Titanium dioxide (TiO2) polymorphs are promising electrode materials for Li-ion and Na-ion batteries, due to their potentially high capacity, cycling stability and charge/discharge rate. The mechanistic understanding of redox processes, involving compounds like TiO2, in terms of formally defined charge transfers between the involved reactants is, by definition, based on the change of the oxidation states of the involved species. Although the way formal oxidation states are determined can be justified by a variety of valid arguments, the implications that come with them, especially with regard to redox processes, might be counter-intuitive and of little practical use. The charge state of titanium in titanium dioxide is commonly assumed to be +4, which is the basis for any description of redox processes involving this material. Previous investigations on the other hand (e.g. Nature 453 (2008) 763) showed that the physical charges on transition metal atoms are generally little affected by changes in their formal oxidation states. We concluded in comprehensive investigations on TiO2 molecules and TiO2 bulk crystals, using different quantum theory of atoms in molecules (QTAIM) approaches like Bader charge analysis or delocalization indices, a lower charge state (+3).
Moreover, a recent investigation (Organometallics 36 (2017) 622) using charge reporter molecules concluded a remarkable stability/similarity of charge states of many transition metal atoms, including titanium, in different environments corresponding to very different formal oxidation states. Ab initio investigations carried out by us on titanium carbonyl complexes support this experimental findings. Furthermore, our QTAIM charge analyses on titanium for its intercalation into p-doped crystals and in titanium halides confirm the similarity of the Ti charge in different environments and do not suggest that the charge state of titanium reaches the ideally assumed Ti4+ in other common Ti(IV) compounds commonly classified as ionic. All of this makes the possibility of further Ti oxidation or O reduction in terms of charge density increase or depletion around the corresponding nuclei, at least in a theoretical framework, possible, as it was seen in our analyses on simple model systems.