Flowerlike CeO₂ used as novel adsorption material for removal of hydrogen fluoride gas from lithium-ion battery during thermal runaway

Abstract

Thermal runaway presents a significant challenge for large-scale application of lithium-ion batteries (LIBs), often leading to the release of flammable, explosive, and toxic gases. In this study, porous flowerlike cerium dioxide microspheres (FL-CeO₂) were investigated to eliminate hydrogen fluoride (HF) gas generated during thermal runaway. A dedicated test device and method were developed for this purpose. The FL-CeO₂ was synthesized via a hydrothermal method and coated onto nickel foam to fabricate a gas filter. During thermal runaway of a 5 Ah lithium iron phosphate (LiFePO₄) battery, the filter—loaded with 1.2 g CeO₂—achieved an instantaneous HF removal rate of up to 82.24% within approximately 40–50 s. X-ray photoelectron spectroscopy (XPS) results indicate that F⁻ ions replace O²⁻ ions in the CeO₂ lattice. Additionally, the potential for reusability of the CeO₂ microspheres was evaluated through multiple HF adsorption and desorption cycles. After 10 cycles, the regenerated CeO₂ microspheres retained a HF adsorption rate of 76.11%, demonstrating promising reusability.

Graphical abstract

Keywords

lithium-ion batteries (LIBs) / thermal runaway / ceria microspheres / hydrogen fluoride (HF) / regeneration

1 Introduction

Lithium-ion batteries (LIBs) have recently gained widespread popularity as energy storage solutions [1]. Although LIBs have great development prospects, safety concerns have become one of the main obstacles restricting their application and development. Many methods have been proposed to enhance the safety of LIBs, such as modeling for health estimation and enhancing the performance of separators to eliminate HF inside the battery [2,3]. However, LIBs are still susceptible to thermal runaway, which can lead to combustion and explosion under mechanical, electrical, or thermal abuse, and the consequences of such incidents are extremely serious [4–7].

During thermal runaway, in addition to the release of intense heat and pressure, a large quantity of dangerous gases is also generated. Current research on gas emission during LIB thermal runaway mainly focuses on their composition, formation mechanisms, and associated hazards. A lithium-ion battery consists of key components such as a cathode, an anode, a separator, and electrolytes [8]. Typically, the electrolyte is composed of a lithium salt, LiPF₆, dissolved in a carbonate solvent [9]. When temperatures within the battery exceed 220 °C, violent reactions can occur among the cathode, anode, electrolyte, and binder [4,10]. These reactions produce substantial volumes of flammable gases including CH₄, H₂, C₂H₄, CO₂, and CO, which are the major gases released by LiBs during thermal runaway [11–13]. Yuan et al. [14] tracked temperature escalations in LIBs of various chemistries using an accelerated rate calorimeter. They discovered that CO₂, CO, and H₂ were the primary gases released, accounting for over half of all emissions. Qin et al. [15] induced thermal runaway in a prismatic LFP battery pack and, using hydrogen probes and an FTIR device, discovered that the predominant gases emitted were H₂, CH₄, CO, CO₂, and C₂H₄. When flammable and explosive gases such as CO and H₂ mix with oxygen in a confined space, they can lead to violent explosions, especially under the high temperatures and open flame conditions.

The hazards of thermal runaway are not limited to flammable and explosive gases, as toxic gases such as hydrogen fluoride (HF) also pose significant dangers. Larsson et al. [16] examined thermal runaway in LIBs and reported the production of hazardous HF gas. Zhang et al. [17] found that CO and HF the primary toxic gases released during such events. The toxicity of HF gas is well-documented: it is extremely irritating to human skin and can penetrate deeply, causing necrosis inside the body, which is extremely difficult to treat. In addition, the occupational exposure limit of HF gas is only 2.5 mg/m³ over a 15-min period [11], suggesting that even low concentrations can cause serious harm. In fact, HF gas is primarily generated through the thermal decomposition of the electrolyte salt lithium hexafluorophosphate (LiPF₆) [18]. LiPF₆ remains thermally stable at 107 °C in dry conditions with water content under 10 parts per million. However, when water levels reach 300 parts per million, it begins to decompose at approximately 87 °C due to a heat-induced reactions with water vapor, producing phosphorous oxyfluoride (POF₃) and HF. Above 117 °C, HF and POF₃ continue to be released via two further reactions (Reactions 2 and 3), until LiPF₆ is completely transformed into lithium fluoride (LiF). The detailed generation process is as follows [19,20].

(1){\mathrm{L}\mathrm{i}\mathrm{P}\mathrm{F}}_6\to {\mathrm{L}\mathrm{i}\mathrm{F}+\mathrm{P}\mathrm{F}}_5

(2){\mathrm{P}\mathrm{F}}_5+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{P}\mathrm{O}\mathrm{F}}_3+2\mathrm{H}\mathrm{F}

(3){\mathrm{L}\mathrm{i}\mathrm{P}\mathrm{F}}_6+{\mathrm{H}}_2\mathrm{O}\to \mathrm{L}\mathrm{i}\mathrm{F}+{\mathrm{P}\mathrm{O}\mathrm{F}}_3 +2\mathrm{H}\mathrm{F}

From the above reactions, it can be seen that the presence of water vapor is very important, as it may also accelerate the formation of HF gas [21]. Although the amount of HF gas released during thermal runaway of a single lithium-ion battery is not much, the risk becomes significantly greater in the case of failure propagation. If a large lithium-ion battery pack undergoes thermal runaway, the emission of toxic gases can be extremely harmful, which has attracted great attention [17,22].

