Conventional battery systems are very complex: they usually consist of several individual cells connected to each other by cables. Not only is this costly, but there is also the danger of hot spots - areas where the cables may become too hot. In addition, each of these cells must be packaged. A large part of the battery therefore consists of inactive material not contributing to battery performance.
Bipolar batteries are designed to solve this problem: In their case, the individual cells are connected to each other by means of flat bipolar plates. However, other challenges arise in this context. These bipolar plates are made of metal and are therefore susceptible to corrosion. Or they are made of a mixture of plastic and carbon, which means that they must be at least several millimeters thick.
Material savings of more than 80 percent
Researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen have now developed an alternative. »We manufacture bipolar plates from electrically conductive polymers«, explains Dr.-Ing. Anna Grevé, department head at Fraunhofer UMSICHT. »This allows us to produce very thin plates and - compared with conventional cells connected by cables - to save more than 80 percent of the material«.
Moreover, this material offers numerous other advantages. First, it does not corrode. At the same time, it can be subsequently formed. For example, it can be used to emboss structures that are important for fuel cells. And the novel bipolar plates can be welded together so that the battery system is absolutely leak-proof.
In contrast, conventional bipolar plates are unsuitable for welding due to the thermal and mechanical stress on the material during production: Seals are required to join them together in such a way that neither gases nor liquids can permeate the joints. However, such seals quickly become porous and they also take up a lot of space. Another advantage of the new material is that researchers can adapt the properties of the bipolar plates to the specific requirements. »We are able to manufacture plates that are both flexible and bendable enough to be wrapped around your finger, as well as plates that are stiff as a board«, specifies Grevé.
Cost-effective production by reel-to-reel process
The major challenge was the development of the material and the manufacturing process. »We use polymers and graphite materials that are commercially available. The key, however, is the recipe«, emphasizes Grevé. Since the material consists of about 80 percent graphite and only about 20 percent polymers, the manufacturing process has little in common with conventional polymer processing. The research team at Fraunhofer UMSICHT opted for the reel-to-reel process, which allows cost-effective production, and adapted it to their needs.
Eventually, the ingredients must be homogeneously distributed in the manufactured plates, while the plates must also be robust and completely leak-proof. This was not easy due to the initial structure of the materials. »We were able to meet all the requirements in a single process. The plates can therefore be used as they come from the line«, explains Grevé. Another advantage of the process is that the plates can be manufactured in any size.
The researchers are already able to produce relevant sample quantities of the new bipolar plates. Together with the company Saueressig, they have translated the previous method of producing bipolar plates for fuel cells into a continuous process. At the Hannover Messe from April 1 to 5, 2019, Grevé and her team will be showing a 3.2 square meter bipolar plate that enables the construction of large-scale redox flow batteries (Hall 2, Booth C22).