Understanding the Lithiation Pathways of Tetrel Clathrates with Synchrotron X-Ray Characterization

Abstract

Tetrel clathrates are materials composed of cage-like frameworks of covalently bonded Group IV (Si, Ge, Sn) elements containing guest atoms in the center of each cage. Our group has been investigating the electrochemical properties of Si and Ge clathrates for potential applications as Li-ion battery anodes. Our studies have found that in some cases the Li-alloying pathways are distinctly different in clathrates compared to those seen in the diamond cubic analogs. For example, the Ba8Al16Si30 Type I clathrate does not undergo a Li-alloying reaction [1] while the Ba8Ge43 clathrate does [2]. In the case of Ba8Ge43, the reaction pathway appears to follow an amorphous phase transformation, in contrast to diamond cubic Ge, which undergoes several phase transformations to form crystalline Li-Ge phases [3].In this work, we employ synchrotron X-ray pair-distribution function (PDF) analysis, X-ray diffraction (XRD), and in situ heating experiments to elucidate the electrochemically induced structural changes in Ge and Sn clathrates and how they differ from those in the elemental analogs. The Type I Ba8Ge43 and Type VIII Ba8Ga16Sn30 clathrates are chosen and compared to diamond cubic Ge and beta-Sn. The PDF method allows us to probe the local structure of the amorphous phases that form after lithiation, while in situ heating experiments enable us to observe structural changes to the amorphous phases upon moderate heating. The insights learned from the structural characterizations are then used to explain the differences in the electrochemistry between the Tetrel clathrates and their elemental analogs.The results suggest that the amorphous phases that are part of the final lithiated state of Ba8Ge43 have local structures similar to the Li-Ge crystalline phases. We conclude that the Ba atoms prevent the bulk nucleation of Li-Ge crystalline phases and restrict the Li-Ge ordering to the scale of 10-15 Å, resulting in a solid solution mechanism and a lower reaction voltage compared to cubic-Ge. Similar analysis of the Type VIII Ga-Sn clathrate (Ba8Ga16Sn30) results in a similar lithiation path showing a solely amorphous phase transformation. These results give insight to the fundamentals of how the initial structure of group IV materials affect their Li-alloying properties and structures. Figure 1 (a) Unit cell of Ba8Ge43, purple atoms are Ge and green atoms are Ba. (b) PDFs of the lithiation of Ba8Ge43 at different states of lithiation. (c) PDFs of cubic-Ge at different states of lithiation.[1] R. Zhao. ACS Appl. Mater. Interfaces. 9 (2017) 41246–41257.[2] A. Dopilka. ACS Appl. Mater. Interfaces. 10 (2018) 37981–37993.[3] H. Jung. Chem. Mater. 27 (2015) 1031–1041. Figure 1