However, there are few studies on the rapid adsorption of toxic gases produced during the thermal runaway of LIBs. In recent years, CeO₂ material has emerged as a novel adsorbent for fluoride removal in the field of water purification due to its non-toxic nature and unique redox properties [23–32]. A comparison of various materials is presented in Table S1, in which the fluoride removal performance of CeO₂, commercially available Al₂O₃, and bone char is compared. It can be seen that the fluoride adsorption performance generally shows a positive correlation with specific surface area. Moreover, the morphology and pore structure of the material also affect its adsorption capacity, potentially leading to differences in performance. Due to the existence of the oxygen vacancies, which may facilitate Ce³⁺-F complexation and ion exchange, CeO₂ also has great potential for fluoride removal [48].

In addition, CeO₂ is known as a unique oxygen storage and release material, and it has demonstrated excellent catalytic activity toward CO. It has been effectively used in the adsorption of CO in automobile exhaust systems [33,34]. Sun et al. [35] developed a hydrothermal method to synthesize nearly uniform, flower-like CeO₂ microspheres with an open, three-dimensional, hollow porous framework, showing great potential for application in catalysis and adsorption.

In this work, a simple test device and method were proposed and designed to evaluate the adsorption performance of gases generated during the thermal runaway of LIBs. Flowerlike CeO₂ microspheres with a three-dimensional porous structure composed of nanosheets were synthesized via a hydrothermal method and successfully applied to the adsorption of toxic HF gas generated during the thermal runaway of LIBs.

2 Experiment

2.1 Synthesis of flowerlike CeOHCO₃ microspheres

All chemicals were purchased from Beijing Chemical Reagent Company and used without further purification. Glucose (0.01 mol), acrylamide (0.015 mol), and cerium nitrate hexahydrate (0.005 mol) were added to approximately 80 mL of deionized water and stirred with a magnetic stirrer until the solution became clear. Then, 25 wt% ammonia was added to this clear solution while stirring continuously, adjusting the pH to around 10. After maintaining agitation for 5 h, the solution was transferred to a Teflonlined autoclave and heated in a blast drying oven at 180 °C for 72 h. After the heating process, the autoclave was allowed to cool naturally, and the resulting orange-tinted suspension was centrifuged to separate the solid phase. The resulting solid was rinsed several times with both distilled water and pure ethanol, then dried at 60 °C for 12 h to obtain flowerlike CeOHCO₃ microspheres (Fig. S1 in Electronic Supplementary Material).

2.2 Synthesis of flowerlike CeO₂ microspheres

The prepared CeOHCO₃ microspheres were calcined in two steps to obtain flowerlike CeO₂ microspheres. First, the prepared product was calcined under an argon atmosphere in a tube furnace at 600 °C for 6 h. Subsequently, the resulting product was further heated in air at 400 °C for 4 h. In both steps, the temperature increased at a rate of 3 °C per min. (Fig. S1).

2.3 Preparation of flowerlike CeO₂ filter

Nickel foam was used as a support because it is resistant to high temperatures and non-flammable, making it well-suited for high-temperature environment generated during thermal runaway of LIBs. Flowerlike CeO₂ was coated onto the nickel foam to fabricate filters. First, a certain amount of flowerlike CeO₂ and 5 wt% (mass fraction) sodium carboxymethyl cellulose (CMC) were added to water and mixed uniformly by sonication for 30 min. Then, the nickel foam was cut into a round filter, and the mixed solution was evenly applied onto the surface. After the solution completely dried, the filter was pressed and held for 2–5 min to form the final structure. Filters coated with different masses of CeO₂ were prepared using the same procedure. As shown in Fig. S2, due to the flexibility of nickel foam and the presence of the binder, the prepared filter is highly flexible and can be folded easily for multiple times, making it suitable for a variety of application scenarios.

2.4 Characterization of materials

X-ray powder diffraction, conducted using a Bruker D8 advance, was employed to analyze the purity and phase composition of the samples. Structural features were observed using scanning electron microscopy (XL30s-FEG at 10 kV) and transmission electron microscopy (JEOL JEM-2100F), which provided insights into the morphology. To determine the surface area and porosity, nitrogen adsorption-desorption isotherm analysis was performed at 77 K using a Micromeritics ASAP 2460. Additionally, the elemental composition and oxidation states within the flowerlike CeO₂ structures were assessed through X-ray photoelectron spectroscopy using a Thermo Scientific K-Alpha device.

2.5 Adsorption experiment

A test device and experimental method were designed to evaluate the HF gas adsorption performance of LIBs during thermal runaway. A sealed box made of 304 stainless steel customized for the experiment. The top of the box was fitted with transparent tempered glass, allowing for observation of the thermal runaway reaction inside. At the rear of the box, a pressure sensor was installed to monitor internal pressure changes, and a set of copper conductors connected the test equipment both inside and outside the box, such as heating plates and thermocouples. As shown in Figs. S3(a) and S3(b), the lithium-ion battery was fixed on the heating plate inside the box, with thermocouples placed on the surface of the battery, the bottom of the heating plate, and the outlet for the thermal runaway gases to measure corresponding temperature changes. The dimensions of the box, as seen in Fig. S3(d), are 550 mm × 400 mm × 400 mm, with all air ports having an outside diameter of 6 mm. Additionally, a simple filter device was designed to securely hold the prepared nickel foam filter inside, as shown in Figs. S3(e) and S3(f).

