Disturbances Due to Short- and Long-Term Diffusion Equalization Effects in Self-Discharge Measurements of Lithium-Ion Batteries
Authors: Roth Thomas , Streck Luiza , Niehoff Philip , Jossen Andreas 
Published: November 2022

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.
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