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 (LixBa0.17Ga0.33Sn0.67, 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 in 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 local structure in the clathrate featuring Ba atoms surrounded by a cage of Ga and Sn atoms 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. Based on the improved electrochemical performance, clathrates could act as tunable precursors to form amorphous Li alloying phases with novel electrochemical properties.
Highlights
Tetrel (Tt = Si, Ge, Sn) clathrates are host−guest materials with the potential for unique properties
Laboratory powder X-ray diffraction (XRD) (PXRD) performed on the as-synthesized clathrate powders confirmed that the product mainly consisted of the type VIII clathrate along with other impurities (Figure S1b)
The diffraction pattern showed reflections corresponding to the type VIII clathrate phase along with a 10.2 mol % phase fraction of βSn0.93Ga0.07, which likely originates from the residual Sn flux from the synthesis
Summary
We recently used X-ray pair distribution function (PDF) analysis to evaluate the local and long-range structure of the intermediates that form during room-temperature lithiation of Ba8Ge43 and Ba8Al16Ge30 clathrates, which both adopt the type I structure but contain vacancies or Al atoms, respectively, on the Ge framework.[19] The PDF analysis showed that the lithiation of the clathrates proceeded through amorphous phase transformations, different from those in diamond cubic Ge (α-Ge), which often progress through crystalline phases.[20] We hypothesized that during the lithiation process, the Ba atoms act as “pillars” that kinetically prevent long-range ordering of the regions rich in Li−Ge bonding, which results in suppression of crystalline phase formation. Despite the destruction of the crystalline clathrate structure after lithiation, the PDF analysis showed that the cage-like local
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