2, Michigan State University, East Lansing, Michigan, United States
Most solid materials have defects such as surfaces, interfaces, and grain boundaries that serve as pathways for transport; as a result, the properties of polycrystalline solids can be vastly different from its intrinsic properties. Diffusion in polycrystalline materials plays an important role in a wide range of electrochemical systems, from batteries to solid oxide fuel cells. Due to the computational expense in explicitly considering the grain boundary network, establishing rational design rules for nanocrystalline materials with desired transport properties remains a challenge. We apply the Smoothed Boundary Method to evaluate the effective diffusivity of polycrystalline materials with a range of morphologies. We find that the anisotropy of grain morphologies plays a critical role in the overall transport behavior, which cannot be quantified using the classical mean field theories. The results are used to obtain an expression for mixed-pathway transport that is capable of universally predicting the effective diffusivity in complex polycrystalline solids without the use of computationally intensive simulations. Applications to battery materials are highlighted.