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 (DNa+) 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 (DLi+ ∼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.