Float Current Analysis for fast calendar aging detection investigating 18650 NCA/ Graphite varying temperature and voltage

Summary:

Converting our electricity and transportation strategy into a sustainable system, we need high-quality battery electric storage. The most popular candidate up to now is the lithium-ion bat-tery family as it provides high energy density combined with long cycle life at low costs. The lifetime is targeted e.g. for e-mobility to 10-20 years. Due to the low aging, the assessment of the lifetime of the cells is however currently a challenge due to the time-intensive characteri-zation of battery cells. Therefore, tests need to have a very high precision or have to be ac-celerated by more severe test conditions increasing the SOC and/or the temperature. Accel-erated tests have the drawback that they often trigger aging mechanisms that would not oc-cur regularly in the dedicated application. Reversible capacity effects such as the anode overhang effect, further superimpose the high precision for calendar life tests (10.1016/j.jpowsour.2017.01.133.). Thus the measurement precision, especially for lower voltages and temperatures, is a challenge with the requirement for short test duration. Moreo-ver, the results at temperature below 20°C is influenced strongly by the reference check-up conditions that may be even higher than the actual aging in the test condition. Besides the precision, the time and the number of test cells to obtain a full characterization is a big hurdle in current test strategies. This high effort is the reason for the low amount of research on path dependence of the aging that might have a strong influence on models.
We present a novel test method evaluating the aging of li-ion cells by means of float currents. These float currents, defined as the steady-state of the self-discharge, are evaluated at de-fined conditions. For LFP/Graphite cells, a correlation with capacity loss rate was shown (10.1016/j.jpowsour.2017.03.136.). For other cell types no additional effects at standard condi-tions are observed as it is reported by Zilberman et al. (10.1016/j.jpowsour.2019.03.109.) evaluating the correlation of capacity loss rate and open circuit drop over time. Thus, there is a need to investigate the float current analysis for more different cell types and to improve the test and evaluation strategy to show the potential including innovative applications and the limitation of the method.
In this poster presentation, we show the results for another cell type (25R Samsung 18650) with NCA vs. graphite for 5 cell potentials and a temperature range from 5 to 60°C in 5K steps using a standard test bench. We demonstrate the ability for a fast characterization and for low temperature characterization in the order of 0-10°C depending on the SOC. We show the Arrhenius relation over SOC and temperature and we discuss the activation energy. Moreo-ver, we discuss path dependence of temperature sequence and the correlation to capacity loss rate for two temperature sequences to improve the understanding of aging and the novel test strategy. The shown results are published here: 10.3390/batteries7020022.