Electrochemical Criticality of Coating Defects in Lithium-Ion Battery Electrodes

Since the lithium-ion battery was established technologically, its applications possibilities have increased massively. Especially in the field of electric mobility the demand for LIBs has magnified extremely. However, to minimize the costs of electric vehicles, it is important to make the production process for LIBs more efficient. Currently, battery production is affected by high scrap rates, which amount to 5-30% of the total cell production or even more [1,2]. The scrap can originate from various steps during the battery production process, e.g. from insufficient quality of the raw materials, electrode production, cell assembly, or even from downstream processes like the conditioning of the cells. Therein, the production of electrodes is a particularly important step. During this process, which in turn includes mixing, coating, drying and calendering, various defects can be introduced into the electrodes. These defects - such as agglomerates, foreign particle contamination (mainly metals), point defects (blisters, divots, pinholes), line defects or inhomogeneities of thickness, porosity or composition can lead to high scarp rates. [3]. These well-known defects can already be easily detected with different methods like camera-based optical systems, thermography, computed thermography, or post-mortem analysis methods. However, the crucial point lies in the quantification of the electrochemical influence of the defects, i.e. in an quantitative evaluation which defect types, sizes and concentrations can be tolerated, and in which case the electrode must be classified as scrap. To evaluate the criticality and establish tolerance limits for different defect types, we produced and studied defect-free electrodes and electrodes with different defect types. Metal contaminations are usually considered the most severe electrode defect, as they can cause chemical or physical short circuits. We found additional redox processes occurring at metal particles in cathodes, quantified the influences of size and concentration of metal impurities on cyclability and established a concentration threshold value for these impurities. Furthermore, we investigated the influence of inhomogeneities of porosity and thickness in electrodes and other defects like line defects. A special experimental setup allows us to analyze the internal dynamics of the interaction between defect-containing and defect-free electrodes. Therein, equalizing currents continuously redistribute the inhomogeneous charges in electrodes with different defects. This equilibrating effect was evaluated in terms of the caused accelerated aging. Our results support the importance of a deep understanding of electrode coating defects as a basis for further developing effective quality assurance strategies and reducing scrap rates. (1) Gaines, L.; Dai, Q.; Vaughey, J. T.; Gillard, S. Direct Recycling R&D at the ReCell Center. Recycling 2021, 6 (2), 31. DOI: 10.3390/recycling6020031. (2) Brückner, L.; Frank, J.; Elwert, T. Industrial Recycling of Lithium-Ion Batteries—A Critical Review of Metallurgical Process Routes. Metals 2020, 10 (8), 1107. DOI: 10.3390/met10081107. (3) David, L.; Ruther, R. E.; Mohanty, D.; Meyer, H. M.; Sheng, Y.; Kalnaus, S.; Daniel, C.; Wood, D. L. Identifying degradation mechanisms in lithium-ion batteries with coating defects at the cathode. Applied Energy 2018, 231, 446–455. DOI: 10.1016/j.apenergy.2018.09.073. Figure 1