Study of local structure and Li dynamics in Li4+xTi5O12 (0 ≤ x ≤ 5) using 6Li and 7Li NMR spectroscopy

We studied the local structure and the Li ion dynamics in electrochemically and chemically prepared Li4+xTi5O12 with x = 0...5. We used magic-angle spinning Li-7 NMR on samples with different Li contents to investigate the sites that are occupied/emptied during Li insertion/removal. While the electrochemical measurements show a lithium insertion in two steps, 1D MAS NMR as a function of the lithium content shows that the overall spectral evolution observed during lithium insertion is inverted during lithium removal. Thereby the second insertion step is associated with an increased structural disorder. For samples with x=0, 2, 3, and about 5, we performed temperature-dependent measurements of the Li-7 NMR relaxation rates T-1(-1), T-2(-1), and T-1 rho(-1) to study the dynamics of the Li ions. For the samples with x=0, 2, and 3, activation energies of (0.45 +/- 0.1) eV were obtained. The highest mobility of the Li ions is observed for the samples with x=2 and 3. Results from Li-6 and Li-7 2D exchange MAS NMR spectroscopy on samples with x=2 and 4 show that magnetization transfer for Li-7 below 323 K is dominated by spin diffusion. (C) 2011 Elsevier Inc. All rights reserved.