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This study explores the optimization of hydrometallurgical processes for recycling lithium-ion batteries (LIBs) containing a mixture of lithium iron phosphate (LFP) and nickel–manganese–cobalt (NMC) cathodes. Two approaches were investigated: acid-excess leaching and acid-deficient leaching with residue recirculation. A design of experiments (DoE) framework was applied to assess the impact of key parameters, including sulfuric acid and hydrogen peroxide concentrations, as well as solid-to-liquid (S/L) ratios, on the dissolution yields of target metals (Ni, Mn, Co, and Li). Acid-excess leaching achieved nearly complete dissolution of target metals but required additional purification steps to remove impurities. Acid-deficient leaching with a 60% recirculation of leaching residue improved dissolution yields by up to 12.5%, reduced reagent consumption, and minimized operational complexity. The study also evaluated separation strategies for manganese and cobalt through solvent extraction. Results indicate that while acid-excess leaching offers higher yields, acid-deficient leaching with residue recirculation is more cost-effective and environmentally friendly. These findings provide valuable insights for developing sustainable LIB recycling technologies. [ABSTRACT FROM AUTHOR]