Study uncovers key degradation insights

Lithium-sulfur batteries have a number of advantages over conventional lithium batteries: they use the abundant raw material sulfur, do not require the critical elements cobalt or nickel, and can achieve extremely high specific energy densities. Prototype cells are already achieving up to 500 Wh/kg, almost twice as much as current lithium-ion batteries.

However, lithium-sulfur batteries have so far been much more susceptible to degradation processes: during charging and discharging, dissolved polysulfides and sulfur phases form on the lithium electrode, gradually reducing the performance and lifetime of the battery.

“Our research aims to elucidate these processes in order to improve this type of battery,” says HZB physicist Dr. Sebastian Risse, who leads a team at HZB working on operando analysis of batteries.

He is focusing on pouch cells, a battery format widely used in industry. HZB’s Institute for Electrochemical Energy Storage (CE-IEES), headed by Prof. Yan Lu, has therefore set up a laboratory specializing in the production of lithium-sulfur batteries in the required pocket format. Here, scientists can produce and investigate a wide variety of lithium-sulfur pouch cells.

As part of the SkaLiS project, coordinated by Risse, a team from the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden has now published a comprehensive study of lithium-sulfur pouch cells in the journal Advanced Energy Materials.

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

“In particular, phase-contrast radiography with coherent synchrotron light at the BAM beamline at BESSY II allowed us to follow the morphology of the only weakly absorbing lithium metal and to correlate it with other measurement data, giving us a comprehensive picture. X-ray analyses in the imaging laboratory at HZB, carried out in collaboration with the imaging group of Dr. Ingo Manke also allowed the formation of strongly absorbing sulfur crystals to be analyzed during battery operation.”

“Our results bridge the gap between basic research and technology transfer, and in particular allow conclusions to be drawn about the scalability of this battery technology and the further development of high-energy battery systems,” says Risse.

Among other things, the team showed that a new design approach by the IWS Dresden is promising: a perforated and thus significantly lighter cathode current collector does not impair 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.