RVE generation and computational homogenization of structural battery electrolytes

Licentiate thesis, 2022

The structural battery is multifunctional in the sense that it is able to carry mechanical loads, and at the same time store and deliver energy. This is made possible due to carbon fibers' ability to act not only as structural reinforcement materials, but also as electrode components. While conventional batteries rely solely on liquid electrolyte to allow for ion transfer between the electrodes, structural batteries exploit the so-called structural battery electrolyte (SBE). The SBE consists of two continuous phases; a porous polymer skeleton and a liquid electrolyte. The role of the liquid electrolyte is to allow for ion transfer, while the porous polymer skeleton contributes to the mechanical properties. In short, the structural battery consists of carbon fibers (acting as electrodes) embedded in an SBE.

In the first part of the thesis, we study the multifunctional performance of various SBE microstructures by performing virtual material testing on artificially generated Representative Volume Elements (RVEs). In particular, we obtain the effective ionic conductivity by solving a diffusion equation with Fick's law, and the effective stiffness by assuming linear elasticity. The generated RVEs and the predicted performance are compared to experimental data.

The second part covers the development of a multiscale modeling framework for electrochemically coupled ion transport in SBEs. After establishing the governing equations, we exploit Variationally Consistent Homogenization (VCH) to obtain a two-scale model. If the subscale RVE problem exhibits negligible transient effects for length scales relevant to the studied application, then an assumption of micro-stationarity can be introduced. This opens up for the possibility to devise a numerically efficient solution scheme for the macroscale problem that is based on a priori upscaling of the effective response. The procedure is demonstrated in a set of numerical examples, including validation toward a (single-scale) reference solution.