Modeling Performance and Degradation of Ni-rich Cathodes

Kurzfassung

Understanding limitations in current Li-ion battery materials is crucial for their further development in order to meet the requirement of a high energy density while maintaining a long life-time of the battery. In commercial Li-ion batteries, NMC with a trend towards a high Ni-content like LiNi0.8Mn0.1Co0.1O2 (NMC811) gains increasing importance as a cathode material due to a very high achievable capacity in a given voltage window. However, those electrodes suffer from structural instability especially when cycled to a high cut-off voltage [1]. Oxygen release and a phase transformation from the layered NMC structure to a spinel- and or rock-salt like structure have been experimentally observed [1, 2], which results in a higher charge transfer resistance and electrolyte decomposition. However, since it is well known that the microstructure of electrodes is very complex and strongly influences the performance of a cell [3], it is evident that the microstructure might also play an important role in the degradation process. In this contribution, we will present a 1+1D modeling approach predicting the degradation of a Ni-rich cathode in terms of phase transformation and layer growth. This approach will then be complemented by 3D microstructure-resolved electrochemical continuum simulations conducted in the simulation framework BEST. 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. This allows us to study effects introduced by the microstructure of the cathode on its aging behavior. In our work, we analyze commercial NMC811/graphite cells in the pristine state and after a dedicated aging test with a high cut-off voltage of 4.4 V. First, we show a very good agreement of our simulations with experimentally obtained rate tests in the pristine state and discuss performance limitations induced by an insufficient electronic network. From this we will extract the geometrical parameters of the electrode to consistently parameterize our 1+1D approach to allow for long-term degradation studies of this electrode. The results can then be used in our microstructure resolved models to investigate the impact of a resistive film such as a rock-salt like phase on the electrode performance. Combining an efficient 1+1D model with our 3D microstructure-resolved simulations provides a valuable toolchain to gain a comprehensive understanding of the impact of electrode microstructure on the degradation of Ni-rich materials. [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, A1361-A1377 (2017) [2] S.-K. Jung et al, “Understanding the Degradation Mechanisms of LiNi0.5Co0.2Mn0.3O2 Cathode Material in Lithium Ion Batteries”, Adv. Energy Mater., 4, 1300787 (2014) [3] S. Hein et al.,“Influence of Conductive Additives and Binder on the Impedance of Lithium-Ion Battery Electrodes: Effect of Morphology”, J. Electrochem. Soc., 167, 013546 (2020)