Recently, the use of gas sensors alongside Fourier transform infrared spectroscopy (FTIR) has become a common practice for monitoring HF emissions from LIBs during thermal runaway [36–38]. While gas sensors tend to be less accurate and more prone to damage, they are far more convenient and suitable for measuring specific gases. Therefore, a gas sensor was selected for this experiment. The HF gas sensor (TB200B) from EC Sense GmbH in Germany was chosen, with a range of 0–500 mg/m³ and an accuracy of 0.1 mg/m³. The gas sensor was recalibrated before each experiment, and repeated tests show that the error could be controlled within a specific range.

The experimental setup, shown in Fig. S3(c), includes a simple filter device connected to the gas exhaust line of the lithium-ion battery during thermal runaway, with gas sensors positioned at the gas outlet. To prevent the potential explosion of the lithium-ion battery inside the box after thermal runaway, a vacuum pump was used to extract part of the air in the box (e.g., reducing the pressure to −50 kPa), and nitrogen was introduced to return pressure to normal levels (with a relative humidity of about 56%). All valves, except the exhaust port, were closed. Finally, an electric heating device was utilized to induce thermal runaway in the lithium-ion battery.

3 Results and discussion

3.1 Physical characterization of flowerlike CeO₂ microspheres

X-ray powder diffraction (XRD) was utilized to identify the crystalline phase of the synthesized cerium dioxide, as shown in Fig. S4. The phase of the calcination product was determined to be face-centered cubic cerium dioxide (JCPDS No. 34-0394) with the space group Fm3m (225) [35].

Both scanning and transmission electron microscopy were employed to observe the size and morphology of the specimens. The images, particularly Fig.1(a) and Fig.1(b), reveal that many of the CeO₂ particles are almost uniform spheres with flowerlike features. These spheres generally range from 1 to 3 µm in diameter. Further analysis, as detailed in Fig.1(c) and Fig.1(d), shows that these flowerlike spheres consist of numerous nanosheets, which resemble petals. The individual nanosheets are approximately 20–30 nanometers thick, and they interlace to form an open, porous structure.

Fig.1(e) and Fig.1(f) show the TEM images of CeO₂ microspheres. The plane spacing of the ordered fringes in Fig.4(h) is approximately 0.16 nm, which corresponds to the (311) lattice plane of CeO₂. Fig.2 shows the nitrogen adsorption‒desorption isotherm of the obtained CeO₂ microspheres, along with the corresponding BJH (Barret-Joyner-Halenda) pore size distribution curves. The material exhibits a type IV adsorption‒desorption pattern with an H3 hysteresis loop, as seen in Fig.2(a), indicative of its mesoporous nature. The area of Brunauer-Emmett-Teller (BET) surface area of the CeO₂ microspheres is approximately 129.8 m²/g. The average pore size, calculated from the nitrogen isotherm adsorption branch in Fig.2(b) using the BJH method, is 10.25 nm. The corresponding cumulative desorption volume from the BJH analysis is 0.29 cm³/g.

3.2 Adsorption experiment for the gas generated during thermal runaway of LIBs

All tested batteries were maintained at 100% state of charge, ensuring that the release of hazardous gases during thermal runaway was representative. Lithium-ion phosphate batteries were chosen for the thermal runaway experiments due to their high safety, making them almost impossible to explode. Additionally, they produce gases during thermal runaway, which is the most critical aspect of this experiment. A 5 Ah lithium-ion phosphate battery was chosen for the thermal runaway experiment, because it generates an appropriate concentration of HF gas. The pressure (≥ 4 kPa gauge) generated in the box by the lithium-ion battery thermal runaway gas ensures that the gases smoothly and quickly exhausted through the only open exhaust port. The filter was placed on the gas path, and the HF gas concentration was measured at the air outlet.

Initially, the experiment was conducted with four pieces of nickel foam filter without CeO₂ coating to rule out the possible effects of the nickel foam itself. As shown in Figs. S8(a) and S8(b), an experiment was also performed without the nickel foam, directly measuring the HF gas concentration from the LIBs thermal runaway. The maximum HF gas concentration is 388.5 mg/m³, suggesting that the presence of nickel foam does not interfere significantly with the experiment.

Next, 0.4 g of CeO₂ was coated onto 2 and 4 pieces of nickel foam, respectively, as filters to study the effect of the number of filters. Adsorption experiments were conducted by coating 0.4, 0.8, 1.2, and 1.6 g of CeO₂ microspheres onto 4 pieces of nickel foam to investigate the effect of mass on adsorption.

Moreover, regeneration and reuse experiments were performed on CeO₂ microspheres which had already undergone adsorption. The filters were sonicated in water for 1–3 h to obtain the CeO₂ coating, then placed in an oven at 60 °C for 2–5 h to remove moisture. The CeO₂-coated filter was then heated in a tube furnace at 800 °C for 60–120 min to remove the internal fluoride ions. Finally, adsorption experiments were carried out following the same method previously described.

Since the thermal runaway gas production of lithium-ion battery is a rapid and dynamic process, four pieces of filter, both without CeO₂ and with 1.2 g CeO₂ microspheres, were placed in the gas path for two groups of experiment. 4 L gas collection bags were used to collect gases generated during thermal runaway in order to estimate the gas production rate. As shown in Videos 2 and 3, the gas collection bags were filled in about 30 s. The estimated gas production rate for both two groups of experiments was approximately 133 mL/s, confirming that the pressure inside the box allows the gases to pass through the filters without any problems.

