@article{AzzamEhrensbergerScheueretal.2023, author = {Azzam, Mohamed and Ehrensberger, Moritz and Scheuer, Reinhard and Endisch, Christian and Lewerenz, Meinert}, title = {Long-Term Self-Discharge Measurements and Modelling for Various Cell Types and Cell Potentials}, volume = {16}, pages = {3889}, journal = {Energies}, number = {9}, publisher = {MDPI}, address = {Basel}, issn = {1996-1073}, doi = {https://doi.org/10.3390/en16093889}, year = {2023}, abstract = {Self-discharge of lithium-ion cells leads to voltage decay over time. In this work, the self-discharge was measured at 30 °C for three cell types at various voltage levels for about 150 days in a constant voltage mode determining the current at a high precision (float current). All cells exhibit a transient part leading to a steady-state, which is no longer influenced by reversible effects. To study the effect of the check-ups on the steady-state float current, the cells, interrupted and not interrupted by check-ups, were compared. The results indicate that both the transient processes and steady-state currents are highly reproducible. In the first period of the float current, the polarization dominates the measured current, followed by the anode overhang effect dominating the process for a period of 5-30 days. After the decline of both processes, a mostly constant steady-state in the order of µA is observed. The check-up interruption generally shows no apparent effect on the reached steady-state and results only in an extended settling time. A model to simulate the transient process and steady-state of float currents was developed. The model shows a high accuracy in reproducing the results and identifying the time needed to reach the steady-state.}, language = {en} } @article{AzzamEhrensbergerEndischetal.2024, author = {Azzam, Mohamed and Ehrensberger, Moritz and Endisch, Christian and Lewerenz, Meinert}, title = {Accelerating float current measurement with temperature ramps revealing entropy insights}, volume = {2024}, pages = {114142}, journal = {Journal of Energy Storage}, number = {102, Part B}, publisher = {Elsevier}, address = {Science}, issn = {2352-1538}, doi = {https://doi.org/10.1016/j.est.2024.114142}, year = {2024}, language = {en} } @article{StrasserWilliamsNgaleuHanzletal.2023, author = {Straßer, Xenia and Williams Ngaleu, Guy and Hanzl, Christian and Azzam, Mohamed and Endisch, Christian and Lewerenz, Meinert}, title = {Influence of Switching on the Aging of High Energy Lithium-Ion Cells}, volume = {9}, pages = {139}, journal = {Batteries}, number = {2}, publisher = {MDPI}, address = {Basel}, issn = {2313-0105}, doi = {https://doi.org/10.3390/batteries9020139}, year = {2023}, abstract = {An AC-battery or multilevel inverter used to increase safety and flexibility is realizable by switching the cells and modules on and off in a defined way and thus can replace the bidirectional converter. Assessing possible additional aging due to switching, the results of a previous study for a high-power optimized cell showed no influence on the current rates or the switching. In this paper, a highly energy-optimized LG 18650-cell is investigated to discuss the influence of switching during the charge and discharge process, respectively, as well as combining both processes together with clear performance differences when applying higher charge and discharge currents. Moreover, the influence of switching is discussed for the two frequencies (50 Hz and 10 kHz) and different duty cycles. The aging is analyzed by capacity loss and resistance increase, by dV/dQ analysis, and by electrochemical impedance spectroscopy. We found no clear negative influence of switching but a positive effect if the cells are switched during charge. The best performance is found for switching during charge as well as during discharge. The cell aging during switching is clearly determined by the average and not the maximum current applied. This work shows no negative effects of multilevel inverter applications on the tested cells.}, language = {en} } @article{AzzamEndischLewerenz2023, author = {Azzam, Mohamed and Endisch, Christian and Lewerenz, Meinert}, title = {Evaluating the Aging-Induced Voltage Slippery as Cause for Float Currents of Lithium-ion Cells}, volume = {10}, pages = {3}, journal = {Batteries}, number = {1}, publisher = {MDPI}, address = {Basel}, issn = {2313-0105}, doi = {https://doi.org/10.3390/batteries10010003}, year = {2023}, abstract = {This paper provides a comprehensive exploration of float current analysis in lithium-ion batteries, a promising new testing method to assess calendar aging. Float currents are defined as the steady-state trickle charge current after a transient part. In the literature, a correlation to capacity loss was reported. Assuming the float current compensates for the voltage decay over time and is linked to calendar aging, effects from voltage slippery must be considered. The dU/dQ analysis suggests solely a loss of active lithium. Therefore, we investigate the solid electrolyte interphase (SEI) growth as the general aging mechanism to explain the origin of float currents. Our results show that the voltage slippery theory holds true within the low to middle test voltage ranges. However, the theory's explanatory power begins to diminish at higher voltage ranges, suggesting the existence of additional, yet unidentified, factors influencing the float current. A shuttle reaction or lithiation of the cathode by electrolyte decomposition are the most promising alternative aging mechanisms at high voltages. The paper proposes a unique voltage slippery model to check for correlations between aging mechanisms, the float current test and the check-up test. For a better understanding, test strategies are proposed to verify/falsify the aging mechanisms beyond SEI.