Microwave-Assisted Recycling of Lithium-Ion Batteries: Linking Process Optimization to Volatile Organic Compounds and Fluorinated Gases Emission Mitigation

Abstract

This work investigates the volatile fraction released from black mass (BM) obtained from spent lithium-ion batteries subjected to microwave (MW) thermal treatment. MW processing is emerging as an alternative to conventional pyrometallurgy for improving energy efficiency and recovery of critical metals such as lithium, yet the associated emission profile remains poorly characterized. However, the studies of the emissions associated with these treatments are quite limited. Here, a multilevel full factorial Design of Experiments is applied for the first time to evaluate the influence of MW power, exposure time, and BM mass on heating dynamics and lithium extraction efficiency. Volatile organic compounds generated during MW processing are identified by headspace solid-phase microextraction coupled to gas chromatography–mass spectrometry (HS-SPME/GC-MS), showing a complex mixture of aliphatic and aromatic hydrocarbons, carbonate esters, and phosphorus- and fluorine-containing species. Multinuclear NMR spectroscopy (¹H, ⁷Li, ¹⁹F, ³¹P) confirms the presence of electrolyte-derived residues such as Li⁺, PF₆⁻, and phosphate esters. The combined analytical approach clarifies degradation pathways during MW heating and highlights the need to monitor and mitigate the formation of potentially hazardous volatile species in future MW-assisted recycling processes. Statistical models reveal that the time to reach 600°C and the maximum temperature depend primarily on power and exposure time, while Li recovery is governed by BM mass and its interaction with power.