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