A Critical Appraisal of Pyrolysis Pretreatment in Lithium-Ion Battery Recycling: From High-Temperature Material Transformations to Environmental Impact

Abstract

Driven by sustainability goals and raw material scarcity issues, recycling of spent lithium-ion batteries is becoming imperative. Pyrolysis is commonly employed for the removal of polymers to improve the liberation of active materials, yet its impact during the thermal degradation remains unexplored. Thus, the present study investigates the effect of pyrolysis time and temperature on the liberation and composition of “black mass (BM).” Thermal decomposition of various size fractions of an industrial BM was studied using thermogravimetric analysis and bench-scale pyrolysis, while characterization before and after pyrolysis was performed using X-ray diffraction and scanning electron microscopy. Furthermore, analysis of gaseous byproducts during pyrolysis using Fourier transform infrared offered insights on the decomposition byproducts and their environmental impact. It is hereby demonstrated that, while pyrolysis results in the partial degradation of polymeric binders, this is not the result of pure thermal degradation. Indeed, graphite, polymers, and Al reduce cathode active materials into metallic species via in situ carbothermic or aluminothermic reduction, producing Al2O3, COx, and alkanes. The experimental results are rationalized into a reaction mechanism for pyrolysis of black mass, revealing that pretreatment via pyrolysis can compromise the preservation of battery active material and needs to be carefully considered during direct recycling routes.