Influence of lattice dynamics on the ionic conductivity and stability of solid-state lithium-ion conductors
Massachusetts Institute of Technology. Department of Materials Science and Engineering.
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Electrochemical energy storage devices are clean and efficient, but their current cost and performance limit their use in many transportation and stationary applications. Lithium-ion batteries are one of the leading candidates for these large applications, however their current use of liquid electrolytes negatively effects their lifetime and safety. Furthermore, the liquid electrolyte's potential stability window, thermal stability, and volatility are of particular concern in these large-scale applications. Solid-state electrolytes are investigated as one of the best solutions to overcome these challenges. However, the ionic conductivity and especially (electro)chemical stability of many solid electrolytes are still problematic. The focus of this thesis is on ionic mobility and stability of solid-state Li-ion conductor and descriptors that correlates with these properties. We first provide a comprehensive review of several important families of Li-ion conductors that have been studied and published in the literature focusing on their and an overview of some descriptors that have been proposed to correlate with the ionic conductivity/activation energy, for instance, the volume of the diffusion pathway, high-frequency dielectric constants and frequencies of low-energy optical phonons. Build upon these previous understandings, we propose a new approach to understand ion mobility and stability against of lithium insertion/removal in ion conductors based on lattice dynamics. By combining inelastic neutron scattering measurements with density function theory computation, greater lithium ion mobility was correlated with decreasing lithium vibration frequency that was quantified using a newly proposed descriptor which we phonon band centers. Known superionic lithium conductors were shown to have not only low lithium phonon center but also low anion phonon band center, which unfortunately reduces stability against electrochemical oxidation. Therefore, the interplay between lattice dynamics and ion mobility and stability highlights the need and opportunities to search for fast lithium ion conductors having low lithium band center but high anion band center which exhibit high ion conductivity and high (electro)chemical stability in lithium ion batteries. We show and discuss that Olivines with low lithium band centers but high anion band centers are particularly promising to explore for lithium ion conductors with high ion conductivity and stability. With this new approach, we were able for the first time to account for the trend in ionic conductivity and electrochemical oxidation stability of lithium ion conductors from one common physical origin, their lattice dynamics. Such findings open new avenues for the discovery of new lithium ion conductors with enhanced conductivity and stability using lattice dynamics. Finally, to study the correlation between the actiation energy and the pre-exponential factor, the ionic conductivity and activation energy of lithium in the Li₃PO₄-Li₃VO₄-Li₄GeO₄ system was systematically investigated as model system. The sharp decrease in activation energy upon Ge substitution in Li₃PO₄ and Li₃VO₄ was attributed to the reduction in the enthalpy of defect formation while the variation in activation energy upon increasing Ge content was rationalized in term of the inductive effect. The series of compound with and without partial lithium occupancy were shown to fall into two distinct lines whose slope was related to the inverse of the energy scale associated with phonon in the systems according to multi-excitation entropy theory and the intercept to the Gibbs free energy of defect formation. Compiled data of pre-exponential factor and activation energy for commonly studied Li-ion conductors shows that this correlation is very general, implying an unfavorable trade-off between high pre-exponential factor and low activation energy needed to achieve high ionic conductivity.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 126-145).
DepartmentMassachusetts Institute of Technology. Department of Materials Science and Engineering.
Massachusetts Institute of Technology
Materials Science and Engineering.