From chemical to physical characterization: Challenges in the analysis of black mass

The growing demand for lithium-ion batteries, together with the increasing volume of end-of-life waste, highlights the need for efficient and sustainable recycling strategies. Black mass, a key intermediate in lithium-ion battery recycling, has been extensively studied from a chemical perspective; however, its physical properties remain insufficiently understood despite their importance for metallurgical processing. In this study, three industrial NMC-based black mass materials produced via different thermal pretreatment routes (pyrolysis or vacuum-drying) were systematically characterised using a multimethod analytical approach. Chemical composition was determined by ICP-OES, crystalline phases were identified by X-ray diffraction and microstructural features by SEM-EDS and TEM-EDS.Thermogravimetric analysis, particle size and shape measurements, nitrogen adsorption, helium pycnometry, powder rheology, and small-angle X-ray scattering (SAXS) were employed to quantify porosity, surface area, density, and flow behaviour. The results show that thermal pretreatment controls the physical state of black mass. Pyrolysed materials are associated with reduced organic and fluorinated residues, enhanced surface accessibility, pronounced nanoscale porosity, and improved powder flowability. In contrast, the vacuum-dried sample retains binder films, elevated fluorine content, and smooth particle surfaces, resulting in poor dispersibility and low interfacial contrast. SAXS analysis reveals that nanoscale interfacial characteristics strongly correlate with surface accessibility relevant for leaching and with macroscopic flow performance, supporting their use as a complementary indicator of processability. Overall, this study demonstrates that physical properties, particularly nanoscale interfacial features, are likely important factors influencingdetermining the suitability of black mass for metallurgical processing and highlights the industrial relevance of controlled thermal pretreatment and multimodal characterization.