An Efficient and Precipitant-Free Approach to Selectively Recover Lithium Cobalt Oxide Made for Cathode Materials Using a Microwave-Assisted Deep Eutectic Solvent

The recovery of cathode materials using deep eutectic solvents (DESs) still requires a long leaching time and relatively high temperatures. The poor selectivity of the leaching process leads to the low purity of the precipitate. Herein, a microwave-assisted choline chlorine–oxalate–water system was designed to selectively leach Li and recover high-purity cobalt oxalate simultaneously, which could greatly shorten the extraction time and decrease the leaching temperature. Fourier transform infrared spectroscopy, X-ray diffraction, inductively coupled plasma–mass spectrometry, and X-ray photoelectron spectroscopy methods were used to explore the mechanism of microwave-assisted DES leaching of valuable metals from lithium cobalt oxide (LCO). It was found that 99.05% of Li could be selectively leached and 99.21% of Co could be precipitated as cobalt oxalate at 100 °C in 10 min simultaneously. The purity of cobalt oxalate precipitated was 95.36% without any purification procedures. A new polymer compound Co2+[(Ch+)x(Cl–)y(HC2O4–)z] was formed during the microwave-assisted leaching process. The compound could be hydrolyzed, and the cobalt oxalate precipitate appeared by increasing the water content in the DES. Finally, the DES for the recovery of LCO could be cycled four times with favorable selectivity. A low-pollution, efficient, process-simple, and precipitant-free technology using a microwave-assisted DES for selective recovery of LCO was proposed, which provided an important reference method for large-scale recovery of valuable metals from spent lithium-ion batteries.