Understanding the Zero-Strain Lithium Insertion Scheme of Li[Li1/3Ti5/3]O4: Structural Changes at Atomic Scale Clarified by Raman Spectroscopy

Abstract

Lithium titanium oxide Li[Li1/3Ti5/3]O4 (LTO) is regarded as an ideal electrode material for lithium-ion batteries because of its “zero-strain” characteristic, high thermal stability, and structural stability. Here, the zero-strain means that the change in cubic lattice parameter is negligibly small during charge and discharge reactions. We performed ex situ Raman spectroscopy on Li1+x[Li1/3Ti5/3]O4 samples with 0 ≤ x ≤ 0.94 to gain information about the relationship between a zero-strain reaction scheme and structural change at the atomic scale. The x = 0 (initial) sample exhibits three major Raman bands at 671, 426, and 231 cm–1 and six minor Raman bands at 751, 510, 400, 344, 264, and 146 cm–1. According to Raman spectroscopy results on other lithium titanium oxides such as Li2TiO3 and TiO2, the Raman bands at 510, 400, and 146 cm–1 are attributed to TiO2 anatase, which is used as a starting material. As x increases from 0 to 0.94, the two major Raman bands at 426 and 231 cm–1 show a blue shift, while the major Raman band at 671 cm–1 maintains frequency. The three major Raman bands at 671, 423, and 231 cm–1 are assigned to the A1g mode of symmetric stretching vibration νsym(Ti–O), the Eg mode of asymmetric stretching vibration νasym(Li–O), and the F2g mode of bending vibration δ(Ti–O), respectively. Thus, the change in the Raman spectrum with x indicates that the bond length between the Ti and O atoms in the TiO6 octahedron is independent of x, while that between the Li and O atoms in the LiO6 octahedron and the bond angle between the Ti and O atoms in the TiO6 octahedron change with x. Raman studies with decreasing x from 0.94 to 0.10 clarified that such local structural changes are reversible, as in the case for the electrochemical reaction. The zero-strain insertion scheme is discussed from the perspective of Raman spectroscopy.