Insights into degradation mechanisms of commercial Li-ion batteries by using a multiscale modelling approach

Abstract

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. In order to maximize battery lifetime, a deep understanding of the degradation processes in the electrodes is needed. Since extensive testing is time-consuming and expensive, predictive simulation tools are needed that are able to evaluate the electrode performance and especially the degradation behavior based on a given electrode structure. In this contribution, we will present 3D micro-structure-resolved electrochemical continuum simulations conducted in the simulation framework BEST that is based on a thermodynamically consistent transport theory for mass and charge in the electrolyte and the active material [1]. Due to the finite volume implementation of the governing equations, real tomographic data obtained by focused ion beam - scanning electron microscopy (FIB-SEM) of the electrode structure is used as the simulation domain. This approach will be complemented by using a pseudo-2D (p2D) model considering different aging modes such as growth of a solid electrolyte interphase and lithium plating. In our work we analyze commercial NMC811/graphite cells. The analysis is based on validated input data for a fresh cell with experimental comparison for both the rate performance as well as impedance. Here, especially the cathode is of interest since its very low content (3 vol-%) of carbon binder domain (CBD) results in a poor network for electronic conduction. Therefore, the electronic conductivity of the active material severely limits the electrode performance at high degrees of lithiation. Combining the 3D-microstructural simulation with the p2D approach gives a comprehensive understanding of the performance limitations as well as the dominant aging modes present in a commercially available cell. Furthermore, limitations induced by possible cathodic degradation will be discussed. The presented method acts as a tool to gain a deeper understanding of the underlying electrochemical processes in the cell during operation and aging.

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 Baden-Württemberg through bwHPC (JUSTUS 2).

References: [1] A. Latz and J. Zausch, “Thermodynamic consistent transport theory of Li-ion batteries”, J. Power Sources, 196, 3296-3302 (2011).