Assessment of coating defect criticality in lithium-ion battery electrodes

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State-of-the-art battery production suffers from high scrap rates, which are reported to range between 5 % and 30 % or even more, depending on the manufacturer [1,2]. Lowering these rates will be required in order to reach strategic targets, such as battery prices of less than 100 € kW h-1. Scrap may originate from various steps in battery manufacturing, such as unsatisfactory raw material quality, the electrode production process, the stacking or winding of cells or even further downstream processes, such as packing or formation. As a step of particular importance, electrode production, including mixing, coating and drying suffers from high scrap rates caused by various defects, such as metal contaminants, pinholes, blisters, line defects, agglomerates or inhomogeneities of thickness or composition [3].
While numerous inspection methods including thermography, computed tomography or post mortem microscopic techniques have been developed to detect such coating defects, there is a general lack of knowledge concerning their actual significance. In order to assess the criticality of different defects and to derive tolerance limits, we investigated model electrodes containing deliberately introduced coating defects of different types.
Metal contaminants, typically seen as the most detrimental type of electrode defect, cause the risk of chemical or physical short-circuits. We identified additional redox processes of metal contaminants in cathodes, as well as influences of the size and concentration of metal contaminants. Similarly, we studied the effect of thickness inhomogeneities within electrodes. Using a specially designed experimental setup, we were able to observe internal dynamics of defect-containing electrodes. The effect of compensatory currents, constantly redistributing inhomogeneous charges in electrodes of varying thickness, was evaluated with respect to the caused accelerated aging.
Our results highlight the importance of a thorough understanding of electrode coating defects as a basis for further development of effective quality assurance strategies and the reduction of 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.

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