Li Ion Diffusion in the Anode Material Li_12Si_7:Ultrafast QuasMD Diffusion and Two Distinct Fast 3D Jump Processes Separately Revealed by ~7Li NMR Relaxometry

摘要

The intermetallic compounds Li_xS_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-difrusivity in crystalline Li_12Si_7 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 Li_12Si_7. The diffusion process characterized by the highest Li difiusivity 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 ~10~s s~(-1) at 150 K and reached 109 s~(-1) at 425 K, indicating an activation energy as low as 0.18 eV.