Inhomogeneity in Li(ion) Battery Electrodes: Causes and Effects

The environmental and climate change concerns are calling for rapid energy transition by the use of more green resources and zero emission solutions. This requires development of mature energy storage systems like rechargeable batteries. For instance, the Li-ion battery technology was first commercialized in 1985, but still is under research and development to increase energy density and cycle life beyond the state-of-the-art. The search for energy dense active-materials like high nickel content transition metal oxides, the use of solid electrolytes or the strive for enabling dendrite-free lithium metal anodes all aim to tackle the limitations of today’s technology. Another strategy is to ensure that the battery is manufactured authentically, devoid of defects and inhomogeneities. These inhomogeneities might exist at different length scales from nanoscale to macroscale or in other words from active-material level to cell pack level. The focus of this thesis is to investigate the causes and effects of inhomogeneities in the cells, especially at the electrode level, like poor carbon distribution or insufficient electrolyte wetting in the electrodes. Such flaws can interfere with electronic and ionic transport in the battery porous electrodes and possibly cause reduced performance, non-uniform degradation and accelerated aging. The first chapter presents a brief review of Li-ion battery technology, porous electrode model, battery aging mechanisms and an introduction to inhomogeneities at different length scales. In the second chapter, we study LiNi0.6Mn0.2Co0.2O2 slurries prepared via different methods and characterize their viscoelastic behavior and dis/charge behavior of electrodes made thereof. We observed disparities among the dis/charge behavior, and ascribed it to the local variations in carbon-binder domain porosity and thickness. In chapter 3, we fabricated bilayer electrodes with a disparity among the two layers as inhomogeneity of porous electrodes. In our system, we had a carbon-deficient or carbon-rich layer near the electrode current collector. The results showed that the former can have a destructive effect on rate performance and aging while the latter appeared to have a constructive effect. Chapter 4 investigates possible aging mechanism in porous NMC electrodes in the presence of heterogeneity in the electrode structure. We observed frequent reductions in the charge transfer resistance of the electrodes during long term cycling of the electrodes which is correlated to the fracture of active-material particles. This hypothesis was further elaborated by cross-sectional imaging as well as observation of an unusual energy recovery which could be explained by intergranular fracture. Finally in chapter 5, we looked at the inhomogeneity in-plane of the battery electrodes. Large format lithium electrodes were made and tested in a home-made setup in symmetric configuration where both working and counter electrodes were lithium. The spatial distribution of degradation over the surface of large electrodes was correlated to non-uniform distribution of current density in-plane of the battery electrodes.

Publication year:2022

Accessibility:Embargoed