New Method Using Floating Currents for Fast and High Resolved Measurement of Calendar Life Aging Rate to Improve Prediction of Aging
Authors: Lewerenz Meinert , Käbitz Stefan , Knips Marcus , Münnix Jens , Schmalstieg Johannes , Warnecke Alexander , Sauer Dirk Uwe 
Published: February 2020

Within this work a method is presented that is a powerful tool to characterize the calendaric aging induced rate of capacity fade and enables to present the Arrhenius relation with a high resolution in a simple test arrangement. This method may be a fast and cheap alternative to state-of-the-art calendaric aging tests. The results have been already published here [1].
In this test 9 cylindrical 8 Ah LiFePO4|Graphite battery cells are analyzed during calendaric aging at 25 °C, 40 °C and 60 °C at 3.6 V which corresponds to about 100% SOC. The 3.6 V are kept by applying constant voltage while the floating currents are logged. The floating of the cells is interrupted by check-up tests to evaluate the capacity and internal resistance. The steady-state of the floating currents reveal to be rather constant at 25 °C, linear increasing at 40 °C and decreasing from a higher level at 60 °C (figure left). Comparing the steady-state values of the floating currents with the derivative of the capacity loss a high correlation is visible (figure right). This can be further optimized by adding the aging during check-up to the measured floating currents.
The advantage of evaluating only the steady-state aging is that it allows neglecting the influence of reversible effects like the anode overhang [2-4] or the homogeneity of lithium distribution (HLD) [5]. Therefore, the signal-to-noise ratio is significantly increased, which helps to predict the capacity fade for moderate aging conditions.
Additional tests with three test cells, varying the temperature from 40 °C to 60 °C in steps of 5 K, exhibit on one side a non-constant floating current and on the other side a non-uniform behavior of the three samples starting from 50 °C on. The cells being once above 50 °C reveal an irreversible increased floating current that is still increased, if the cell is brought back to temperatures lower than 50 °C. Thus accelerated aging is only reasonable until 45 °C as the aging above this threshold will have a different origin. The floating current tests are evaluated in a comparison to a regular calendaric aging test. The 2 months float-measurement gives an impressively higher resolution of the Arrhenius relation of the capacity fading rate than the calendaric aging tests after 2 years of testing.
This method, including both strategies, has now been applied to 25 Ah prismatic Li(Ni1/3Mn1/3Co1/3)O2|Graphite cells used for automotive applications. The scope is to determine the stability of the cell to accelerate the aging in a reasonable way and to predict the aging without being influenced by the anode overhang, internal resistance or influence of homogeneity of lithium distribution (HLD). Moreover, the stability of the cell with respect to cell voltage and temperature is investigated.

References

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[5] M. Lewerenz, A. Marongiu, A. Warnecke, D.-U. Sauer, Differential voltage analysis as a tool for analyzing inhomogeneous aging: a case study for LiFePO4|Graphite cylindrical cells, J.Power Sources. 368 (2017) 57–67. doi:https://doi.org/10.1016/j.jpowsour.2017.09.059.

Figure 1