Résumé
The transport properties of Li+ and Co2+ from metal nitrate salts in supercritical carbon dioxide (scCO2) at 40 °C and 250 bar using a crown-ether containing copolymer, poly(N-(2-(2,3,5,6,8,9-hexahydrobenzo[b][1,4,7,10]tetraoxacyclododecin-12-yl)ethyl) methacrylamide-grad-1,1,2,2-tetrahydroperfluorodecyl acrylate) (P(B12C4EMAAm-grad-FDA)) were studied and correlated with density functional theory (DFT) binding-energy calculations. The lithium and cobalt recovery yields from LiNO3 and Co(NO3)2·6H2O mixtures, used here as a simplified model system representative of LCO cathodes, were quantified by ICP-OES. The copolymer exhibited higher extraction efficiency for Co2+ than for Li+, consistent with DFT predictions, implying greater stability of Co-B12C4EMAAm complexes. While the introduction of small amounts of water (molar ratio H2O/Li = 7.8) enhanced Li+ binding and transport from 25.7% to 83%, in agreement with solvated DFT models, the polymer remained non-selective for Li+ extraction once in the presence of Co2+. To overcome this competition, bipyridine (bpy) was employed as a cobalt-selective ligand. The solubility of bpy in scCO2 was determined by cloud point measurements. Starting from LiNO3 and Co(NO3)2·6H2O mixtures in the presence of bpy, the precipitation of a scCO2–insoluble Co–bpy complex was observed, removing cobalt from the extraction medium, therefore enabling selective lithium extraction. A lithium transport efficiency of 40% was obtained, whereas the efficiency of cobalt transport was less than 2%, resulting in a Li-Co separation factor of 34.8. Overall, the combined experimental and theoretical results show that, while the copolymer alone does not enable selective lithium transport in mixed-ion systems, cobalt sequestration via strongly chelating ligands is a viable strategy to shift selectivity toward lithium.