Researchers at the Fraunhofer Institute for Material and Beam Technology IWS in Dresden have developed a new battery cell production process that coats the electrodes of the energy storage cells with a dry film instead of liquid chemicals. This simplified process saves energy and eliminates toxic solvents. A paper on the Fraunhofer process, with colleagues from Samsung R&D in Japan, is published in the journal Energy Strorage Materials.

Electrodes coated with the new dry transfer coating technology. Fraunhofer IWS process enables battery electrodes to be produced on a pilot scale without using toxic solvents. © Fraunhofer IWS

Better and more cost-efficient production methods for energy storage are increasingly in demand, especially in Germany: All major automobile manufacturers have launched ambitious electric vehicle programs that will ensure a sharp rise in demand for batteries. So far, German companies have been purchasing the cells for this purpose in Asia.

There are two main reasons driving this trend: one, Asian technology groups have many years of experience in the mass production of battery cells and, two, the processes are energy-intensive. Production at locations with high electricity prices—such as Germany—is, therefore, very high-cost.

The Fraunhofer engineers want to change this.

Our dry transfer coating process aims to noticeably reduce the process costs in electrode coating. Manufacturers can eliminate toxic and expensive solvents and save energy costs during drying. In addition, our technology also facilitates the use of electrode materials that are difficult or even impossible to process wet-chemically. For all these reasons, we think that our technology can help to achieve internationally competitive battery cell production in Germany and Europe.

—IWS project manager Dr. Benjamin Schumm

The Finnish battery company BroadBit Batteries, together with IWS, has commissioned a pilot plant in its Espoo factory, which coats electrodes with dry electrode material instead of wet pastes, as has been common in industry up to now. BroadBit uses it to produce new types of sodium-ion batteries.

On a laboratory scale, the IWS can already coat electrode foil with a remarkable production speed of several meters per minute. In this respect the Dresden engineers can show the potential for transferring the technology to the production scale.

Until now, cell producers have mostly coated their battery electrodes in a complex wet-chemical process. First, they mix the active materials, intended later to release the stored energy, with additives to create a paste. In this process they add organic solvents, which are expensive and usually toxic. In order to protect operators and the environment, elaborate precautions for occupational safety and reprocessing are necessary. Once the paste has been applied to thin metal foils, a further expensive process step begins: Dozens of meter long heating sections dry the coated films before they can be further processed. This drying procedure usually causes high electricity costs.

The new film transfer technology for dry electrode coating, on the other hand, operates without these ecologically damaging and expensive process steps: The IWS engineers mix their active material with binding polymers. They process this dry mixture in a rolling mill. The shear forces in this system tear entire molecular chains out of the binder polymers. These fibrils join with the electrode particles as in a spider web. This provides the electrode material with stability. The result is a flexible dry electrode material layer. In the next step, the rolling mill laminates the 100 micrometer thick film directly onto an aluminum foil, thus creating the battery electrode.

Free-standing NCM sheets with a high areal loading of 6.5 mAh cm⁻² were prepared showing even at room temperature the same rate performance like binder-free electrodes with 2.5 mAh cm⁻². The impact of binder content on cell performance has been studied revealing significantly reduced impedance at contents below 0.7 wt%. To realize a practical cell, the cell composition was optimized and a 9 cm² sized rocking-chair type all-solid-state battery was prepared without any solvents underlining the sustainability of the DF process. The battery was cycled for 100 cycles without any artificial pressure, demonstrating the versatility and potential of the DF process.

—Hippauf et al.

In this way, we are also able to process materials for new battery generations where classical processes fail.

—Benjamin Schumm

These new generation systems include, for example, energy storage systems that use sulfur as active material or solid-state batteries which employ ion-conducting solids instead of flammable liquid electrolytes.

On the way of processing electrodes for all solid state batteries the researchers have reached one important milestone by applying their dry film technology using extremely low binder contents.

The Dresden engineers now aim at enhancing their technology in cooperation with industrial partners. In the BMBF-funded “DryProTex” project, for example, they are further developing the dry transfer coating process together with the companies Saueressig, INDEV, Netzsch Trockenmahltechnik and Broad-Bit Batteries.

In the DryProTex project material, process and equipment developments are conducted with the aim of realizing process design for industrial scale dry cathode production.

Resources

• Felix Hippauf, Benjamin Schumm, Susanne Doerfler, Holger Althues, Satoshi Fujiki, Tomoyuki Shiratsushi, Tomoyuki Tsujimura, Yuichi Aihara, Stefan Kaskel (2019) “Overcoming binder limitations of sheet-type solid-state cathodes using a solvent-free dry-film approach,” Energy Storage Materials, doi: 10.1016/j.ensm.2019.05.033