Unveiling scale differences in the two-phase transformation of Li4Ti5O12 via operando X-ray absorption and diffraction studies

Lithium titanium oxide Li4Ti5O12 (Li[Li1/3Ti5/3]O-4) has been receiving significant attention as an ideal electrode material for lithium-ion batteries, because its cubic lattice parameter (a(c)) changes negligibly during the electrochemical reaction. However, this same characteristic hinders in-depth understanding of the reaction mechanism, restricting widespread applications, such as use in electric vehicles. We thus employed the recently developed operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) technique, combined with multivariate curve resolution by alternating least squares (MCR-ALS) method. The MCR-ALS analyses of both the XAS and XRD data confirmed a two-phase reaction scheme consisting of Li[Li1/3Ti5/3]O-4 and Li-2[Li1/3Ti5/3]O-4, but differences in the structural transformations clearly appeared at the two different scales. Linear, reversible changes in the bond distance between Ti and O atoms were observed at the atomic scale (similar to 1 nm), whereas hysteresis in a(c) between the discharge and charge reactions was observed at the microscale (similar to 100 nm). This difference was found to originate from the difference in spatial resolutions between the XAS and XRD techniques.