Grain Boundary Contributions to Li-Ion Transport in the Solid Electrolyte Li7La3Zr2O12 (LLZO)
S Yu and DJ Siegel, CHEMISTRY OF MATERIALS, 29, 9639-9647 (2017).
The oxide with nominal composition Li7La3Zr2O12 (LLZO) is a promising solid electrolyte thanks to its high (bulk) Li-ion conductivity, negligible electronic transport, chemical stability against Li metal, and wide electrochemical window. Despite these promising characteristics, recent measurements suggest that microstructural features, specifically, grain boundaries (GBs), contribute to undesirable short-circuiting and resistance in polycrystalline LLZO membranes. Toward the goal of understanding GB-related phenomena, the present study characterizes the energetics, composition, and transport properties of three low-energy (Sigma 3 and Sigma 5) symmetric tilt GBs in LLZO at the atomic scale. Monte Carlo simulations reveal that the GB planes are enriched with Li, and to a lesser extent with oxygen. Molecular dynamics simulations on these off-stoichiometric boundaries were used to assess Li-ion transport within and across the boundary planes. We find that Li transport is generally reduced in the GB region; however, the magnitude of this effect is sensitive to temperature and GB structure. Li-ion diffusion is comparable in all three GBs at the high temperatures encountered during processing, and only 2-3 times slower than bulk diffusion. These similarities vanish at room temperature, where diffusion in the more compact Sigma 3 boundary remains relatively fast (half the bulk rate), while transport in the Sigma 5 boundaries is roughly 2 orders of magnitude slower. These trends mirror the activation energies for diffusion, which in the Sigma 5 boundaries are up to 35% larger than in bulk LLZO, and are identical to the bulk in the Sigma 3 boundary. Diffusion within the Sigma 5 boundaries is observed to be isotropic. In contrast, intraplane diffusion in the Sigma 3 boundary plane at room temperature is predicted to exceed that of the bulk, while transboundary diffusion is similar to 200 times slower than that in the bulk. Our observation of mixed GB transport contributions (some boundaries support fast diffusion, while others are slow) is consistent with the limited GB resistance observed in polycrystalline LLZO samples processed at high temperatures. These data also suggest that higher energy GBs with less-compact structures should penalize Li-ion conductivity to a greater degree.
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