Study of local structure and Li dynamics in [formula omitted] ([formula omitted]) using 6Li and 7Li NMR spectroscopy

Abstract

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 7Li 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 7Li NMR relaxation rates T1−1, T2−1, and T1ρ−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 6Li and 7Li 2D exchange MAS NMR spectroscopy on samples with x=2 and 4 show that magnetization transfer for 7Li below 323K is dominated by spin diffusion.