Different Oxygen Desorption Durabilities of Lithium Titanium Oxides as Confirmed by Transmission Electron Energy-Loss Spectroscopy Analysis of Li4Ti5O12 and β-Li2TiO3 Biphase Specimens

Spinel-type Li4Ti5O12 and monoclinic β-Li2TiO3 phases are well known as typical functional materials in lithium titanium oxide. The former phase is practically applied as a negative electrode material for Li-ion battery systems, while the latter phase is used as a tritium breeder blanket material for thermonuclear reactors; thus, the material stability of these phases is essential for their functions. In this study, we investigate the oxygen desorption durability of oxide materials with different structures and chemical formulas by transmission electron microscopy-based electron energy-loss spectroscopy. To compare the different properties of these two phases, biphase specimens that consisted of Li4Ti5O12 and β-Li2TiO3 phases with a homogeneous thickness were successfully prepared by a two-step thermal process to perform suitable analysis by transmission electron microscopy. Irradiation with a scanning transmission electron microscopy probe, followed by electron energy-loss spectroscopy analysis, was performed in the Li4Ti5O12/β-Li2TiO3 interphase region of the biphase specimen, revealing that the oxygen desorption feature of β-Li2TiO3 was approximately one-third that of the Li4Ti5O12 phase. The stability of the cation–oxygen bond in each oxide crystal was also simulated by reaction enthalpy analysis based on density functional theory calculations, and the lattice stability of the Li4Ti5O12 phase was estimated to be three times higher than that of the β-Li2TiO3 phase. The oxygen vacancy formation energy of the β-Li2TiO3 lattice was approximately 0.8 eV higher than that of the Li4Ti5O12 phase. Thus, the reason for the different oxygen desorption durabilities of these oxide phases is not the stability of the cation–oxygen bond but the ease of reduction phase formation via oxygen desorption.