(41e) Structural and Mechanical Characterization of the Calendaring Process of Lithium-Ion Electrodes Via Discrete Element Method Simulations

Lithium-ion batteries are widely used as electrochemical power sources for mobile telephones, personal computers, cameras and other modern-life appliances. They are also remarkably successful in the electric power vehicle market due to their long live cycles and high-rate capabilities.

However, as lithium-ion batteries appear to be gaining in popularity, there is still room to improve their performance and durability. In fact, the relevance of electrode structural stability and mechanical integrity has been already pointed out by several research groups [1]. Lithium-ion battery electrodes consist of porous composite materials (active material, conductive additives and binder) coated on a substrate. Microstructural characteristics of such particulate coatings, namely porosity and particle size distribution are of major importance in determining bulk properties. For instance, the performance of a lithium-ion battery can be significantly enhanced by selecting the adequate electrode porosity or by adjusting the correct amount of binder. Concerning their mechanical aspects, it is well-known that stress generation within the electrodes is one of the main causes for capacity fade and eventual failure of lithium-ion batteries. For this reason, mechanical instabilities, including particle fragmentation, structure disintegration and fracturing, loss of contact between the electrode and the current collector or plastic deformation are still a major subject of extensive research activities [2].

Within this framework, the present study proposes a discrete element method (DEM) procedure to create particulate coatings with defined product characteristics and to give insight into the influence of microstructure with regard to the mechanics. Operating the open source software LIGGGHTS with additional programming code, particulate structures can be computer-generated taking into consideration a specific particle size distribution, bulk porosity and coating thickness. This process is automatized and fulfilled by an iterative adjustment of input parameters. The application of this effective designed tool is put into practice for the numerical generation of lithium-ion electrode structures. A key component of such coatings is the binder, which ensures the cohesion throughout the particulate network and the adhesion between the coating and the substrate. The role of the binder in the mechanics of the system is therefore essential and must also be considered for the simulations. Bearing this fact in mind, a Hertzian-bond contact model was also developed in this study in order to capture the elasto-plastic behavior of the electrode. This contact model combines both particle and binder stiffness by computing bonds between particles under certain conditions with regard to interparticle distance and particle radii.

In this study, different lithium-ion battery electrodes were experimentally produced as well as numerically generated in order to validate the implemented procedure. Furthermore, nanoindentation experiments and corresponding simulations were carried out with the aim of calibrating and validating the DEM contact model. Nanoindentation is a useful technique that coupled with DEM simulations, contributes to get a deeper look into the mechanical properties of the electrodes. This outcome provided an essential comprehensive understanding of how the interaction among the electrode components may affect the mechanical and electrochemical properties of the whole structure.

The validated computer-generated structures were subsequently used to simulate the calendaring process. This relevant manufacture step consists of compacting the electrodes with two rolls directly after coating and drying. Calendaring has been proved to have a substantial impact on the pore structure and, as a result, on the electrochemical performance of the lithium-ion battery. Hence, it is of extreme importance to comprehensively control this process in order to achieve targeted battery performances [3]. Within this work, an additional electrode was produced and calendered at different line loads by varying the gap size between rolls. Respective simulations enabled to provide detailed knowledge concerning the influence of this process on stress generation by calculating the macroscopic stress associated to each calendered structure. Moreover, simulations brought the advantage of contributing to a better understanding regarding the evolution of the microstructure and, in particular, the gradual change of particle contacts. This feature is directly related to the conductivity of the electrode, which is an essential aspect with regard to the performance of the battery. Ultimately, this research may be valuable not only to get deeper knowledge into the manufacture process but also to optimize electrode design with the aim of achieving specific battery requirements

[1] S. W. Peterson, D. R. Wheeler, Direct Measurements of Effective Electronic Transport in Porous Li-Ion Electrodes, Journal of The Electrochemical Society, 161 (14) A2175-A2181 (2014)

[2] A. Mukhopadhyay, B. W. Sheldon, Deformation and stress in electrode materials for Lithium-ion batteries, Progress in Materials Science 63 58-116 (2014)

[3] C. Meyer, H. Bockholt, A. Kwade, Characterization of the Calendering Process for Compaction of Electrodes for Lithium-Ion Batteries, Journal of Materials Processing Technology 249 (2017)