High-performance lithium-ion cells, such as those used in power tools, deliver very high electrical currents for short periods. Until now, measurements have typically been taken only at the surface of the cell housing, which systematically underestimates the actual temperature inside the cell.

For the study, thermocouples were attached to the electrodes of a high-power pouch cell (4.5 Ah, NCM/graphite) that had been manufactured and formed externally, and the cell was resealed. Additional sensors were attached to the top and bottom of the cell housing. The cells were then tested at discharge rates of up to 22 C (ratio of charge/discharge current to rated capacity). This corresponds to an electrical discharge current that would completely discharge the cell in less than three minutes.

The measurements show that at a discharge rate of 20 C, the core temperature averaged about 2 °C higher than the temperature measured at the surface. The sensor integration itself had only a minor effect on cell behavior; the deliverable capacity decreased by about two percent. The temperature data obtained are therefore transferable to unmodified cells.

Computed tomography (CT) scans confirmed the sensor's position at the center of the cell and revealed only locally limited structural effects. However, postmortem analyses using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) revealed chemical changes in the immediate vicinity of the implantation site, indicating potential for optimization in the selection of sensor materials.

“The fact that we can conduct high-current tests, cell disassembly under an inert gas atmosphere, and the subsequent post-mortem analysis all in one location is what made this rapid development possible in the first place. It is precisely this combination under one roof at the ‘Center for Electrical Energy Storage’ that allows us not only to develop new measurement methods but also to validate them reliably.” says Daniel Nusko, who led the project at Fraunhofer ISE.

This work arose from a commissioned research project by Hilti AG and serves as an example of how industrial challenges are addressed scientifically at Fraunhofer ISE. In this case, the findings were also published in a scientific journal, ensuring that the insights gained are not limited to the project but are made openly available to the research community. The method can be applied to other cell formats and cell chemistries. For larger cells, even more pronounced temperature differences between the core and the surface are expected.

“For Hilti, this work creates a robust database on the actual thermal stress experienced by high-performance cells, thereby expanding the foundation for the safe and optimal design of the battery management system,” explains Dr. Sebastian Maletti, project manager at Hilti Entwicklungsgesellschaft mbH.

The measurements were made possible by the unique infrastructure at the Fraunhofer ISE’s “Center for Electrical Energy Storage.” Safety-critical tests at high C-rates and temperatures were conducted in an explosion-proof, fire-resistant test environment under controlled conditions by qualified technical personnel. Sensor integration and cell opening took place in an argon-filled glove box; analytical instruments such as SEM and EDS are available on-site. This combination of high-current and abuse testing, cell handling under inert gas, and advanced post-mortem analysis is essential for developing and validating methods of this kind.