Cation Diffusion Facilitated by Antiphase Boundaries in Layered Intercalation Compounds

Facile ion diffusion is critical to the performance of layered intercalation compounds commonly used in battery electrode applications. The diffusion behavior of layered Na and K intercalation compounds is less well understood than that of their Li counterparts and is complicated by the tendency of those ions to order more strongly. Several such materials are predicted to adopt ion-vacancy orderings based on antiphase boundaries (APBs) between ordered domains, with the density of APBs determining the local intercalant concentration. A mechanism for APB migration has been identified in the canonical layered Na intercalation compound P3-Na x CoO2, allowing for the collective motion of Na or vacancies through the two-dimensional intercalation layers. We report on a kinetic Monte Carlo study examining the macroscopic implications of this APB migration mechanism. We find that the mechanism enables fast long-range diffusion and that the simulated behavior is consistent with Fick’s laws. The resulting diffusion coefficients exhibit a significant dependence on Na concentration due to changes in APB density and migration barriers. This study highlights APB migration as a unique transport mechanism, in that it relies on extended defects inherent to the stable phases rather than vacancy cluster defects, which often lead to highly correlated diffusion in related materials.