}, language = {en} } @article{AzzamEhrensbergerEndischetal.2024, author = {Azzam, Mohamed and Ehrensberger, Moritz and Endisch, Christian and Sauer, Dirk Uwe and Lewerenz, Meinert}, title = {Comparison of dU/dQ, Voltage Decay, and Float Currents via Temperature Ramps and Steps in Li-ion Batteries}, volume = {8}, pages = {e202400627}, journal = {Batteries \& Supercaps}, number = {1}, publisher = {Wiley}, address = {Weinheim}, issn = {2566-6223}, doi = {https://doi.org/10.1002/batt.202400627}, year = {2024}, language = {en} } @article{KiranAzzamEndischetal.2025, author = {Kiran, Iqra and Azzam, Mohamed and Endisch, Christian and Butt, Nauman Zafar and Lewerenz, Meinert}, title = {Evaluation of Calendar Aging in Cells with Graphite: Silicon Anode Using Float Current Analysis Under the Influence of Voltage Hysteresis}, volume = {172}, pages = {100529}, journal = {Journal of The Electrochemical Society}, number = {10}, publisher = {IOP Publishing}, address = {Bristol}, issn = {1945-7111}, doi = {https://doi.org/10.1149/1945-7111/ae0fe8}, year = {2025}, abstract = {This study investigates the calendar aging of lithium-ion batteries with graphite-silicon anodes using float current analysis. While float current analysis is already a proven method for assessing aging in cells with graphite-based anodes, the presence of silicon introduces additional complexities due to its voltage hysteresis. We address this by comparing the results for the scaling factor separately for charge and discharge. The scaling factor is initially derived from charge and discharge GITT measurements on fresh cells, including an aging-induced shift among both electrode curves. This approach enables quantification of SEI growth ISEI growth, and cathode lithiation current ICL bridging measured results for float currents with capacity loss rate. As a result, the scaling factor during charge delivered the most meaningful results regarding fitted aging currents. By extending the estimation method based on the Arrhenius equation across temperatures from 5 °C to 50 °C, our model is validated against measured float currents, improving the predictive accuracy of long-term aging trends in silicon-containing anodes. Electrochemical impedance spectroscopy provided further insights into degradation mechanisms, revealing a strong correlation between cathode lithiation by salt decomposition and resistance increase at high voltages (⩾4.15 V), confirmed by pulse tests at 100\% SOC showing a sharp resistance increase at elevated voltages.}, language = {en} } @article{AzzamSauerEndischetal.2025, author = {Azzam, Mohamed and Sauer, Dirk Uwe and Endisch, Christian and Lewerenz, Meinert}, title = {Comprehensive Analysis of Float Current Behavior and Calendar Aging Mechanisms in Lithium-Ion Batteries}, volume = {9}, pages = {e202500349}, journal = {Batteries \& Supercaps}, number = {1}, publisher = {Wiley}, address = {Weinheim}, issn = {2566-6223}, doi = {https://doi.org/10.1002/batt.202500349}, year = {2025}, abstract = {Aiming to quantify degradation currents from solid electrolyte interphase formation (ISEIgrowth) and gain of active lithium due to cathode lithiation (ICL), resulting from electrolyte decomposition, the float current behavior of lithium-ion batteries is investigated with different cathode materials. The float current, IFloat , represents the recharge current required to maintain the cell at a fixed potential during calendar aging. This current arises as lithium is irreversibly consumed at the anode or inserted into the cathode, shifting the electrode potentials. To account for the asymmetric response of the electrodes, a voltage-dependent scaling factor, SF, is introduced, derived from the slopes of the electrode-specific voltage curves. Using this factor in combination with measured float currents and capacity loss rates from check-up tests, ISEIgrowth and ICL is quantified at 30 °C across various float voltages. Although the SF and capacity data are limited to 30 °C, the model is extended to a range of 5-50 °C using only float current measurements. The results show that using capacity loss rates alone underestimate ISEIgrowth and that ICL, contributes significantly to the observed float current at elevated voltages, indicating that cathode lithiation plays an increasingly important role in high-voltage calendar aging.}, language = {en} }