound, and that solubilized species are electrochemically active, giving rise to specific signatures in both CVs and galvanostatic experiments. Raman spectroscopy enabled us to characterize these species, which are derived from the Ti-Cl system. The solubilized species, however, eventually precipitated, as shown by a purple deposit observed on the separator and tentatively assigned to TiCl3, known to be insoluble in this medium. By providing further information on the Al3+ intercalation chemistry and a better understanding of the electrochemical and chemical reactivities of electrode materials, this work will enable progress to be made in the development of aluminum-ion batteries.