The battery is a key component in deciding the success of e-vehicles. This is why manufacturers have set themselves ambitious targets: They want to double the energy density from currently around 250 watt hours per liter (Wh/l) to 500 Wh/l. At the same time, production costs are to fall well below € 200 per kilowatt hour. Within the "ePadFab” research project, IAV is working with partners thyssenkrupp System Engineering and Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) to reach this target.
Two approaches lend themselves to increasing battery capacity and hence the cruising range of e-vehicles: new storage materials with a higher energy density and new integration concepts for getting more storage material into the car. Above all, the “ePadFab” project partners are opting for large, planar-type lithiumion batteries that can be integrated directly into the vehicle’s chassis. The central innovation is a new type of structure for the energy storage system. In future, the batteries are to consist of stacked cells, with sets of two cells sharing an electric surge arrester. One side serves as an anode, the other as a cathode.
Today’s lithium batteries are set up differently. They consist of a wound or stacked structure that is inserted into cylindrical or prismatic cell housings or sealed in a pouch. The energy density falls to 40 to 60 percent of the value at cell level merely by packaging cells individually and additionally installing the cells in the system. As a result, today’s battery systems only reach between 140 and 300 Wh/l.
Double the share of active material in the volume of the battery system
This is why the project partners want to make fundamental changes to the structure of battery systems. The planned planar setup of bipolar electrodes with a surface area of up to two square meters makes it possible to break up the conventional cell and module boundaries and integrate the energy storage system as a component into the vehicle chassis. The consortium calls the concept EMBATT (“Chassis- embedded Energy”). “Our aim is above all to break up the cell, module and battery boundaries of energy storage systems and, in this way, double the share of active storage material to 80 percent of battery volume”, says Michael Clauß, battery system developer at IAV. “We want to integrate the batteries into the vehicle’s underfloor. Depending on package, voltages of 850 volts are possible at present and up to 1.200 volts in future. This will provide electric traveling ranges of up to 1,000 kilometers.”
To get there, however, numerous challenges still have to be overcome. These include the production of bipolar electrodes with anode and cathode that need to be coated with different materials on both sides. Stacking the individual layers is no trivial matter either. “We must get the electrolyte into the large areas of the electrodes. At a cell layer thickness of approximately 300 micrometers, this is a complex process”, Clauß says. The battery’s mechanical stability is to be guaranteed by an endoskeleton and an exoskeleton which, for example, could consist of aluminum, a composite material or, quite simply, vacuum. Besides the battery system’s innovative structure, the project partners use particularly efficient materials. Lithium titanium oxide will be used for the anode, lithium nickel manganese oxide for the cathode. This will make it possible to achieve a cell voltage of 3.2 volts, and the material is also resistant to overcharging and behaves less critically in the event of damage. A major challenge will also be to find the right electrolyte for this.
At the end of the “ePadFab” project, new production processes are to be available that will make it possible to manufacture the large EMBATT batteries industrially and cost-effectively. IAV is providing its entire development expertise from devising vehicle concepts, vehicle safety, battery layout and design to calibrating control unit software. The Fraunhofer IKTS is contributing its knowledge of developing tailor-made materials and special processes for manufacturing electrodes, whereas ThyssenKrupp System Engineering will mainly be involved in producing the batteries later on.
At the end of the project in 2017, a demonstrator is to be available in a scale of 1 : 5 that will let the project partners run safety and performance tests. The launch could follow around 2025. Yet the EMBATT technology is not only suitable for electric vehicles – it can also be used as a decentralized energy storage unit in photovoltaic systems. The “ePadFab” project is being sponsored with ERDF funds and by the Free State of Saxony.