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Anna-Caroline Lavergne Bril

Evaluation of MOFs (metal organic frameworks) as precursors for Li-ion batteries

Published on 19 July 2023
Thesis presented July 19, 2023

The experimental works and modelling DFT approaches conducted in this thesis aim at understanding the selectivity mechanism toward cobalt of a ligand from the imidazole family from a multi metallic nickel, manganese and cobalt (Ni2+, Mn2+, Co2+) solution, mimicking a leached battery solution. The as synthetized ZIF material (Zeolitic-Imidazolate Framework) can then be used in a closed loop process as precursor for cathode components. Experimental work using 2-methylimidazole (2-MeIm) ligand has been led, to study the Co2+ selectivity through ZIF-67 synthesis from Ni2+, Mn2+,Co2+ solution. This work has then been extended to other ligands from the same family (imidazole, 4-methylimidazole, benzimidazole), in order to use this concept for the synthesis of other ZIFs.
Nickel-based co-products can crystallized when reacting with 2-MeIm, leading to Nix2-MeImy(MeOH)z complexes of unresolved stoichiometry. Those complexes are soluble in the solvent used for the synthesis, and can be washed away. Those complexes are not soluble when the synthesis temperature is superior to 70°C. Nickel can also react with the ligand by precipitating as an amorphous component, of which the kinetic of precipitation are lower than the one of the ZIF-67. DFT modelling has elucidated the coordination of the Ni centers as square planar, which is most likely found in this amorphous component. In a multi-metallic media of Ni2+ and Co2+, 2-MeIm ligands coordinates preferentially with Co2+, inhibiting the precipitation of Nix2-MeImy(MeOH)z in the favor of ZIF-67 synthesis. Manganese does not react with Imi ligands, but its oxidation followed by its precipitation as oxides is triggered by high-pKa ligands. Ligand modelling helped to calculate their pKa and their values follows this trend pKa2-MeIm>pKaImi>pKa4-MeIm>pKaBzIm. 2-MeIm ligand appears to be enhancing Mn2+ oxidation, when BzIm is the ligand achieving the best selectivity (inhibiting Mn2+ oxidation). This approach is to be used to evaluate the best ligand to be used to enhance the selectivity of the process, which is in part determined by its ability to not trigger Mn2+ oxydation. For now, the yield of the selectivity process with BzIm is of 70 %mol.

batteries, MOF, Li-ion