This study proposes a sustainable and selective hydrometallurgical route for the recovery of lithium (Li) and cobalt (Co) from spent lithium-ion batteries (LIBs), integrating sodium sulfite-assisted sulfation roasting with water and organic acid leaching. The sulfation roasting process at 600 °C facilitated the transformation of LiCoO2 into soluble LiNaSO4 while preserving cobalt predominantly in oxide form. Subsequent water leaching at 50 °C selectively dissolved lithium (up to 74.4%), leaving cobalt in the residue. To achieve complete metal recovery, oxalic acid—a biodegradable and environmentally benign organic acid—was used as a leaching agent. Under optimized conditions (1 mol·L−1 oxalic acid, 90 °C, 90 min, S/L ratio 1/160), leaching efficiencies reached 99.96% for Li and 99.15% for Co. Notably, cobalt was recovered directly as a cobalt oxalate precipitate, while lithium remained in solution, eliminating the need for additional separation steps. In the water-leaching stage, lithium selectively dissolves while cobalt largely remains in oxide form. In the separate oxalic-acid leaching stage, both metals dissolve; however, cobalt immediately precipitates as cobalt oxalate whereas lithium remains soluble. Thus, although the two leaching approaches are independent processes, each exhibits a distinct Li/ Co separation behavior. Kinetic modeling revealed that nucleation and growth mechanisms, best described by the Avrami equation, governed the leaching behavior. This combined pyro-hydrometallurgical process offers a high-efficiency, lowimpact solution for critical metal recovery from LIB waste and represents a viable alternative to conventional mineral acid-based methods.