Photo: ZSW

Go-ahead for lightweight and safe lithium-ion batteries for electric flying

Mobility and thus also air traffic cause a large CO2 footprint. In order to be able to reduce this significantly in the future, lightweight, safe and cost-effective high-energy batteries are needed. And these should, as far as possible, only contain materials that conserve resources and are environmentally compatible. A research project of the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) with three industrial companies starts exactly here. The project will develop new active materials with high specific energy and safety, as well as the processes to turn them into battery electrodes. Critical and expensive materials are to be substituted. Both anode and cathode are processed aqueous instead of with the harmful solvent NMP. The knowledge gained is to be used for the production of round cells suitable for industrial use. The project started at the end of 2021.

In addition to ZSW, the material manufacturer Johnson Matthey Battery Materials GmbH, the mechanical engineering company Coperion GmbH and the cell manufacturer VARTA AG are also involved. VARTA is coordinating the project, which is being funded by the German Federal Ministry of Education and Research (BMBF) with 1.6 million euros. The research project will run for three years until October 31, 2024.

Today, commercial high-energy cells still contain significant amounts of the expensive and carcinogenic metal cobalt, which is also sometimes mined under precarious conditions. “An important project goal will be to use cobalt-free cathode materials in future batteries by developing suitable process conditions,” says Prof. Dr. Markus Hölzle, ZSW board member and head of the Electrochemical Energy Technologies business unit in Ulm. “On the anode side, the use of silicon oxide is expected to significantly increase the energy content. Another new feature is that the production of both electrodes is water-based, i.e. without the use of the toxic solvents that are common today.”

Environmentally friendly materials with more specific energy

The consortium aims to increase the specific energy of the battery cells by up to 20 percent – compared to an already available sustainable cell chemistry. This would significantly reduce the weight of the batteries for the same energy content. This is to be achieved through four measures. First, the use of intrinsically safe lithium manganese iron phosphate (LMFP) in place of the established lithium iron phosphate (LFP) as the cathode material. Both materials are free of the critical raw materials nickel and cobalt, but LMFP contains more energy than LFP. The second measure is derived from this: The partners want to increase the surface capacity by 40 percent compared to an LFP cathode.

Third, on the anode side, graphite, which is becoming increasingly scarce and expensive, is to be replaced by the abundantly available silicon oxide (SiOx). Because the energy content of SiOx is significantly higher than that of graphite, this can save weight and volume of the batteries.
The fourth measure is to substitute water for the hazardous solvent NMP, which has been used almost exclusively to coat the electrodes.

Develop low-cost cell production suitable for industrial use

All improvements are to be translated into processes suitable for industrial use in order to be able to build high-performance and safe batteries on a pilot scale. To do this, the process requirements for very high capacity electrodes – anode and cathode – during mixing, coating, drying and calendering must be investigated and understood. To this end, the scientists and engineers are researching, among other things, the process technology of extrusion as an innovative, highly efficient way of continuous processing. Based on the findings, it should ultimately be possible to manufacture thick water-based electrodes in an industry-relevant roll-to-roll process and wind them for use in round cells.

At the ZSW in Ulm, the processes are being tested on a dedicated pilot line and validated in small laboratory batteries as a demonstrator. In parallel, VARTA is introducing the results of the project into the production of wound button cells and 21700 round cells.

The batteries developed in this way can be used in numerous applications. An important application in mobility is aviation. Battery-powered aircraft are currently being developed by a number of companies, and high energy content combined with maximum safety are key factors in the success of the high-performance batteries required for this purpose.