Structural and Electrochemical Properties of the Type VIII Ba8Ga16–δ Sn30+δ (δ ≈ 1)clathrate during Lithiation

Abstract

Clathrates of the Tetrel (Tt = Si, Ge, Sn) elements are host-guest structures that can undergo Li alloying reactions with high capacities. However, little is known about how the cage structure affects the phase transformations that take place during lithiation. To further this understanding, the structural changes of the type VIII clathrate Ba8Ga16–δSn30+δ (δ ≈ 1) during lithiation are investigated and compared to those in β-Sn with ex situ X-ray total scattering measurements and pair distribution function (PDF) analysis.The results show that the type VIII clathrate undergoes an alloying reaction to form Li-rich amorphous phases (Li x Ba0.17Ga0.33Sn0.67, x = 2 – 3) with local structures similar to those in the crystalline binary Li-Sn phases that form during the lithiation of β-Sn. As a result of the amorphous phase transition, the type VIII clathrate reacts at a lower voltage (0.25 V vs. Li/Li+) compared to β-Sn (0.45 V) and goes through a solid-solution reaction after the initial conversion of the crystalline clathrate phase. Cycling experiments suggest that the amorphous phase persists after the first lithiation and results in considerably better cycling than β-Sn. Density functional theory (DFT) calculations suggest that topotactic Li insertion into the clathrate lattice is not favorable due to the high energy of the Li sites, which is consistent with the experimentally observed amorphous phase transformation. The clathrate structure is hypothesized to kinetically circumvent the formation of Li-Sn or Li-Ga crystalline phases, which results in better cycling and a lower reaction voltage.The electrochemical lithiation pathway of Ba8Ga16–δSn30+δ (δ ≈ 1) is similar to that of the previously investigated type I Ba8Ge43 1 suggesting that the amorphous lithiation could be a common mechanism for guest-filled Tetrel clathrates. As amorphous phase transformations are a beneficial property for alloying anodes, Tetrel clathrates could act as precursors for obtaining amorphous phases with tunable electrochemical properties. With this assumption of a common mechanism for guest-filled clathrates, we will then discuss design rules for clathrate based alloying anodes in terms of optimizing the gravimetric capacity and reaction voltage. References (1) Dopilka, A.; Childs, A.; Bobev, S.; Chan, C. K. Understanding the Amorphous Lithiation Pathway of the Type I Ba8Ge43 Clathrate with Synchrotron X-Ray Characterization. Chem. Mater. 2020, 32, 9444–9457.