Effects of mechano-thermal conditioning on PVDF decomposition and fluorine distribution in spent lithium-ion batteries.

During thermal treatment of black mass from spent lithium-ion batteries (LIBs), fluorine-bearing species such as polyvinylidene fluoride (PVDF) can generate secondary fluorinated compounds that adversely affect downstream recovery processes and pose environmental challenges. This study evaluates the effects of dry and cryogenic grinding as mechanical pre-treatment prior to slow pyrolysis on PVDF decomposition and fluorine speciation in mixed black mass derived from spent LIBs. Black mass samples were subjected to identical milling conditions (30 Hz, 7 min) followed by slow pyrolysis under an inert atmosphere up to 550 °C. The resulting materials were evaluated using advanced characterization techniques to assess thermal decomposition, phase transformations, and fluorine-containing species. The cryogenically treated sample exhibited slightly higher organic-related mass loss in the 350-500 °C range (1.90 wt% versus 1.74 wt% for dry grinding), indicating enhanced decomposition of PVDF and associated organics. Spectroscopic analyses revealed lower concentrations of PVDF-related functional groups following cryogenic treatment, while surface characterization showed a greater reduction of fluorocarbon species and a corresponding increase in inorganic fluoride formation after slow pyrolysis. In particular, PVDF-derived fluorine decreased to approximately 19.2% in the cryogenic sample compared with 23.6% in the dry-ground sample. These results demonstrate that mechanical pre-treatment influences PVDF decomposition and fluorine redistribution during slow pyrolysis, providing insight into impurity transformation mechanisms during lithium-ion battery recycling.