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The saponification and extraction mechanism drives Mg(HCO 3) 2 as a substitution of NaOH to achieve higher separation factor and greener technique. [Display omitted] • Mechanism-driven saponifying agent tuning achieves β increase from 148.98 to 3189.95. • The extraction mechanism of nickel and cobalt has been supplied and visual. • The released alkalinity in saponification-extraction is the key factor in separation. The carbon neutralization urgently requires massive applications of energy storage materials such as lithium-ion batteries. As the critical metal for lithium-ion batteries, the sustainable utilization of nickel and cobalt still takes a long process due to the low separation factor resulting in high costs and environmental concerns, especially in spent lithium-ion batteries (LIBs). In this work, based on the further revealing whole-process mechanism of the typical saponification-extraction process, a new saponifying reagent is proposed to achieve the separation ability of a multistage countercurrent through single-stage extraction solely along with shorter processes and less pollution. The separation factor of cobalt/nickel enhances from 148.98 to 3189.95. and the new saponification-extraction process applies in the recycling of LIBs successfully. The mechanism shows that the alkalinity introduced by saponification is the key factor affecting the separation and purity. Saponification by traditional NaOH releases all putting-in-organic-phase strong alkalinity to aqueous, providing excessive extraction capacity but hindering separation. Therefore, the mild released alkalinity tuning instructs seeking Mg(HCO 3) 2 as an alternative saponifying agent to achieve the balance between released alkalinity and capacity. A cleaner avenue for the high-purity recovery of cobalt and nickel is proposed by mechanism driving, which has the ideological guiding significance to other process improvements. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lactic acid addition increased the selectivity of Co/Ni separation. • The complexing of lactic acid is the main driving force for effective separation. • QSAR modeling of the complexing group guides the design of better complexing agents. • The non-saponified extraction process was designed based on the stripping strategy. Ternary cathodes account for more than half of the Lithium-Ion Batteries (LIBs) cathode market share and their recycling draws the most attention. Solvent extraction is the most efficient and common method to separate and recover Co and Ni from spent ternary cathode leachates. However, the traditional extraction process produces a large amount of saline wastewater, thus will seriously pollute the environment. In this work, an environmental approach is proposed by adding a water-soluble complexing agent into the aqueous phase, the separation factor is 2 times higher and the consumption of NaOH is 90% reduced compared to the Na-saponified counterpart, which was attributed to the differences of the complexation between metal ions and complexing agent in aqueous phase. The differences in the complexing ability are enhanced with an increased local nucleophilicity index of the active sites along with a reduced molecular volume in complexing agent, thus leading to superior cobalt extraction efficiency (∼98.9%) and separation factor (∼345). In addition, because of the fact that the whole-process pollution control is needed for cleaner chemical production, a green process is developed by recovering additives through Tri-n-butyphosphate. This excellent separation performance suggests that adding complexing agent in solvometallurgical recovery process may aid in the future development of high-purity raw metals for advanced fields. [ABSTRACT FROM AUTHOR]