Density Functional Theory-based Modeling of Cathode Materials for Electronic and Electrochemical Systems

Materials functioning as cathodes in electronic devices such as low work function electron emitters and electrochemical devices such as solid oxide fuel cells and lithium-ion batteries have become ubiquitous in modern technology. For electron emission applications, we have studied scandate cathodes with Ba and BaO and proposed the most probable mechanism responsible for the low work functions observed in experimental scandate cathodes. Next, we considered a representative set of transition metal-containing perovskite oxides as new potential electron emission materials. We have explained trends in perovskite work functions via band filling, bond hybridization, and surface dipoles. In addition, we computationally predicted that SrVO3, particularly when doped with Ba, may function as an ultra-low work function material and also exhibit a very long thermionic emission lifetime. Our work on solid oxide fuel cell cathodes used high-throughput Density Functional Theory methods to screen approximately 1300 distinct perovskite oxide compositions for new fuel cell cathodes. We used first principles-based bulk electronic structure descriptors to screen for high oxygen reduction and oxygen evolution reaction activity, and multicomponent phase stability analysis to assess the stability of all compounds under realistic operating conditions. This study resulted in a list of several new high activity, high stability perovskite materials that are promising for next generation fuel cell cathodes. Our research of lithium-ion batteries focused on protective cathode coatings and the conversion cathode material FeF3. For coatings, we developed an electrolyte model and have shown that practical battery coatings need to be amorphous or otherwise highly defected to facilitate sufficiently fast lithium diffusion. For FeF3, we have combined Density Functional Theory with X-ray absorption spectroscopy to determine the sequence of material phases occurring during charge and discharge cycles, and have shown that the reaction pathway of FeF3 during charge and discharge proceeds through the same set of phases. Our results demonstrate that rational nanostructuring of the FeF 3 cathode can most likely mitigate a sizeable fraction of the overpotentials resulting from nucleation of new phases and compositional inhomogeneity in the battery, thus making this material one step closer to being a viable option for future high energy density lithium-ion batteries.