Researchers at the Fraunhofer Institute for Material and Beam Technology IWS in Dresden have developed a new production process which is aimed at future efficient and environmentally friendly battery production. They coat the electrodes of the energy storage cells with a dry film instead of liquid chemicals. This simplified process saves energy and eliminates toxic solvents. BroadBit Batteries, a Finnish company, is currently successfully putting the new IWS technology into practice.

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 in Asia for this purpose. There are two main reasons driving this trend: Asian technology groups have many years of experience in the mass production of battery cells and a lot of energy is consumed in these processes. Production at locations with high electricity prices, such as Germany, is therefore extremely expensive.

It is exactly this fact that the Saxon Fraunhofer engineers want to change: “Our dry transfer coating process aims to significantly reduce the process costs in electrode coating”, emphasizes IWS project manager Benjamin Schumm. “Manufacturers are able to eliminate toxic and expensive solvents and save energy costs during the drying process. In addition, our technology also facilitates the use of electrode materials which are difficult or even impossible to process chemically in a liquid form.” But exactly these materials are needed for future batteries with higher energy density. “For all these reasons, we think that our technology can help to achieve internationally competitive battery cell production within Europe.”

A pilot plant in Finland

This potential has also been noticed by Fraunhofer’s Nordic partners: The Finnish battery company ‘BroadBit Batteries’ has commissioned a pilot plant in its Espoo factory which coats electrodes with dry electrode material instead of wet paste, as has been common in the industry up until now. BroadBit uses it to produce new types of sodium-ion batteries. “The demand for our technology is high, even in Germany,” Benjamin Schumm reports. 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 are able to demonstrate the potential for transferring the technology to production scale.

Until now, cell producers have mostly coated their battery electrodes in a complex liquid chemical process. First, they mix the active materials, (which will later release the stored energy) with additives to create a paste. In this process, they also 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 required. Once the paste has been applied to the thin metal foils, another expensive step in the process begins: dozens of meter-long heating sections dry the coated films before they can be further processed. This drying procedure usually causes higher electricity costs.

Binding molecules form a cobweb

On the other hand, the latest film transfer technology for the dry electrode coating process operates without these ecologically damaging and expensive process steps. The IWS engineers mix their active material with metal-binding polymers. They process this dry mixture in a roll-to-roll process known as ‘calendar’. The sheer forces in this system tear entire molecular chains out of the binder polymers. These ‘fibrils’ join the electrode particles a bit like a spider web. This provides the electrode material with more stability. The result is a flexible dry electrode layer of material. In the next step, the calendar laminates the 100-micrometre thick film directly onto an aluminium foil, thereby creating the battery electrode.

“In this way, we are also able to process materials for new battery generations where classical processes fail,” says Benjamin Schumm. These include, for example, energy storage systems that use sulfur as the active material or solid-state batteries which employ ion-conducting solids instead of flammable liquid electrolytes. “These batteries will be able to store more energy within the same volume as today’s lithium-ion batteries.”