Generation and Finite Element Analysis of Synthetic Microstructures for Structural Battery Electrolytes

Examensarbete för masterexamen
Boekhout, Per
Fredriksson, Daniel
The structural battery composite is a new multifunctional material that combines the mechanical features of a carbon fiber composite with the energy storing capabilities of a lithium-ion battery by exploiting the electrochemical properties of the carbon fiber. Further development of this technology can lead to significant improvement of efficiency in a variety of applications, with vehicular applications being the most immediate and promising. The aim of the project is to model the porous electrolyte polymer matrix inside the structural battery composite by generating synthetic microstructure, serving as statistical volume elements, and applying first order homogenization. The evaluation is made by solving boundary problems for the generated structures with the finite element method where strong periodic boundary conditions are used for the modeling. Using this numerical evaluation as a basis, the effective diffusive and elastic properties are estimated for the generated microstructures such that the overall performance of the battery is derived. Four types of fully periodic synthetic electrolyte microstructures are presented. The Cahn-Hilliard structure based on solving the Cahn-Hilliard phase separation equation. A dense sphere packing structure made from a swelled distribution of spheres generated with the Lubachevsky-Stillinger algorithm. The Voronoi structure where a damage phase field is solved with essential boundary conditions on the cell edges of a Voronoi tessellation. A novel convex hull structure created through a Voronoi tessellation around a sparse sphere packing where random points are generated within the cells to create convex hull particles, with particle connectivity enforced through a connectivity algorithm. A modification to the convex hull microstructure allows the implementation of a three-phase structure where some of the connections were redefined as binder material as an attempt to create a synthetic positive structural battery electrode microstructure. Furthermore, the passivation layer that forms along the negative fiber electrode is also modeled and evaluated in terms of delamination. Comparison with previous research into synthetic electrolyte microstructures finds a good match between the presented results and those from previous work, which validates the methodology of both studies. Furthermore the convex hull microstructure manages to emulate the effective properties of certain experimental values better than microstructures in previous research. Overall both the effective stiffness and diffusivity were found to correlate linearly with the porosity, but the different topologies had a clear effect on the performance. Investigating the mechanical performance of the electrode structure found that the stiffness of the binder material has a significant impact on the effective stiffness of the electrode, but the volume fraction of the binder within the domain did not seem to have any effect on the stiffness. A toolbox is developed for modeling the interface between the polymer matrix and the passivation layer in 2D, and a study with arbitrarily material parameters is conducted. From this study it is shown that the passivation layer may have major influence on the mechanical properties of the generated microstructure if the porosity is high and that the delamination is highly dependent on assigned material parameters. The final result is a fully automated process where statistical volume elements are generated for a type of microstructure and analyzed through computational homogenization. The automation is achieved through interfacing external open-source software packages and the commercial finite element software COMSOL with MATLAB.
Structural batteries, Synthetic microstructures, Computational Homogenization
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