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Realising an ideal lithium-ion battery (LIB) cell characterised by entirely homogeneous physical properties poses a significant, if not an impossible, challenge in LIB production. Even the slightest deviation in a process parameter in its production leads to inhomogeneities and causes a deviation in performance parameters of LIBs within the same batch. The greater the number and/or intensity of inhomogeneities, the more they need to be avoided. Severe inhomogeneities (defects), such as metal particle contamination, significantly impact the cell’s performance. Besides electrical measurements, image-based measurement methods can be used to identify defects and, thus, ensure the production quality and safety of LIBs. While the applicability of computed tomography (CT) as an image-based measurement method for detecting defects has been proven, the limitations of this method still need to be determined. In this study, a systematic analysis of the capabilities of CT imaging was conducted. A multilayer pouch cell without an electrolyte was reassembled with several defects on one of the middle anodes. To investigate the boundaries of CT, defects such as a partial and complete removal of the coating, a cut, or a kink, as well as particle contaminations of various sizes and materials (aluminium, copper, iron) were chosen. By comparing the CT images of the cell using laser scanning microscope images of the defective anode, it could be proven that all selected defects except the kink were detectable.

For the research and development of new battery materials, achieving high reproducibility of the performance parameters in the laboratory test cells is of great importance. Therefore, in the present work, three typical small-format lithium-ion cells (coin cell, Swagelok cell and EL-CELL ECC-PAT-Core) were tested and compared with regard to the reproducibility of their performance parameters (discharge capacity, internal resistance and coulombic efficiency). A design of experiments (DOE) with the two factors separator type and anode–cathode ratio (N/P ratio) was carried out for all cells. For the quality features discharge capacity, internal resistance and coulombic efficiency, the coefficient of variation is used as a measure of reproducibility. The statistical evaluation shows that in 83% of all cases, higher reproducibility is achieved when the Freudenberg separator is used instead of the Celgard separator. In addition, higher reproducibility is achieved in 78% of all cases if the anode and cathode are the same size. A general statement about which test cell format has the highest reproducibility cannot be made. Rather, the format selection should be adapted to the requirements. The examined factors seem to have an influence on the reproducibility but are more insignificant than other still-unknown factors. Since the production of small-format test cells is a manual process, the competence of the assembler seems to prevail. In order to mitigate the influence of as many unknown variables as possible, assembly instructions are proposed for each cell type.

Due to the simple structure and the possibility of manual production, coin cells enable fast and, compared to larger cell formats, an inexpensive examination option in battery research. The comparability and traceability of coin cell structures in literature are only feasible to a limited extent due to the lack of a standard in manual production. Since the findings from the literature are barely building up on each other and have not been repeated, a full factorial Design of Experiments (DoE) was performed to investigate the significance of earlier findings in terms of their influence on the reproducibility of the performance. The parameters studied were the anode-to-cathode ratio, the amount of electrolyte, the spring type and the separator count. To quantify the reproducibility of coin cell assembly, the number of functional cells (here: successful formation followed by 30 cycles) and the empirical coefficient of variation for the performance parameters discharge capacity, internal resistance and coulombic efficiency were compared. The critical parameters found in prior literature have no statistically significant influence on reproducibility when focusing on the number of functional cells. Instead, other uninvestigated parameters seem to influence the system coin cell more. By further examining the parameter settings that produced the most functional cells (≥75% of 8 cells), guidance for constructing coin cells (type R2032) was suggested, and other potential influencing parameters are discussed for further study.