Mo, 7.9.2026 | Original article from: Process Safety and Environmental Protection, Volume 214, 2026, 109134
This study characterizes particle size, elemental composition, and morphology of emissions from NMC and NCA lithium-ion battery fires using spectrometry and SEM-EDS.Size distribution and elemental analysis of fine particles from Li-ion battery cell fire
Process Safety and Environmental Protection, Volume 214, 2026, 109134: Fig. 2. Setup of the experimental methods including particle capture and further analysis.
This study investigates fine particle emissions generated during thermal runaway of cylindrical NMC and NCA lithium-ion battery cells. Particles ranging from 14 nm to 9.7 μm were collected with a 14-stage cascade impactor and analyzed for size distribution, elemental composition, and morphology.
Atomic absorption and atomic emission spectrometry showed that lithium was most abundant in particles below 1 μm, while nickel dominated larger particles. SEM-EDS provided complementary information on particle morphology and composition, supporting assessment of the environmental and health risks associated with lithium-ion battery fires.
The original article
Size distribution and elemental analysis of fine particles from Li-ion battery cell fire
Lukáš Preislera, Simona Šachrováb, Jiří Pospíšila, Tomáš Siteka, Renata Komendováb
Process Safety and Environmental Protection, Volume 214, 2026, 109134
licensed under CC-BY 4.0
Selected sections from the article follow. Formats and hyperlinks were adapted from the original.
Electromobility has emerged as a prominent representative of environmentally friendly transportation in recent years. Electric vehicles (EVs) have high engine efficiency in comparison to internal combustion engines and generate zero tailpipe emissions. Despite the significant potential of electromobility, numerous unresolved issues remain, such as electricity production, the availability of rare materials, and the production of batteries, with the main current focus on lithium-ion batteries (LIBs). The growing demand for EVs highlights the increasing importance of LIB safety. While these batteries enable advanced technology, they also pose significant risks, particularly during thermal runaway (TR) events that can lead to fires or explosions. The fire of LIBs emits elements that are hazardous to human health, including cobalt, aluminum, copper, and lithium. Fine particles released during such incidents present substantial health hazards when inhaled into the lower respiratory tract.
In the event of an accident or malfunction of a battery cell, TR may occur. The initiation mechanisms of TR are electrical, mechanical, or thermal abuse (Liu et al., 2022, Larsson et al., 2014). Electrical damage occurs after overvoltage, overcurrent, overcharge, over-discharge, or short circuit (Larsson and Mellander, 2014). Thermal initiation of TR occurs after local overheating, or extreme cold (Luo et al., 2022). Overheating can lead to the vaporization of the liquid electrolyte, resulting in the formation of gas (Essl et al., 2021). Damage to the separator can result in the cathode and the anode of a battery cell becoming electrically connected, leading to strong exothermic reactions (Pfrang et al., 2017, Loveridge et al., 2018). The process leading to thermal runaway is generally divided into three distinct phases (Golubkov et al., 2014): Phase I is the initial heating stage, where the cell is heated by an external source while internal structural changes, such as endothermic separator melting, begin to occur. During Phase II, the cell becomes its own heat source due to internal exothermic reactions, causing the temperature to rise until the TR trigger point. Phase III is the final stage, defined by a sharp increase in the heating rate accompanied by massive venting and particle generation. This rapid escalation significantly increases internal pressure and degrades electrolyte stability. A significant amount of heat is generated, causing the temperature of the surface of a battery cell to rise to 400 °C (Zhang et al., 2019a, García et al., 2024). Furthermore, the sparks generated during an explosion may reach temperatures up to 1200 °C (Wang et al., 2020). The intensity of TR is dependent on state of charge (SOC). When a battery cell is fully charged, the reaction is significantly more intense compared to a battery cell with a low charge (Li et al., 2024a, Essl et al., 2020, Chen et al., 2020). Due to the TR of a single battery cell, there is a high risk of fire propagation to other cells (Niu et al., 2022, Zhu et al., 2023).
The process of TR has the potential to result in the occurrence of fire and explosion, releasing smoke into the atmosphere. The composition of the smoke is a mixture of gaseous and particulate matter, including solid and liquid particles. This particulate matter (PM) contains a mixture of elements and substances, such as heavy metals (Wang et al., 2020). The release of gas can be attributed to three primary mechanisms: electrolyte decomposition, the reaction between anode and electrolyte, or the decomposition of cathode materials (Wang et al., 2020) and binders (Spotnitz and Franklin, 2003, Wang et al., 2023a). A comprehensive analysis of the gaseous toxic substances was conducted by Wang et al. (2020) and Zhang et al. (2019b). The composition of the gaseous emissions primarily consists of CO2, CO, H2, H2O, hydrocarbons, and other substances, depending on the composition of the cathode material. These substances include highly toxic gases such as HF, PF5, and POF3 (Larsson et al., 2014). Claassen et al. (2024a) found a significant emission of gaseous H2SO4 from LCO and LFP battery cells. In addition, Nedjalkov et al. (2016) identified the presence of benzene, toluene, styrene, and biphenyl. TR of a NCA battery cell emits CO and HF at concentrations significantly higher than IDLH (Ubaldi et al., 2023). These emissions may trigger a range of health concerns, including respiratory irritation, asthma attacks, respiratory infections, cardiovascular diseases, pneumonia, persistent cough, respiratory failure, and even death (Valavanidis et al., 2008).
