Lithium Migration at High Temperatures in Li4Ti5O12 Studied by Neutron Diffraction

The structure of Li4Ti6O12 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 degrees C. Up to 500 degrees C, linearly increasing values for the unit cell parameter, the isotropic atomic displacement parameters, and the oxygen position are observed. At 700 degrees 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 Li4Ti6O12 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 Li4Ti6O12 at high temperatures, which has been observed by impedance spectroscopic studies. Further heating to 1000 degrees C resulted in a partial decomposition of Li4Ti6O12 into the ramsdellite-type Li2Ti3O7 and the cubic gamma-Li2TiO3, and at 1100 degrees C, the Li4Ti5O12 spinel was fully decomposed.