Lithium-ion batteries can now be found almost everywhere. "In addition to phones and laptops, they are used in power tools and electric cars, as well as in larger stationary energy systems, such as battery banks for storing solar power," said Reio Praats, a junior researcher at the National Institute of Chemical Physics and Biophysics.

While valuable metals hidden inside batteries, such as cobalt and nickel, can already be recycled, the graphite they contain remains problematic waste. This carbon-based material plays a key role in storing charges and typically makes up about a quarter of a battery's mass. "Today, more than 80 percent of natural graphite is mined in China and over 90 percent of graphite used in batteries is produced in China. For us in the European Union, this poses a supply security issue," Praats pointed out.

In his recently defended doctoral thesis at Tallinn University of Technology (TalTech), he set out to find a new use for recycled battery graphite. Specifically, he worked with the leftover fraction from lithium-ion battery recycling, which consists mainly of graphite but also contains small amounts of metal residues. As a result of his research, Praats was able to produce electrocatalysts — key materials for manufacturing a new type of metal-air battery.

Compared to lithium-ion batteries, metal-air batteries rely on fewer scarce and strategically sensitive materials, the junior researcher explained. This makes them cheaper and less harmful to the environment. However, metal-air batteries would not be possible without electrocatalysts. "The results were quite unexpected: we managed to turn waste into a valuable material," Praats said.

From old to new

Up to now, the graphite refined by Reio Praats has essentially been treated as waste. "Once we've extracted a mineral, we don't want it to go to waste right away. That's why it makes sense to recycle it — either for new batteries or for other energy technologies," he said.

Old lithium-ion batteries can be recycled using pyro- or hydrometallurgical methods. In the first case, the graphite is simply burned. In the second, the batteries are crushed and some compounds are separated out. What remains is the so-called black mass. "It mainly consists of two battery elements: the active cathode material, which contains valuable metals such as lithium, cobalt, manganese and nickel, and graphite," Praats explained. Once the more valuable metals are recovered through leaching, the leftover residue of the black mass remains. "Processing this residual fraction was the main focus of my doctoral thesis," the junior researcher recalled.

In his work, he sought to produce electrocatalysts from the black mass residue. These are materials that help speed up certain chemical reactions. Praats was particularly interested in the oxygen reduction and oxygen evolution reactions. "The oxygen reduction reaction can be harnessed to produce electricity, for example, in hydrogen fuel cells or the new type of metal-air batteries I've been working to develop," he explained.

Both of the reactions Praats focused on are essential for metal-air batteries to function. When used to generate electricity, the battery undergoes oxygen reduction. When recharged, the battery undergoes oxygen evolution. "These electrocatalyst materials are needed to make those reactions happen more efficiently and quickly," the junior researcher said.

Tons of waste lying in wait

Currently, most electrocatalysts are based on extremely expensive and rare precious metals, such as platinum and iridium. "These are very scarce in the Earth's crust and the goal is to replace them with catalyst materials made from non-precious metals," Reio Praats explained. In his doctoral thesis, he set out to provide a cheaper and more environmentally friendly alternative.

According to him, it was actually a good thing that the black mass residue was not pure graphite but also contained traces of battery metals. "Whereas battery recyclers usually see these impurities as a problem that is hard to separate, for me they were useful. Electrocatalyst materials require exactly the kinds of metals that are found in this residue," he said.

In fact, Praats's catalysts even outperformed precious-metal-based ones in his experiments. "Of course, many question marks remain, because the development of metal-air batteries is still in its infancy and fully established standards don't exist yet," he acknowledged.

Praats did not recycle batteries himself for the project but used existing industrial waste. According to him, hundreds of thousands of tons of black mass residue are currently awaiting processing, with no clear use. "One of the goals of this work was to advance the circular economy so we don't lose so many valuable resources. At the same time, it also helps develop new energy storage technologies that allow us to store renewable energy sources — such as solar and wind power — more effectively," he said.

Reio Praats will defend his doctoral thesis, "Upcycling Li-Ion Battery Waste into Sustainable Electrocatalysts for Zinc-Air Battery Application," on September 29 at Tallinn University of Technology. The dissertation was completed at the National Institute of Chemical Physics and Biophysics (KBFI) under the supervision of KBFI senior researchers Kerli Liivand and Ivar Kruusenberg. The opponents are Tim-Patrick Fellinger of the German Federal Institute for Materials Research and Testing (BAM) and Associate Professor Cristina Pozo Gonzalo of the Spanish National Research Council.

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