Unveiling scale differences in the two-phase transformation of [formula omitted][formula omitted][formula omitted] via operando X-ray absorption and diffraction studies

Abstract

Lithium titanium oxide Li4Ti5O12 (Li[Li1/3Ti5/3]O4) has been receiving significant attention as an ideal electrode material for lithium-ion batteries, because its cubic lattice parameter (ac) 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]O4 and Li2[Li1/3Ti5/3]O4, 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 (∼1 nm), whereas hysteresis in ac between the discharge and charge reactions was observed at the microscale (∼100 nm). This difference was found to originate from the difference in spatial resolutions between the XAS and XRD techniques.