A synergistic solvent extraction (SSX) system consisting of tung oil-based carboxylic acid (TFCA) and isonicotinic acid ester (4PC) was proposed to selectively extract Ni over Li and Mg from acid leach solutions of spent lithium-ion batteries (LIBs). The SSX system indicates a synergistic effect of Ni which can be mainly attributed to the multiple chelate ring structure formed between TFCA and Ni, therefore resulting in strong selectivity of Ni. With optimal conditions, the extraction of Ni reaches 97.3% in a single stage, while the extractions of Mg and Li are nil and 2.29%, respectively. Further, after scrubbing with 300 mg/L NiSO₄, the loaded organic solution is stripped using 0.2 mol/L H₂SO₄. The strip liquor containing 11.52 g/L Ni and only less than 6 mg/L Li is obtained.