The recovery of nickel from spent lithium-ion batteries presents critical sustainability challenges as demand for battery materials escalates. Three dominant nickel recovery methods—hydrometallurgical, pyrometallurgical, and direct recycling—exhibit distinct life cycle assessment outcomes when evaluated under ISO 14040/44 frameworks. Regional operational differences in China, the EU, and the US further influence environmental performance due to variations in energy grids, regulatory standards, and waste handling practices.

Hydrometallurgical processes employ aqueous chemistry to leach nickel from black mass, typically using acids like sulfuric or hydrochloric, followed by solvent extraction or precipitation. Energy consumption ranges between 15–25 kWh per kg of recovered nickel, heavily dependent on leaching efficiency and chemical regeneration. GHG emissions average 8–12 kg CO2-eq/kg Ni in regions with coal-dominated grids (China), dropping to 4–6 kg CO2-eq/kg Ni in the EU, where renewable energy penetration is higher. Water usage is significant at 200–300 L/kg Ni due to rinsing and neutralization steps, with acidic wastewater requiring treatment for heavy metal residuals. Toxicity impacts arise from reagent use, particularly if hydrofluoric acid is employed for silicon removal. Recovery rates can exceed 95%, but tailings containing unrecovered lithium or cobalt require stabilization to prevent leaching.

Pyrometallurgical methods rely on high-temperature smelting (above 1400°C) to reduce nickel oxides into a metallic alloy. Energy inputs are substantially higher at 30–40 kWh/kg Ni, with emissions reaching 15–20 kg CO2-eq/kg Ni in China and 10–14 kg CO2-eq/kg Ni in the US. The process avoids liquid waste but generates slag, which may encapsulate up to 5% of unrecovered nickel unless further processed. SOx and NOx emissions necessitate scrubbers, adding to operational costs. The advantage lies in simplicity—pyrometallurgy tolerates mixed feedstocks without precise sorting, but it sacrifices lithium recovery entirely, as lithium reports to the slag phase. Toxicity is lower than hydrometallurgy regarding water pollution but higher in airborne particulate matter.

Direct recycling, an emerging approach, preserves the cathode crystal structure by relithiating and reprocessing nickel-rich cathodes without full breakdown. Energy use is the lowest at 5–10 kWh/kg Ni, with emissions of 2–3 kg CO2-eq/kg Ni in the EU where clean energy supports low-temperature processing. Water use is minimal (under 50 L/kg Ni), and toxicity is reduced by avoiding harsh reagents. However, direct recycling requires pristine feedstock separation and struggles with contaminants, limiting recovery rates to 85–90% unless supplemented by hydrometallurgical refining for purity. Tailings are negligible, but the process is less adaptable to varied battery chemistries.

Regional disparities highlight policy dependencies. China’s reliance on coal inflates emissions for all methods, but its centralized facilities achieve economies of scale. The EU’s carbon pricing favors hydrometallurgy with renewable energy, while US operations balance lower grid intensity with higher transport emissions due to dispersed recycling infrastructure.

Tradeoffs between recovery efficiency and environmental burden are stark. Pyrometallurgy maximizes nickel yield but at high carbon cost. Hydrometallurgy offers balanced recovery with manageable emissions if wastewater is treated. Direct recycling is the cleanest but cannot yet meet 95% recovery without hybrid systems. Tailings management adds complexity—slag from pyrometallurgy is inert but voluminous, while hydrometallurgical residues demand chemical stabilization.

Future improvements hinge on process integration, such as coupling direct recycling with mild hydrometallurgy for impurity removal, or using renewable heat for pyrometallurgy. Regional policies must incentivize low-carbon energy for recycling plants and standardize tailings handling to mitigate long-term risks. The LCA outcomes underscore that no single method is universally superior; optimal nickel recovery strategies must align with local resources, regulatory frameworks, and circular economy priorities.