Burning electric vehicles, smartphones that have become hot and burst open, or exploding stationary storage units are horror scenarios that no one wants to experience firsthand. The cause of such dangerous incidents can be malfunctions and the associated uncontrollable exothermic reactions with strong heat generation of the installed batteries or battery cells. This thermal run-away triggers a chain reaction in the battery cell due to the energy released: from a drop in performance to swelling and outgassing to a fire in the cell. In order to keep the risk of such safety-critical incidents as low as possible with appropriate safety measures, manufacturing companies are legally obligated to at least comply with the Product Safety Act or, ideally, to subject their battery products to standardized safety tests. KIT has been working on an improved and fair test procedure together with SOLARWATT GmbH, CTC advanced GmbH, and AVL Deutschland GmbH, among others, in the joint project ProLIB since the beginning of 2020.

Dr. Anna Smith from the Institute for Applied Materials (IAM) at KIT coordinates the joint project and explains: "There are some standardized tests, such as the so-called propagation test. The standardized test procedure involves provoking, or more clearly harassing, a battery cell in a battery on the basis of external influences – such as heat, overcharging, or mechanical damage, e.g., nailing of the cell – until defects occur inside that lead to a thermal run-away. This is implemented in test laboratories under freely selectable, usually hardly reproducible circumstances. "For example, in the area of stationary storage, the propagation test is considered to have been passed if the battery housing does not break at the end of the test and no flames escape from the housing. The cell behavior and the severity of the damage during the entire course of the test ultimately provides the testers with information about the expected risk. Accordingly, a storage manufacturer can then take measures for more safety, such as housing adjustments up to the selection of other cells.

New approach for lithium-ion battery testing

It is not only those involved in the project who have recognized the need to optimize the propagation test: "Other research teams around the world are working on making the test procedure more reproducible, for example with lasers as the manipulation medium used. However, the basic problem remains: The thermal run-away is brought about with all its might, regardless of whether the scenario could actually occur in practice," Timke emphasizes. ProLIB is the only research project so far to explore real-world, cell-specific failures. Smith explains the approach: "In the project, we worked out which internal cell errors could realistically occur. In our research, so-called lithium plating turned out to be a common cause of internal cell problems."

Lithium plating describes the phenomenon that during charging under unfavorable boundary conditions, e.g. low temperature, excessive battery cell currents, metallic lithium can be deposited on graphite-based anodes, which is normally embedded in the anode material. This unwanted deposition causes small wires of lithium to form on the surfaces of the anode material. If these structures grow to a critical size, they penetrate the separator as a separating layer and cause a fine circuit when the cathode makes contact. In practice, there are a wide variety of factors influencing the plating behavior. The task in the project was therefore to develop a method with which massive lithium plating can be produced on the graphite anode independently of cell type and cathode chemistry.

Smith reports with satisfaction: "We have managed to specifically induce the defect pattern of lithium plating and dendrite growth in several cell types and chemistries. And it can be done relatively quickly and reproducibly, making it a suitable methodology for a safety test. The effects on the cells being tested can be quite different. Interestingly, the dendrite growth and associated fine closure was not itself the problem that led to safety-critical behavior. Rather, it was the consequence of freshly deposited lithium: chemical reactions occur, for example of electrolyte with reactive lithium, which lead to gas formation and thus to a significant increase in the cell internal resistance. The higher this internal resistance, the more the battery cell then heats up during further energization. "The gradation of the fault characteristics that occur (no visual changes in the cell up to swelling, opening and even thermal run-away) in turn allows a gradation in terms of safety. This is comparable to benchmarking, as is the case with bicycle locks, for example. Here, there are certified security classes that provide information about the theft resistance of a lock.