MOS-based Gas Sensors for Early Alert of Thermal Runaway in Lithium-ion Batteries

1. INTRODUCTION

Lithium-ion batteries (LIBs) have been extensively used as high-capacity rechargeable energy systems in electric vehicles and portable electronic devices owing to their high energy density, light weight, and long cycling life [1-3]. However, their high energy density and flammability pose safety concerns. In particular, under abusive conditions such as physical damage, overcharging, and extreme temperatures, they can undergo thermal runaway [4]. This process involves a rapid increase in temperature and gas venting, which can lead to fire or explosion. Considering preventing thermal runaway remains challenging, current strategies focus on delaying its onset and offering early alerts to allow users to respond. Thus, developing a promising system for early-stage monitoring of thermal runaway is essential to ensure user safety.

Indicators of thermal runaway include changes in temperature, voltage, pressure, and gas composition. As the temperature of the LIB increases, the materials within the cell begin to decompose, generating various gases (e.g., CO, H₂, CO, and C_xH_y) [5,6]. The accumulation of flammable gases in a cell, accompanied by increased temperature and pressure, can eventually result in an explosion or fire. Therefore, early detection of these gases is a promising pathway for providing warnings of thermal runaway. Moreover, providing a sensor platform with fast response speed and high reliability is crucial. Metal-oxide-semiconductor (MOS)-based gas sensors are promising platforms for gas detection owing to their high sensitivity, fast response/recovery speeds, small size, and cost-effectiveness [7-12]. These advantages render them suitable for detecting gases generated during the LIB thermal runaway process, thereby enabling early-stage alerts.

In this review, we present the factors that induce LIB thermal runaway and the gas generation process and demonstrate H₂, CO, and C_xH_y as thermal runaway indicators. We propose MOS-based gas sensors for the reliable detection of these gases owing to their inherent advantages. In addition, we discuss the operating mechanisms, current research trends, and development strategies of MOS-based sensors to provide early warnings of LIB thermal runaway.

2. THERMAL RUNAWAY IN LI-ION BATTERIES

2.2 Generation mechanism of gases during thermal runaway

LIBs undergo multiple reactions during the thermal runaway. When the temperature of a cell or a specific region inside the cell reaches a critical range, the components, including the solid electrolyte interface (SEI), separator, anode, cathode, and electrolyte, begin to decompose, resulting in exothermic breakdown. The rates of these reactions are precisely related to the exothermic self-heating rate, which increases exponentially with increasing temperature. The thermal runaway process in LIB generally involves six reaction steps: decomposition of the SEI, reactions between the anode and electrolyte, melting of the separator, decomposition of the cathode, decomposition of the electrolyte, and reactions between the binder and active materials in the cathode and anode (Fig. 1) [4,15].

Fig. 1.

Reaction process and release of gases during LIB thermal runaway.

Decomposition of SEI: At the initial stage of thermal runaway, the temperature of the LIB continued to increase. When the temperature reaches 70–90°C, the thin passivating SEI layer (e.g., ROCOOLi, Li2CO3, and ROLi) on the anode starts decomposing exothermically and releases a large amount of heat and gases (e.g., CO₂, O₂, and C_xH_y) [16,17]. The following equation takes ROCOOLi as an example:

$\[ {\left(\mathrm{R}\mathrm{O}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{L}\mathrm{i}\right)}_2\to {\mathrm{L}\mathrm{i}}_2{\mathrm{C}\mathrm{O}}_3+{\mathrm{C}}_2{\mathrm{H}}_4\uparrow +{\mathrm{C}\mathrm{O}}_2\uparrow +\frac{1}{2}{\mathrm{O}}_2\uparrow \]$

(1)

Reaction between anode and electrolyte: When the temperature range reaches 120–140°C, the decomposition of the SEI layer is almost complete, leading to the direct exposure of the graphite anode to the electrolyte [18]. The heat generated from SEI decomposition causes a reaction between the intercalated Li ions in the anode and the organic solvents in the electrolyte, such as ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC), releasing flammable C_xH_y gases as follows [19-21]:

$\[ 2\mathrm{L}\mathrm{i}+{\mathrm{C}}_3{\mathrm{H}}_4{\mathrm{O}}_3\left(\mathrm{E}\mathrm{C}\right)\to {\mathrm{L}\mathrm{i}}_2{\mathrm{C}\mathrm{O}}_3+{\mathrm{C}}_2{\mathrm{H}}_4\uparrow \]$

(2)

$\[ 2\mathrm{L}\mathrm{i}+{\mathrm{C}}_4{\mathrm{H}}_6{\mathrm{O}}_3\left(\mathrm{P}\mathrm{C}\right)\to {\mathrm{L}\mathrm{i}}_2{\mathrm{C}\mathrm{O}}_3+{\mathrm{C}}_3{\mathrm{H}}_6\uparrow \]$

