Origin of High Grain Boundary Resistance in Anisotropic Polycrystalline Ion Conductors

Abstract

For polycrystalline ion conductors with anisotropic conduction, revealing the origin of high grain boundary resistance and determining the space charge potential and grain boundary thickness are crucial for their applications in clean energy fields, such as batteries and fuel cells. This work uses La_9.33+xSi₆O_26+3x/2 (LSO)-based anisotropic polycrystalline oxide ion conductors with apatite structure as a model system. The results demonstrate that the primary cause of high grain boundary resistance is the significant increase in electrical grain boundary thickness, which arises from electrical anisotropy and random grain orientation. The study also proposes a method to determine the space charge potential at grain boundaries based on the electrical grain boundary thickness, yielding a potential of approximately 0.12 eV for LSO-based conductors. The approach for determining space charge potential can be extended to other anisotropic ion conductors. This study's understanding of the grain boundary conduction mechanism improves the ionic conductivity of materials, thereby enhancing the performance of energy devices such as batteries and fuel cells.