Materials Science & Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/2260
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Item First Principles Computational Design of Solid Ionic Conductors through Ion Substitution(2019) Bai, Qiang; Mo, Yifei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.Item First Principle Computational Study of Fast Ionic Conductors(2018) He, Xingfeng; Mo, Yifei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fast ionic conductors have great potential to enable novel technologies in energy storage and conversion. However, it is not yet understood why only a few materials can deliver exceptionally higher ionic conductivity than typical solids or how on can design fast ion conductors following simple principles. In this dissertation, I applied first principles computational method to understanding the fast ionic diffusion within fast ionic conductors and I demonstrated a conceptually simple framework for guiding the design of super-ionic conductor materials. I studied Na0.5Bi0.5TiO3 (NBT) as the model material for oxygen ionic conductor. The structure-property relationship of the NBT materials is established. Based on the newly gained materials understanding, our first principles computation predicted that Na and K were promising dopants to increase oxygen ionic conductivity. The newly designed NBT materials with A-site Na and K substituted A sites exhibited a many-fold increase in the ionic conductivity at 900K comparing to that in the experimental compound. We demonstrated that the concerted migration mechanism with low energy barrier is the universal mechanism of fast ionic diffusion in a broad range of ionic conducting materials. Our theory provides a conceptually simple framework for guiding the design of super-ionic conductor materials, that is, inserting mobile ions into high-energy sites to activate concerted ion conduction with lower migration barriers. We demonstrated this strategy by designing a number of novel fast Li-ion conducting materials to activate concerted migration with reduced diffusion barrier. We identified the common features of crystal structural framework for lithium SICs. Based on the determined attributes, we performed a high-throughput screening of all lithium-containing oxide and sulfide compounds. The screening revealed several crystal structures that are potential to be fast ion conductors. Through aliovalent doping, we modified the Li content of these structures which resulting in different Li sublattice within the structure and we found a number of lithium super- ionic conductors that are predicted to have Li+ conductivities greater than 0.1 mS/cm at 300K.