Sustainable recovery of lithium, nickel, manganese, and cobalt from NMC811 cathodes using propionic and ascorbic...

An environmentally benign leaching process for recovering valuable metals from LiNi_0.8Mn_0.1Co_0.1O₂ (NMC-811) cathodes was developed using propionic acid as the leaching agent and ascorbic acid as the reducing agent. Propionic acid as a mild monocarboxylic acid allows continuous metal dissolution in the leaching process. Ascorbic acid was employed as a green reducing agent owing to its low ionization potential and strong electron-donating ability. The UV-visible analysis revealed an absorption band at λ_max ≈ 390 nm in the leachates containing propionic acid or its mixture with ascorbic acid, attributed to, Ni(II), and Mn(II) Co(II)-propionate complexes while the ascorbic acid-containing leachate showed an exclusive peak at λ_max ≈ 323 nm. The persistence of the metal-propionate band was further confirmed by LC-MS, indicating the role of ascorbic acid as a reducing agent under the current leaching conditions. A superior performance was observed for leaching by (propionic acid + ascorbic acid) compared to that by (propionic acid + H₂O₂), which was justified by considering the reductants' standard potentials. The leaching efficiencies by (propionic acid + ascorbic acid) were 12-79% higher than those by conventional H₂SO₄/HCl and H₂O₂, owing to more efficient consumption of the ligand and the reductant in the reaction mechanism. Using the response surface methodology, the ratio of ascorbic acid to propionic acid (Asc/Pro) was detected as the most influential parameter followed by solid-to-liquid (S/L) and temperature (T). Under the optimal operational conditions ((Asc/Pro) = 0.146, T = 70 °C, and S/L = 13.86 g L⁻^¹), the yielded leaching efficiencies were 95.53% (Li), 86.03% (Ni), 87.07% (Mn), and 92.43% (Co). The leaching results by the proposed leaching system as well as the favorable preliminary tests for subsequent purification steps highlight the application of propionic acid /ascorbic acid system as a sustainable and industrially promising route for recycling of Li-ion battery cathodes.