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

Abstract

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) cm(2) 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) cm(2) 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.

A highly reversible, low-strain Mg-ion insertion anode material for rechargeable Mg-ion batteries

“…It is well known that there is no obvious change in the XRD pattern during Li þ insertion/extraction in LTO, which is the 'zero-strain' characteristic of Li insertion. [30][31][32][33] The reason that no obvious new peaks were captured during the Mg 2 þ insertion/ extraction in the LTO might be attributed to the very closed ionic radii of the Mg ion (0.062 nm) and Li ion (0.068 nm). We believe that the excellent cycling stability of LTO in a Mg battery could benefit from the small volume changes during the charging-discharging process.…”

“…Thus, Mg 2 þ insertion is expected to be analogous to the insertion process of Li þ /Na þ . [30][31][32][33][34][35] At the beginning of the first Mg insertion process, Mg ions are more likely to occupy the 16c sites of the Li4 phase (where the Li 16d ions are nearly fixed) to form the Mg4Li phase. At the same time, the Li 8a ions, accompanying the other Li ions in the 8a sites of the nearest-neighbor Li4 phase, are pushed by Mg 2 þ to the 16c sites of the nearest-neighbor Li4 phase to form the new Li7 phase.…”

“…The transformation from the Mg4Li to the Mg3.25Li phase is similar to the reaction from Li7 to Li4. 29,31 However, for the Li7 phase, the Li 16c ions are extracted but cannot fill the 8a sites. Then, Mg ions instead of Li ions fill the 8a sites and 16c sites, forming the Mg3Li phase.…”

“…This mechanism is different from the classical Li þ /Na þ insertion/ extraction mechanism in LTO. [30][31][32][33][34][35] Furthermore, the most desirable property of this material is that after the initial activation the volume change during the Mg-ion insertion and extraction is only B0.8%, which shows the 'zero-strain' characteristics, thereby ensuring a long cycle life, as demonstrated by a good capacity retention of 495% after 500 cycles. Our finding is a new mechanism of Mg-ion storage in the LTO host lattice and offers an alternative Mg-ion insertion material for rechargeable Mg ion batteries.…”

Recent Progress in Electrode Materials for Sodium‐Ion Batteries

“…[ 362 ] According to their report, the Na intercalation behavior in the LTO anode strongly depends on the particle size. In 440-nm-sized LTO particles, a small amount of Na ions can be intercalated and provided a specifi c capacity of ≈16 mA h g −1 (corresponding to 0.27 Na per LTO).…”