A study of ionic materials for the energy applications through first-principles calculations and calphad modeling

Computational simulations play an important role in materials designs. In particular, computational thermodynamics have been steadily demonstrated to provide invaluable insight to identify materials exhibiting desirable properties. Especially, application to the ionic materials as energy materials are subject to great interest due to their importance in alternate energy source and their complex defect structures. In the present dissertation, three examples using CALPHAD (CALculation of PHAse Diagram) and first-principles calculations to aid materials design for energy applications are presented:;i) (La1--xCa x)FeO3--delta perovskite is a potential candidate material for gas separation membrane and cathodes in solid oxide fuel cells. Its performance is dictated by its defect structures under service conditions. In the present work, the defect chemistry and the energetics of (La1-- xCax)FeO 3--delta are studied owing to their importance in understanding its electrochemical performance. The interactions of multi-component and multiple-defects are modeled by the CALPHAD method using the thermochemical data obtained from oxygen nonstoichiometry and the experimental phase equilibrium data. The calculated phase diagrams are in good agreement with experimentally reported phase equilibrium data. Based on the models developed, the defect chemistry and the underlying energetics of (La1--xCa x)FeO3--delta perovskite under various service conditions are predicted in terms of gas composition, pressure, temperature, and Ca content and are compared with experimental validations. The developed models hence provide guidance on operational parameters of membranes, solid oxide fuel cells and other applications involving (La1-- xCax)FeO3--delta perovskite.;ii) LiBH4, owing to its high hydrogen density (18wt%), has been widely studied as a possible candidate for the hydrogen storage material. However, it desorbs hydrogen gas at a relatively high temperature (400 °C) due to the strong ionic bonding. Since the hydrogen atoms form covalent bond with boron atom, its effect is questionable, and as an effort to elucidate doping effect on the hydrogen desorption temperature, divalent metal-dopants, Mg, Ca, and Zn, on the stability of LiBH4 is studied. First-principles calculations have been used to study the relative energies of hypothetical doping compounds, and then their dehydriding reactions are estimated using the CALPHAD approach to understand the energetically-preferred reaction mechanism of metal with LiBH4.;iii) LiFePO4 is an electrode material for Li-ion battery. Phase transformation and electrochemical cell potential during insertion/deinsertion of Li+ ions are crucial properties which dictate the use of this material as an electrode. By using first-principles calculations, discharge voltage and enthalpy of formation are obtained, and phase diagram of the LiFePO 4-FePOu pseudo-binary system and cell voltage change at a given condition are predicted by thermodynamic modeling.