Abstract <p>The rapid growth of the energy industry has resulted in the large-scale accumulation of spent ternary lithium battery cathodes (LiNi_<i>x</i>Co_<i>y</i>Mn_{1−<i>x</i>−<i>y</i>}O₂, NCM) and copper slag, causing significant resource loss and environmental pressure. To enable the synergistic high-value utilization of these two solid wastes, a novel reduction smelting strategy is proposed in which iron from copper slag serves as a natural metal collector. Carbothermal synergistic reduction smelting was employed to recover valuable metals from both waste streams. The pyrolysis and phase transformation behavior of NCM during reduction roasting were systematically investigated, with particular focus on the effects of smelting temperature and slag binary basicity (CaO/SiO₂) on Ni and Co capture and metal recovery. Under optimal conditions (1550&#xa0;°C, binary basicity 1.1, 60&#xa0;min), iron efficiently alloyed with reduced nickel and cobalt, forming well-separated Fe–Ni–Co alloys. Iron recovery reached 99.05%, nickel 98.31%, and cobalt 98.8%. Mechanistic analysis indicates that the newly formed iron droplets not only act as active metal carriers for physically capturing Ni and Co but also promote further reduction of their oxides through displacement reactions. Moreover, owing to the good lattice compatibility among Fe, Ni, and Co, a stable γ-(Fe, Ni, Co) solid solution readily forms, thermodynamically driving the continuous migration of Ni and Co from the slag to the metal phase. This research provides a feasible “treating waste by waste” technical pathway for the cotreatment of metallurgical solid waste and spent lithium batteries, laying a theoretical foundation for the synergistic recovery of valuable metals from multi-component solid wastes, with significant industrial application potential and environmental implications.</p> Graphical Abstract <p></p>