Recycling and optimized resource cycles, second use, and knowledge-based cell design are expected to enhance sustainability and safety of lithium-ion batteries in future. The basis is now provided by process engineers and materials scientists of Karlsruhe Institute of Technology (KIT), who jointly study the battery lifecycle. The new research projects are carried out within the battery research clusters “greenBatt“ and “BattNutzung“ funded by the Federal Ministry of Education and Research (BMBF).
Battery cells of constant high performance can considerably reduce the ecological footprint of applications, such as electric mobility. In addition, second use of such cells is considered, e.g. in large networks of storage systems. But not all cells are suited for such “second-life scenarios.” Long-term operation requires perfect interaction of a number of components and materials. “Permanent charging and discharging of the battery is inevitably associated with undesired side reactions,” says Professor Hans-Jürgen Seifert from the Applied Materials Physics Department of KIT’s Institute for Applied Materials. “When this has an adverse effect on the battery’s behavior, it is referred to as degradation or aging. Degradation cannot be prevented completely, but it can be delayed and mitigated by an adequate cell design.” Seifert and his team analyze degradation mechanisms in the highly reactive electrolyte by the associated gas formation. They carry out highly precise calorimetric measurements to balance heat quantities during battery operation and model them thermodynamically. The project is aimed at precisely predicting the cell behavior during use, Seifert explains: “With our models, safe and sustainable batteries can be developed in a knowledge-based manner and quick commercialization will be possible.”
Understanding and Controlling Degradation
Better understanding of degradation processes also helps predict the lifetimes of lithium-ion cells more reliably. The corresponding test series, however, are very time-consuming. “We need test methods, in which aging is accelerated,” says Professor Thomas Wetzel from the Institute of Thermal Process Engineering. “The comfort range of the cells is about 25° C. If cells are exposed to heat or cold, they age more quickly.” Still, aging processes and thermal conditions in the cells are highly complex and make it difficult to transfer results of accelerated testing to conventional processes. Wetzel and his team are now identifying conditions and parameters that cause as few additional aging mechanisms as possible and, hence, may serve as suitable markers. With the help of this “thermal fingerprint” of a battery cell, reliable prediction of aging will also be possible by accelerated test series.
New Approaches to Battery Recycling
The new clusters also focus on recycling-friendly battery design and further development of recycling processes and resource cycles. “At the moment, two recycling processes of lithium batteries exist. With the pyrometallurgical method, the cells are melted at high temperatures. This is robust and safe, but the achievable recycling rate is limited,” says Professor Hermann Nirschl from KIT’s Institute for Mechanical Process Engineering and Mechanics (MVM). “Mechanical methods, such as crushing and sorting, promise to reach higher recycling rates. However, they are associated with higher safety risks and separation of materials is of moderate selectivity.” At MVM, single process parameters and process chains of mechanical recycling are simulated with high resolution, compared, and optimized to ensure economically acceptable, environmentally friendly, and function-preserving battery recycling. Researchers also consider innovative approaches, such as shockwaves, ultrasonic methods, or wet grinding, which guarantee high material selectivity, preservation of functional materials, and high safety due to the use of water. In future, favorable design features of batteries will be derived directly from simulation results.
Recovery of Valuable Resources
When current battery recycling processes reach their limits, the yield can be further increased by a better combination of mechanical and thermal methods. The team of Professor Wilhelm Schabel of the Thin Film Technology (TFT) Group of KIT is working on thermal recycling processes for volatile organic components in electrode layers. “We want to recover valuable resources that have not yet been considered sufficiently when reprocessing battery cells,” Schabel says. “Together with our project partners, we will optimize treatment of shredded materials at temperatures below 500° C.” Experiments with new spectroscopic methods are supposed to provide basic understanding of micro- and macroprocesses in electrode layers during recycling. In addition, an advanced thermal treatment strategy shall be developed to separate high-boiling components that slowly diffuse in the layer structures. “Our experimental findings will then be transferred to simulation models,” Schabel emphasizes. “This is the only way to optimize future recycling processes.”
Smart Monitoring of Battery Systems
Apart from sustainability, safety of battery systems is in the focus of the new research clusters. Safety-critical defects on the cell level are very rare, but may have serious impacts, an example being lithium plating. “This effect is caused by accumulation of metallic lithium in the anode,” says Professor Ulrike Krewer from the Institute for Applied Materials – Electrochemical Technologies. “This may result in a massive loss of capacity and, in extreme cases, in short circuits and even cell fires.” To prevent this, cells can be monitored and checked during operation. However, such online processes have mainly been used at the laboratory so far and are hardly sensitive on the system level. Krewer and her team are now developing better analysis algorithms. “We also take into account non-linear processes during battery operation. These data have hardly been used for diagnosis so far,” Krewer says.
Joint Research on Behalf of the Federal Government
Within the overarching concept “Battery Research Factory,” the German Government recently established four new competence clusters for battery research, which are funded with a total of EUR 100 million. KIT coordinates Germany-wide research into flexible production systems in the competence cluster “InZePro“ (Intelligente Batteriezellproduktion, intelligent battery cell production) and into high-performance batteries in the cluster “AQua“ (Analytik/Qualitätssicherung, analytics/quality assurance). In the research clusters ”greenBatt“ (Recycling / Grüne Batterie, recycling / green battery) and “BattNutzung“ (Batterienutzungskonzepte, battery use concepts), KIT also collaborates closely with various institutions. Among the partners are institutes of Fraunhofer-Gesellschaft, Hochschule Ingolstadt, RWTH Aachen University, TU Braunschweig, Clausthal University of Technology, TU Freiberg, Technical University of Munich (TUM), University of Münster (WWU), and the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW). (mhe)
More Information: https://www.bmbf.de/de/batterieforschung-in-deutschland-662.html (in German)
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