Frank Lienesch

The performance and aging behavior of lithium-ion pouch cells are influenced by mechanical compression and also by the separator properties. This study investigates the electrochemical effects of the external pressure on lithium-ion pouch cells using different separator materials. The tested separators include a dry-processed PP/PE/PP separator, a ceramic-coated separator and a wet-processed ceramic coated separator. External pressure was applied at pressures ranging from 0 MPa to 0.45 MPa. The impact of compression and separator type on electrochemical performance was assessed through capacity tests, internal resistance measurements and cycling tests over 300 cycles. It is demonstrated that the effect of external pressure on cell performance depends on the separator behavior. The optimum external pressure level is influenced by the type of separator, its wettability, material specifications and porosity. Excessive external pressure results in increased internal resistance, reduced capacity and premature aging effects due to pore deformation.

The formation of lithium‐ion batteries is a time‐consuming and important process during manufacturing. During the formation, surface layers on the electrodes are formed that affect the cell performance. State‐of‐the‐art formation procedures consist of several slow current cycles as it is believed to guarantee good surface layer properties while preventing degradation processes. Herein, five different formation strategies with process times between 52.79 and 1.68 h for coin cells with a lithium reference electrode are assessed. The fastest method is based on a fast charging approach that prevents lithium‐plating. Based on a real‐time negative electrode voltage control to a threshold of 20 mV, lithium‐plating is successfully prevented while ensuring a fast formation process. The formation is finished after just one cycle and results to similar cell and electrode resistance, impedance, and capacity retention compared to the other strategies. The fast charging formation approach leads to the lowest degradation when storing the cells at 80% state of charge and 60 °C for 28 days. The results conclude that the fast charging formation method with real‐time control of the negative electrode voltage is a beneficial method as it leads to faster process times while ensuring durable cell properties.

Battery safety tests are defined by several international standards in different ways and with heterogenous termination and failure criteria. In this work, lithium-ion cells were examined regarding their behavior in the event of overcharging and also in the event of an external short circuit with varied parameters specified by standards. The voltage, current, and temperature curves were evaluated. In addition, the changes in the cells were analyzed using electrochemical impedance spectroscopy (EIS). It is shown that the cells exhibit reproducible behavior in a clamped state. Further, it could be determined that the position of the cell opening during an overcharge has an influence on the further behavior of the cell. EIS data showed that the cells have a significantly higher internal resistance after an overcharge. The short-circuit tests at different ambient temperatures indicated that the internal resistance of the cell decreases with increasing temperature. However, no reproducible effects in impedance spectra were present after the short-circuit test. This work illustrates that the choice of test parameters and termination criteria undoubtedly influence the test results and thus may change the classification of cells as either safe or unsafe. Thus, cells may be classified as safe regarding a certain standard but unsafe regarding another.

Fast charging of lithium-ion batteries is often related to accelerated cell degradation due to lithium-plating on the negative electrode. In this contribution, an advanced electrode equivalent circuit model is used in order to simulate fast-charging strategies without lithium-plating. A novel parameterization approach based on 3-electrode cell measurements is developed, which enables precise simulation fidelity. An optimized fast-charging strategy without evoking lithium-plating was simulated that lasted about 29 min for a 0–80% state of charge. This variable current strategy was compared in experiments to a conventional constant-current–constant-voltage fast-charging strategy that lasted 20 min. The experiments showed that the optimized strategy prevented lithium-plating and led to a 2% capacity fade every 100 fast-charging cycles. In contrast, the conventional strategy led to lithium-plating, about 20% capacity fade after 100 fast-charging cycles and the fast-charging duration extended from 20 min to over 30 min due to increased cell resistances. The duration of the optimized fast charging was constant at 29 min, even after 300 cycles. The developed methods are suitable to be applied for any given lithium-ion battery configuration in order to determine the maximum fast-charging capability while ensuring safe and durable cycling conditions.

Zusammenfassung Elektrische Kontaktöffnungsentladungen sind eine potenzielle Zündquelle für brennbare Gasgemische. Betriebsmittel, in denen gegebenenfalls Kontaktentladungen auftreten, können mit einem sogenannten Funkenprüfgerät der internationalen Norm IEC 60079-11 (6te Ed.) geprüft werden. Nachteile dieser Methode sind die schlechte Reproduzierbarkeit der Ergebnisse und die Verwendung von toxischem Cadmium. Für die Entwicklung einer Alternative wurden bisher die elektrischen Entladungen charakterisiert. Der Übergang von der Entladung zur Zündung wurde dabei noch nicht ausreichend untersucht. In diesem Artikel werden die ersten Ansätze zur Abschätzung des Temperaturfeldes mit Schlieren- und örtlich aufgelösten spektroskopischen Untersuchungen dargestellt.

The reuse and repurposing of lithium-ion batteries for transportation in stationary energy systems improve the economic value of batteries. A precise suitability test at the beginning of the second life is therefore necessary. Common methods such as electrochemical impedance spectroscopy (EIS) and current interrupt (CI) analysis, as well as capacity analysis, can be used for testing. In this paper, these methods are studied from the aspects of test duration, sensitivity and acquisition costs of the measuring instruments. For this purpose, tests are carried out on battery modules, which were used for transportation. It is shown that subtle differences are better detected with EIS and less accurately with the CI method. The test duration is fastest with the CI method, followed by EIS and the capacity test. Strongly aged modules are reliably detected with all methods.

The cell characterization in the incoming inspection is an important but time and cost intensive process step. In order to obtain reliable parameters to evaluate and classify the cells, it is essential to design the test procedures in such a way that the parameters derived from the data allow the required statements about the cells. Before the focus is placed on the evaluation of cell properties, it is therefore necessary to design the test procedures appropriately. In the scope of the investigations two differently designed incoming inspection routines were carried out on 230 commercial lithium-ion battery cells (LIBs) with the aim of deriving recommendations for optimal test procedures. The derived parameters of the test strategies were compared and statistically evaluated. Subsequently, key figures for the classification were identified. As a conclusion, the capacity was confirmed as an already known important parameter and the average cell voltage was identified as a possibility to replace the usually used internal resistance. With regard to capacity, the integration of CV steps in the discharging processes enables the determination independently from the C-rate. For the average voltage cycles with high C-rates are particularly meaningful because of the significant higher scattering due to the overvoltage parts.