• Grain boundaries: Polycrystalline materials, as employed in batteries, contain grain boundaries, which can act as barriers for ion migration, as schematically shown in Figure . These boundaries can also impede ion mobility and increase resistance to ion transport, limiting the overall performance and stability of the battery. Controlling and minimizing these boundaries is crucial for improving ion mobility. Note that the formation energies of point defects in the grain boundary are lower compared to the bulk lattice. Vacancies and interstitials have similar formation energies, indicating their equal importance in grain-boundary diffusion. Additionally, the diffusion anisotropy is influenced by the grain-boundary structure, with faster or slower diffusion occurring along the tilt axis in tilt boundaries compared to diffusion normal to the tilt axis.