Theoretical studies on the ion migration through crystalline materials

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Migration and diffusion of ions or atoms ontop or through solid materials is of interest not only for our fundamental understanding of mobility but also for various applications ranging from charge storage devices over fuel cells, membranes, polymers, sensors, and many more. While different experimental techniques have been developed to gain even atomically-resolved insights on such events, atomistic modelling is often restricted to rather limited and idealized model systems. However, to really capture the whole complexity of ion migration in realistic materials requires a multi-scale approach that is capable of describing concentration-dependent migration processes as well as structural diversities, e.g. defects or grain boundaries.Focusing on Li-borates as well as on perovskites as model systems that are available as well-defined crystalline materials (P1 Weitzel), the goal of this project is to study the relationship between structure (and composition) on the diffusion behaviour of cationic species and to resolve the complex potential energy landscape in these solid-state materials. Based on structural information on the bulk materials as well as well-defined grain boundaries (bicrystals) provided by the HR-TEM (P4 Jooss) and atom probe tomography (P3 Volkert) we will first study the morphology and electronic structure of the respective bulk systems using first principles methods (in particular DFT). The obtained information will then be used to optimize reactive forcefields for subsequent grand-canonical molecular dynamics (GC-MD) simulations on the system dynamics under varying conditions (e.g. temperature, ion loading, lattice defects, etc.). Here we will already be able to study both bulk systems as well as grain-boundaries in contact with an ion reservoir. These simulations will provide insights into the potential energy landscape of the material as well as the role of ionic and electronic charge mobility, information that can be compared to the experimental ToF-SIMS studies (P1 Weitzel) studies. In addition, the observed ion distribution within the material can be compared to the structural APT (P3 Volkert) and NMR (P2 Vogel) analyses, while the energetics will enter the mean-field simulations performed within P1 (Weitzel) and the Monte-Carlo studies of P5 (Maass). Finally, we will perform kinetic Monte-Carlo simulations in order to follow the CAIT experiment (P1 Weitzel) on larger time- and length-scales, where ion migration is induced by a concentration gradient. Again, the outcome will be readily comparable to the experiments performed in P1 (Weitzel) and the structural analyses of P3 (Volkert), P4 (Jooss) and P2 (Vogel).