Fig. 3: Environmental impacts of conventional and circular refining technologies.

a Energy consumption, b CO₂-eq emissions, and c water consumption of gate-to-gate refinement by different pathways for lithium nickel cobalt aluminum oxide (NCA) battery-grade salts. Numbers in parentheses labeled on the top of stacked bars denote the refinement methods summarized in Fig. 2. The conventional mined pathway (Conv. Mined) refines natural deposits and produces discrete salts (Method (1) in Fig. 2); note that Al is presented on the top of each stacked bar but its contributions are too small to be seen; specific environmental impacts of each element contributor were detailed in Supplementary Table 3. Circular supply chains refine from either mixed energized end-of-life lithium-ion batteries collected from consumers (Recycled Battery, Method (4) in Fig. 2) or non-energized battery scrap from a production facility (Recycled Scrap, Method (5) in Fig. 2), producing mixed metal sulfates. Multi-step circular refinement pathways include mechanical processing (Me, gray), reductive calcination (RC, red), and hydrometallurgy (Hy, blue). RC is an additional processing step for energized batteries and was not used for non-energized recycled scrap. Open bars in the panels “Representative Circular” denote the environmental impacts of recycling NCA batteries with representative existing pyrometallurgical (Py*), hydrometallurgical (Hy*), and direct recycling (Direct*) methods as comparison, with data obtained from the literature¹⁹. Literature data were normalized by the same functional unit in this study, and uncertainties were determined by combining two different battery form factors: pouch and cylindrical. The vertical dashed line in each graph demarcates different data types, where the model-based conventional and representative existing pathways were summarized in the left panel, operational data from Redwood Materials were presented in the middle panel (“industrial circular”), and literature data in the right panel (“representative circular”). Note that water consumption has generally not been quantified in previous studies, leading to no literature data panel for (b). Environmental impacts of material extraction and transport in the supply chains were not included.