Ultrasound-induced delamination of solvent-based lithium-ion battery cathodes for direct recycling: Investigating process parameters and green solvent alternatives

Ultrasound is a commonly used tool to aid cleaning of microscopic or soluble contamination. Transferred to battery electrodes, it can be applied to remove active material and binder from the current collector foil. Many direct recycling studies make use of it as an effective delamination method, but rarely investigate in the process parameters. This work considers frequency, temperature, residence time and type of solvent as important adjusting screws to gain a valuable recycling product.
Triethyl phosphate as a non-critical solvent alternative reached fast and similarly good delamination results to the common N-methyl-2-pyrrolidone close to 100 %, while dimethyl sulfoxide caused undesirable structural changes of the lithium-nickel-manganese-cobalt-oxide cathode active material observed by X-ray diffractometry. Others like cyrene, ethyl acetoacetate and acetone did not reach sufficient delamination degree, which was below 31%. In general, elevated temperatures were necessary to enable dissolution of the binder, but no significant difference between 60 °C and 80 °C was observed.
29 kHz ultrasound frequency generates strong cavitation micro jets, which delaminated the electrode efficiently. ... mehrIt reached a delamination degree of (101.4 ± 0.6) %, but also caused plenty pitting of the current collector foil. This resulted in 4478 ppm of additional aluminum in the inductively coupled plasma optical emission spectroscopy of the recyclate and would make battery manufacturers reject the material. A frequency of 120 kHz reached complete delamination ((100.5 ± 0.3) %) without pitting and exceeded the initial aluminum content by just 121 ppm. The cavitation micro jets at higher frequencies are of greater number concentration, but less energetic, and spare the foil from pitting with simultaneously high delamination degree.
In literature, ultrasound treatment has been described as a promising way to regain active material during direct battery recycling. This study carries on these investigations and gives further insights in the process step. It demonstrates the importance of the process parameters, especially the so far often neglected frequency, and the solvent phase, for a successful recycling.