Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Abdolvand, Hamidreza A.

2nd Supervisor

Sun, Xueliang A.

Co-Supervisor

Abstract

The all-solid-state-battery (ASSB) serves as a promising candidate for next generation lithium ion batteries for significant improvements in battery safety, capacity, and longevity. Of the material candidates researched to replace the conventionally used liquid electrolyte, the garnet oxide Ta-LLZO (Li6.4La3Zr1.4Ta0.6O12) has received much attention thanks to its high chemical and electrochemical stability, and ionic conductivity which rivals that of liquid electrolytes. While much investigation has taken place regarding the electrochemical performance of Ta-LLZO, much less is known about the micromechanics, including microstructural characterization, stress and strain development, and material failure which can lead to significant performance degradation of the ASSB. The work presented in this thesis outlines the development of a standard procedure for the microstructural characterization of Ta-LLZO by Electron Backscatter Diffraction (EBSD). It is shown that the microstructure of Ta-LLZO can be altered by manufacturing process and further investigated using EBSD. This method can be used to optimize the grain boundary and microstructure for both lithium-ion conduction improvement and stress development mitigation. Finite element modelling (FEM) is used to determine stress and strain localization which may occur during battery operation. A parametric study of a single grain is performed which found that the elastic anisotropy of Ta-LLZO is such that E<111> > E<110> > E<100> and E<111>\E<100> = 1.22 , E<110>\E<100> = 1.16. The elastic responses of the anisotropic behaviour of Ta-LLZO bicrystals are studied. Results indicate the stress distribution near the grain boundary is strongly dependent on crystal orientation. Stress development far from the grain boundary follow the prediction of serial and parallel spring systems. For the first time, an experimentally derived microstructure of Ta-LLZO is implemented into a FEM model to evaluate the influence of grain size, texture, morphology, and neighbouring grains on the state of Ta-LLZO. Grain neighbourhood effects are found to be as important to grain stress and strain development as grain orientation. The grain neighbourhood can alter the grain averaged strain state by up to 15.3% of the total applied strain. From this work, it is clear that the use of experimentally derived models will be required for the computational micromechanical analysis of Ta-LLZO.

Summary for Lay Audience

The All-Solid-State-Battery (ASSB), a lithium-ion battery system that uses a solid-state electrolyte (SSE) rather than a liquid electrolyte, has received significant attention to address safety issues as well as enable chemistries of much greater energy density that are otherwise unsuitable for liquid electrolyte systems. During the operation of the ASSB, electrode materials experience a change in volume as lithium is inserted or extracted. If a liquid electrolyte is used, these volume changes are accommodated, however in the ASSB, volume changes result in material stress development which, if great enough, can result in fracture and irreversible performance degradation. This thesis examines one of the most promising SSE materials, Ta-LLZO, from a mechanical perspective. The elastic properties of Ta-LLZO are anisotropic meaning the elastic response is dependent on the direction it is loaded. For a solid material such as Ta-LLZO, the material is composed of individual crystals termed grains which are each oriented differently than the neighbouring grains and can differ in size. Characterizing the distribution of grain orientation and size determines the materials microstructure. The region where grains meet is called the grain boundary and may not necessarily exhibit the orderly structure of atoms which is observed in the grain body. The atomic disorder influences how easily ionic lithium can diffuse and can be higher or lower compared to the grain body depending on the orientations of the grains relative to each other. In addition, since Ta-LLZO is elastically anisotropic, the development of stress in the material will be influenced depending on how the grains are oriented relative to the loading direction and to neighbouring grains. A technique called Electron Backscatter Diffraction (EBSD) is used to determine the microstructure of the material. By changing the manufacturing method of Ta-LLZO, it is found that the grain orientation distribution can be influenced. The microstructure is then introduced to a computational modelling technique called the Finite Element Method (FEM) to predict the stress and strain the material experiences during battery operation. The modelling predicts that changes in microstructure which are manufacturing process dependent can significantly alter the stress and stress distribution of the material. The results of this thesis have wide ranging implications in terms of SSE material design for the optimization of material microstructure. Future study is suggested to develop an improved understanding of manufacturing processing on microstructural development as well as modelling to address other critical regions of the ASSB including material degradation at the interface of electrode and SSE materials.

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Available for download on Saturday, December 11, 2021

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