The released stream of aerosols contains approximately 17% by mass of solid particles of various sizes (Zhang et al., 2019a). In a heterogeneous aerosol environment, complex processes of nucleation and particle growth take place, which are schematically depicted in Fig. 1 as described below. The emission of particles may occur directly into the atmosphere or via firefighter water (Zhang et al., 2019a). It has been observed that nearly 90% of the total particles, including settleable particles, by weight, are smaller than 500 μm (Li et al., 2024b). Particles larger than 100 μm are predominantly formed from the disintegration of the electrodes and the separator. These particles are characterized by a significant settling velocity, resulting in a relatively short persistence in the atmosphere (Alves et al., 2020). When the gas stream carrying particles is directed toward a solid surface, impaction leads to particle deposition on the surface.
Process Safety and Environmental Protection, Volume 214, 2026, 109134: Fig. 1. Scheme of particle processes during TR
2. Methods
2.2. Experimental setup
The heating of battery cells was conducted within a sealed testing chamber, with a volume of 200 liters and equipped with a pressure valve to prevent significant pressure build-up during the thermal runaway event, see Fig. 2. The battery cell was clamped by a 10 mm thick aluminum heating block attached to its center. The heating element involves a resistance heating with the power of 40 W. The surface temperature of the cell was monitored by a K-type thermocouple. The thermocouple was connected at the center of the vertical section of the battery cell using a steel ring.
Process Safety and Environmental Protection, Volume 214, 2026, 109134: Fig. 2. Setup of the experimental methods including particle capture and further analysis.
The interior of the testing chamber is continuously exposed to outdoor air through a 10 mm diameter opening equipped with a HEPA filter, ensuring the incoming air remains uncontaminated. This connection maintains the interior of the test chamber at atmospheric pressure throughout the experiments. For the homogenous distribution of particles within the volume of the testing chamber, two PC fans are placed in both vertical and horizontal directions to mix the air with the particles inside the test chamber. During the TR event, the gas and the particles leave the battery cell in an upward direction along the longitudinal axis of the battery cell, as shows Fig. 1. Big particles are captured on an impaction inflammable surface to protect the rest of the equipment in the test chamber. Sampling of particles for subsequent analysis is conducted in the upper portion of the test chamber. Inside the test chamber a high-frame-rate camera was placed to determine the length of TR and to capture images of TR phases.
3. Results and discussion
Visual documentation of the TR process is presented in Fig. 3, illustrating its distinct evolutionary phases. The left image shows phase II (the venting phase) and was captured after the gas release through a pressure valve few seconds before the jet flames appeared. The middle image captures the violent eruption of phase III and is the most exothermic part of TR. The image on the right shows the cooling period of phase III the heat release of particles which were deposited on a surface above the battery cell. The red flames are indicative of lithium combustion. From the video capture, the duration of phase III was quantified, with TR lasting 2.5 s, 24.3 s, 19.3 s, and 35.3 s in Tests 1–4, respectively. These results highlight the large variability in TR timescales. All measured durations remained well below the total particle sampling interval.
Process Safety and Environmental Protection, Volume 214, 2026, 109134: Fig. 3. Capture of TR phases: (a) venting phase, (b) violent eruption and (c) cooling of battery cell No.2.
The surface temperature profiles of the battery cells clearly marked the initiation of Phase III immediately prior to the rapid thermal spike. For battery cell No. 1, the initiation temperatures were recorded at 174.5, 169.3, and 164.7 °C for tests 1, 3, and 5, respectively, with corresponding maximum temperatures peaking at 392.4, 389.9, and 467.5 °C. Conversely, battery cell No. 2 exhibited Phase III initiation temperatures of 186.9, 176.0, and 189.0 °C during tests 2, 4, and 6, respectively, subsequently reaching maximum surface temperatures of 432.9, 416.6, and 307.3 °C. The temperature profiles confirm TR stochasticity: stable initiation temperatures contrast with unpredictably fluctuating maximums. This physical randomness drives the variability in total aerosol mass, proving that generalizability lies in the thermodynamic mechanisms, not exact mass totals.
