Sustainable recovery of lithium, nickel, manganese, and cobalt from NMC811 cathodes using propionic and ascorbic acids through optimization and kinetic analysis

An environmentally benign leaching process for recovering valuable metals from LiNi 0.8 Mn 0.1 Co 0.1 O 2 (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 + H 2 O 2 ), 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 H 2 SO 4 /HCl and H 2 O 2 , 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.