Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) and its derivatives are considered promising candidate materials for all-solid-state Li-ion batteries as solid electrolytes due to their high Li⁺ conductivities of around 10^-4 S cm^-1 at room temperature and wide electrochemical window. Despite the larger number of detailed studies utilizing various experimental and theoretical approaches, the experimentally evaluated Li⁺ conductivities did not precisely match the computationally predicted values. The discrepancy between the ion conductivities can be attributed to the ambivalent properties of grain boundaries (GBs). GB structure is a critical parameter that controls the macroscopic properties of materials. However, various technological limitations of both the experimental and theoretical approaches utilized for elucidating the Li⁺ conduction behavior of GBs have restricted our ability of reaching a deeper understanding of the processes occurring at the atomic level. In this study, we investigated the Li⁺conducting behaviors of various tilted GBs of cubic LLZO with a garnet framework by using molecular dynamics approaches.

Nine stoichiometric equilibrium GB models were used to determine Li⁺conductivity. It was found that the GB conductivity was smaller than the bulk one regardless of the orientation. In particular, the conductivities perpendicular to the GBs were characterized by the lowest values.

Some factors affecting Li conductivity near the tilted GBs may be related to their stability. The relationship between the structural distortions at the GBs and the Li⁺ conducting characteristics were evaluated in terms of the GB energies and radial distribution function for the La-La, Zr-Zr, and O-O interactions in the LLZO bulk and GBs. We found that the Li⁺ conductivities are highly correlate with the structural distortions at the GBs.

In order to elucidate the Li⁺ diffusion characteristics at the GBs in more detail, the variations of the atomic concentration near the GBs were calculated along the axis perpendicular to the GB surface. The atomic population at bulk region was constantly modulated, while that at GB region was anomalously dispersed. The average atomic concentration at the GBs was lower than that at the bulk. In particular, the observed decrease in the Li⁺ concentration suggests the formation of Li-deficient sites across the GB center, which represents the primary reason for the degraded ionic conductivity in the GB region. In fact, the Li ionic conductivity perpendicular to the GBs decreased with decreasing Li⁺ concentration at the GBs, which was caused by the slower diffusion of Li⁺.

Thus, preventing the distortion of the GB crystallographic structure and the related drop in the Li⁺concentration should be considered one of the top priorities when designing GBs with desirable properties.

Acknowledgement

This work was partially supported by CREST, JST Grant Number JPMJCR1322, Japan.