Modelling performance and degradation of Ni-rich cathodes

Abstract

To further increase the achievable capacity and thus the energy density of commercial Li-ion batteries, NMC with an increasing Ni-content like LiNi0.8Mn0.1Co0.1O2 (NMC811) gains increasing importance as a cathode material. While a higher Nickel content is favourable regarding the achievable capacity in a given voltage window, 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 modelling 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 behaviour. In our work, we analyse 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. From this, we will extract the geometrical parameters of the electrode to consistently parametrize our 1+1D approach to investigate long-term degradation 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.