Element Distribution and Phase Analysis of Positive Electrode Active Materials in Black Mass

Significance of the topic

Recycling lithium-ion batteries (LIBs) is critical for securing strategic metals (Li, Co, Ni, Mn, etc.), reducing supply risk and lowering environmental impact. Black mass, the powdered product from battery disassembly and comminution, is the primary feedstock for recovery of these metals. Accurate characterization of black mass composition — including active-material types, formation products from thermal history, and contaminants such as current-collector foils, casings and electrolyte residues — is essential to optimize downstream hydrometallurgical and pyrometallurgical processes, improve metal recovery yields, and minimize waste treatment costs.

Objectives and study overview

• Demonstrate how electron probe microanalysis (EPMA) can be used to map element distributions and perform phase analysis on black mass from used LIBs.
• Identify and distinguish multiple positive electrode active-material chemistries (e.g., LFP, NMC variants, NCA) and detect impurities and heat-treatment products relevant to recycling process design.
• Show how combined elemental mapping and phase-clustering approaches enable semiquantitative identification of active-material populations within heterogeneous black mass.

Methodology

• Sample preparation: Cross sections of black mass pellets or consolidated powders were prepared for EPMA imaging to reveal particle internal composition and adhered contaminants.
• Elemental mapping: EPMA compositional (COMPO) and secondary electron (SE) imaging and focused X-ray mapping were performed to visualize distributions of major elements relevant to LIBs (C, O, F, P, S, Al, Cu, Fe, Si, Ni, Co, Mn).
• Analytical parameters: Mapping and point analyses were performed at an accelerating voltage of 15.0 kV; mapping areas presented included ca. 480 × 360 µm and 120 × 90 µm field sizes. Results were expressed as mass percentage concentrations (wt%).
• Phase analysis approach: Ternary scatter diagrams (Ni–Co–Mn and Ni–Co–Al) were constructed from pixel-wise composition data. Reference composition marks for known compounds (NMC811, NMC622, NMC523, NMC111, NCA) were plotted, clusters were identified, and filters (cluster regions) were defined to produce binary/phase maps and a color-coded phase diagram.

Used instrumentation

• Electron Probe Microanalyzer: EPMA-8050G (Shimadzu).
• Imaging modes: COMPO (quantitative composition imaging), SE (secondary electron imaging) and element-specific X-ray maps (Ka lines).
• Operational details reported: 15 kV accelerating voltage; mapping fields of view of roughly 480 × 360 µm and 120 × 90 µm; compositional outputs presented in wt% and count-rate maps for each element.

Main results and discussion

• Elemental distributions: Mapping clearly resolved positive-electrode transition metals (Ni, Mn, Co) and negative-electrode carbon (graphite). Electrolyte-related elements (F, P) and structural materials (Al, Cu, Fe, Si, S) were also detected and spatially associated with different particle populations and matrix regions.
• Impurities and adhered phases: Aluminum (from current collectors or laminated films) and copper (fibrous Cu foil fragments) were observed; Cu showed partial oxidation and local adherence to P- and Fe-rich regions. Phosphorus and fluorine mappings indicated residues from LiPF6 electrolyte salt; sulfur-rich regions suggested electrolyte additives. Silicon was largely present as oxides associated with Al.
• Multiple active-material populations: Ni–Mn–Co mapping revealed heterogeneous distributions with particle-to-particle and intra-particle compositional variation. Overlay images (e.g., Mn–Co–Ni and Al–Mn–Ni) suggested the presence of two to three distinct positive-active-material populations within the same black mass sample.
• Phase identification via scatter diagrams: Ternary Ni–Co–Mn and Ni–Co–Al scatter plots showed distinct clusters aligning with theoretical compositions for NMC811, NMC622 and NCA. Filters placed around these clusters produced a binary-color-coded phase map demonstrating spatial distributions of these compound classes. Binary Ni–Co plots confirmed that cluster populations are compositionally consistent and trend toward the origin consistent with mixture/heterogeneity effects.
• Implications for recycling: Detecting and spatially resolving multiple NMC/NCA chemistries and LFP-type signals enables tailored downstream processing — for example, segregated hydrometallurgical routes or adjusted leaching conditions — and informs thermal pretreatment strategies to avoid undesirable formation products that impede recovery.

Benefits and practical applications

• Impurity detection: EPMA reliably identifies metallic foil residues (Al, Cu), casing-derived Fe, and electrolyte decomposition products (F, P, S), important for feedstock quality control.
• Phase discrimination: Scatter-diagram-based phase analysis enables identification of different positive-electrode chemistries (NMC variants, NCA, LFP) within mixed black mass streams, supporting process routing and resource valuation.
• Process optimization: Knowledge of particle composition and adherent impurities informs selection of heating, oxidation, and leaching parameters, improving metal extraction efficiency and lowering reagent/waste-treatment demands.
• R&D and QA/QC: High-resolution mapping supports technology development for recycling and provides a diagnostic tool for plant feed characterization and incoming material specification enforcement.

Future trends and potential applications

• Automation and high-throughput mapping: Integration of automated image segmentation and compositional clustering will allow rapid screening of many black mass samples for industrial QC and sorting.
• Machine-learning classification: Training ML models on EPMA-derived scatter patterns could enable automated identification of active-material classes and prediction of downstream leaching behavior.
• Multimodal correlative analysis: Combining EPMA with bulk techniques (XRD, ICP-MS), micro-Raman, or focused ion beam cross-sectioning will improve phase identification and quantify amorphous/oxidized phases formed during thermal pretreatment.
• Standardization across recycling supply chains: Development of shared phase maps and reference datasets for common cathode chemistries will improve consistency in feedstock valuation and processing choices.
• Advanced depth-sensitive approaches: Cryogenic or low-voltage EPMA and complementary nano-analytical techniques can be used to study delicate phases or surface layers (electrolyte residues, SEI fragments) without inducing artefacts.

Conclusion

EPMA-8050G-based elemental mapping combined with ternary/binary scatter-diagram phase analysis provides a powerful, spatially resolved approach to characterize the heterogeneous composition of black mass from spent LIBs. This methodology identifies active-material classes (multiple NMC variants and NCA), detects metallic and electrolyte-derived impurities, and reveals compositional heterogeneity within and between particles. Such insights are directly applicable to optimizing pretreatment, leaching and recovery workflows, reducing operating costs and environmental impact in battery recycling operations.

References

1. Manabu Nishimura et al., Secondary Batteries, Kobelco Research Institute, No. 58, Apr. 17, 2024.
2. Taku Shiraishi, Guide to Understanding State-of-the-Art Secondary Batteries, Gijutsu-Hyoron Co., Ltd., 2020, pp. 151, 163.