Lithium-sulphur pouch cells investigated at BESSY II
Photomontage: the diagonal line divides the image into a photo of the lithium-sulfur pouch cell (left) and the corresponding X-ray image (right) during the multimodal measurement with force sensor (golden) and temperature sensors. The perforated honeycomb structure of the current collector can be clearly seen on the X-ray image. This new design approach reduces the weight of the cell without compromising performance. © R. Müller/ HZB
A team from HZB and the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden has gained new insights into lithium-sulphur pouch cells at the BAMline of BESSY II. Supplemented by analyses in the HZB imaging laboratory and further measurements, a new picture emerges of processes that limit the performance and lifespan of this industrially relevant battery type. The study has been published in the prestigious journal Advanced Energy Materials.
Lithium-sulphur batteries have a number of advantages over conventional lithium batteries: they use the abundant raw material sulphur, 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.
Degradation processes examined
However, lithium-sulphur batteries have so far been much more susceptible to degradation processes: during charging and discharging, dissolved polysulphides and sulphur 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.
The pouch cell lab at HZB
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 specialising in the production of lithium-sulphur batteries in the required pocket format. Here, scientists can produce and investigate a wide variety of lithium-sulphur pouch cells. As part of the BMBF-funded 'SkaLiS' project, coordinated by Sebastian Risse, a team from the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden has now published a comprehensive study of lithium-sulphur pouch cells in the prestigious journal Advanced Energy Materials.
Multimodal setup
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 sulphur crystallites during multi-layer battery operation,' says Dr Rafael Müller, HZB chemist and first author of the study.
Phase-contrast radiography at BESSY II
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 sulphur crystals to be analysed during battery operation.
Outlook: High-energy battery systems
‘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 optimise the performance and lifetime of lithium-sulphur batteries, so that this promising battery type can meet the requirements of mobile and stationary energy storage systems.
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.
- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.