Our society is becoming increasingly electrified – and electrochemical energy storage in the form of batteries is playing a central role in providing the energy we need. At the same time, however, demands are increasing with regard to the sustainability and safety of the battery materials used. In September 2022, the joint project “CaSino” therefore started to determine the potential of the so-called calcium-sulfur (Ca-S) battery as an alternative to lithium-ion batteries. The project consortium consists of five research institutions, two industrial companies and an industrial advisory board and is coordinated by DLR Stuttgart. It is being funded by the German Federal Ministry of Education and Research (BMBF) with 3 million euros as part of the “Battery 2020 Transfer” announcement.
Calcium as a battery material
Compared to the lithium-ion batteries currently used by industry, the Ca-S battery offers a number of advantages, but also poses challenges for researchers.
Lithium, which has been used in conventional batteries to date, represents the perfect element for an electrochemical cell in many respects: it combines high current storage capacity and cell voltage with fast ion migration. This enables compact batteries and fast charging and discharging. However, lithium-based systems have an increased fire hazard because dendrites can form during repeated charging, leading to an internal short circuit in the worst case. In addition, lithium deposits on earth are limited and not available on all continents, mining is controversial and it can only be recycled at great expense. Calcium – a multivalent metal like aluminum, magnesium and zinc – is 400 times more abundant in comparison and therefore inexpensive, as well as being available worldwide and equally distributed. Like lithium, it has a high storage capacity and cell voltage and is also safer in terms of short circuits, since calcium does not form typical dendrites during operation.
The greatest challenge in the use of calcium is its reactivity and the formation of surface layers, whether in contact with air or moisture – or even with the electrolyte used in the battery. The oxidized surfaces block ion diffusion down the road, preventing efficient charging and discharging. The development of a compatible electrolyte thus assumes a key function. In addition, the use of a sulfur cathode generates soluble polysulfides, which can also block the Ca anode – this must also be prevented.
Project consortium with complementary competencies
The “CaSino” project has now set itself the goal of achieving significant progress in terms of cycle stability and energy density of calcium-sulfur batteries through innovative material development. These steps are accompanied by experimental investigations, advanced analytical methods and extensive modeling work. The BMBF-funded joint project will be coordinated by the Institute of Engineering Thermodynamics of the German Aerospace Center in Stuttgart during the three-year project period.
From the consortium, the Karlsruhe Institute of Technology (KIT) with its Helmholtz Institute Ulm (HIU) plays a pioneering role in the field of research on calcium batteries. With a non-corrosive boron-based electrolyte, KIT has laid the foundation for further investigation of Ca batteries. This recent development now enables more stable charging and discharging of Ca batteries over hundreds of cycles. Building on this, KIT together with IoLiTec GmbH, the specialist for ionic liquids, is striving to further improve the already established KIT electrolyte.
The Research Institute for Precious Metals and Metal Chemistry (fem) is addressing the need for thin and structured Ca anodes, while the DLR Institute for Technical Thermodynamics is developing tailored coatings for this purpose to protect against passivation. The electrochemical characterization of the new materials will be extended by innovative structural and morphological analysis methods (analytical electron and ion microscopy) at the Natural and Medical Sciences Institute (NMI) at the University of Tübingen. Furthermore, experimental activities will be supported by atomistic simulations at the University of Ulm and continuum simulations and modeling of the Ca-S cell at the DLR Institute of Technical Thermodynamics. Finally, a comprehensive consideration of economic and ecological aspects by EurA AG will highlight the potential of the Ca-S system in terms of sustainability, cost and recyclability compared to state-of-the-art battery materials.
The project continues to be supported by an industry advisory board consisting of the companies Alantum, Varta, CustomCells and Accurec. All involved institutions and companies are working together towards the goal of demonstrating the performance of this sustainable energy storage technology based on calcium and sulfur in an industry-compatible battery cell for use as stationary storage.