Li and Na migration in Guest-free, type I clathrates evaluated via ab initio methods

Open, cage-like clathrates Tt₄₆ (Tt = Si, Ge, Sn) has a potential as anodes in Li/Na-ion batteries. Density-functional theory calculations were performed to explore ionic mobility of Li/Na through clathrate crystals. Local energy minima of Li/Na locations inside the clathrates were determined, the Li/Na migration paths/barriers were calculated using nudged elastic band method. It was found that it is favorable for Li to occupy the Tt₂₀ cage center while preferring the off-center positions in larger Tt₂₄ cages. The lowest Li migration barriers were found to be 0.35, 0.13 and 0.37 eV for Si₄₆, Ge₄₆, and Sn₄₆, respectively. In contrast, Na shows preference for the cage centers and higher migration barriers than Li. In general, the Tt₂₄ channels in the guest-free, type I clathrates are ideal for fast Li diffusion, while Na is too large to migrate effectively between cages. The energy landscape for Li inside the clathrates is uniquely different than that in diamond allotropes, leading to significantly lower energy barriers for Li migration. These results suggest that open frameworks of intermetallic clathrates may enable facile Li migration and have potential as anodes in Li-ion batteries.
[Reference] J. Phys. Chem. C 123, 22812-22822 (2019)