Li-Ion Conduction Characteristics at Grain Boundaries\nin Garnet Li_7–<i>x</i>La₃Zr_2–<i>x</i>Nb_<i>x</i>O₁₂ (0 ≤ <i>x</i> ≤ 2)

Abstract

Understanding\nand control of atomic-scale materials design for\nboth bulk and grain boundaries (GBs) of solid electrolytes are essential\nfor developing solid-state batteries. However, the in-depth insight\ninto ion transport characteristics in the GB region is still far from\nunderstood. The ionic conductivity of solid electrolytes is often\nexperimentally measured by electrochemical impedance spectroscopy\nand computationally evaluated by atomic scale modeling. However, there\nis a large gap in conductivity between experiment and simulation,\nand one of the factors is the difficulty of modeling to accurately\nunderstand the relationship between disturbed atomic arrangement and\nionic conduction in the GB region. Therefore, to minimize technological\ngaps, we have demonstrated that molecular dynamics (MD) calculations\nof tilted GBs with various symmetries are a very powerful approach\nto understanding the ion conduction behavior specific to GB having\nan amorphous phase-like disturbance atomic arrangement. We extend\nthis approach to investigate the effect of Nb-substitution on ionic\nconduction in the GB region. In this study, the effect of Nb-substitution\non the ion conduction behavior at GBs of garnet-type solid electrolytes\nof Li_7–<i>x</i>La₃Zr_2–<i>x</i>Nb_<i>x</i>O₁₂ (0\n≤ <i>x</i> ≤ 2) is evaluated via MD calculations\nand multivariate analyses. Higher Li-ion conductivity observed in\nthe thermodynamically stable Σ3 (2 – 1 – 1) =\n(1 – 21) GB structure (relatively lower GB formation energy)\nis characterized by both a high Nb concentration in the GB region\nand a partial rotation of the Zr/NbO₆ octahedron. Further,\nthe analysis of the atomic arrangement and corresponding Li trajectories\nreveals that the Nb substitution promoted the formation of new Li\nconduction paths through partial rotation of the Zr/NbO₆ octahedron and enhanced the ionic conductivity. The results reported\nhere enhance our understanding of new material design strategies,\nincluding conductivity enhancement of LLZO-based solid electrolytes\nand element substitution at Zr sites.