An emerging risk: Hazardous ultrafine particles and their metal-PAH enriched emissions from lithium-ion battery fires

Nickel-rich lithium-ion batteries (LIBs) with LiNi₀.₈Mn₀.₁Co₀.₁O₂ (NMC811) cathodes are central to global electrification, yet thermal runaway (TR) generates chemically complex and poorly characterized particulate emissions with potential carcinogenicity. This study quantifies size-resolved particulate emissions from 18650 NMC811 cells subjected to 50 kW/m² radiant heating under normoxic conditions, comparing states of charge (SOC) from 0% to 70%. Increasing SOC from 0% to 70% intensified TR, raising peak temperature from 680 to 806 °C and mass loss by 156% (7.59-19.47 g). Total particle number concentration (PNC) decreased by 69% (3.17 × 10⁸ to 9.88 × 10⁷ particles/cm³), yet ultrafine particles (UFPs≤0.1 µm) dominated at both conditions, decreasing from 94% at 0% SOC to 87% at 70% SOC. Conversely, the total PM mass increased by 50% (0.78-1.17 mg), while UFP PM mass decreased by 25%, and the fine (+66%) and coarse (+233%) fractions increased. Metal concentrations surged 531% (1.14-7.20 µg/m³), driven primarily by Ni and Co released from cathode lattice collapse. UFPs-bound high-molecular-weight PAHs increased 451% (945-5205 µg_PAH/g_PM), reflecting high-temperature polymer pyrolysis and radical-driven aromatic ring growth. Morphology progressed from discrete spheres to metal-decorated fractal agglomerates containing up to 8.24 wt% nickel, capable of penetrating the layers of a firefighter's personal protective jacket. These findings establish SOC as a pivotal modulator of toxicologically synergistic ultrafine metal-PAH hybrids during battery TR, underscoring the need for SOC-informed exposure guidelines, enhanced respiratory protection, and regulatory prioritization of these particles in occupational and environmental risk frameworks.