Experimental and Computational Study of the Lithiation of Ba₈Al _yGe_{46- y} Based Type I Germanium Clathrates

In this work, we investigate the electrochemical properties of Ba₈Al _yGe_{46- y} ( y = 0, 4, 8, 12, 16) clathrates prepared by arc-melting. These materials have cage-like structures with large cavity volumes and can also have vacancies on the Ge framework sites, features which may be used to accommodate Li. Herein, a structural, electrochemical, and theoretical investigation is performed to explore these materials as anodes in Li-ion batteries, including analysis of the effect of the Al content and framework vacancies on the observed electrochemical properties. Single-crystal X-ray diffraction (XRD) studies indicate the presence of vacancies at the 6c site of the clathrate framework as the Al content decreases, and the lithiation potentials and capacities are observed to decrease as the degree of Al substitution increases. From XRD, electrochemical, and transmission electron microscopy analysis, we find that all of the clathrate compositions undergo two-phase reactions to form Li-rich amorphous phases. This is different from the behavior observed in Si clathrate analogues, where there is no amorphous phase transition during electrochemical lithiation nor discernible changes to the lattice constant of the bulk structure. From density functional theory calculations, we find that Li insertion into the three framework vacancies in Ba₈Ge₄₃ is energetically favorable, with a calculated lithiation voltage of 0.77 V versus Li/Li⁺. However, the calculated energy barrier for Li diffusion between vacancies and around Ba guest atoms is at least 1.6 eV, which is too high for significant room-temperature diffusion. These results show that framework vacancies in the Ge clathrate structure are unlikely to significantly contribute to lithiation processes unless the Ba guest atoms are absent, but suggest that guest atom vacancies could open diffusion paths for Li, allowing for empty framework positions to be occupied.