Li-Ion Conduction Characteristics at Grain Boundaries in Garnet Li7–xLa3Zr2–xNbxO12 (0 ≤ x ≤ 2)

Abstract

Understanding and control of atomic-scale materials design for both bulk and grain boundaries (GBs) of solid electrolytes are essential for developing solid-state batteries. However, the in-depth insight into ion transport characteristics in the GB region is still far from understood. The ionic conductivity of solid electrolytes is often experimentally measured by electrochemical impedance spectroscopy and computationally evaluated by atomic scale modeling. However, there is a large gap in conductivity between experiment and simulation, and one of the factors is the difficulty of modeling to accurately understand the relationship between disturbed atomic arrangement and ionic conduction in the GB region. Therefore, to minimize technological gaps, we have demonstrated that molecular dynamics (MD) calculations of tilted GBs with various symmetries are a very powerful approach to understanding the ion conduction behavior specific to GB having an amorphous phase-like disturbance atomic arrangement. We extend this approach to investigate the effect of Nb-substitution on ionic conduction in the GB region. In this study, the effect of Nb-substitution on the ion conduction behavior at GBs of garnet-type solid electrolytes of Li_7–xLa₃Zr_2–xNb_xO₁₂ (0 ≤ x ≤ 2) is evaluated via MD calculations and multivariate analyses. Higher Li-ion conductivity observed in the thermodynamically stable Σ3 (2 – 1 – 1) = (1 – 21) GB structure (relatively lower GB formation energy) is characterized by both a high Nb concentration in the GB region and a partial rotation of the Zr/NbO₆ octahedron. Further, the analysis of the atomic arrangement and corresponding Li trajectories reveals that the Nb substitution promoted the formation of new Li conduction paths through partial rotation of the Zr/NbO₆ octahedron and enhanced the ionic conductivity. The results reported here enhance our understanding of new material design strategies, including conductivity enhancement of LLZO-based solid electrolytes and element substitution at Zr sites.