Lithium diffusion in Li4Ti5O12 at high temperatures

Synthesis of the spinel lithium titanate Li4Ti5O12 by an alkoxide-free sol–gel method is described. This method yields highly pure and crystalline Li4Ti5O12 samples at relatively low temperature (850 °C) and via short thermal treatment (2 h). 6Li magic angle spinning nuclear magnetic resonance (MAS NMR) measurements on these samples were carried out at high magnetic field (21.1 T) and over a wide temperature range (295–680 K). The temperature dependence of the chemical shifts and integral intensities of the three 6Li resonances demonstrates the migration of lithium ions from the tetrahedral 8a to the octahedral 16c sites and the progressive phase transition from a spinel to a defective NaCl-type structure. This defective structure has an increased number of vacancies at the 8a site, which facilitate lithium diffusion through 16c → 8a → 16c pathways, hence providing an explanation for the reported increase in conductivity at high temperatures. Molecular dynamics simulations of the spinel oxides Li4+xTi5O12, with 0 ≤ x ≤ 3, were also performed with a potential shell model in the temperature range 300–700 K. The simulations support the conclusions drawn from the NMR measurements and show a significant timescale separation between lithium diffusion through 8a and 16c sites and that out of the 16d sites.