Souzan Hammadi: Mesoscopic and Atomistic Insights into the Microstructural Evolution of Energy Storage Materials

Abstract

Energy storage is possible through the incorporation of Li-electron pairs into battery electrodes. Depending on the nature of the interactions between the species within electrode materials, different atomic arrangements can form. Weak interactions result in an even distribution of Li forming a solid solution phase, while strong interactions result in short range order and ultimately in phase separation. A material that undergoes phase separation during battery operation is LiFePO4 and structural mismatch at the Li-rich (LFP) and Li-poor (FP) phase boundaries create strain fields that, over time, lead to mechanical damage. This material showcases both phase separation and solid solution behaviour at different conditions due to its complex short and long range order at the atomic scale. Understanding the factors underlying Li order is, therefore, important to avoid unwanted aging effects and in the design of more efficient electrode materials. This is explored in this thesis using computational modelling at many scales.

A phase-field model is developed to evaluate the effect of the charge transfer rate on the microstructural evolution at the mesoscale. Two models of charge transfer are confronted, the Butler-Volmer model of ion transfer and the Marcus-Hush-Chidsey model of electron transfer. Depending on the model and the chosen input, different discharge rates and microstructures can be expected. The microstructure evolution also depends on the free energy landscape which generally is approximated with the regular solution model. In contrast, many free energy descriptions parameterized on experimental data showcase an asymmetric form with a third local minima. This minimum represents a metastable solid solution that evolves at room temperature when integrated into the phase-field model.

Such an asymmetric energy landscape is also to be expected as the short range order of LFP is lower than in FP. This indicates that accessing the solid solution phase is easier during Li extraction.  Addition of dopants such as Mn further decreases the short range order in the material. Finding routes to stabilize the solid solution phase in LFP will ultimately enhance the battery cycling rate and mechanical stability as seen in the phase-field model, where higher rates and lower strains are now possible.