Accurate and characterization of mismatches in cell characteristics is essential for quality battery production, handling and recycling. In this paper the key parameters cell impedance and open-circuit voltage (OCV) are analyzed in their variance as they are vital for assessing the state of health and lifespan of lithium-ion cells. Variations in these parameters reflect the quality of the manufacturing process,while sufficiently large dataset can also identify outliers. Early detection of such outliers is particularly important for future cell-to-pack concepts, where individual cells are practically non-replaceable and the associated risks and costs of failures are significant.
This study focuses on the quasi-open-circuit voltage (qOCV) and impedance measurements of 200 brand-new commercial cylindrical 21700 cells. Established models are applied to analyze the variance in the measurement results. Additionally, eight representative cells were selected for a more detailed analysis of their impedance across the full state of charge (SoC) range at several selected temperature levels. Impedance was measured using the multi-sine electrochemical impedance spectroscopy (EIS). Measurements for all new cells were conducted at an OCV of 3.6V and a constant temperature of 25 °C. The results were then divided into frequency bands based on variance to identify relevant regions of interest (RoI) in the frequency spectrum. The reference cells were further characterized across the entire SoC range in 1% increments at 7 temperature levels. To account for these dependencies, an analysis of the variance across SoC and temperature was performed. The qOCV measurement were carried out for all cells with a precision better than 20µV and 100nA across the full SoC range. The variance of these results was analyzed with respect to the lithiations processes interphases and specific RoIs were identified.