Fig.3 shows the thermal runaway process of the lithium-ion battery. In Fig.3(b), it can be seen that as the temperature rose, the battery started to expand due to the gas production at around 95 °C. From Fig.3(c)–Fig.3(e), it can be observed that when the temperature reached approximately 120–130 °C, the battery started to undergo thermal runaway and generated a large volume of gas within 40–50 s. Figure S6 shows the temperature changes of the gases measured by the thermocouple placed at the gas outlet during the thermal runaway of the lithium-ion battery. It can be seen that the gas temperature reaches a maximum of 88.9 °C.

The removal rate (R%) of HF gas at a given moment is calculated by

(4)R\mathrm{\%}=\frac{C_i-C_n}{C_i}\times 100,

where C_i and C_n are the HF gas detection concentration of the filters both without and with CeO₂ coating at a certain time respectively, mg/m³.

Experiments investigating thermal runaway in LIBs reveal that gas production during such events is dependent on the battery’s thermal state. As a battery enters full thermal runaway, gas generation accelerates abruptly. Monitoring typically involves tracking the battery’s surface temperature. A sudden spike in temperature, peaking at its maximum value, coincides with the most intense phase of thermal runaway. During this phase, hazardous gases are produced rapidly, with their concentrations peaking when the battery temperature reaches its zenith.

According to the experiments, LIBs experience thermal runaway between 120 and 130 °C, followed by the production of large amounts of gas. Since the venting valve remains open, the chamber pressure gradually rises and stabilizes between 4 and 7 kPa as the lithium-ion battery temperature reaches its maximum. At this point, the volume of released gas is at its largest, and the concentration of HF gas generated also peaks. Therefore, the ability of the CeO₂ material is evaluated to rapidly remove HF gas based on the detected HF concentration at this time. The highest instantaneous removal rate is calculated accordingly.

Fig.4(a) shows the HF concentration detected with four pieces of nickel foam filter without CeO₂ coating, reaching a maximum concentration of 349 mg/m³. Fig.4(b) and Fig.4(c) show the HF concentrations detected on two and four pieces of nickel foam filter, each coated with 0.4 g of CeO₂. The highest instantaneous HF concentrations are similar: 112.2 and 117.8 mg/m³, indicating little difference in filtration efficiency between the two and four filters. Therefore, in later experiments, four pieces of filter were used as the substrate, as they could hold more adsorbent material.

Fig.4(c)–Fig.4(f) show HF concentrations when 0.4, 0.8, 1.2, and 1.6 g of CeO₂ were coated on four pieces of nickel foam filter. The highest instantaneous HF concentrations are 117.8, 67, 61.3, and 50.7 mg/m³, with the corresponding instantaneous removal rates of 66.25%, 80.81%, 82.24%, and 85.47%, respectively.

Due to various factors, such as differences in the production of LIBs and the inherent randomness of the thermal runaway process, the concentration of thermal runaway gas produced by LIBs can vary. Therefore, the experiment was repeated by coating 0.4, 0.8, 1.2, and 1.6 g of CeO₂ on four pieces of nickel foam filters. As shown in Fig.4(g) and Fig.4(i), and Tab.1, the corresponding highest HF concentrations were 109.4, 73.8, 48.6, and 41.5 mg/m³, with instantaneous HF removal rates of 68.66%, 78.86%, 86.08%, and 88.11%, respectively. These results are basically consistent with the findings of the first experiment.

From the experiments, it can be seen that the rapid adsorption effect of the filters for HF gas improves as the mass of CeO₂ coating increases. However, the enhancement in the adsorption becomes less significant once the CeO₂ coating reaches a certain point. As shown in Tab.1, the increase in instantaneous HF removal rate is relatively small when the CeO₂ mass increases from 1.2 to 1.6 g (82.24% to 85.47%). Based on these results, 1.2 g of CeO₂ appears to be an optimal choice in terms of cost-effectiveness.

In addition to adsorption properties, the reuse of adsorbents is also crucial. To advance the potential application of CeO₂ materials for the rapid adsorption of toxic gases from LIBs during thermal runaway, it is important that they function effectively in high temperature environments. The ability of CeO₂ to regenerate at high temperatures was briefly investigated, with the hope that it would be useful for future experiments. Therefore, 1.2 g of CeO₂ coated on four pieces of filters, after one adsorption cycle, was regenerated and reused. The adsorbed CeO₂ was heated at 800 °C in air for 60 min to remove the fluorine element.

Fig.5(a) and Fig.5(b), along with Tab.2 and Tab.3, show the SEM images and corresponding EDX data after one adsorption and regeneration cycle of the flowerlike CeO₂ microspheres. These images reveal that the CeO₂ particles still maintained their spherical structure. However, the pores seem to be partially blocked by some substances, which may include CMC binders or products produced by the adsorption of thermal runaway gas of lithium-ion battery and vaporized electrolyte. In addition, the EDX data (Tab.2Tab.3, Figs. S2 and S3) show that the 60-min treatment at 800 °C effectively removed fluorine from the CeO₂ microspheres, thus achieving regeneration.

As shown in Fig.6 and Tab.4, the regenerated flowerlike CeO₂ microspheres still continue to demonstrate effective adsorption, with the highest HF detection concentration detected being 74.3 mg/m³ and the highest instantaneous HF removal rate reaching 78.71%. This simple investigation confirms that, after HF adsorption, the fluorine element can be removed by heating, enabling the regeneration of adsorbent. Furthermore, the regenerated CeO₂ maintains strong adsorption performance.

