Understanding the Drying Process and Mud Cracking of Li-Ion Battery Electrodes through Synchrotron X-Ray Computed Tomography

Abstract

The drying process of slurry cast Li-ion battery electrodes is a subject with significant impacts on the performance of the battery but where detailed understanding is lacking(1). Mud cracking, opening of cracks in the surface due to stress from the slurry shrinking as it dries, is the most dramatic phenomenon during the process. Cracks may be detrimental to electrode performance due to delamination, which makes sections of the electrode unusable, or generally poor electrode cohesion. However, cracks could provide channels for ion transport to deeper levels of the electrode, similar to templated electrodes (2) but without the extra manufacturing steps. Cracking is prominent in thick (3-5) or water processed electrodes (3), which are desirable for greater energy density and replacement of the toxic solvent N-methyl pyrrolidone (NMP), respectively. Thus, understanding mud cracking during drying can lead to more energy dense (thicker), greener (without toxic NMP), and better performing electrodes (fast charging).Li-ion electrodes are industrially produced by casting a slurry on a metal foil current collector by a slot die or doctor blade method in a roll-to-roll machine. In the case of the positive electrode this slurry typically contains active metal oxide (e.g. LiNiMnCoO2, LiNi0.6Mn0.2Co0.2O2) particles, polyvinylidene fluoride (PVDF) binder, and carbon black. Drying is composed of three stages illustrated in Fig. 1 a-d, with the binder, carbon black settling and migrating at random (1),(6, 7). Crack formation is thought to occur during the third stage when capillary forces cause stress in the drying slurry opening cracks as solvent menisci form between particles. However, the process of crack nucleation and their 3-dimensional growth has never been directly observed in-situ(1),(4). X-ray computed tomography (Fig. 1 f-j) is able to show the morphology of cracks in an electrode and can be used to characterize properties like delamination, crack volume, crack network connectedness or crack network tortuosity, which can be linked to electrode performance. However, to understand how these crack networks form requires observation of the time evolution of the cracks and the movement of active material particles near the cracks, which requires fast measurements. Here, we present synchrotron x-ray computed tomography of drying electrode which enable the fast measurements required and allows us to more fully explain mud cracking in drying electrodes, and the drying process in general.References Y. S. Zhang, N. E. Courtier, Z. Zhang, K. Liu, J. J. Bailey, A. M. Boyce, G. Richardson, P. R. Shearing, E. Kendrick and D. J. L. Brett, Advanced Energy Materials, 2102233 (2021). C. Huang, M. Dontigny, K. Zaghib and P. S. Grant, Journal of Materials Chemistry A, 7, 21421 (2019). Z. Du, K. M. Rollag, J. Li, S. J. An, M. Wood, Y. Sheng, P. P. Mukherjee, C. Daniel and D. L. Wood, Journal of Power Sources, 354, 200 (2017). B. Lu, C. Ning, D. Shi, Y. Zhao and J. Zhang, Chinese Physics B, 29, 026201 (2020). R. Sahore, D. L. Wood, A. Kukay, K. M. Grady, J. Li and I. Belharouak, ACS Sustainable Chemistry & Engineering, 8, 3162 (2020). S. Jaiser, L. Funk, M. Baunach, P. Scharfer and W. Schabel, Journal of Colloid and Interface Science, 494, 22 (2017). Y. S. Zhang, A. N. Pallipurath Radhakrishnan, J. B. Robinson, R. E. Owen, T. G. Tranter, E. Kendrick, P. R. Shearing and D. J. Brett, ACS Applied Materials & Interfaces, 13, 36605 (2021). Figure 1