The recovery of valuable metals from spent lithium-ion batteries is beneficial to protect the environment and avoid resource depletion. Based on the synergistic effect of the reducing ability of oxalic acid and the acidic strength of sulfuric acid, this study was conducted to recover valuable metals from spent LiNi0.8Co0.1Mn0.1O2 lithium-ion battery cathode materials with the compound leaching agents of sulfuric acid and oxalic acid. Under the optimized conditions of sulfuric acid concentration at 2.5 mol·L−1, oxalic acid concentration at 20 g·L−1, liquid-to-solid ratio at 10 mL·g−1, reaction temperature at 85 °C, and reaction time at 100 min, the leaching rate of Li, Ni, Co, and Mn measured by ICP-OES was, respectively, 99.26%, 98.41%, 96.95%, and 97.54%. It was further validated that the valuable metals were almost completely leached when combined with the XRD and SEM-EDS analysis of spent cathode materials before and after leaching. The leaching of Li, Ni, Co, and Mn was all in accordance with the Avrami model with their activation energies of 31.96 kJ·mol−1, 41.01 kJ·mol−1, 47.57 kJ·mol−1, and 42.95 kJ·mol−1, indicating that the diffusion was the control of the Li leaching process, and the surface chemical reaction was the control of the other three metals. This work provides a new idea and method for the recycling of spent lithium-ion batteries.

Manganese has excellent performance in removing metal ions from aqueous solutions, but there are few studies on the adsorption and removal of heavy metal impurities in metal salt solutions. In this paper, the adsorption of cobalt and nickel ions in MnSO4 solution by δ-MnO2 prepared from two different manganese sources was studied. The optimum adsorption conditions were as follows: When the concentration of Mn2+ was 20 g/L, δ-MnO2 addition was 10 g/L, Co2+ concentration was 80 mg/L, Ni2+ concentration was 80 mg/L, reaction time was 60 min, reaction temperature was 80 °C, and pH value was 7, the adsorption rate of Co2+ and Ni2+ reached more than 80%. The manganese dioxide adsorbed by heavy metals was analyzed and detected. The results showed that MnOOH appeared in the phases of both kinds of δ-MnO2, and their morphologies were dense rod-like structures with different lengths and flake-like structures of fine particles. Co and Ni were distributed on the surface and gap of MnO2 particles, and the atomic percentage of Co was slightly higher than that of Ni. The new vibration peaks appeared near wave numbers of 2668.32, 1401.00, and 2052.19 cm–1, which were caused by the complexation of cations such as Co2 + and Ni2 + with hydroxyl groups. Some cobalt and nickel appeared on the surface of δ-MnO2, and the surface oxygen increased after adsorption. The above characterization revealed that the adsorption of cobalt and nickel in manganese sulfate by δ-MnO2 was realized by the reaction of its surface hydroxyl with metal ions (M) to form SOMOH.