Lithium-Sulfur batteries: First multimodal analysis in pouch cell format

Lithium-sulphur (Li/S) batteries have significantly higher energy densities than conventional lithium-ion batteries, but age very quickly. Now, for the first time, a team at HZB has investigated Li/S batteries in the industry-relevant pouch cell format with different electrolytes during operation. Teams from TU Dresden and the Fraunhofer IWS were also involved in the study. With a specially developed measuring cell, impedance, temperature and pressure can be recorded at different times and combined with radiographic images. The evaluation shows how the electrolyte affects the formation of unwanted sulphur particles and polysulphides. The study has been published in the renowned journal Advanced Energy Materials.

Lithium-sulphur (Li/S) batteries theoretically have an energy density of 2500 watt-hours/kg, which is significantly higher than in conventional lithium-ion batteries.  In addition, Li/S batteries use more environmentally friendly cathode materials compared to lithium-ion batteries. But there is a problem with Li/S batteries: with increasing number of charging cycles, the active material changes, the metallic lithium anode corrodes, the capacity decreases rapidly. With innovative electrolytes and refined additives, attempts are being made to slow down this ageing. So far, however, mainly Li/S batteries in coin cell design have been investigated, where these reactions take place soaked in plenty of electrolyte.

Format matters

For industry, however, other formats such as round cells (Tesla), prismatic cells (BMW Group) or pouch cells (Volkswagen) are of particular interest. In these formats, the amount of electrolyte is extremely small, which enables particularly high energy densities. At HZB, multimodal operando investigations on Li/S pouch cells have now been carried out for the first time as part of the BMBF-funded project "HiPoLiS". In collaboration with teams from the TU Dresden as well as the Fraunhofer IWS, a team led by Dr. Sebastian Risse investigated single-layer Li/S cells with different electrolytes. "We first need to understand the processes in monolayer cells before we can also optimise multiple layers in pouch cells," Risse is convinced.

Radiography and sensors combined

For their study, they combined measurement data with X-ray radiography analyses, which were created in close cooperation with the group led by HZB imaging expert Dr. Ingo Manke. "This allowed us to draw conclusions about the formation and deposition of sulphur particles and polysulphides over the course of the charging cycles," says Dr Rafael Müller, a postdoctoral researcher in Risse's electrochemistry group. Their results also allowed to evaluate the influence of the electrolyte on particle formation.

The multimodal measuring cell, which Müller developed together with Risse, contains various sensors: they record the electrochemical impedance, the temperature, but also mechanical forces on the electrodes. In addition, the pouch cell is illuminated with X-rays during the entire discharging/charging cycle (operando) this creating several radiographies from which the chemical deposition processes can be deduced. The figure here shows the arrangement of the sensors and the different states of the Li/S pouch cell. A video of the conversion processes can be seen in the publication in the much-cited journal "Advanced Energy Materials".

The pouch cell laboratory at HZB

In order to make further progress on the basis of this cell format, a pouch cell laboratory was set up last year in Prof. Yan Lu's Electrochemical Energy Storage department.  To produce these cells, rectangular electrodes in credit card format are stacked on top of each other and - separated only by a thin separator foil - placed in a sealable pocket (pouch). Compared to coin cells, pouch cells require only a small amount of electrolyte to ensure charge transport. All electrochemical processes therefore take place under much drier conditions. "The necessary lack of electrolyte has a very strong effect on these processes and must therefore be investigated directly in an industrially relevant cell format," says Risse.

Energy for the wingcopter

One goal of the HiPoLiS project is to increase the range of a logistics drone of the project partner Wingcopter from Darmstadt with the improved pouch cells from Dresden. For this purpose, the Fraunhofer IWS produces Li/S cells with up to 40 layers, which are then integrated into the existing energy supply of the drone.

Outlook: Battery research projects at HZB

Nanostructures for cathodes

Building on this work, another BMBF project with a total of six partners called "SkaLiS" was successfully obtained, which started in July 2021 and is coordinated by Sebastian Risse. The research activities here now focus on multilayer pouch cells with optimised cathode structure and improved electrolyte composition (TU Dresden). The material approach ranges from specially synthesised nanoscopic particles (Prof. Yan Lu) to macroscopic cathode structuring (Fraunhofer IWS). The new material approaches are being analysed by the Manke and Risse working groups in collaboration with the TU Berlin.

Development of EU standards and ISO standards.

In addition, a new EU project coordinated by PTB on the metrology of Operando measurements and the definition of EU standards and ISO norms has been acquired and is expected to start in September 2022. Sebastian Risse is leading a work package for operando impedance spectroscopy.