Published on January 11, 2023
Setting new standards: Technology transfer through norms
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.
The ProLIB project consortium agrees that such manipulation tests do not adequately represent realistic stress, since the trigger in the propagation test often does not represent a real cell-internal fault. But another aspect in the test scenario weighs even more heavily: the varying quality of battery cells is not taken into account. The cell test expert explains the discrepancy: "High-quality cells and battery systems with integrated safety features, such as powerful battery management systems, are at a disadvantage in such tests. Because of the high basic safety, much more massive damage has to be inflicted on these cells to literally blow them up. Specifically, this means much more energy is added to the entire system in order for a reaction to occur, compared to battery cells with low-quality cell components or cell designs. But at the same time, the more energy means that the damage ends up being much more massive."
Thomas Timke, Senior Battery Expert at Solarwatt and previously an expert at KIT, makes it clear: "Distributors and manufacturers like us, who value safe cells, have to oversize their entire system as a result in order to pass the applicable tests without complaint. But until improved testing procedures are established and proven, we believe the existing propagation testing with thermal run-away is necessary. It is currently the only consensus-based procedure for all manufacturers to demonstrate compliance with certain protection goals, specifically no fire outside the storage tank." Solarwatt, the Dresden-based manufacturer of high-quality photovoltaic systems and home storage units, is directly affected and therefore advocates for a fair testing procedure in the ProLIB project. At the company, Timke keeps an eye on incorporating the applicable normative requirements into the products. He has been involved in committee work in standardization processes for product safety testing for some time, so he is always up to date with the latest developments in standards. Smith, a researcher, sees another drawback to battery development: "Testing procedures that are too crude make it difficult for battery technology to evolve. Every performance improvement of a battery cell has to prove itself in propagation testing, even though the test conditions don't represent reality."
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.
Cells under observation
For the new test procedure, the project partners are not only looking at replicating a real cell fault, but also making the risk assessment from a completely different angle. "In contrast to the propagation test, we are not trying to force the thermal run-away of the cell, but we are evaluating the behavior that the cell exhibits during the stress test – a triggered real cell fault. Based on reference values, this would allow classification into safety levels," Smith, the cell expert, says with foresight. It was important to the ProLIB partners that the test procedure be applicable in the real installation situation. The individual cell in the battery pack is electrically contacted and measured directly at its destination. This is the only way to ensure that behavior patterns are not influenced by changes in the underlying conditions. Ultimately, many factors in the installation environment, such as heat sinks, directional opening and blow-off of cells, play an important role that can only be properly evaluated in the real installation situation.
Despite the promising project results, establishment as a recognized testing technique is still a few steps away. Smith provides an outlook: "Now that we have been able to show that more realistic testing of internal cell defects in lithium-ion cells in battery modules is possible in a new way, we are working on a comprehensive database for future safety levels. This will require further evaluation of different cell types and formats." Industry partner Timke adds, "We have laid a promising foundation for new standards with ProLIB. The medium-term goal is to elaborate the test method developed for a new standard for safety testing of lithium-ion batteries in stationary and mobile applications." For this purpose, a working group has already been established at the German Commission for Electrical, Electronic & Information Technologies (DKE), which is concerned with the development of standards, norms and safety regulations in the fields mentioned.
The founding of the working group s is an important milestone for ProLIB: entry into a standardization process. This entry into the expert circle and the community's constructive discussion of the new test procedure are decisive for acceptance in the market. The DKE itself sums it up as follows: Standards. Make. Future. As electrification progresses, the applications of lithium-ion cells continue to grow rapidly. Therefore, it is time to introduce fairer, more realistic test conditions for manufacturing companies. In the project, stakeholders have shown that there are realistic evaluation criteria for the safety and quality of Li-ion batteries. The new standard should enable fairer competition, help reduce raw material use, development and product costs by avoiding overdesign, and increase safety in lithium-ion battery operation.
The way to the norm
Although the term "norm" has a formal, bureaucratic appearance, it is not to be equated with an official process. Rather, it is a collaborative effort of many stakeholders who decide on a consensus. It always starts with one person or interest group submitting a proposal for a standard. This should be based on verified results of science, technology and experience. The German Institute for Standardization organizes experts to come together to set technical rules.
Standardization organizations such as this exist at national, European as well as international level. In standardization projects, the proposal is publicly discussed and revised in the expert group until all participants reach a consensus. The agreed standards, or more precisely rules for technical circumstances and procedures, are then published in the form of a norm. The entire standardization process usually takes three years from the time the proposal is submitted. As a rule, standards are only created if there is an economic interest in their application.