Solid ionic conductors are key components of energy storage and conversion devices. To achieve high efficiency in these energy devices, solid ionic conductors should demonstrate high ionic or electronic conductivity. While pristine materials often suffer from poor conductivity, substituting ions in materials can tailor their electronic and ionic transport to fulfill requirements of transport properties in energy devices. In this dissertation, I applied first-principles computational techniques to elucidate the effect of ion substitution on electronic and ionic transport properties of solid materials. Therefore, three representative materials SrCeO3, La2-x-ySrx+yLiH1-x+yO3-y, and Li6KTaO6 are investigated as model systems to elucidate how ion substitution can affect the transport of electron, anion, and cation, respectively.

I studied SrCeO3 as a model material to uncover the effects of B-site dopants on electronic transport. Based on theoretical calculations, I confirmed a polaron mechanism, including polaron formation and hopping, contributed to the electronic conductivity of SrCeO3. I found different dopants exhibit distinct capabilities for localizing electron polarons, and therefore result in different electronic conductivities in doped SrCeO3. The study demonstrated the capabilities of first principles computation to design new materials with desired polaron formation and migration.

I studied La2-x-ySrx+yLiH1-x+yO3-y oxyhydrides as a model material to investigate H- diffusion mechanism in a mixed anion system and its relationship with the cation substitution of Sr2+ to La3+. I found the substitution of Sr2+ to La3+ can alter the H- diffusion mechanism from 2D to 3D pathways. Increasing H- vacancies through Sr2+ to La3+ substitution can also expedite the H- conductivity of the oxyhydrides. Based on the new understanding, a number of promising dopants in Sr2LiH3O were predicted to enhance H- transport.

Fast Li-ion conductor materials as solid electrolytes are crucial for the development of all-solid-state Li-ion batteries. I systematically studied Li+ diffusion mechanisms in Li6KTaO6 predicted by our computational study. I found that different carrier defects such as Li vacancies or interstitials can induce distinct Li+ transport mechanisms. In addition, I developed a computational workflow to predict a wide range of materials in a prototype structure. By employing the workflow, I computationally predicted a group of Li superionic conductors with good stabilities by substituting the Li6KTaO6 structure.