Mechanics of Electrodes in Lithium-Ion Batteries

Abstract

This thesis investigates the mechanical behavior of electrodes in Li-ion batteries. Each electrode in a Li-ion battery consists of host atoms and guest atoms (Li atoms). The host atoms form a framework, into which Li atoms are inserted via chemical reactions. During charge and discharge, the amount of Li in the electrode varies substantially, and the host framework deforms. The deformation induces in an electrode a field of stress, which may lead to fracture or morphological change. Such mechanical degradation over lithiation cycles can cause the capacity to fade substantially in a commercial battery. We study fracture of elastic electrodes caused by fast charging using a combination of diffusion kinetics and fracture mechanics. A theory is outlined to investigate how material properties, electrode particle size, and charging rate affect fracture of electrodes in Li-ion batteries. We model an inelastic host of Li by considering diffusion, elastic-plastic deformation, and fracture. The model shows that fracture is averted for a small and soft host—an inelastic host of a small feature size and low yield strength. We present a model of concurrent reaction and plasticity during lithiation of crystalline silicon electrodes. It accounts for observed lithiated silicon of anisotropic morphologies. We further explore the microscopic deformation mechanism of lithiated silicon based on first-principles calculations. We attribute to the microscopic mechanism of large plastic deformation to continuous Li-assisted breaking and reforming of Si-Si bonds. In addition, we model the evolution of the biaxial stress in an amorphous Si thin film electrode during lithiation cycle. We find that both the atomic insertion driven by the chemomechanical load and plasticity driven by the mechanical load contribute to reactive flow of lithiated silicon. In such concurrent process, the lithiation reaction promotes plastic deformation by lowering the stress needed to flow. Li-ion battery is an emerging field that couples electrochemistry and mechanics. This thesis aims to understand the deformation mechanism, stresses and fracture associated with the lithiation reaction in Li-ion batteries, and hopes to provide insight on the generic phenomenon that involves interactive chemical reactions and mechanics.