Simulation and experiments to understand the microstructure of and transport limitations in lithium-ion battery electrodes

Abstract

It is well accepted that the electrode microstructure plays a pivotal role in determining the cell performance (e.g., energy and power density), lifetime, and final cost of lithium-ion batteries (LIBs). In a typical lithium-ion battery electrode, particle-particle interactions inside the electrode solid matrix (active-material (AM), carbon-AM, carbon-binder, and AM-binder) on one hand and the pore structure on the other hand dictate the microstructure of the electrodes. Electrode microstructure is a strong function of the manufacturing processes including mixing, coating, drying, and calendaring steps. This work attempts to decipher the interplay between the electrode manufacturing processes, electrode microstructure, and battery rate-limiting factors for the LiNixMnyCo1-x-yO2 (NMC)- based cathodes. First, the existing literature on the role of electrode microstructure as a central concept linking the electrode manufacturing processes and the battery performance is reviewed. It is shown that there is always a trade-off between the effective ionic and electronic transport properties of the battery electrodes. This necessitates a comprehensive understanding of the interdependencies of the electrode structure and the manufacturing processes to optimize the final battery performance. Next, we introduce an approach to assess the electrode inhomogeneity based on different electrode formulations and thicknesses using a combination of systematic experimentation and mathematical modeling. The electrochemical behavior of lithium-ion batteries will be studied by analyzing the experimental rate-capability data of the cells with the aid of a macro non-homogeneous physicsbased model which is a modified version of the so-called Newman’s model. This is possible only when the electrode microstructure is fully characterized. For this reason, the tortuosity of the electrodes was measured using a modelexperimentation approach. The rate limitations were identified based on the longrange and short-range limitations. It is shown that the spatial distribution of the carbon-binder domain controls the relative contribution of the short- and longrange limitations. A similar model-experimentation platform was also developed to study the performance limitations in situations where the electrode microstructure properties cannot be precisely measured. This introduces an efficient approach to scrutinize the battery behavior since a thorough characterization of the battery electrodes is not always feasible and practical. In this regard, the overall polarization of a battery undergoing constant-current charge and discharge cycles was divided into the particle-level, short-range, and long-range polarization categories. This classification is meaningful since each of these polarization groups is influenced distinctly by the material modification (e.g., functionalization) and components’ (e.g., electrode thickness) design strategies. In this regard, the concept of polarographic map was introduced to correlate the share of different polarization groups in the rate limitation of NMC porous electrodes. Lastly, a general methodology to evaluate the advanced porous electrodes was discussed. Here, surface-modified active material particles were used to prepare a series of advanced porous electrodes with improved electronic and ionic conductivities. The electrochemical performance of these electrodes was evaluated using the insights gathered from the model-based studies of the earlier chapters. It was shown that our model-based methodology can simplify the screening and evaluation of the advanced novel porous electrodes without sacrificing accuracy.