Experimental Characterization of Electrodes and Multi-Scale Modeling of Swelling Induced Stresses in Lithium-ion Batteries

QC 230620

Abstract

Over the last few decades, rechargeable lithium-ion batteries have been extensively used in portable instruments due to their high energy density and low self-discharge rate. Recently, lithium-ion batteries have emerged as the most promising candidate for electric vehicles and stationary energy storage. However, the maximum energy that lithium-ion batteries store decreases as they are used because of various irreversible degradation mechanisms. The mechanical properties of the electrode layers inside the battery highly influence the battery's performance. There is, however, a fundamental lack of understanding of the mechanical properties of electrodes and how they evolve during electrochemical cycling, which makes it a necessity to characterize their mechanical behavior for mesoscopic and macroscopic level modeling. Lithium-ion batteries are complex systems to understand, and various processes and their interactions make battery modeling challenging. This thesis contributes to understanding the mechanical behavior of electrodes in lithium-ion batteries and provides methods for the design and efficient modeling of battery systems.

        In Paper A and Paper B, the macroscopic mechanical behavior of active layers in the electrodes is investigated using U-shape bending tests. The active layers are porous and a different tensile and compressive behavior is captured by performing tests on single side coated dry electrodes. The experiments reveal that the active layer is stiffer in compression as compared to tension. The compressive stiffness increases with bending strain whereas the tensile stiffness is almost independent of the bending strain. A very low value of modulus of the active layer (1-5 GPa) is measured in comparison to the metal foils (70-110 GPa) and the active particles (50-200 GPa) which shows that the electrode properties are governed majorly by the binders present in the active layers. The time-dependent and hysteresis effects are also captured by the method which circumvents the flaws of many other test methods presented in the literature.  

        Paper C focuses on characterizing the layer-level evolution of mechanical and electrochemical properties of a Ni-rich positive electrode during early-stage electrochemical cycling, along with complementary cross-section analyses to understand the relationship between macroscopic and microscopic changes. Macroscopic constitutive properties were measured using the U-shaped bending test method developed in papers A and B, which revealed that the compressive modulus was primarily influenced by the porous structure and binder properties. It decreased notably with electrolyte wetting but increased with cycling and aging. Electrochemical impedance spectra showed an increase in local resistance near the particle-electrolyte interface with early-stage aging, which was likely due to secondary particle grain separation and carbon black redistribution. Cross-section analyses reveal significant variations in particle properties between pristine and cycled samples, including particle swelling, compression of the binder phase, and increased particle contact, contributing to the rise in the elastic modulus of the porous layer during cycling.

        In Paper D and Paper E, we present a multiscale homogenization method that couples mechanics and electrochemistry at the particle, electrode, and battery scales. The active materials of lithium-ion battery electrodes exhibit volume change during lithium intercalation or deintercalation. A lithium concentration gradient develops inside particles, as well as inside the active layer. The developed stress due to deformations further affects solid diffusion.  We utilized models that have already been developed to couple particle and electrode layer levels. Electric vehicle battery packs consist of numerous battery modules, each of which includes multiple battery cells composed of electrode, separator, and current collector layers. A finite element model capable of capturing stresses at the layer level would need to be very large to account for all the details. The mechanical coupling between the electrode and the battery level is achieved by homogenization of the layered battery using three-dimensional laminate theory, which greatly reduces the number of finite elements required for stress simulations in batteries. After obtaining a homogenized solution, layer-level stresses can be determined in a post-processing step. The method accurately predicts stresses on various scales and captures the effects of external battery loadings, cycling rates, and mechanical parameters. The efficiency of the method is demonstrated by comparing it to detailed finite element computations. The simulations indicate that layer-wise stresses, such as pressure, can be predicted as functions of position and time, providing insights into the inhomogeneous aging state of the battery.

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