Leaching of Metals from Spent Ni-Rich Cathode Materials (NCM811) Using Green Deep Eutectic Solvent of Hydrated Choline Chloride:Citric Acid: Water Presence Effect, Mechanism, Optimization, and Kinetic Study

Leaching of metals from Ni-rich cathode materials (NCM811) was performed using hydrated choline chloride:citric acid (ChCl:CA) deep eutectic solvent. The formation of hydrated DES as well as its intermolecular interactions was confirmed using Raman spectroscopy. ChCl:CA DES possesses a higher acidity compared to CA solution induced by the high electronegativity of chloride anions in the DES structure and availability of more acidic hydrogens that take part in the destruction of metal oxide bonds in the cathode structure. The increment of water content within 25 to 30 wt % increases the mass transfer rate, resulting in an increase in the overall leaching efficiency within the experimental time interval while adding more water renders the solution acidic and reduces the metal leaching efficiencies. In addition, considering the effects of adding water on impeding the esterification reaction and preservation of the DES structure that ultimately affect the leaching efficiencies, an optimum water content of 30 wt % in the hydrated DES (DES-30) was determined. The alterations in the DES structure and the new interactions due to the presence of water were identified in FT-IR spectra of the anhydrous and hydrated DESs. The LC-MS analysis of the leachate revealed that the ligands in the metal complexes are partially replaced by water. The main leaching parameters including temperature, time, and liquid to solid ratio (L/S) were optimized using the experimental design set as 90 °C, 160 min, and 15 g/g, respectively, that yield 100, 97.8, 91.09, and 90.15% leaching efficiencies for Li, Ni, Co, and Mn, respectively. The experimental design indicated that within the selected range, the temperature has the most significant impact on the leaching efficiency. The study of leaching kinetics showed that the overall reaction rate is controlled by a combination of chemical reaction and diffusion through the product layer at low to moderate temperatures up to 70 °C, while it is controlled by diffusion at higher temperatures. The metal leaching efficiencies by DES-30 were higher than those by (HCl + H2O2) and (CA + H2O2) in the optimum operational conditions. Comparable leaching efficiencies in either a lower temperature or shorter time with those reported in the literature confirmed the promising potential of the hydrated DES for leaching of metals from waste Ni-rich LIBs.