Insights into Performance and Degradation of Commercial Ni-rich Cathodes: A Modeling Perspective

Kurzfassung

Li-ion batteries are widely used in consumer electronics due to their high energy density and currently gain further importance with regards to future mobility and their application in electric vehicles. While a high nickel content in stoichiometries like LiNi0.8Mn0.1Co0.1O2 (NMC811) increases the achievable capacity in a given voltage window, it also poses challenges to structural stability during cycling [1]. Therefore, application and further development of those materials requires a deep understanding of their performance limitations as well as their degradation. Since extensive testing is time-consuming and expensive, predictive simulation tools are needed that are able to describe the electrochemical behavior of the cell. In this contribution, we will present 3D microstructure-resolved electrochemical continuum simulations conducted in the simulation framework BEST. It is based on a thermodynamically consistent transport theory for mass and charge in the electrolyte and the active material [2]. Owing to the finite volume discretization of the governing equations, it is straightforward to use voxel-based image data obtained by focused ion beam - scanning electron microscopy (FIB-SEM) as the simulation domain. Previous studies have shown the importance of microstructural simulations with regards to battery operation [3] and contributed to a deeper insight into the cell aging [4]. This approach will be complemented by a pseudo-2D (p2D) model considering different aging modes, thus providing additional predictions on the cell aging. In our work we analyze commercial NMC811/graphite cells. We demonstrate a very good agreement at the beginning of life between our simulations and experimental data obtained in rate tests and electrochemical impedance spectroscopy (EIS). In the high-energy cathodes we found a very low content (3 vol-%) of carbon binder domain (CBD), which is insufficient to establish a conductive network. Hence, the electronic conductivity of the active material determines the effective conductivity of the electrode and limits the performance at high degrees of lithiation. Additionally, we evaluate the degradation of the NMC active material using our simulation approach. Combining the 3D microstructure-resolved simulations with an efficient p2D model provides a valuable toolchain to gain a comprehensive understanding of the performance limitations as well as the relevant aging modes in commercially available high-energy cathodes. Acknowledgement: This work has been funded by the ‘Bundesministerium für Bildung und Forschung’ within the project MiCha under the reference number 03XP0317D. The authors acknowledge support by the state of BadenWürttemberg through bwHPC (JUSTUS 2). References: [1] R. Jung et al., “Oxygen Release and Its Effect on the Cycling Stability of LiNixMnyCozO2 (NMC) Cathode Materials for Li-Ion Batteries”, J. Electrochem. Soc., 164 (7), A1361- A1377 (2017). [2] A. Latz and J. Zausch, “Thermodynamic consistent transport theory of Li-ion batteries”, J. Power Sources, 196, 3296-3302 (2011). [3] T. Danner et al., “Thick electrodes for Li-ion batteries: A model based analysis”, J. Power Sources, 334, 191–201 (2016). [4] L. Bolay et al., “Microstructure-resolved degradation simulation of lithium-ion batteries in space applications” J. Power Sources Advances, 14,100083 (2022)