Molecular Dynamics Simulation of Li-Ion Conduction at Grain Boundaries in NASICON-Type LiZr₂(PO₄)₃ Solid Electrolytes

Abstract

Na superionic conductor-type LiZr2(PO4)3 (LZP)-related materials are considered promising solid electrolytes that can assist in realizing rechargeable all-solid-state Li-ion batteries with high Li-ion conductivity and electrochemical stability. However, the grain boundary (GB) resistance considerably reduces the total Li-ion conductivity of the sintered polycrystalline body, which is observed in LZP and several other Li-ion conductive oxides. In this regard, the rational design of solid–solid interfaces is known to improve the ionic conductivity. Therefore, examining the ion conduction mechanism at GBs is important from the viewpoints of practical usability and elucidation of the fundamental knowledge on dynamics in crystalline solids. In this study, 32 GB models were constructed, consisting of various Miller indices and terminations, and the corresponding GB Li-ion conductivities were evaluated using molecular dynamics simulations with density functional theory-derived force-field parameters. A few of the GB models exhibited improved Li-ion conductivities compared to the bulk ionic conductivity. Machine learning analysis using descriptors derived from interfacial structure characteristics suggested that the size of cavities around the original Li 6b sites significantly affected the GB ionic conductivity, which could enable the rational design of GB structures.