Glycine-Assisted Hydrothermal Leaching of LiCoO₂/LiNiO₂Cathode Materials with High Efficiency and Negligible Acid Corrosion Employing Batch and Continuous Flow System

Glycine was applied as the leachant for the hydrothermal leaching of lithium-ion battery (LIB) cathode materials, LiCoO2 and LiNiO2, at 90-180 °C for 5-90 min. LiCoO2 was completely leached at 180 °C for 30 min. It was revealed that Co(III)-glycine complex formed first and was gradually reduced to Co(II)-glycine complex. Compared to LiCoO2, LiNiO2 required a lower temperature and shorter time for complete leaching. Different from using inorganic acids or organic acids like citric acid, pH values of the glycine solution before or after hydrothermal leaching of LiCoO2 or LiNiO2 was in the range 5.8-9.0, closer to neutral. In the mechanism analysis, the consumption of glycine was attributed to the oxidation of glycine and the formation reaction of Co-glycine complex; during the hydrothermal leaching at 180 °C for 30 min, more than half of leached Co was calculated to be reduced from trivalent to divalent. The shrinking unreacted core model was adopted to study the kinetics. As a result, hydrothermal leaching of Li and Co with glycine was well fitted by the interface reaction control, and the activation energies of Li and Co leaching were 87.5 and 72.2 kJ/mol, respectively. With the successful running of continuous hydrothermal leaching of LiCoO2 with glycine using a specifically customized flow system, LIB cathode materials were leached through the green and continuous process with a high efficiency close to 100%, and the acid corrosion was detected to be as light as to be almost negligible, for the first time.