Battery-go-round

Everyone knows the phenomenon from their cell phone or laptop: Over time, the capacity of the battery decreases so that you have to reach for the charging cable more and more often. The same is true of the much larger batteries in electric vehicles: although vehicle manufacturers can now guarantee a service life of eight to ten years for lithium-ion batteries in electric vehicles, sooner or later these too will have to be recycled.

In a project supported by the Swiss Federal Office of Energy (SFOE), Swiss electric vehicle manufacturer Kyburz Switzerland AG and Empa have set themselves the goal of recycling discarded batteries from electric vehicles. For this purpose, Kyburz, with the support of Empa, developed a recycling system that breaks down old batteries into their components.

Before a battery ends up in the recycling plant, it can be given a second, sometimes even a third life. After its first use in the yellow electric tricycle scooters that Kyburz manufactures for Swiss Post AG and are known throughout Switzerland, it can still be used, for example, in "second-life" vehicles powered by batteries that have already been used. If the capacity of the battery continues to drop, this is not necessarily the end. Batteries with reduced capacity could be installed in stationary applications for storing solar energy, for example. This "multi-life" concept is intended to significantly reduce the demand for primary raw materials in the future.

Precise separation

If the capacity of the battery is no longer sufficient for this further use, it is finally sent to the recycling plant. "In this type of battery, the cathode, separator and anode are installed in several layers in a plastic housing," explains Empa researcher Andrin Büchel from the "Technology and Society" department. By cleverly unrolling the separator, the cathodes and the anodes - metal foils coated with particles to allow lithium ions to be stored - are sorted into two separate containers.

The next step is the recovery of the electrode materials. The cathode, an aluminum foil coated with lithium iron phosphate particles, is placed in a water bath where the particles detach from the foil and are recovered as powder after decantation and drying. The same procedure is used for the anode, which consists of a copper foil coated with graphite particles. In this case, however, a homogeneous suspension is formed, making an extra step in a centrifuge necessary to separate the particles.

"At the end of the recycling process, we get back the housing, the separator, the aluminum and copper foils, and the electrode materials sorted by type," Büchel says. This type of recycling process is called direct recycling. "In direct recycling, the battery is only disassembled as far as necessary to preserve the functional properties of the materials. This allows us to keep the number of necessary steps as low as possible, including for further processing," says Büchel.

Accurate analysis

But the work is not yet done with the recovery of the materials. In order for these to be used again in a new battery, they must be regenerated. This is precisely what Büchel is currently working on across departments with his Empa colleague Edouard Quérel. In the battery laboratory of the Materials for Energy Conversion department, they have already uncovered the mechanism behind the aging of the cathode material. "The lithium iron phosphate has a crystalline structure that releases and reabsorbs lithium ions during each charge and discharge cycle," Büchel explains. "This structure remains, but the amount of active lithium ions decreases over time." The researchers are currently working on "refreshing" the cathode material by selectively adding lithium. The ultimate goal: to build the most efficient new batteries possible from the recycled material and close the loop.

In conventional recycling processes, batteries are shredded and the recyclables are separated using thermal and wet chemical processes. Direct recycling is said to be more resource efficient in comparison, using less energy and no chemicals. However, the process developed by Kyburz and Empa is currently only suitable for the specific design and cell chemistry of batteries such as those used in Kyburz vehicles. "We are currently investigating whether and how this process can be transferred to other cell types as part of the Innosuisse project 'CircuBAT', which involves 23 other partner companies in addition to Kyburz," says Büchel.