Layered Lepidocrocite Type Structure Isolated by Revisiting the Sol-Gel Chemistry of Anatase TiO2: A New Anode Material for Batteries

Searches for new electrode materials for batteries must take into account financial and environmental costs to be useful in practical devices. The sol gel chemistry has been widely used to design and implement new concepts for the emergence of advanced materials such as hydride organic inorganic composites. Here, we show that the simple reaction system including titanium alkoxide and water can be used to stabilize a new class of electrode materials. By investigating the crystallization path of anatase TiO2, an X-ray amorphous intermediate phase has been identified whose local structure probed by the pair distribution function, H-1 solid-state NMR and density functional theory (DFT) calculations, consists of a layered type structure as found in the lepidocrocite. This phase presents the following general formula Ti-2_x square O-x(4_4),(OH)(4x) center dot nH2O (x similar to 0.5) where the substitution of oxide by hydroxide anions leads to the formation of titanium vacancies (square) and H2O molecules are located in interlayers. Solid-state H-1 NMR has enabled us to characterize three main hydroxide environments, Ti square-OH, Ti-2 square(2)-OH, and Ti-3 square-OH, and layered H2O molecules. The electrochemical properties of this phase were investigated vs lithium and were shown to be very promising with reversible capacities of around 200 mAh.g(-1) and an operating voltage of 1.55 V. We further showed that the lithium intercalation proceeds via a solid-solution mechanism. Li-7 solid-state NMR and DFT calculations allowed us to identify lithium host sites that are located at the titanium vacancies and interlayer space with lithium being solvated by structural water molecules. The easy fabrication, the absence of lithium, easier recycling, and the encouraging properties make this class of materials very attractive for competitive electrodes for batteries. We thus demonstrate that revisiting an "old" chemistry with advanced characterization tools allows one to discover new materials of technological relevance.