Based on the results from the above experiments, additional adsorption and corresponding regeneration experiments were conducted to further evaluate the reuse ability of flowerlike CeO₂. In this experiment, 0.8 g of CeO₂ was uniformly distributed on two pieces of nickel foam to create two filters which were used to adsorb HF gas produced during thermal runaway of a single 5 Ah lithium iron phosphate (LiFePO₄) soft-pack battery over several cycles, in order to investigate the adsorption capacity of the filters. As shown in Fig.7, the highest instantaneous HF gas concentrations observed during thermal runaway of the first to 10th 5 Ah LiFePO₄ soft-pack batteries were 65.5, 67.5, 75.9, 87.2, 109.8, 110.9, 125.1, 195.5, 208.6, and 220.2 mg/m³, respectively. The corresponding highest instantaneous removal rates were 81.23%, 80.66%, 78.25%, 75.01%, 68.54%, 68.22%, 64.15%, 43.98%, 40.23%, and 36.91%, respectively.

Repeated experiments are shown in Fig. S9, where the highest instantaneous HF gas concentrations during thermal runaway of the first to 10th 5 Ah LiFePO₄ soft-pack batteries were 62.8, 69.7, 74.1, 89.1, 101.8, 110.5, 137.6, 221.7, 234.6, and 242.5 mg/m³, respectively. The corresponding highest instantaneous removal rates were 82.01%, 80.03%, 78.77%, 74.47%, 70.83%, 68.22%, 60.57%, 36.48%, 32.78%, and 30.52%, respectively.

Following this, the 0.8 g of CeO₂, which had undergone multiple adsorption cycles, was placed in an air environment at 800 °C for 60–120 min for regeneration. Figure S10 shows the XPS data of the flowerlike CeO₂ after multiple adsorptions and regeneration. From Table S4, it can be seen that maintaining flowerlike CeO₂ at 800 °C for 60–120 min in air can effectively remove the fluorine element. After 120 min, the fluorine content inside the flowerlike CeO₂ was reduced to 0.25%, which was selected for resorption experiments.

As shown in Fig.8(a) and Fig.8(b), after one adsorption cycle, the flowerlike structure of the CeO₂ microspheres is still visible, though there are noticeable agglomerates inside. However, as observed in Fig.8(d) and Fig.8(e), after ten adsorptions, the pores of the flowerlike CeO₂ microsphere seem to be completely blocked, and the structure looks more like a solid sphere. Upon regeneration (Fig.8(f)), the flowerlike structure is retained after one adsorption, but after 10 adsorptions and regeneration (Fig.8(g), (h), (j), and (k)), the flowerlike CeO₂ structure is transformed into smaller particles, which aggregate into spherical formations. This transformation is likely due to the removal of the fluorine element from the CeO₂ lattice. After 10 adsorptions, the elemental fluorine content inside the flowerlike CeO₂ is higher (9.55%), which may account for the significant morphological changes upon regeneration. The mechanism of this structural transformation remains unclear and requires further investigation.

The 0.8 g of CeO₂ coated on two pieces of filters, after 10 adsorption cycles, was regenerated at 800 °C for 120 min. The HF gas concentration detection during the adsorption experiment after CeO₂ regeneration is shown in Fig.9. The highest instantaneous HF gas concentrations during the thermal runaway of the 1st to 3rd 5 Ah LiFePO₄ soft-pack batteries were 83.4, 127.6, and 130.7 mg/m³, respectively. The corresponding highest instantaneous removal rates were 76.11%, 63.44%, and 62.55%. These results provide further in-depth evidence of the regeneration and reuse potential of flowerlike CeO₂ for HF gas adsorption.

3.3 Possible adsorption mechanism analysis

CeO₂ typically has catalytic activity for CO oxidation above 200 °C [39]. However, as shown in Fig. S6, the gas temperature during thermal runaway can reach a maximum of 88.9 °C, well below 200 °C. Therefore, CO oxidation catalysis by CeO₂ is not considered relevant in this context. Related experimental studies will be conducted in the future.

The possible adsorption mechanism of flowerlike CeO₂ microspheres for HF gas in the high-temperature hazardous gas generated by LIBs during thermal runaway is briefly discussed. In the field of polluted water purification, both outer and inner sphere complexation are known to play an important role in fluoride removal by CeO₂ [40]. To further study the removal mechanism of flowerlike CeO₂ for HF gas, XPS was performed on CeO₂ samples before and after a single adsorption.

Fig.10(a) and Fig.10(e) show the 3D spectra of Ce before and after a single adsorption, respectively. The peaks at 885, and 903.4 eV correspond to the Ce³⁺ oxidation state (blue line), while the peaks at 882.5, 889, 898.9, 901.4, 908, and 917.2 eV correspond to the Ce⁴⁺ oxidation state (red line). This observation suggests a certain oxygen deficiency in the flowerlike CeO₂ structure [41]. The ionic radii of F⁻ (0.133 nm) and O²⁻ (0.140 nm) are nearly identical in size [42], implying that F⁻ ions from HF gas could displace O²⁻ ions in the cerium matrix via an ion exchange process. This ion exchange may disrupt the charge equilibrium, promoting the reduction of Ce⁴⁺ to Ce³⁺, which is confirmed by the increased proportion of Ce³⁺ in the flowerlike CeO₂ after adsorption (Table S5). Surface defects, particularly oxygen vacancies from the Ce³⁺/Ce⁴⁺ transition, significantly impact charge transfer and surface chemisorption, thus enhancing fluoride adsorption [43].

