Researchers at the Fraunhofer Institute for Materials and Beam Technology IWS in Dresden have developed a new battery cell production process that covers the electrodes of energy storage cells with a dry film instead of liquid chemicals. This simplified process saves energy and eliminates toxic solvents. An article on the Fraunhofer process, with colleagues from Samsung R&D in Japan, is published in the journal Energy storage materials.
Electrodes coated with the new dry transfer coating technology. The Fraunhofer IWS process enables the production of pilot-scale battery electrodes without the use of toxic solvents. © Fraunhofer IWS
Better and more economical production methods for energy storage are increasingly in demand, especially in Germany: all major car manufacturers have launched ambitious electric vehicle programs that will ensure a sharp increase in demand for batteries. So far, German companies have purchased cells for this purpose in Asia.
There are two main reasons for this trend: First, Asian technology groups have many years of experience in mass production of battery cells, and second, the processes are energy intensive. Producing in places with high electricity prices, such as Germany, is therefore very expensive.
Fraunhofer engineers want to change that.
Our dry transfer coating process aims to significantly reduce electrode coating process costs. Manufacturers can eliminate toxic and expensive solvents and save energy costs during drying. In addition, our technology also makes it easier to use electrode materials that are difficult or impossible to wet process. For all these reasons, we believe that our technology can contribute to internationally competitive battery cell production in Germany and Europe.
—IWS Project Manager Dr. Benjamin Schumm
Finnish battery company BroadBit Batteries, together with IWS, has commissioned a pilot plant at its Espoo plant, which coats the electrodes with dry electrode material instead of wet pastes, as was common in the industry so far. BroadBit uses it to produce new types of sodium-ion batteries.
On a laboratory scale, the IWS can already coat an electrode foil with a remarkable production speed of several meters per minute. In this regard, the engineers in Dresden can show the potential for technology transfer at the production scale.
Until now, cell producers have mainly coated their battery electrodes in a complex wet chemical process. First, they mix the active ingredients, later intended 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, another expensive process step begins: dozens of heating sections several meters long dry the coated films before they can be processed further. This drying procedure usually results in high electricity costs.
New film transfer technology for dry electrode coating, on the other hand, works without these environmentally damaging and expensive process steps: IWS engineers mix their active ingredient with binder polymers. They process this dry mixture in a rolling mill. The shear forces in this system tear entire molecular chains from the binder polymers. These fibrils join with the electrode particles like in a spider’s web. This gives the electrode material stability. The result is a layer of flexible dry electrode material. In the next step, the rolling mill laminates the 100 micron thick film directly onto aluminum foil, creating the battery electrode.
Self-supporting NCM sheets with a high surface load of 6.5 mAh cm-2 were prepared showing even at room temperature the same rate performance as the electrodes without binder with 2.5 mAh cm-2. The impact of the binder content on the performance of the cells was studied, revealing a significantly reduced impedance at contents below 0.7% by weight. To achieve a practical cell, the cell composition has been optimized and a 9 cm2 The all-solid rocking-chair type drums have been prepared without any solvents, which emphasizes the durability 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 generations of batteries where conventional processes fail.
These next generation systems include, for example, energy storage systems that use sulfur as the active material or solid state batteries that use ion-conducting solids instead of flammable liquid electrolytes.
Regarding the processing of electrodes for all solid state batteries, the researchers took an important step by applying their dry film technology using extremely low binder contents.
The engineers in Dresden are now aiming to improve their technology in cooperation with industrial partners. In the BMBF-funded “DryProTex” project, for example, they are further developing the dry transfer coating process with the companies Saueressig, INDEV, Netzsch Trockenmahltechnik and Broad-Bit Batteries.
In the DryProTex project, material, process and equipment developments are carried out with the aim of achieving process design for dry cathode production on an industrial scale.
Felix Hippauf, Benjamin Schumm, Susanne Doerfler, Holger Althues, Satoshi Fujiki, Tomoyuki Shiratsushi, Tomoyuki Tsujimura, Yuichi Aihara, Stefan Kaskel (2019), ” Energy storage materials, do I: 10.1016 / j.ensm.2019.05.033