Inhibition Role of Solvation on the Selective Extraction of Co(II): Toward Eco-Friendly Separation of Ni and Co

Abstract

Ternary cathodes account for more than half of the market share for lithium-ion battery cathodes, and recycling draws considerable attention. The traditional extraction process used to separate and recover Co and Ni from spent ternary cathode leachates produces a large amount of saline wastewater; thus, a green method is urgently needed. In this work, we underscore the crucial role played by metal solvation during extraction, and this provides a new perspective for achieving green separation. The extraction mechanism can be described as follows: metal cations M2+ (M–Ni and Co) generate stable hydrated ions M(H2O)62+ upon solvation. However, Ni(H2O)62+ and Co(H2O)62+ exhibit different activities in reactions with acidic extractants (HA) and form different complexes, NiA2·2H2O and CoA2, respectively. Correspondingly, the coordinated water increases the electron density and steric hindrance of the center metal ions and thus inhibits the subsequent extraction. Therefore, a means of reducing solvation (adding lactic acid) was developed, and this approach exhibited good performance. The separation factor was improved by a factor of 192. These results open a new avenue for high-performance selective extraction of Co, which features a green recovery process and is suitable for future use in industrial production.