Understanding the Amorphous Lithiation Pathway of the Type i Ba₈Ge₄₃Clathrate with Synchrotron X-ray Characterization

Tetrel (Tt = Si, Ge, and Sn) clathrates have highly tunable host-guest structures and have been investigated as novel electrode materials for Li-ion batteries. However, there is little understanding of how the clathrate structure affects the lithiation processes and phase evolution. Herein, the electrochemical lithiation pathway of type I clathrate Ba8Ge43 is investigated with synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analyses and compared to the lithiation of germanium with a diamond cubic structure (α-Ge). The results confirm previous laboratory XRD studies showing that Ba8Ge43 goes through a solely amorphous phase transformation, which contrasts with the crystalline phase transformations that take place during lithiation of micrometer-sized α-Ge particles. The local structure of framework-substituted clathrate Ba8Al16Ge30 after lithiation is found to proceed through an amorphous phase transformation similar to that in Ba8Ge43. In situ PDF and XRD during heating show that the amorphous phases derived from lithiation of Ba8Ge43 are structurally related to various Li-Ge phases and crystallize at low temperatures (350-420 K). We conclude that the Ba atoms inside the clathrate structure act to break up the long-range ordering of Li-Ge clusters and kinetically prevent the nucleation and growth of bulk crystalline phases. The amorphous phase evolution of the clathrate structure during lithiation results in electrochemical properties distinct from those in α-Ge, such as a single-phase reaction mechanism and lower voltage, suggesting possible advantages of clathrates over elemental phases for use as anodes in Li-ion batteries.