Powder-Based Evaluation of Anode Active Materials for Mechanically Flexible Electrodes for Fast-Charging Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are a key technology for the electrification of the transportation sector. Despite their numerous advantages, their performance remains a critical challenge. In particular the fast charging capability is an essential factor for increasing the acceptance of battery electric vehicles in the population. The performance of LIBs is significantly influenced by the electrode microstructure, as it affects the ion transport kinetics through the porous electrode structure [1,2]. Hence, optimizing the electrode microstructure has been a major focus for enabling enhanced fast charging capabilities [2-5]. This is especially important in cylindrical cells, where tightly wound electrodes with limited space for expansion can lead to issues like electrolyte motion-induced salt inhomogeneity (EMSI), which can cause localized lithium plating [6-7]. This effect can be considered as a result of the interplay between cell design, electrode mechanics and microstructure [6]. During the electrode production process, the microstructure and mechanics is significantly influenced by the calendering step, that involves the compression of electrodes between rollers [8,9]. This step alters properties like density, porosity and particle orientation as well as the tortuosity, and impacts the overall microstructure of the electrode. However, the extend of the microstructure and mechanics altering during the calendering process is highly dependent on the electrode composition, especially from the morphology and mechanical properties of the active materials. Since electrodes have to withstand mechanical stresses arising during fast-charging and discharging, they are required to be mechanically flexible in order to accommodate these stresses, enhancing the fast-charging performance and cycle life of LIBs. This highlights the importance of a fundamental understanding of the calendering process of powders with different properties for the development of high-performance LIBs, especially when aiming fast charging capabilities within a limited cell volume. In order to gain a better understanding for the calendering process, the focus of this work is on directly relating the compressive stresses experienced during calendering to the powder-level compaction behavior of anode active materials. By correlating powder compaction characteristics with the resulting electrode mechanics and microstructural properties, such as tortuosity, particle orientation, and porosity, suitable anode materials for fast charging can be efficiently identified. The study investigates two different graphite powders as model anode active materials. First, their compaction behavior is analyzed using a custom powder compaction method. Electrodes are then fabricated from these graphites, calendered to different densities, and the resulting microstructural and mechanical properties are characterized and linked back to the powder properties. The resulting mechanics and microstructure, including tortuosity, particle orientation and porosity, was analyzed and correlated with the powder compaction characteristics. It was found that the powder-based approach is a promising tool for evaluating the relationship between active material characteristics and electrode properties, particularly for fast charging applications. References: [1] Y. Yang, N. Li, B. Wang, N. Li, K. Gao, Y. Liang, Y. Wei, L. Yang, W.-L. Song, H. Chen, Electrochemistry Communications 2022 , 136 , 107224. [2] B. T. Habte, F. Jiang, Solid State Ionics 2018 , 314 , 81–91. [3] S. Ahmadi, D. Maddipatla, B. J. Bazuin, M. Z. Atashbar, Adv Energy and Sustain Res 2024 , 5 , 2300213. [4] J. Li, C. Guo, L. Tao, J. Meng, X. Xu, F. Liu, X. Wang, Adv Funct Materials 2024 , 34 , 2409097. [5] X. Lu, A. Bertei, D. P. Finegan, C. Tan, S. R. Daemi, J. S. Weaving, K. B. O’Regan, T. M. M. Heenan, G. Hinds, E. Kendrick, D. J. L. Brett, P. R. Shearing, Nat Commun 2020 , 11 , 2079. [6] S. Solchenbach, C. Tacconis, A. Gomez Martin, V. Peters, L. Wallisch, A. Stanke, J. Hofer, D. Renz, B. Lewerich, G. Bauer, M. Wichmann, D. Goldbach, A. Adam, M. Spielbauer, P. Lamp, J. Wandt, Energy Environ. Sci. 2024 , 17 , 7294–7317. [7] H. Pegel, O. von Kessel, P. Heugel, T. Deich, J. Tübke, K. P. Birke, D. U. Sauer, Journal of Power Sources 2022 , 537 , 231443. [9] D. Wang, G. Wang, C. Xu, H. Liu, Journal of Energy Storage 2024 , 87 , 111521.