State Analysis of Positive Electrode Active Materials and Compounds in Black Mass

Significance of the Topic

Understanding the microchemical state and phase composition of "black mass" — the heterogeneous powder derived from dismantled and shredded lithium-ion batteries — is critical for efficient and sustainable battery recycling. Accurate identification of positive electrode active materials, formation products, and impurities (e.g., Al, Cu, F, P, S) directly informs downstream processing decisions (thermal treatment, leaching, and purification), enabling higher recovery yields for critical metals (Li, Co, Ni, Mn), lower processing costs, and reduced environmental burdens from wastewater and gaseous emissions.

Objectives and Overview of the Study

This application study used an electron probe microanalyzer (EPMA) to (1) characterize spatial distributions of major and minor elements in black mass cross-sections, (2) perform phase identification of coexisting positive electrode chemistries (notably NMC variants and NCA), and (3) perform state (valence/compound) analysis of selected elements and formation products. The aim was to demonstrate how combined element mapping, ternary scatter/phase plotting, and spectral state analysis can resolve multiple active-material types and thermally/chemically formed compounds within microregions of black mass.

Methodology and Instrumentation

• Elemental mapping: high-resolution EPMA mapping of cross-sections capturing distributions of Ni, Co, Mn, Al, Cu, O, P, F, S, and C to reveal compositional heterogeneity and particle-level variations.
• Phase identification: generation of ternary scatter diagrams (Ni–Co–Mn and Ni–Co–Al) from pixel-wise compositional data to find clusters corresponding to theoretical compositions of NMC variants (NMC811, NMC622, NMC523, NMC111) and NCA. Spatial filters were then applied to segregate these phases and reconstruct a phase map.
• State analysis (chemical state/valence): wavelength-dispersive spectral analysis using characteristic emission lines — Lα for Ni/Co/Mn and sKα 3,4 for Al and P — to detect wavelength shifts, intensity ratios, and waveform differences indicative of oxidation state and compound class (oxides, phosphates, phosphides, etc.).
• Data integration: combining mapping, ternary clustering and spectral state signatures to attribute both phase identity and chemical state at micrometer scale.

Instrumentation Used

• EPMA-8050G electron probe microanalyzer (Shimadzu) used for quantitative element mapping and wavelength-dispersive spectroscopy.
• Representative operating conditions reported in the study: mapping at ~15.0 kV accelerating voltage with mapped areas on the order of 60 × 45 µm and spatial features resolved at the micrometer scale; characteristic lines used included Lα lines for transition metals and high-resolution sKα 3,4 lines for light elements (Al, P).

Main Results and Discussion

• Heterogeneous composition: Element maps revealed co-distribution of Ni, Mn, and Co consistent with positive electrode active materials, plus graphite C from negative electrodes and metallic residues (Al, Cu) from current collectors and casings.
• Coexistence of multiple positive electrode chemistries: Ternary Ni–Co–Mn and Ni–Co–Al scatter plots showed distinct clusters attributable to NMC622 and NCA phases. Cluster-based filtering enabled a phase map clearly separating these two dominant positive-electrode materials within the analyzed microregion.
• Detection of formation products: An O–P–Al ternary and overlay imaging indicated localized Al–P–O-rich regions interpreted as formation products arising from reactions between electrolyte salt (LiPF6) and Al-containing components (current collector, casing, or laminate). Phosphorus distribution and spectral features suggested P present in phosphide-like environments (spectrally similar to Ca5(PO4)3 in the study) rather than classical phosphate alone.
• Valence/state changes in transition metals: Wavelength shifts and spectral waveform differences in Ni, Co, and Mn Lα lines showed that Ni, Co, and Mn in black mass regions (both NMC622-like and NCA-like) are partially reduced relative to the tetravalent state found in some raw NMC811 precursor material. The observed peak shifts toward longer/shorter wavelengths and altered line shapes are consistent with mixed valence states resulting from battery use and thermal/chemical processing.
• Element-specific observations: Fluorine from electrolyte became incorporated into some high-Co particles; sulfur distributions align with some positive electrode particles and likely derive from electrolyte additives; aluminum was present both as metalaceous particles and oxidized Al2O3-like states in Al–P–O regions.

Benefits and Practical Applications of the Method

• Accurate phase identification at micrometer scale enables sorting of feedstock by chemistry (e.g., isolating NCA vs. NMC622), optimizing downstream hydrometallurgical and pyrometallurgical routes.
• State analysis of elements supplies information on oxidation state and compound class (oxide, phosphide-like, fluoride-containing phases) that influence leaching behavior, reagent choice, and thermal processing parameters.
• Mapping of impurities and formation products supports targeted pretreatment strategies to minimize secondary wastes (e.g., controlling P- and F-bearing species that complicate wastewater treatment).
• Integration of mapping, ternary phase plotting, and wavelength-dispersive state spectroscopy provides a robust diagnostic workflow for black mass quality control, feedstock certification, and process optimization in battery recycling facilities.

Future Trends and Potential Applications

• Broader adoption of combined spatial/chemical-state EPMA workflows to build statistical databases of black mass compositions across diverse recycling streams, enabling feedstock prediction and automated sorting.
• Coupling EPMA data with machine-learning classification on pixel/particle-level compositions to automate phase identification and predict optimal recovery routes for mixed chemistries.
• Integration with complementary microscopies (SEM–EBSD, nanoSIMS) and X-ray spectroscopies to refine identification of complex formation products and trace element distributions (e.g., Li mapping, light element quantification).
• Application of in-situ heating or controlled-atmosphere EPMA experiments to simulate thermal treatments and track phase transformations relevant to recycling process design.

Conclusion

Combining EPMA elemental mapping, ternary scatter-based phase discrimination, and wavelength-dispersive state analysis enables unambiguous identification of multiple positive electrode materials (notably NMC622 and NCA) and the chemical states of impurities and formation products within black mass microregions. This multimodal microanalysis approach delivers actionable insight to optimize thermal and chemical recycling steps, improve metal recovery efficiency, and reduce environmental impacts associated with battery recycling.

References

1. Manabu Nishimura, et al.: Secondary Batteries, Kobelco Research Institute, No. 58, Apr., 17 (2024).
2. Masashi Jinno: The Battery Book, Sogo Kagaku Publishing, 2019, p. 113.
3. Shimadzu Application News: Element Distribution and Phase Analysis of Positive Electrode Active Materials in Black Mass, Application News No.01-01121-EN.