A Size-Dependent Sodium Storage Mechanism in Li₄Ti₅O₁₂ Investigated by a Novel Characterization Technique Combining in Situ X‑ray Diffraction and Chemical Sodiation

A novel characterization technique using the combination of chemical sodiation and synchrotron based in situ X-ray diffraction (XRD) has been detailed illustrated. The power of this novel technique was demonstrated in elucidating the structure evolution of Li₄Ti₅O₁₂ upon sodium insertion. The sodium insertion behavior into Li₄Ti₅O₁₂ is strongly size dependent. A solid solution reaction behavior in a wide range has been revealed during sodium insertion into the nanosized Li₄Ti₅O₁₂ (∼44 nm), which is quite different from the well-known two-phase reaction of Li₄Ti₅O₁₂/Li₇Ti₅O₁₂ system during lithium insertion, and also has not been fully addressed in the literature so far. On the basis of this in situ experiment, the apparent Na⁺ ion diffusion coefficient (D_Na+) of Li₄Ti₅O₁₂ was estimated in the magnitude of 10^–16 cm² s^–1, close to the values estimated by electrochemical method, but 5 order of magnitudes smaller than the Li⁺ ion diffusion coefficient (D_Li+ ∼10^–11 cm² s^–1), indicating a sluggish Na⁺ ion diffusion kinetics in Li₄Ti₅O₁₂ comparing with that of Li⁺ ion. Nanosizing the Li₄Ti₅O₁₂ will be critical to make it a suitable anode material for sodium-ion batteries. The application of this novel in situ chemical sodiation method reported in this work provides a facile way and a new opportunity for in situ structure investigations of various sodium-ion battery materials and other systems.