<p>Developing an efficient system to leach valuable metals from spent lithium-ion batteries (LIBs) is crucial for saving important mineral resources and lowering their hazardous waste environmental impact. In this study, the effect of sulfuric and DL-malic acid mixture on the leaching efficiency of valuable metals from mixed cathode materials of spent LIBs was investigated and optimized using Response Surface Methodology (RSM). The collected cathode materials were characterized by X-ray diffraction, X-ray fluorescence, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and inductively coupled plasma to determine its phase composition, chemical composition, and elemental distribution. The mixed black mass was calcined at 700&#xa0;°C to burn off the polymeric binder and residual electrolyte before leaching experiments. Leaching experiments were designed and analyzed using the Box–Behnken Design approach leading to a quadratic response surface model that established the relationship between the leaching efficiency and key process variables, including the concentrations of both acids, temperature, and leaching time. The developed mathematical model is capable of satisfactorily estimating the leaching efficiencies of Li, Co, Ni, and Mn based on key operational variables. Under the optimized conditions determined in this work, the leaching efficiencies of Li, Co, Ni, and Mn reached 98.00, 92.87, 89.88, and 95.15%, respectively, with less than 1% deviation from the model-predicted values.</p> Graphical Abstract <p></p>