Insights into $\text{Li}^{+}$, $\text{Na}^{+}$ and $\text{K}^{+}$ Intercalation in Lepidocrocite-type Layered $\text{TiO}_{2}$ Structures

Abstract: A lamellar lepidocrocite-type titanate structure with ~25% $\text{Ti}^{4+}$vacancies was recently synthesized, and it showed potential for use as an electrode in rechargeable lithium-ion batteries. In addition to lithium, we explore this material's ability to accommodate other monovalent ions with greater natural abundance (e.g. sodium and potassium) in order to develop lower-cost alternatives to lithium-ion batteries constructed from more widely available elements. Galvanostatic discharge/charge curves for the lepidocrocite material indicate that increasing the ionic radius of the monovalent ion results in a deteriorating performance of the electrode. Using first-principles electronic structure calculations, we identify the relaxed geometries of the structure for various positions of the ion in the structure. We then use these geometries to compute the energy of formations. Additionally, we determine that all ions are favorable in the structure, but interlayer positions are preferred compared to vacancy positions. We also conclude that the exchange between the interlayer and vacancy positions is a process which involves the interaction between interlayer water and surface hydroxyl groups next to the titanate layer. We observe a cooperative effect between structural water and $\text{OH}$ groups to assist alkali-ions to move from the interlayer to the vacancy site. Thus, the as-synthesized lepidocrocite serves as a prototypical structure to investigate both the migration mechanism of ions within a confined space along with the interaction between water molecules and the titanate framework.