For the last few years, many automobile industries have been trying to develop better energy storage materials for the paradigm shift to eco-friendly vehicles. For this purpose, lithium-ion batteries are first in line to achieve high energy density with respect to smaller-size and light-weight designs. However, capacity degradation poses a major hindrance to develop improved performance electrode materials. Mechanical failures associated with the continuous expansion/contraction of the electrode active materials during charging/discharging processes are the main cause of the capacity degradation of lithium-ion batteries. Electrode material consists of a heterogeneously complex network of electrochemically inactive binders and electrochemically active particles. The microstructural features of an electrode (for example, particle size, particle/binder interface properties, and binder morphological characteristics) and lithiation kinetics (for example, charge rate and lithium diffusion coefficient) play a key role in increasing the battery performance. Therefore, refinement of micro-structural design through eliminating the initiation of mechanical failures holds the key for developing improved performance electrode material.
Other than active particle disintegration and binder yielding, these mechanical failures also include the particle/binder interface debonding associated with the lithiation/delithiation induced micro-structural deformations which appears to have been studied less in the literature. The particle/binder interface debonding leads to the enhanced electrical impedance at the interface and ultimately complete electrical isolation of the active particle, thereby, creating an electrochemically inactive particle which can no longer be used for the energy storage. The aim of this dissertation is to understand the fundamental mechanisms driving the chemo-mechanical response of particle/binder electrode systems, investigate the lithium diffusion-induced debonding of active particle from binder network, study the effects of interface failure on stress and lithium concentration fields, and assess the effects of binder’s characteristics like thickness, mechanical property, lithiation kinetics, and coverage area with single and multiple binder connections on the chemo-mechanical response of composite electrodes.
To achieve the goals, a fully coupled electrochemical-mechanical model is presented to investigate the chemo-mechanical response of electrode systems consisting both the primary (electrochemically active particles) and secondary (electrochemically inactive binders) material phases. Within this framework, first, a series of two (2D) and three (3D) dimensional finite element method (FEM) simulations are performed to systematically investigate the effects of particle size, charge rate and binder features on the initiation of the most probable mechanical failures in electrode materials; inner active particle fracture, yielding of the binder domain and debonding at the particle-binder interface.
Next, to correlate the lithium diffusion and the subsequent growth of particle/binder interface debonding, mixed-mode cohesive zone model is implemented sequentially coupled with the electrochemical-mechanical model. Both models are further solved in a fully coupled manner to investigate the implementations of interface damage on the particle and binder stress levels, tractions at the particle-binder interface, and the lithium concentration distributions inside the active particle. The basic insights gained from the intensive discussions of simulation results to explore the factors promoting/inhibiting interface failure will certainly help in developing more robust and mechanically stable electrode materials.