A second life for batteries

Data from the Graz University of Technology (TU Graz) indicates that approximately 1.2 million lithium-ion batteries of electric cars, buses and construction machinery will be decommissioned worldwide by 2030. Around 14 million discarded batteries are expected to be discarded by 2040. Recycling these materials makes sense, but as it can be expensive and technically demanding, it would be more sustainable to reuse the batteries, for example as stationary electricity storage units. However, this requires an assessment of their remaining performance capability and safety.

Now, researchers from TU Graz have established the first parameters that can be used to assess the condition of the discarded batteries. So far, the reduced charging capacity and an increase in internal resistance have served as indicators of the condition of a used battery. However, that is not sufficient for a decision on possible second-life use.

For this reason, the researchers analysed lithium-ion cells that had been used in vehicles under real conditions and identical new ones in the laboratory. During repeated charging and discharging cycles, they recorded 31 different parameters and then checked how well they represented the aging condition of the batteries. 13 of these indicators turned out to be meaningful, including the charging and discharging capacity, the temperature difference between the poles during the charging process and the relaxation behaviour of the battery cell after the charging process.

Jörg Moser, head of the Battery Safety Centre Graz at the Vehicle Safety Institute at TU Graz, said the researchers used these indicators to draw conclusions about the aging status of lithium-ion batteries and different usage profiles without having to rely on data that is sensitive in terms of data protection, like the usage history of the batteries. “On this basis, we can decide whether a battery is, in principle, suitable for further use in a particular area of application,” Moser said.

However, the safety status of the batteries still needs to be assessed to ensure low-risk reuse. In the course of their first life, chemical changes occur in the batteries, which can affect their safe use. Christian Ellersdorfer, the head of the COMIT project SafeLIB at the Vehicle Safety Institute, said it is crucial to understand battery cells and the processes, reactions and changes that take place in them in order to determine their safety behaviour.

It could be several years before the batteries are reused on a broad basis in subsequent applications, making electromobility even more sustainable over its entire lifecycle. After all, new storage materials, the safety of different battery technologies, warranties and liability must also be taken into consideration.

“This results in an interdisciplinary field of research that we want to work on at TU Graz together with national and international partners in further research projects,” Ellersdorfer said.

Image caption: Jörg Moser (left) and Christian Ellersdorfer from the Vehicle Safety Institute at TU Graz. Image credit: Lunghammer - TU Graz.