Improved test procedures for safe battery systems

Between worst case and reality: KIT works together with partners on new test methods and standards for lithium-ion cells in battery systems

The temperature-controlled safety housing developed at KIT and equipped with sensors for testing lithium-ion cells under critical conditions. (Photo: Amadeus Bramsiepe, KIT)

The temperature-controlled safety housing developed at KIT and equipped with sensors for testing lithium-ion cells under critical conditions. (Photo: Amadeus Bramsiepe, KIT)

The development of innovative battery systems requires reliable safety tests of the lithium-ion cells used under real conditions. In the ProLIB research project, the Karlsruhe Institute of Technology (KIT) is now working together with testing and standardization institutes as well as with partners from industry to develop improved standards that should provide more security, but also more flexibility in battery design. The Federal Ministry for Economic Affairs and Energy (BMWi) is funding this research with more than 1.2 million euros.

If you don't remember the last time you found your notebook pierced by a nail, you can be sure that something like this rarely happens. Nevertheless, this is exactly what is common practice today for safety tests for lithium-ion batteries: "In order to carry out certain tests, the cells first have to be massively abused," says Dr. Anna Smith from the KIT Institute for Applied Materials (IAM). "The goal of the efforts is to trigger assumed worst-case defects inside the cells in order to observe their progress in the battery." In addition to piercing with nails, it is also common to overcharge or overheat cells.

The discrepancy between test methods and realistic stress that arises with such methods does not remain without consequences: Actual error profiles are not the subject of the investigation and so manufacturing errors of low-quality cells and their risks remain undetected, while actually safe cells are disadvantaged. "If battery systems are designed for realistic worst-case scenarios, regardless of their cell quality, it doesn't make them safer, but more voluminous, heavier, less sustainable and also more expensive than necessary," said Smith. Your team at the KIT Battery Technology Center works together with the network partners AVL Deutschland GmbH, CTC advanced GmbH and TÜV Rheinland LGA Products GmbH and the associated partners ads-tec Energy GmbH, Intilion GmbH, Jungheinrich Norderstedt AG & Co.

More security through more realistic test procedures

Basically, the concern about dangerous defects in lithium-ion cells is not unfounded: dendrites, i.e. pointed lithium deposits, can form on the anode. The likelihood that these will then trigger short circuits and thus ultimately also lead to a thermal runaway (an exothermic reaction with strong, self-accelerating heat development) is particularly given in cells that contain low-quality cell components. In the worst case, spreading this error to neighboring cells (propagation) results in a chain reaction and a fire in the battery. And any increase in energy density, for example to extend the range of a fully electric vehicle or to improve sustainability through the use of fewer raw materials, is made more difficult by overly rough test procedures. “The resistance of the lithium-ion cell to really dangerous defects, which can vary greatly depending on the manufacturer, for example due to the cell structure or cell components, is far too little in focus. Starting with the thermal runaway is like judging the safety of a lighter solely by its explosion behavior, ”says Smith.

Research is currently underway around the world to improve propagation tests. In other research projects, however, the focus is on triggering a thermal runaway more reproducibly (e.g. with lasers) - regardless of whether a cell would actually do it in practice. ProLIB is so far the only research project in which realistic and cell-specific errors are being researched. An improved test procedure for new standards for lithium-ion batteries in stationary and mobile applications is now to be developed in order to close the existing gaps in standardization with regard to realistic evaluation criteria for the safety and quality of lithium-ion batteries. The new standards should enable fairer competition, to reduce the use of raw materials,

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