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FIRST PRINCIPLES INVESTIGATIONS OF ELECTROLYTE MATERIALS IN ALL-SOLID-STATE BATTERIES

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title
FIRST PRINCIPLES INVESTIGATIONS OF ELECTROLYTE MATERIALS IN ALL-SOLID-STATE BATTERIES
author
Li, Yan
abstract
Research into solid electrolytes has recently attracted significant interest along with the emerging demands of developing all-solid-state batteries with major benefits of superior safety, high energy density, and long operation life. The general purpose of this dissertation work is to apply first principles calculations and other computational techniques to reliably explain and predict the detailed structural and ionic transport properties of known and designed solid electrolyte materials. On this basis, five research projects are incorporated into the present work. In the first project, we have focused on Na4P2S6 and Li4P2S6 for which the structures optimized within the framework of density functional theory with PBEsol and harmonic phonon approximations agree with the experimental findings. Compared to the poor ionic conductor Li4P2S6, Na4P2S6 appears to be a competitive electrolyte for Na ion batteries. Of particular interest is that the Na ion conductivity can be enhanced by alloying Na4P2S6 with Li to form new favorable electrolyte Li2Na2P2S6. In an effort to understand the discontinuities in the phonon dispersion curves of anisotropic ionic materials, the second part of this work investigates in detail the coupling of long wavelength electromagnetic waves with the vibrational modes of ionic crystals from some fresh perspectives. We then modeled the stability and ionic conduction mechanisms of pure and doped Li3BO3 and Li3BN2 crystals. By performing extended quasi-harmonic phonon analysis, the X-ray diffraction data of the corrected alpha form of Li3BN2 presents improved agreement with the experimental measurements. Given that the Li ion migration in the high ionic conducting phases of both materials proceeds most likely via vacancy mechanisms, we computationally substituted F for O in Li3BO3 and B for C in beta-Li3BN2 to introduce excess Li vacancies, finding increased ionic conductivity relative to the pure compounds. Continuing computational efforts are devoted to exploring the structural stability and diffusion mechanisms of lithium haloboracites Li4B7O12Cl and Li5B7O12.5Cl. Although the two materials, both characterized by rigid B-O frameworks, have remarkable similarities in atomic arrangements, our simulations show that they exhibit distinct performances as electrolytes. Further attempt finds the Li ion conductivity can be promoted by substituting S for O in Li4B7O12Cl to form Li4B7S12Cl. Our current efforts focus on the crystalline members of the Li7.5B10S18X1.5 (X = Cl, Br, I) family, which are identified to be superionic conductors with measured room-temperature ionic conductivity on the order of 1 mS/cm. The preliminary simulations are consistent with the experimental report, indicating that these materials are very promising solid electrolytes for possible use in solid-state Li ion batteries.
subject
Alkali ion batteries
First principles calculations
Solid electrolytes
contributor
Holzwarth, Natalie (committee chair)
Lachgar, Abdessadek (committee member)
Jurchescu, Oana (committee member)
Kerr, William C. (committee member)
Thonhauser, Timo (committee member)
date
2022-01-15T09:35:37Z (accessioned)
2022-01-15T09:35:37Z (available)
2021 (issued)
degree
Physics (discipline)
identifier
http://hdl.handle.net/10339/99397 (uri)
language
en (iso)
publisher
Wake Forest University
type
Dissertation

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