Synergistic Pathways for Pollution and Carbon Reduction in End-of-Life Nickel Cobalt Manganese Oxide Battery Recycling Processes

Synergistic pollution and carbon reduction in end-of-life (EOL) lithium-ion battery recycling are essential for the green and low-carbon transition of the battery recycling industry. However, systematic comparisons of different recycling processes from a synergy perspective remain limited. This study applies life cycle assessment to compare four representative recycling routes for EOL lithium nickel cobalt manganese oxide (NCM) batteries in China: pyrometallurgical recycling, hydrometallurgical recycling, pyro-hydrometallurgical recycling, and direct recycling. Environmental performance is evaluated across 18 impact categories using the ReCiPe 2016 midpoint method, and contribution hotspot analysis, sensitivity analysis, scenario analysis, and a synergy index (SI) are integrated to identify key burden sources and potential for synergistic optimization. The results show that direct recycling performs best under the baseline scenario, with the lowest burdens in 12 of the 18 impact categories and the lowest global warming potential (GWP) of 1.35 kg CO2-eq EOL NCM battery. Key influencing parameters differ markedly among processes. Scenario analysis shows that technological progress yields the most stable synergistic benefits; under this scenario, direct recycling achieves nearly synchronized reductions in pollution and carbon, with pollution-related SI values ranging from 0.96 to 0.99. In contrast, energy structure transition substantially reduces GWP, with a 64.43% reduction for pyro-hydrometallurgical recycling, but may induce non-carbon environmental burden shifting, particularly in terrestrial ecotoxicity and ozone depletion. The combined scenario achieves high absolute reduction potential, but its synergy depends on whether pollution-related impacts decrease synchronously with GWP. Process-specific strategies that integrate source-oriented technological improvements, electricity decarbonization, and key input reduction are essential to achieve robust pollution-carbon synergies in EOL NCM battery recycling.