Clathrates of the Tetrel (Tt = Si, Ge, Sn) elements are potential anode materials for Li-ion batteries with host–guest structures. However, how the cage structure affects the phase transformations during lithiation, or how the lithiation pathways affect the cycling performance of the materials, is not well understood. To further this understanding, the electrochemical lithiation pathways of three types of clathrates, guest-free type-II clathrate (Si136), type-I clathrate Ba8Ge43, and type-VIII clathrate Ba8Ga16−δSn30+δ (δ ≈ 1), are investigated with synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analyses and compared to those of conventional Si, Ge, and Sn electrodes. We find distinct lithiation mechanisms for each type of clathrate. In the type-II Si136 clathrates, Li-ions are first topotactically inserted into the vacant clathrate cages, followed by an alloying reaction that forms an amorphous lithium silicide phase. PDF analysis shows that the clathrate becomes completely amorphous at an earlier stage of lithiation when compared to diamond Si, forming a phase with larger amounts of Si–Si bonding. The initial insertion of Li into the clathrate cages establishes important Li diffusion paths that lead to more homogeneous lithiation throughout the electrode, which is beneficial for Li-ion battery applications. The Ba8Ge43 clathrate and type-VIII Sn clathrates both undergo alloying reactions to form Li-rich amorphous phases, different from the crystalline phase transformations seen in α-Ge and β-Sn. In situ PDF and XRD during heating show that these amorphous phases are structurally related to various Li–Tt. We conclude that the Ba atoms inside the clathrate structure act to break up the long-range ordering of Li–Tt clusters and prevent the nucleation and growth of bulk crystalline phases, resulting in better cycling and a lower reaction voltage. Our results show how clathrates could act as tunable precursors to form amorphous Li alloying phases with novel electrochemical properties, such as a single-phase reaction mechanism and lower voltage, suggesting possible advantages of clathrates over elemental phases for use as Li-ion battery anodes.

Understanding the amorphous lithiation pathways of clathrate anode with synchrotron X-ray characterization and local structure analysis

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