<p>Effective recycling techniques are required to recover important metals like Li and Co from spent lithium-ion batteries (LIBs) cathode material due to the growing need for LIBs as well as environmental concern. This present investigation explores the solvent extraction of Li and Co from used LIB leachate using tetrabutylammonium chloride (TBAC) and decanoic acid (DecA) DES. FTIR and ¹H NMR spectroscopy have been implemented to characterize the DES and TG–DTA analysis was undertaken to assess its thermal behavior. The thermo-physical properties like density (<i>ρ</i>), refractive index (<i>n</i>_D), viscosity (<i>η</i>) and conductivity (<i>κ</i>) at different temperatures. Using design of experiment (DOE), the extraction of Li and Co was methodically optimized while taking pH, time, and organic-to-aqueous phase volume ratio (O/A) into account. Time and O/A ratio variations have positive impact on extraction, while pH negatively influences the extraction efficiency. The results demonstrated that TBAC:DecA DES selectively extracted cobalt over lithium. Under optimized conditions of shaking time 40&#xa0;min, pH 1.1, and O/A ratio 2:1, extraction efficiency of 68.54% was obtained for Co with no extraction of Li. The 100% Co stripping from the loaded DES was achieved with water. Complete separation of Co and Li from leachate was achieved after eight extraction-stripping stages.</p> Graphical Abstract <p></p>