Dynamic Structural Transformations in a Series of Zero-Strain Lithium-Ion Battery Materials: Almost Simultaneous Operando X-ray Diffraction and X-ray Absorption Spectroscopy on Li2ZnTi3O8 and Related Compounds

Abstract

A series of Li_4/3-2x/3Zn_xTi_5/3-x/3O₄ (LZTO) with 0 ≤ x ≤ 0.5 have received considerable interest as a negative electrode material for long-cycle-life lithium-ion batteries. However, their dynamic structural transformations under operating conditions have remained unknown, making an in-depth understanding essential for further improving the electrochemical performance. We, thus, performed almost simultaneous operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) studies on x = 0.125, 0.375, and 0.5. The x = 0.5 sample, Li₂ZnTi₃O₈, indicated differences in the cubic lattice parameter between the discharge and charge reactions (δa_cs), corresponding to the reversible movement of Zn²⁺ ions between the tetrahedral and octahedral sites. δa_c was also observed for x = 0.125 and 0.375, although the capacity region exhibiting δa_c decreased with a decrease in x. For all of the samples, there is no significant difference in the nearest-neighbor distance of the Ti-O bond (d_Ti-O) between the discharge and charge reactions. We also demonstrated different structural transformations between the micro- (XRD) and atomic (XAS) scales. In the case for x = 0.5, for instance, the maximum microscale change in a_c was within +0.29(3)%, whereas at the atomic scale, d_Ti-O changed by up to +4.8(3)%. Combined with our previous results for ex situ XRD and operando XRD/XAS measurements on other x compositions, the whole structural nature of LZTO, such as correspondence between a_c and d_Ti-O, origins for voltage hysteresis, and zero-strain reaction mechanisms, has been unveiled.