HF gas adsorption increases the proportion of Ce³⁺ within the ceria structure, indicating the formation of additional oxygen vacancies. Thus, it is likely that oxygen vacancies promote easier Ce³⁺-F complexation and ion exchange. When fewer oxygen vacancies are present, the ability of F⁻ to enter these oxygen vacancies and form complexes with Ce³⁺ is reduced, which may slow down the adsorption rate [45,46]. This phenomenon has also been reported previously [40,44].

The peaks at 529.4 eV in Fig.10(b) and Fig.10(f) correspond to the Ce-O band, mainly associated with surface lattice oxygen (Ce-O). It can be observed that the Ce-O ratio decreases after a single adsorption (Table S5), further supporting the idea that F⁻ replaces O²⁻ in CeO₂. Fig.10(h) shows the F1s XPS spectrum after adsorption by flowerlike CeO₂ microspheres, where the F 1s content increased from 0% to 3.36%. The fluorine peak at 684.1 eV, with binding energies between 684 and 685.5 eV, is characteristic of the metallic fluorine bond [45], which confirms the adsorption of fluorine on the flowerlike CeO₂ microspheres.

The flowerlike CeO₂ microspheres also have an open 3D porous structure with a large specific surface area and pore volume (129.8 m²/g, 0.29 cm³/g), which is conducive to gas transport.

Based on the following two premises: a low percentage of Ce³⁺ (e.g., below 20%) suggests that few oxygen vacancies are present, and the adsorbed substance is HF gas or other fluoride containing H⁺ ions, it can be inferred from the X-ray photoelectron spectroscopy results that the adsorption of HF gas by flowerlike CeO₂ occurs in two steps. As shown in Fig.11, first, HF gas molecules enter the channel of the 3D porous structure of flowerlike CeO₂, where they are physically adsorbed. This serves as the foundation for subsequent chemical adsorption. Second, as the F and H atoms in the HF molecules dissociate and migrate, some F⁻ ions enter the oxygen vacancies to complex with Ce³⁺. Meanwhile, the majority of F⁻ replace O²⁻ and bind to the cerium oxide lattice, releasing two electrons in the process. These free electrons initiate the reduction of Ce⁴⁺ to Ce³⁺, which then associates with F⁻. The H atoms from HF molecules bond to O atoms, leading to the production and release of H₂O molecules.

3.4 Discussion of potential adsorbent materials

Currently, few studies have focused on the rapid adsorption of toxic gases produced by LIBs during thermal runaway. When selecting adsorbent materials for such applications, three key factors should be considered:

(1) Temperature resistance: The adsorbent material must withstand high temperatures without desorbing. For example, carbon materials, though widely used for gas adsorption, can easily react in high-temperature environments, while NaF, commonly used for HF gas adsorption, desorbs above 250 °C.

(2) Economic considerations: The economic viability of the adsorbent material is crucial. This includes not only the cost of the material itself but also its regeneration performance and effectiveness. Alumina, for instance, is currently a promising material for HF gas adsorption due to its affordability and efficiency.

(3) Adsorption potential for major toxic gases: The material must exhibit strong adsorption capabilities for the toxic gases, such as CO and HF from LIBs during thermal runaway. CeO₂ is a promising candidate due to its ability to form oxygen vacancies, which enable it to oxidize CO to CO₂ above 200 °C [39]. The temperature required for this reaction can be further lowered when Pt-loaded catalysts are used on Cu-modified CeO₂ supports (below 100 °C) [47].

Given these considerations, future research on materials for the adsorption of toxic gases from LIBs during thermal runaway may focus on two areas points:

(1) Material selection: Materials like CeO₂, Al₂O₃, and molecular sieves should be further explored. These materials either have a well-established research base with wide industrial applications or exhibit unique potential for use in LIBs gas adsorption during thermal runaway. The complex nature of the gases emitted during thermal runaway necessitates an in-depth investigation into the adsorption capabilities of these materials, with an emphasis on their capacity to adsorb a variety of toxic gases.

(2) Material structure and modification: The structure and modification of the materials are crucial for enhancing their adsorption performance. Improving the specific surface area through optimal structural design can increase the contact area between the material and the gas, enhancing its adsorption capacity. For instance, ball milling techniques can effectively reduce particle size, increasing the surface area and porosity, thereby improving adsorption. However, smaller particle sizes may lead to agglomeration, which reduced the material’s activity and adsorption capacity [48]. For certain materials, such as flowerlike CeO₂, further ball milling may not be effective, as it could damage the structure and reduce the material’s specific surface area and porosity. Furthermore, loading materials like Cu, Mg, or Pt can enhance the material’s activity, further boosting its adsorption capacity.

4 Conclusions

This study investigates the rapid adsorption of HF gas generated during the thermal runaway of LIBs and the regeneration of the adsorbed CeO₂ microspheres. First, a simple test device and method were designed to evaluate the gas adsorption performance of LIBs during thermal runaway. Then, flowerlike CeO₂ microspheres were synthesized via a hydrothermal method. These microspheres possess an open 3D porous structure with a large specific surface area and pore volume (129.8 m²/g, 0.29 cm³/g¹), demonstrating significant adsorption capability for toxic HF gas produced during thermal runaway of LIBs.

