Ultrasonic toothbrushes could be the key to a novel, highly efficient battery recycling method.
New research from the U.K.'s Faraday Institution has suggested ultrasonic delamination could offer a fast, sustainable approach. Not only is the technique apparently quicker and more eco-friendly than hydrometallurgical and pyrometallurgical processes, but researchers say it could also yield higher-purity materials.
The key to unlocking battery recycling lies in reclaiming active materials from the cathodes, which contain the bulk of a device's precious metals. For instance, it is the cathodes which contain the nickel, manganese and cobalt which give NMC batteries their initialism.
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Those components are glued in a porous layered film 200 μm thick onto copper and aluminum foils – the current collectors – using a special binder. Commonly used binders include polyvinylidene difluoride, mixed carboxymethyl cellulose, and styrene-butadiene rubber.
Recyclers typically shred batteries and treat them with fire or aqueous solvents to recover such precious metals, with attendant energy consumption and toxic waste drawbacks.
Researchers from the universities of Birmingham and Leicester, working under Faraday Institution battery recycling research project ‘ReLiB', however, have suggested a new mode of recycling. They published their findings in the paper Lithium-ion battery recycling using high-intensity ultrasonication, in Green Chemistry.
The team used a commercial ultrasonic system with a 20mm sonotrode, operating at 20 kHz with a power of up to 2,200 W. At that frequency and power, vapor-filled cavitation bubbles formed randomly, oscillating heavily and expanding before collapsing again. Bursting at the surface of the active components, the cavitation bubbles' blasts were stronger than the polymeric binder, resulting in complete delamination of the current collector.
“This novel technique works in the same way as a dentist's ultrasonic descaler, breaking the adhesive bonds between the coating layer and the substrate,” said Andrew Abbott, from the University of Leicester, who led the research. “It is likely that the initial use of the technology will use production scrap from battery manufacturing facilities as the feedstock and feed recycled material straight back into the battery production line, possibly at the same site. This could be a real step-change in battery recycling.”
For the new process, the electrode is submerged in a solution of de-ionized water and moved past the, similarly immersed, sonotrode at a distance of less than 3mm. It takes around ten seconds to clean an electrode sheet using the method, the academics found, translating into an improvement of the amount of material that can be processed in a given time by a factor of around 100.
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The researchers retrieved 99.5% of the active materials of the anode and cathode in the experiment, with the remainder comprising particles of less than 3 μm – too small for filtration recovery. The researchers assumed those particles were carbon black additives. By comparison, current hydrometallurgical processes using sulfuric acid solutions achieve a recovery efficiency of around 67% of active materials.
The academics said the efficacy of ultrasonic delamination is dependent on the binder used but modifying the pH of the solution, and potentially adding wetting agents to the process, could optimize it for any binder.
The team from the English Midlands said ultrasonic recycling would cost $0.10 per 150 gram of recovered electrode material but may require more expensive pre-treatment as the batteries need to be disassembled very carefully and the fully intact electrode sheets fed into the ultrasonic device. However, added the researchers, 5-20% of battery waste comes as production scrap, which needs no disassembly. For that sort of application, ultrasonic recycling is already suitable, according to the Faraday Institution team. The researchers have filed for a patent for their technique.
The researchers had published a separate paper in Green Chemistry in October, describing how battery manufacturers could apply a ‘design-for-recycling-approach,' featuring comprehensive labeling, easy-to-open structures, and solid busbars to enable robots to perform dangerous disassembly. The paper also advocated connecting cells in a bulk structure rather than multiple modules. That has already been realized with cell-to-pack and blade battery devices which have been brought to market, albeit primarily with the aim of improving the capacity and power of lithium-iron-phosphate products.
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