Modeling Chemo-Mechanical Processes and Degradation in Battery Materials

Abstract

Nowadays, society requires batteries for multiple daily uses, fromelectronic devices to electric mobility. In the coming years, with the spread of renewable systems and electric vehicles, batteries will become fundamental in decarbonizing society. Regarding renewable energy systems, batteries stand out as a promising solution to mitigate grid instabilities and storing energy surpluses, improving the overall efficiency of these systems. Moreover, batteries are necessary to transition towards more sustainable and free-CO2 mobility fully. Thus, developing advanced battery technologies is necessary to transition effectively towards more sustainable technologies. In this regard, research in battery materials is ongoing to develop battery materials with improved performance and durability, while reducing cost and the environmental impact. Lithium-ion batteries (LIBs) are the most used and mature battery technologies, but their development is ongoing. Unfortunately, the materials of LIBs experience degradation that limits the durability of the cells. Also, dual-graphite batteries (DGBs) are a promising type of battery due to their outstanding performance and high voltage, capacity, and energy density values. Unfortunately, these systems present some degradation limitations that need to be solved. The design of these batteries often requires a compromise between performance and durability in the material, opening a broad field for research. Understanding the complex processes in the electrodes of batteries is needed to improve the designs and achieve enhanced performance and limited degradation. During the battery operation, the active material experiences a mass transport phenomenon, causing volume variations. These volume changes generate mechanical solicitations in the active material, which can result in the cracking of the material, a common issue in the electrodes. The fracture of battery materials has severe consequences on the performance and safety of the batteries, limiting their durability. Computational material science has proven to be an efficient tool in understanding the behavior ofmaterials under different conditions, considering multiple physical and chemical processes and length scales. In this regard, modeling and simulation can be used to design and optimize battery materials and electrodes and limit their degradation, providing insights into the influence of the different complex phenomena that occur in the electrode. The scope of this thesis is to provide computational frameworks to understand chemomechanical coupling and mechanical degradation in battery materials, including the most relevant multiphysics processes that cause the cracking of thematerials. This thesis proposes two different finite-element models to analyze chemo-mechanical coupling. Both models are coupled with a stress-based phase-field fracture model to represent the material damage and understand mechanical degradation. Moreover, a novel implementation to represent the effect of heterogeneous and disordered battery materials is proposed, consisting of a stochastic representation of the material properties, which enables the capture of realistic crack patterns that mimic those obtained during electrode operations. The first model proposed, broadly used in the literature, benefits from the advantages of the infinitesimal strain theory and is valid for materials experiencing small volumetric changes. Based on a finite strain framework, the second model includes the equations’ derivation froma free-energy potential to ensure thermodynamical consistency. This model accurately captures the stress of materials experiencing large volumetric changes. This thesis encompasses the analysis of mechanical degradation in LIB andDGBactivematerials. Regarding the analysis of LIB materials, the infinitesimal strain theory-based model is used to analyze mechanical degradation in some of the most used anode and cathode materials: graphite and lithium nickel manganese cobalt oxides (NMC), respectively. The analysis of graphite is performed considering actual graphite active particles obtained by digitizingmicroscopy images available in the literature. It also includes the effect of C-rate and particle interaction with the surroundings in mechanical degradation. The analysis of NMC consists of a comprehensive investigation of the main factors promoting cracking in the material: particle size, C-rate, and depths of charge and discharge. Moreover, the influence of differentmaterial compositions in withstanding large C-rate is analyzed. Additionally, the design of tailored functionally-graded NMC materials with limited degradation is proposed, including three strategies: core-shell, core-shell with composition gradient, and composition gradient. Regarding the mechanical degradation of DGBs, the finite strain-based model captures the large volume expansions that graphite cathodes experience during the intercalation. The analysis focuses on understanding the influence of the C-rate, particle size, and different heterogeneous materials in the mechanical degradation of graphite active particles with PF− 6 intercalation. In this case, the influence of the cracking patterns in some electrochemical measurements (e.g., decrease in effective diffusivity and increase in surfaces) is also analyzed. Moreover, design and operation strategies guidelines are proposed to minimize mechanical degradation in DGBs. Finally, a methodology for the design of composite silicon graphite electrodes, an outstanding anode solution in LIBs, is proposed. In this case, the electrochemical performance of the electrode is analyzed using a pseudo-two-dimensional model. The effect of chemomechanical coupling is included in the analysis, emphasizing the mechanical solicitations of both active materials. The model is applied to understand the implications of graphite particle size and calendering in electrochemical performance and the stress of active materials.

Tesis embargada (tesis por compendio de publicaciones)