A Size-Dependent Sodium Storage Mechanism in Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> 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 Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> upon sodium insertion. The sodium insertion behavior into Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> is strongly size dependent. A solid solution reaction behavior in a wide range has been revealed during sodium insertion into the nanosized Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> (∼44 nm), which is quite different from the well-known two-phase reaction of Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>/Li<sub>7</sub>Ti<sub>5</sub>O<sub>12</sub> 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<sup>+</sup> ion diffusion coefficient (D<sub>Na+</sub>) of Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> was estimated in the magnitude of 10<sup>–16</sup> cm<sup>2</sup> s<sup>–1</sup>, close to the values estimated by electrochemical method, but 5 order of magnitudes smaller than the Li<sup>+</sup> ion diffusion coefficient (D<sub>Li+</sub> ∼10<sup>–11</sup> cm<sup>2</sup> s<sup>–1</sup>), indicating a sluggish Na<sup>+</sup> ion diffusion kinetics in Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> comparing with that of Li<sup>+</sup> ion. Nanosizing the Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> 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.