Understanding the Zero-Strain Lithium Insertion Scheme of LiLi1/3Ti5/3O4: Structural Changes at Atomic Scale Clarified by Raman Spectroscopy

Lithium titanium oxide LiLi 1/3 Ti 5/3 O 4 (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 Li 1+ x Li 1/3 Ti 5/3 O 4 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 Li 2 TiO 3 and TiO 2, the Raman bands at 510, 400, and 146 cm –1 are attributed to TiO 2 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 A 1 g mode of symmetric stretching vibration ν sym (Ti–O), the E g mode of asymmetric stretching vibration ν asym (Li–O), and the F 2 g 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 TiO 6 octahedron is independent of x, while that between the Li and O atoms in the LiO 6 octahedron and the bond angle between the Ti and O atoms in the TiO 6 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.