Characterization and Computational Modelling for the Garnet Oxide Solid State Electrolyte Ta-LLZO

Colin A. Versnick, The University of Western Ontario

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 (Li_6.4La₃Zr_1.4Ta_0.6O₁₂) 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.