Li ion diffusion in the anode material Li₁₂Si₇: Ultrafast quasi-1D diffusion and two distinct fast 3D jump processes separately revealed by ⁷Li NMR relaxometry

The intermetallic compounds Li_xSi_y have attracted considerable interest because of their potential use as anode materials in Li ion batteries. In addition, the crystalline phases in the Li - Si phase diagram turn out to be outstanding model systems for the measurement of fast Li ion diffusion in solids with complex structures. In the present work, the Li self-diffusivity in crystalline Li₁₂Si₇ was thoroughly probed by ⁷Li NMR spin - lattice relaxation (SLR) measurements. Variable-temperature and -frequency NMR measurements performed in both the laboratory and rotating frames of reference revealed three distinct diffusion processes in Li₁₂Si₇. The diffusion process characterized by the highest Li diffusivity seems to be confined to one dimension. It is one of the fastest motions of Li ions in a solid at low temperatures reported to date. The Li jump rates of this hopping process followed Arrhenius behavior; the jump rate was 10⁵ s^{- 1} at 150 K and reached 10 ⁹ s^{- 1} at 425 K, indicating an activation energy as low as 0.18 eV.