Disturbances Due to Short- and Long-Term Diffusion Equalization Effects in Self-Discharge Measurements of Lithium-Ion Batteries

Quality assurance is essential for controlling lithium-ion cell properties during and after manufacturing. The largest contributors to process manufacturing costs of lithium-ion batteries are the cell formation and aging, when cell quality parameters are determined [1]. While voltage, capacity and impedance are determined reasonably fast, cell aging and self-discharge monitoring may take two to three weeks [2]. Apart from economical aspects for manufacturing and cell aging, accurately measuring the self-discharge may take up to several month [3, 4]. The main reason for the long duration of electrical self-discharge measurements are disturbances due to short-term and long-term diffusion equalization effects [5]. Short-term effects up to several hours include solid phase and liquid phase diffusion, leading to open circuit voltage relaxation [6]. Long-term effects up to several weeks are mainly based on the same diffusion phenomena in conjunction with inactive anode overhang areas [7]. Therefore, the storage condition has significant impact on the state-of-charge of the anode overhang, inducing anomalies with regard to capacity [8] and coulombic efficiency [9]. Consequently, it is expected that the aforementioned disturbances might have a similar effect on the electrical measurement of self-discharge. In this work, various cell preconditionings were applied to different methods for electrical self-discharge measurement, such as the capacity loss, the voltage decay and the voltage hold methods. The preconditioning allowed a distinction between undisturbed cells, short-term and long-term disturbed cells, or cells affected by a combination of disturbances. The investigated cells were commercial Samsung INR21700-50E cylindrical cells (NCA/silicon-graphite) and non-commercial pouch cells (NMC622/graphite) with comparable capacity and anode overhang areas. The results showed significant impact of preconditioning on the self-discharge measurements. Self-discharge of undisturbed cells was accurately measured within a few days. Short-term disturbances subsided after several hours, which is in good agreement with the findings from open circuit voltage relaxation [6]. Long-term disturbances due to the anode overhang areas decreased over the measurement period even though the self-discharge current was still many times higher than the undisturbed self-discharge after several weeks. Comparison to initial testing of non-commercial cells showed an equivalence of voltage decay method for fresh cells after formation and long-term disturbed cells, indicating anode overhang charge equalization as the main contributor to self-discharge measurements during cell aging step. The utilized non-commercial pouch cells were designed and produced within the scope of the project “FormEL”, funded by the German Federal Ministry of Education and Research (BMBF) under grant number 03XP0296D. Literature [1] Liu, Y.; Zhang, R.; Wang, J.; Wang, Y.: Current and future lithium-ion battery manufacturing, In: iScience 24 (4) , p. 102332–102332, 2021 [2] Kwade, A.; Haselrieder, W.; Leithoff, R.; Modlinger, A.; Dietrich, F.; Droeder, K.: Current status and challenges for automotive battery production technologies, In: Nature Energy 3 (4) , pp. 290–300, 2018 [3] Zilberman, I.; Sturm, J.; Jossen, A.: Reversible self-discharge and calendar aging of 18650 nickel-rich, silicon-graphite lithium-ion cells, In: Journal of Power Sources 425 (9) , pp. 217–226, 2019 [4] Theiler, M.; Endisch, C.; Lewerenz, M.: Float Current Analysis for Fast Calendar Aging Assessment of 18650 Li(NiCoAl)O2/Graphite Cells, In: Batteries 7 (2) , p. 22–22, 2021 [5] Deutschen, T.; Gasser, S.; Schaller, M.; Siehr, J.: Modeling the self-discharge by voltage decay of a NMC/graphite lithium-ion cell, In: Journal of Energy Storage 19 (8) , pp. 113–119, 2018 [6] Kindermann, F.M.; Noel, A.; Erhard, S.V.; Jossen, A.: Long-term equalization effects in Li-ion batteries due to local state of charge inhomogeneities and their impact on impedance measurements, In: Electrochimica Acta 185 , pp. 107–116, 2015 [7] Wilhelm, J.; Seidlmayer, S.; Keil, P.; Schuster, J.; Kriele, A.; Gilles, R.; Jossen, A.: Cycling capacity recovery effect: A coulombic efficiency and post-mortem study, In: Journal of Power Sources 365 , pp. 327–338, 2017 [8] Burrell, R.; Zulke, A.; Keil, P.; Hoster, H.: Communication—Identifying and Managing Reversible Capacity Losses that Falsify Cycle Ageing Tests of Lithium-Ion Cells, In: Journal of The Electrochemical Society 167 (13) , p. 130544–130544, 2020 [9] Gyenes, B.; Stevens, D.A.; Chevrier, V.L.; Dahn, J.R.: Understanding Anomalous Behavior in Coulombic Efficiency Measurements on Li-Ion Batteries, In: Journal of The Electrochemical Society 162 (3) , A278-A283, 2015