For a 5 Ah LiFePO₄-based lithium-ion battery undergoing thermal runaway, the highest instantaneous removal rate of HF gas reached 82.24% using four pieces of nickel foam filter coated with 1.2 g of CeO₂. The adsorption process is driven by the ion exchange mechanism, where F⁻ ions in the HF gas replace O²⁻ ions in the cerium lattice. In addition, the regeneration and reuse capabilities of flowerlike CeO₂ microspheres were studied. The fluorine element can be effectively removed by maintaining CeO₂ filters at 800 °C for 60–120 min. After a single adsorption and subsequent regeneration, the highest instantaneous removal rate of HF was approximately 78.71% for the 1.2 g of CeO₂, and after 10 adsorption and regeneration cycles, the 0.8 g of CeO₂ maintained a removal rate of 76.11%.

This study provides valuable insights into the prevention of toxic gas diffusion during lithium battery fires, offering a potential solution for protecting firefighters’ respiratory health.

References

[1] Grey C P, Hall D S. Prospects for lithium-ion batteries and beyond—A 2030 vision. Nature Communications, 2020, 11(1): 6279

[2] ZhangX, Feng J, CaiF, et al. A novel state of health estimation model for lithium-ion batteries incorporating signal processing and optimized machine learning methods. Frontiers in Energy, 2024 , early access,doi:10.1007/s11708-024-0969-x

[3] Ding L, Chen Y, Sheng Y. . Eliminating hydrogen fluoride through piperidine-doped separators for stable Li metal batteries with nickel-rich cathodes. Angewandte Chemie International Edition, 2024, 63(51): e202411933

[4] Wang H, Xu H, Zhang Z. . Fire and explosion characteristics of vent gas from lithium-ion batteries after thermal runaway: A comparative study. eTransportation, 2022, 13: 100190

[5] Zhou W, Li G, Zhao H. . Experimental study of explosion parameters of hybrid mixture caused by thermal runaway of lithium-ion battery. Process Safety and Environmental Protection, 2023, 178: 872–880

[6] Wei G, Huang R, Zhang G. . A comprehensive insight into the thermal runaway issues in the view of lithium-ion battery intrinsic safety performance and venting gas explosion hazards. Applied Energy, 2023, 349: 121651

[7] Zhao L, Hou J, Feng X. . The trade-off characteristic between battery thermal runaway and combustion. Energy Storage Materials, 2024, 69: 103380

[8] Feng X, Ren D, He X. . Mitigating thermal runaway of lithium-ion batteries. Joule, 2020, 4(4): 743–770

[9] Wang Q, Mao B, Stoliarov S I. . A review of lithium ion battery failure mechanisms and fire prevention strategies. Progress in Energy and Combustion Science, 2019, 73: 95–131

[10] Larsson F, Bertilsson S, Furlani M. . Gasexplosions and thermal runaways during external heating abuse of commercia lithium-ion graphite-LiCoO₂ cells at different levels of ageing. Journal of Power Sources, 2018, 373: 220–231

[11] Bugryniec P J, Resendiz E G, Nwophoke S M. . Review of gas emissions from lithium-ion battery thermal runaway failure—Considering toxic and flammable compounds. Journal of Energy Storage, 2024, 87: 111288

[12] Claassen M, Bingham B, Chow J C. . Characterization of lithium-ion battery fire emissions—Part 1: Chemical composition of fine particles (PM_2.5). Batteries, 2024, 10(9): 301

[13] Claassen M, Bingham B, Chow J C. . Characterization of lithium-ion battery fire emissions—Part 2: Particle size distributions and emission factors. Batteries, 2024, 10(10): 366

[14] Yuan L, Dubaniewicz T, Zlochower I. . Experimental study on thermal runaway and vented gases of lithium-ion cells. Process Safety and Environmental Protection, 2020, 144: 186–192

[15] Qin P, Jia Z, Wu J. . The thermal runaway analysis on LiFePO₄ electrical energy storage packs with different venting areas and void volumes. Applied Energy, 2022, 313: 118767

[16] Larsson F, Andersson P, Blomqvist P. . Toxic fluoride gas emissions from lithium-ion battery fires. Scientific Reports, 2017, 7(1): 10018

[17] Zhang L, Duan Q, Meng X. . Experimental investigation on intermittent spray cooling and toxic hazards of lithium-ion battery thermal runaway. Energy Conversion and Management, 2022, 252: 115091

[18] Krebs R, Owens J, Luckarift H. Formation and detection of hydrogen fluoride gas during fire fighting scenarios. Fire Safety Journal, 2022, 127: 103489

[19] Yang H, Zhuang G V, Ross P N Jr. Thermal stability of LiPF₆ salt and Li-ion battery electrolytes containing LiPF₆. Journal of Power Sources, 2006, 161(1): 573–579

[20] Kawamura T, Okada S, Yamaki J. Decomposition reaction of LiPF₆-based electrolytes for lithium ion cells. Journal of Power Sources, 2006, 156(2): 547–554

[21] Larsson F, Andersson P, Blomqvist P. . Characteristics of lithium-ion batteries during fire tests. Journal of Power Sources, 2014, 271: 414–420

[22] Larsson F, Anderson J, Andersson P. . Thermal modelling of cell-to-cell fire propagation and cascading thermal runaway failure effects for lithium-ion battery cells and modules using fire walls. Journal of the Electrochemical Society, 2016, 163(14): A2854–A2865

[23] Kahya N, Erim F B. Removal of fluoride ions from water by cerium-carboxymethyl cellulose beads doped with CeO₂ nanoparticles. International Journal of Biological Macromolecules, 2023, 242: 124595

[24] Kang D, Yu X, Ge M. Morphology-dependent properties and adsorption performance of CeO₂ for fluoride removal. Chemical Engineering Journal, 2017, 330: 36–43

