Chemo-mechanics of lithium-ion battery electrodes

Abstract

Mechanical deformation plays a crucial role both in the normal operation of a Lithium-Ion battery, as well as in its degradation and ultimate failure. This thesis addresses the theoretical formulation, numerical implementation, and application of fully-coupled deformation-diffusion theories aimed at two different classes of electrode materials: (i) phase-separating electrodes, and (ii) amorphous Silicon electrodes, which are elaborated on next. Central to the study of phase-separating electrodes is the coupling between mechanical deformation and the Cahn-Hilliard phase-field theory. We have formulated a thermodynamically consistent theory which couples Cahn-Hilliard species diffusion with large elastic deformations of a body. Through a split-method, we have numerically implemented our theory, and using our implementation we first studied the diffusion-only problem of spinodal decomposition in the absence of mechanical deformation. Second, we studied the chemomechanically- coupled problem of lithiation of isotropic spheroidal phase-separating electrode particles. We showed that the coupling of mechanical deformation with diffusion is crucial in determining the lithiation morphology, and hence the Li distribution, within these particles. Amorphous silicon (a-Si), when fully lithiated, has a theoretical capacity ~~ 10 times larger than current-generation graphite anodes. However, the intercalation of such a large amount of Li into a-Si induces very large elastic-plastic deformations. We have formulated and numerically implemented a fully-coupled deformation-diffusion theory, which accounts for transient diffusion of lithium and accompanying large elastic-plastic deformations. We have calibrated our theory, and applied it to modeling galvanostatic charging of hollow a- Si nanotubes whose exterior walls have been oxidized to prevent outward expansion. Our predictions of the voltage vs. state-of-charge (SOC) behavior at various charging rates (Crate) are in good agreement with experiments from the literature. Through simulation, we studied how plastic deformation affects the performance of a-Si-based anodes by reducing stress, thus enabling higher realizable capacities, and introducing dissipation. Finally, in order to design a-Si-based anodes aimed at mitigating failure of the solid electrolyte interphase (SEI), we have formulated and studied a continuum theory for the growth of an SEI layer - a theory which accounts for the generation of growth stresses.