Multi-Stage Structural Transformations in Zero-Strain Lithium Titanate Unveiled by in Situ X-ray Absorption Fingerprints*

Zero-strain electrodes, such as spinel lithium titanate (Li_4/3Ti_5/3O₄), are appealing for application in batteries due to their negligible volume change and extraordinary stability upon repeated charge/discharge cycles. This same property, however, makes it challenging to probe their structural changes during the electrochemical reaction. We report combined studies of lithiation-driven structural transformations in Li_4/3Ti_5/3O₄ using in situ X-ray absorption spectroscopy and ab initio calculations. Based on excellent agreement between computational and experimental spectra of Ti K-edge, we identified key spectral features as fingerprints for quantitative assessment of structural evolution at different length scales. Results from this study indicate that, despite the small change in the crystal lattice, pronounced structural transformations occur in Li_4/3Ti_5/3O₄, both locally and globally, giving rise to a multi-stage kinetic process involving mixed quasi-solid solution/macroscopic two-phase transformations over a wide range of Li concentrations.