Facile Formation of Two-Phase Domains in a Single Crystalline Li₇₋ₓTi₅O₁₂ Particle

Spinel-type lithium titanium oxide, Li4Ti5O12 (LTO), is well known as a negative electrode material for Li-ion batteries with a two-phase reversible reaction mechanism of Li4Ti5O12 ⇔ Li7Ti5O12. This two-phase reaction has no memory effect, unlike in the prototypical LiFePO4 ⇔ FePO4 two-phase reaction system. The absence of the memory effect contributes to the extremely stable charge–discharge performance of LTO. The origin of this stable electrochemical performance is elucidated as a two-phase coexistence in a single crystal, according to theoretical and microscopic analysis. Neural network potential (NNP) and density functional theory (DFT) calculations were conducted in a 16(Li7–xTi5O12) supercell with various Li compositions. More than 18,000 Li/vacancy configurations were evaluated. The low-formation-energy models with an intermediate Li content (0 ≤ x ≤ 3) showed partial phase domain formation in the lattice that is clearly stable against separation into pure Li4Ti5O12 and Li7Ti5O12 phases. Furthermore, a partially reacted single crystalline LTO specimen was observed by scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS). The STEM–EELS mapping of the Li K-edge region clearly showed the existence of partial phase domains less than 50 nm in size in the single crystalline specimen. Both theoretical and microscopic studies strongly suggest that the partial phase domains can be formed in the single crystalline LTO particles, and two-phase coexistence is also acceptable at the single-particle level. This should be a reason no memory effects are found in the LTO electrode.