Elucidating Polymer–Cathode Interfacial Interactions in Pyrolytic Pretreatment of Spent Lithium-Ion Batteries

Pyrolysis has been widely employed as a pretreatment strategy to remove organic components from spent lithium-ion batteries, which is traditionally considered to have minimal influence on subsequent metal recovery. By investigating the synergistic pyrolysis behavior of organic composites with spent lithium nickel cobalt manganese oxide (NCM) cathode materials, it is found that the carbonaceous residues formed during pyrolysis can significantly impact the microstructure and interfacial chemistry of the cathode material and substantially hinder the lithium migration during subsequent low-temperature roasting recovery. It is demonstrated that the functional groups released from organic matter selectively adsorb on the cathode surface under thermal conditions, altering the thermal decomposition pathways of organic components, including poly(vinylidene fluoride) (PVDF) and the separator, and impeding their complete volatilization. This interfacial interaction between PVDF/separator and cathode active materials promotes the activation and release of lattice oxygen, which further interacts with the partially decomposed carbonaceous species and forms oxygen-rich carbonaceous composites. These migrate into the particle interior and form dense, conformal coatings along the grain boundaries, yielding a physical barrier that obstructs lithium diffusion. During sulfur-assisted roasting for selective lithium extraction from ternary black mass, this interfacial architecture significantly suppresses lithium release, reducing extraction efficiencies to below 80%. In this case, enhancing the oxygen partial pressure facilitates complete oxidation of the carbonaceous species, effectively removing interfacial barriers and restoring the lithium leaching efficiency to 96.88%. This work underscores the critical influence of pyrolytic pretreatment on the efficiency of pyrometallurgical recycling and suggests reconsideration of its presumed inertness.