[25] Wang Y, Luo X. Efficient adsorptive removal of low concentration fluoride ions from water by cellulose beads with trapped CeO₂ nanoparticles. Journal of Environmental Chemical Engineering, 2024, 12(5): 113180

[26] Singh S K, Pahi S, Behera A. . Studies of hybrid ceria polypyrrole CPP-2 composite for effective defluoridation from potable water. Colloids and Surfaces. A, Physicochemical and Engineering Aspects, 2024, 702: 135026

[27] Liu X, Rehman D, Shu Y. . Selective fluoride removal from groundwater using CNT-CeO₂ electrodes in capacitive deionization (CDI). Chemical Engineering Journal, 2024, 482: 149097

[28] Wang T, Gang H, Cao Y. . Hydrous CeO₂ polypyrrole nanocomposite as a stable and efficient adsorbent for defluoridation of acidic wastewater. Journal of Materials Science, 2023, 58(19): 7895–7914

[29] Jian S, Shi F, Hu R. . Electrospun magnetic La₂O₃–CeO₂–Fe₃O₄ composite nanofibers for removal of fluoride from aqueous solution. Composites Communications, 2022, 33: 101194

[30] Chen F, Lv F, Li H. . Evaluation of fluoride adsorption in solution by synthetic Al₂O₃/CeO₂: A fixed-bed column study. Water Environment Research, 2021, 93(11): 2559–2575

[31] Wang K, Lei H, Muhammad Y. . Controlled preparation of cerium oxide loaded slag-based geopolymer microspheres (CeO₂@ SGMs) for the adsorptive removal and solidification of F^– from acidic waste-water. Journal of Hazardous Materials, 2020, 400: 123199

[32] Wang F, Wang K, Muhammad Y. . Preparation of CeO₂@ SiO₂ microspheres by a non-sintering strategy for highly selective and continuous adsorption of fluoride ions from wastewater. ACS Sustainable Chemistry & Engineering, 2019, 7(17): 14716–14726

[33] Wilklow-Marnell M, Jones W D. Catalytic oxidation of carbon monoxide by α-alumina supported 3 nm cerium dioxide nanoparticles. Molecular Catalysis, 2017, 439: 9–14

[34] Cha X, He J, Fu Y. . Carbonate-mediated Mars-van Krevelen mechanism on Cu_xO/CeO₂ catalysts for boosting CO oxidation. Chemical Engineering Journal, 2024, 501: 157643

[35] Sun C, Sun J, Xiao G. . Mesoscale organization of nearly monodisperse flowerlike ceria microspheres. Journal of Physical Chemistry B, 2006, 110(27): 13445–13452

[36] Liao Z, Zhang S, Li K. . Hazard analysis of thermally abused lithium-ion batteries at different state of charges. Journal of Energy Storage, 2020, 27: 101065

[37] Andersson P, Blomqvist P, Lorén A. . Using Fourier transform infrared spectroscopy to determine toxic gases in fires with lithium-ion batteries. Fire and Materials, 2016, 40(8): 999–1015

[38] Sun J, Li J, Zhou T. . Toxicity, a serious concern of thermal runaway from commercial Li-ion battery. Nano Energy, 2016, 27: 313–319

[39] Vincent J L, Crozier P A. Atomic level fluxional behavior and activity of CeO₂-supported Pt catalysts for CO oxidation. Nature Communications, 2021, 12(1): 5789

[40] Rashid U S, Das T K, Sakthivel T S. . GO-CeO₂ nanohybrid for ultra-rapid fluoride removal from drinking water. Science of the Total Environment, 2021, 793: 148547

[41] Seal S, Jeyaranjan A, Neal C J. . Engineered defects in cerium oxides: tuning chemical reactivity for biomedical, environmental, & energy applications. Nanoscale, 2020, 12(13): 6879–6899

[42] Slater J C. Atomic radii in crystals. Journal of Chemical Physics, 1964, 41(10): 3199–3204

[43] Vento-Lujano E, González L A. Defect-induced modification of band structure by the insertion of Ce³⁺ and Ce⁴⁺ in SrTiO₃: A high-performance sunlight-driven photocatalyst. Applied Surface Science, 2021, 569: 151044

[44] Ahmad S, Gopalaiah K, Chandrudu S N. . Anion (fluoride)-doped ceria nanocrystals: Synthesis, characterization, and its catalytic application to oxidative coupling of benzylamines. Inorganic Chemistry, 2014, 53(4): 2030–2039

[45] Nair S, Bose R, Durani S. . An eco-friendly and low-cost methodology for fluoride adsorption using synthesized and characterized ceria-impregnated activated graphite (CIAG). Chemicke Zvesti, 2024, 78(6): 3883–3906

[46] Pan Y, Liu X, Zhang M. . The impact of redox annealing on intrinsic properties and fluoride adsorption performance of CeO₂ nanomaterials. Chemical Engineering Journal, 2023, 476: 146347

[47] Liu X, Jia S, Yang M. . Activation of subnanometric Pt on Cu-modified CeO₂ via redox-coupled atomic layer deposition for CO oxidation. Nature Communications, 2020, 11(1): 4240

[48] Hamaloğlu K Ö, Tosun R B, Ulu S. . Monodisperse-porous cerium oxide microspheres as a new support with appreciable catalytic activity for a composite catalyst in benzyl alcohol oxidation. New Journal of Chemistry, 2021, 45(4): 2019–2029

RIGHTS & PERMISSIONS

Higher Education Press 2025