Molecular Dynamics Studies on the Lithium Ion Conduction Behaviors Depending on Tilted Grain Boundaries with Various Symmetries in Garnet-Type Li₇La₃Zr₂O₁₂

Abstract

Grain boundary (GB) structure is a critical parameter that significantly affects the macroscopic properties of materials; however, the evaluation of GB characteristics by modern analytical methods remains an extremely challenging task. In this work, Li+ conductivity degradation at the GBs of cubic Li7La3Zr2O12 (LLZO) with a garnet framework (which represents the most promising candidate material for solid electrolytes utilized in all-solid-state batteries) has been investigated by various molecular dynamics approaches combined with newly developed analytical techniques. It was found that the transboundary diffusion of Li ions was generally slower than their diffusion in the bulk regardless of the GB symmetry; however, this effect strongly depended on the concentration of Li-deficient sites (trapping Li vacancies) in the GB layer. Furthermore, the compactness and density of the combined GB regions represent the key parameters affecting the overall Li+ conductivity of polycrystalline LLZO films.

Reduced Energy Barrier for Li+ Transport Across Grain Boundaries with Amorphous Domains in LLZO Thin Films

“…1 a) [ 21 ]. Typically, EB gb (~ 0.7 eV) is much higher than EB g (~ 0.3 eV) for the case of LLZO [ 20 ].

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The Intrinsic Structural Resistance of a Grain Boundary to Transverse Ionic Conduction

“…They could hence provide data that cannot, at least at present, be obtained experimentally. Molecular dynamics (MD) studies of ion transport in polycrystalline Li‐ion and Na‐ion electrolytes suffer, however, from monitoring the mean‐square‐displacements (MSD) of ions [40–42] . Consequently, the transport coefficient obtained is the tracer diffusion coefficient D *.…”

“…The conversion from D * to σ for grain boundaries is, therefore, far from trivial, even for dilute solutions. The second problem from which simulation studies suffer is that diffusivities are resolved locally based on the MSDs of ions in a specific region of the supercell [40,41] . While a local resolution is in principle ideal for accurately extracting the properties of a grain boundary, the MSD analysis is restricted by the displacements being defined relative to the starting positions of the ions (at the beginning of the simulation).…”

“…Molecular dynamics (MD) studies of ion transport in polycrystalline Li-ion and Na-ion electrolytes suffer, however, from monitoring the mean-square-displacements (MSD) of ions. [40][41][42] Consequently, the transport coefficient obtained is the tracer diffusion coefficient D*. In order to convert D* into a conductivity σ (with the Nernst-Einstein equation), one needs for a dilute solution of charge carriers the tracer correlation coefficient f*.…”

“…The second problem from which simulation studies suffer is that diffusivities are resolved locally based on the MSDs of ions in a specific region of the supercell. [40,41] While a local resolution is in principle ideal for accurately extracting the properties of a grain boundary, the MSD analysis is restricted by the displacements being defined relative to the starting positions of the ions (at the beginning of the simulation). After a simulation run of several nanoseconds, as performed in the reported studies, [40,41] the ions that were originally in the grain-boundary region of the supercell are very likely to also have spent certain amounts of time in the bulk regions of the supercell.…”