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Title:Fast ion conduction in solid electrolyte materials: An investigation of the relationship between lattice dynamics, geometric frustration induced disorder, and ionic conductivity
Author(s):Gustafson, William
Advisor(s):Ertekin, Elif
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Ionic Conductivity
Lattice Dynamics
Geometric Frustration
Abstract:Countless industries rely on lithium ion batteries for dependable energy storage. As society continues to evolve and move towards integration of more electric powered devices and vehicles, battery technologies must innovate to provide these many industries with larger voltage solutions while maintaining or reducing battery size and weight. Examples of large industries that would benefit from improved energy storage technology include consumer electronics, electric vehicles, and renewable energy. All solid state batteries have the potential for higher voltages and safer use as they are more resistant to combustion. Solid electrolytes are currently an obstacle preventing wide scale deployment of solid state batteries as a material has yet to be found that satisfies the required properties of high ionic conductivity and voltage stability against both the cathode and anode. Computational methods can be used as a relatively fast way to discern material properties that could potentially be used to predict new solid electrolytes. However, current theories and known material properties that describe ionic conductivity are not practical for use in high-throughput (HT) prediction algorithms. In order to find better descriptors of ionic conductivity for use in a HT screening one must first have a better physical understanding of superionic conductors and how they differ from regular ion conductors. Pursuing this topic, we studied the lattice dynamical properties and cation disorder of a known superionic conductor, sodium beta-alumina, and compared the results to similar studies conducted on recently synthesized lithium halide materials, Li3YCl6 and LiCs2YCl6. Density functional theory was used to find optimum geometries and calculate the energies of each configuration and the forces experienced by each atom. The harmonic approximation was then used to extrapolate from these results and calculate phonon properties of each material. The superionic conductor, sodium beta alumina, was found to have a soft sodium sub-lattice that is isolated from a stiff anion framework of alumina. In contrast, the fast lithium conducting halide material Li3YCl6 has a soft lithium sublattice that is not isolated from its soft anion framework made up of Y and Cl. Lastly, LiCs2YCl6 is found to have similar phonon properties to Li3YCl6, but exhibits no discernible lithium conductivity. Both sodium beta alumina and Li3YCl6 exhibit frustration in their respective mobile ion sublattices, which leads to disorder of the mobile ion, unlike LiCs2YCl6 which has no frustration or observable disorder in the crystal lattice. Thus having disorder in the mobile ion sublattice is important when searching for a fast ion conductor, whereas having a soft lattice and weak bonding in the material does not always imply fast ion conduction. In addition to disorder, superionic conductors often possess a soft mobile ion sublattice accompanied by a stiff anion framework in which the mobile ion sublattice is free to move about.
Issue Date:2020-07-22
Rights Information:Copyright 2020 William Gustafson
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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