Excellent Performance of Glycine in Isolating Mn during Hydrothermal Leaching of LiMn2O4 Cathode Materials

In this study, hydrothermal leaching of spent LiCo x Ni y Mn1–x–y O2 (NMC) and commercial LiMn2O4 (LMO) cathode materials was conducted by applying glycine as a leaching agent. During the hydrothermal leaching of spent NMC, the effects of reaction temperature, reaction time, glycine concentration, and pulp density on the leaching efficiencies of Li, Co, Ni, and Mn were investigated. With the increase in reaction temperature and time, the leaching efficiencies of Co, Ni, and Mn significantly increased, but even at 200 °C for 15 min, the leaching efficiency of Mn was as low as 16%. The order of the reaction rate was confirmed to be Mn < Co < Ni < Li. Compared with Li, Co, and Ni, the leaching performance of Mn was much poorer, but this special property makes it possible for glycine to isolate Mn directly from the LMO cathode material. During the hydrothermal leaching of the commercial LMO cathode material with glycine, the leaching efficiency of Li significantly increased with reaction temperature and time and was much higher than that of Mn under the same condition. A kinetic study using a shrinking unreacted core model was conducted. The results revealed that the diffusion within the product layer was the rate-limiting step for leaching Li and Mn. Based on the determined reaction rates, the activation energy of Li leaching was calculated to be 57.1 kJ/mol within 0–15 min and 69.6 kJ/mol within 15–90 min, respectively, while that of Mn leaching was 86.7 kJ/mol. At 200 °C for 120 min, the leaching efficiencies of Li and Mn achieved 99.5 and 7.6%, respectively, which means that 92.4% of Mn was successfully isolated and kept in the solid residue. After calcinating at 600 °C for 1 h, the final product was obtained and identified to be Mn2O3 with a purity of 99.0%. This was the first report on the excellent performance of glycine in isolating Mn and achieving Mn isolation in a single-step hydrothermal leaching process of LMO cathode materials with such a high recovery and product purity.