<p>Leaching is a critical step in the recycling of cathode materials, such as lithium cobalt oxide (LCO) and nickel-cobalt-manganese (NCM) oxide, from spent lithium-ion batteries (LIBs). Conventional processes typically use aqueous mineral acids with hydrogen peroxide (H₂O₂) as a reductant. Recently, there is a growing attention on the development of non-aqueous leaching, which works by replacing water-based lixiviants with non-aqueous/water-limited systems, such as acidic organic extractants dissolved in non-polar diluents like kerosene. Metallic reductants (e.g., Cu and Al) are attractive alternatives to H₂O₂, with potential for co-recovery of reductant metals and improved transport and handling. This study evaluates the effectiveness of metallic reductants (Cu, Al, Ni, Mn, Fe, and Zn) in the leaching of cathode materials (LCO and NCM) under both aqueous and non-aqueous leaching systems. In aqueous leaching (using H₂SO₄), all the tested metallic reductants demonstrated effectiveness in advancing the leaching of cathode materials, and their performance was influenced by three key factors: (i) redox reactions, (ii) hydrogen gas (H₂) formation, and (iii) cementation processes. In contrast, non-aqueous leaching exhibited a more complex mechanism, with Cu identified as the only effective reductant. This result prompted the proposal of leaching equilibria specific to the non-aqueous system. The study further investigated the effects of several parameters, including the mass ratio of metal reductant to cathode material, the presence of graphite, and, in the case of non-aqueous leaching, the impact of water addition and the choice of organic extractants.</p>