IntroductionTo enhance the lithium leaching rate, carbon thermal reduction can be employed to convert lithium into lithium carbonate, thereby enabling selective recovery through water leaching. However, due to the relatively low solubility of lithium carbonate, it is challenging to significantly improve the efficiency of water leaching. Furthermore, the extent of carbon thermal reduction is highly dependent on factors such as the amount of added carbon source, reaction temperature, and reaction time. Consequently, there is an urgent need for advanced technologies that are characterized by low energy consumption, a simplified process, and high efficiency for the recovery of active cathode materials from spent lithium-ion batteries. While traditional metallurgical recovery processes can effectively extract most valuable metals from cathode waste, the current lithium extraction process typically requires multi-stage extraction or stepwise precipitation to produce lithium carbonate. This results in a lengthy back-end lithium extraction process, with intermediate steps prone to lithium metal loss, leading to a recovery rate of only 60% to 80%. In the context of comprehensive utilization recycling technologies, efficient lithium extraction and selective leaching of target elements remain bottleneck issues. To address the challenge of efficient lithium extraction, this study proposes the use of waste ternary positive and negative electrode mixed materials as raw materials, employing a process flow of "reduction roasting for priority lithium extraction" followed by water leaching and acid leaching to comprehensively recover valuable metals from the waste.MethodsWeigh accurately an appropriate amount of experimental raw materials and transfer them into a mortar for thorough grinding. Subsequently, transfer the ground materials into a corundum crucible. Place the crucible containing the raw materials into a tube furnace for roasting. Prior to initiating the program, introduce argon gas at a flow rate of 200 mL/min for 30 min to ensure an inert atmosphere. Heat the materials at a controlled heating rate of 5 ℃/min until reaching the set temperature. Once the temperature naturally decreases to room temperature, carefully weigh the mass of the roasted product and record the mass difference before and after roasting. Transfer the roasted product into a beaker, add pure water at a liquid-to-solid ratio of 80 mL/g, and stir using a mechanical stirrer at a constant speed of 500 r/min. Systematically investigate the effects of roasting temperature (500-700 ℃), roasting time (30-150 min), and stirring time (water leaching time of 2-10 h) on the leaching rate of lithium. Calculate the mass differences of various elements based on the recorded mass differences of the roasted products before and after treatment, combined with the leaching efficiency.Results and discussionUsing the positive and negative electrode mixtures from waste lithium-ion batteries as raw materials, lithium was selectively extracted via an in-situ reduction roasting-water leaching process without requiring additional reagents. Single-factor experiments were conducted to determine the optimal process conditions, which were identified as a roasting temperature of 600 ℃, a roasting time of 30 min, and a stirring time of 4 h. Under these conditions, the leaching rate of Li reached 72.23%, while Ni, Co, and Mn exhibited negligible leaching. The elemental contents before and after carbon-thermal reduction were quantitatively analyzed using SEM-EDS. In conjunction with ICP-OES detection results, it was determined that the raw materials contained impurities such as Al, F, and P. Carbon-thermal reduction reactions of commercial ternary cathode materials free of impurities were performed under identical conditions. The leaching rate of Li from the pure ternary material reached 96.6%, significantly higher than that achieved with impure materials (72.23%). Roasting products under various conditions were characterized by X-ray diffraction. It was observed that as the roasting temperature increased from 500 ℃ to 600 ℃, the characteristic peaks of NCM disappeared, indicating the complete destruction of the NCM (LiNi_xCo_yMn_1-x-yO₂) crystal structure. The resulting roasting products primarily consisted of Li₂CO₃, Co, Ni, CoO, NiO, and unreacted graphite.ConclusionsThe optimal process were identified to be roasting temperature of 600 ℃, roasting duration of 30 min, and stirring time of 4 h, through single-factor experiments. The lithium leaching efficiency of 72.23% was achieved, while nickel, cobalt, and manganese exhibited negligible dissolution. Notably, under identical conditions, commercially sourced pure ternary cathode materials demonstrated significantly enhanced lithium recovery at 96.6%, maintaining minimal co-leaching of transition metals (<0.5%). To optimize the comprehensive lithium recovery efficiency in spent lithium-ion battery recycling, strategic introduction of additives capable of exerting synergistic reduction effects during the carbon thermal reduction stage is proposed. Subsequent research should focus on systematic evaluation of additive economics coupled with rigorous quantification of their lithium extraction enhancement potential.