Lithium Migration at High Temperatures in Li₄Ti₅O₁₂ Studied by Neutron Diffraction

The structure of Li₄Ti₅O₁₂ was investigated by neutron powder diffraction, and the study revealed unprecedented details about lithium migration at high temperatures. A commercial sample of the battery anode material Li₄Ti₅O₁₂ (spinel-type) was measured from room temperature to 1100 °C. Up to 500 °C, linearly increasing values for the unit cell parameter, the isotropic atomic displacement parameters, and the oxygen position are observed. At 700 °C, a change of slope occurs, which is assigned to the beginning migration of lithium. Previous investigations identified the octahedral 16c site in the spinel structure as the migration position of lithium upon heating to high temperatures, and because of that, several phase transitions of Li₄Ti₅O₁₂ at high temperatures have been proposed. Here, we unambiguously identify that the lithium atoms occupy split sites around the 16c positions and order–disorder phase transitions of Li₄Ti₅O₁₂ were not observed. One-particle potential shows that the occupancy of 16c is an unstable configuration and that the split-site structure leads to a more favorable migration position. Occupation of the lithium sites (32e) results in the same long-range diffusion path in all ⟨110⟩ directions. The onset of lithium migration can explain the change of the ionic conductivity of Li₄Ti₅O₁₂ at high temperatures, which has been observed by impedance spectroscopic studies. Further heating to 1000 °C resulted in a partial decomposition of Li₄Ti₅O₁₂ into the ramsdellite-type Li₂Ti₃O₇ and the cubic γ-Li₂TiO₃, and at 1100 °C, the Li₄Ti₅O₁₂ spinel was fully decomposed.