Pocket-sized lithium-sulfur batteries examined at BESSY II

A team from HZB and the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden has gained new insights into lithium-sulfur pouch cells at BESSY II's BAMline. Complemented by analyses in the HZB imaging laboratory and other measurements, a new and insightful picture of processes that limit the performance and service life of this industrially relevant battery type has emerged. The study has been published in the renowned specialist journal "Advanced Energy Materials".

Lithium-sulfur batteries have several advantages over traditional lithium batteries: They use sulfur as an abundant raw material, do not require the critical element cobalt or nickel, and can achieve an extremely high specific energy density. Prototype cells already achieve up to 500 Wh/kg, almost twice as much as current lithium-ion batteries.

However, lithium-sulfur batteries are currently much more susceptible to degradation processes: When charging and discharging, dissolved polysulfides and sulfur phases form on the lithium electrode, which increasingly reduce the performance and service life of the battery. "We want to clarify these processes through our research with the aim of specifically improving this type of battery," says HZB physicist Dr. Sebastian Risse, who heads a team at the HZB for the operando analysis of batteries.

He is concentrating on so-called pouch cells, a battery format that is widely used in industry. At the HZB Institute for Electrochemical Energy Storage (CE-IEES), which is headed by Prof. Yan Lu, a laboratory has been set up that specializes in the production of lithium-sulfur batteries in this "pocket format". Here, a wide variety of variants of lithium-sulfur batteries in pocket format can be manufactured and examined. As part of the BMBF project "SkaLiS", which is coordinated by Risse, the team led by Risse, together with a group from the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden, has now published a comprehensive study on lithium-sulfur pouch cells in the journal Advanced Energy Materials .

For this purpose, battery cells were examined in a setup developed at the HZB using various methods such as impedance spectroscopy, temperature distribution, force measurement and X-ray imaging (synchrotron and laboratory source) during charging or discharging. "For the first time, we were able to observe and document both the formation of lithium dendrites and the dissolution or formation of sulfur crystallites during multilayer battery operation," says Dr. Rafael Müller, HZB chemist and lead author of the study.

" In particular, phase contrast radiography with coherent synchrotron light at the BAM beamline at BESSY II enabled us to track the low-absorbing lithium metal morphology. We were able to correlate these insights with other measurement data and thus create a comprehensive picture." With the help of X-ray analyses in the HZB imaging laboratory in cooperation with Dr. Ingo Manke's group, it was also possible to analyze the formation of strongly absorbing sulfur crystals during battery operation.

"Our results build a bridge between basic research and technology transfer, and in particular they also enable conclusions to be drawn regarding the scalability of this battery technology and for the further development of high-energy battery systems ," says Risse. Among other things, the team showed that a new design approach from the IWS Dresden is promising: the perforated and thus significantly lighter cathode current collector does not affect the performance of the cell.

The results of this study will help to optimize the performance and lifetime of lithium-sulfur batteries so that this promising battery type can meet the requirements of mobile and stationary energy storage systems.