3.3. Morphology
The morphology of the collected particles was investigated using SEM. The particles utilized for tests 5 and 6 were once again captured through the use of a cascade impactor. The duration of this process was 15 s, with an airflow rate of 10 liters per minute, ensuring the formation of a thin layer of particles on the aluminum foils. The SEM images from tests 5 and 6 are displayed in Fig. 6. These images offer insights into the morphology of the particles and the structure of the agglomerates that formed. For the analysis, stages 6 and 14 were utilized. Stage 6 exhibited a size range of 94 nm to 152 nm. Stage 14 ranged from 3.62 μm to 5.32 μm.
Process Safety and Environmental Protection, Volume 214, 2026, 109134: Fig. 6. SEM images of particles collected on foils by cascade impactor a) stage 14 battery cell No.2; b) stage 14, battery cell No.2; c) stage 6, battery cell No.2; d) stage 6, battery cell No.2.
It is evident that each stage of the impactor collects particles with distinct morphologies, indicative of the alterations in chemical composition, as illustrated in Fig. 6. Particles measuring below 1000 nm exhibited higher lithium (Li) content, while larger particles contained higher heavy metal content. The images obtained from stage 14 reveal a non-uniform distribution of particles, with smooth spherical particles being juxtaposed against rough surfaces. The smooth spheres form via localized melting of specific metallic components (Cu, Fe) during peak temperatures, followed by rapid quenching upon ejection. Conversely, the rough, irregular particles originate from the mechanical fragmentation of non-melted materials. Furthermore, particles from the sixth stage demonstrated a propensity for agglomeration, resulting in the formation of plate-like structures. In comparison, the composition of stage−6 particles exhibits greater uniformity than that of stage−14 particles. The primary finding derived from Fig. 6 is the morphological confirmation of two distinct particle generation mechanisms: the submicron agglomerates are formed via the condensation of vaporized elements, whereas the coarser particles (> 1 μm), irregular particles are generated through the mechanical fragmentation and ejection of solid cell components.
4. Conclusions
This study characterized the size distribution, elemental composition, and morphology of aerosol particles emitted during the thermal runaway of 18650 cylindrical lithium-ion batteries. While these cylindrical cells provide critical baseline insights, their specific high-velocity venting dynamics may differ from pouch or prismatic formats. To accurately assess these emissions, the aerosols were systematically collected and fractionated using a cascade impactor alongside single filters. This comprehensive methodology reveals the fundamental formation mechanisms of hazardous battery aerosols.
- Gravimetric analysis revealed a critical dependence of particle size distribution on cathode chemistry. NMC cells exhibited a unimodal distribution peaking in the submicron range (252–377 nm). In contrast, the more reactive NCA chemistry produced a bimodal profile (peaks at 152–252 nm and 0.94–1.62 μm), driven by its lower thermal stability and the subsequent violent mechanical ejection of coarser fragments during venting.
- Elemental distribution is strictly size-dependent and governed by thermodynamic boiling points. Volatile alkali metals (Li, Na) vaporize during peak TR temperatures and subsequently nucleate into ultrafine particles (< 500 nm). Conversely, heavy metals (Ni, Co) with high boiling points do not vaporize; they are predominantly ejected as larger (> 1 μm) solid or molten fragments.
- Particle morphology strongly correlates with chemical composition and formation pathways. Submicron particles exhibit uniform composition and tend to agglomerate into plate-like structures. Coarser fractions display extreme heterogeneity, containing a mix of rough, irregular fragments with smooth spheres (e.g., Cu, Fe), which indicate localized melting and rapid quenching.
- The emission profiles directly reflect the pristine cell composition, releasing substantial quantities of deeply respirable, hazardous heavy metals (Ni, Co, Cu) and lithium. While highly toxic trace metals (Pb, Cd) were largely undetected in fractionated samples, their occasional presence on single filters indicates risk of trace contamination.
While this study reveals the fundamental size-dependent vaporization mechanism of battery aerosols, several limitations must be acknowledged. Although the highly stochastic nature of thermal runaway inherently limits the exact quantitative repeatability of total aerosol mass emissions, the fundamental size-dependent elemental partitioning is driven by invariant thermodynamic properties, making this core mechanism generalizable even with a limited sample size per experimental setup. The current findings are specifically bound to the tested cylindrical cell formats and the semi-quantitative limits of SEM-EDS analysis.
Hazardous submicron aerosol emission is a universal feature of Li-ion battery cell thermal runaway. These findings are critical for accurately assessing the acute health hazards associated with battery fires and for driving the development of advanced safety measures to reduce these emissions at the source. Given the prevalence of these deeply respirable, toxic particles, immediate mitigation strategies are imperative. The mandatory use of Self-Contained Breathing Apparatus for first responders and the integration of robust HEPA filtration in indoor EV and battery storage facilities are essential to prevent deep-lung deposition and occupational exposure.