(3)

$\[ 2\mathrm{L}\mathrm{i}+{\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3\left(\mathrm{D}\mathrm{M}\mathrm{C}\right)\to {\mathrm{L}\mathrm{i}}_2{\mathrm{C}\mathrm{O}}_3+{\mathrm{C}}_2{\mathrm{H}}_6\uparrow \]$

(4)

The intensity of the reactions in the process correlates with the extent of Li ion insertion into the anode, releasing more heat with more pronounced reactions. Additionally, Li graphite reacts with the binder, resulting in heat generation.

Melting of separator: When the temperature reaches the melting point of the separator, the separator shrinks and melts, resulting in short circuits between electrodes. Commonly used separator materials include polyethylene (PE), polypropylene (PP), or ceramic-coated separators, which have melting points of ~135°C, 166°C, and 200°C, respectively [22]. This internal short circuit triggers the instantaneous release of a substantial amount of energy, exacerbating battery thermal runaway.

Decomposition of cathode: As the temperature increases and reaches a certain point during LIB thermal runaway, the active materials of the cathode become vulnerable to disproportionation, which causes their breakdown [23,24]. This process results in the release of significant amounts of reaction heat and oxygen, which induces the burning of the electrolyte and flammable gases inside the cell. Transition metal oxides such as LiCoO₂, LiMn₂O₄, LiFePO₄, and LiNiO₂ are commonly used as cathode materials. These decomposition reactions can be described using LiCoO₂ and LiMn₂O₄ as examples.

$\[ {\mathrm{L}\mathrm{i}}_{\mathrm{x}}{\mathrm{C}\mathrm{o}\mathrm{O}}_2\to {\mathrm{x}\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{o}\mathrm{O}}_2+\left(\frac{1-\mathrm{x}}{3}\right){\mathrm{C}\mathrm{o}}_3{\mathrm{O}}_4+\left(\frac{1-\mathrm{x}}{3}\right){\mathrm{O}}_2\uparrow \]$

(5)

$\[ {\mathrm{L}\mathrm{i}\mathrm{M}\mathrm{n}}_2{\mathrm{O}}_4\to {\mathrm{L}\mathrm{i}\mathrm{M}\mathrm{n}\mathrm{O}}_2+\frac{1}{3}{\mathrm{M}\mathrm{n}}_3{\mathrm{O}}_4+\frac{1}{3}{\mathrm{O}}_2\uparrow \]$

(6)

Decomposition of electrolyte: The decomposition of electrolyte salts (e.g., LiPF₆) accelerates when the temperature exceeds 150°C [25]. The generated PF₅ further accelerates the decomposition of the electrolyte and simultaneously reacts with H₂O, generating toxic HF. Using LiPF₆ as the electrolyte salt, the reaction is described as follows:

$\[ {\mathrm{L}\mathrm{i}\mathrm{P}\mathrm{F}}_6\to \mathrm{L}\mathrm{i}\mathrm{F}+{\mathrm{P}\mathrm{F}}_5 \]$

(6)

$\[ {\mathrm{P}\mathrm{F}}_5+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{P}\mathrm{O}\mathrm{F}}_3+2\mathrm{H}\mathrm{F}\uparrow \]$

(7)

Moreover, the electrolyte solvent (e.g., C₆H₄O₃) can react with the previously released oxygen at approximately 200°C, intensifying the thermal runaway by generating substantial heat and gases. During this process, considerable amounts of CO₂ and CO are released, as per the following equations:

$\[ \begin{array}{l}\frac{5}{2}{\mathrm{O}}_2+{\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{O}}_3\to 3{\mathrm{C}\mathrm{O}}_2\uparrow +{\mathrm{H}}_{2}O\\ \left(\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{ }\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\right)\end{array} \]$

(8)

$\[ \begin{array}{l}{\mathrm{O}}_2+{\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{O}}_3\to 3CO\uparrow +2{\mathrm{H}}_{2}O\\ \left(\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{ }\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\right)\end{array} \]$

(9)

As gas accumulates, the internal pressure of the battery cell increases rapidly, causing the battery to swell and eventually flushing the safety valve.

Binder reaction: When the temperature exceeds 260°C, binders can react with the anode or cathode, releasing a significant amount of H₂ and heat, as described in the following reactions (Eqs. 10–12) [26,27]. Commonly used binder materials include polyvinylidene fluoride (PVDF) and carboxymethyl cellulose (CMC), which stabilize the electrode structure.

$\[ -{\mathrm{C}\mathrm{H}}_2-{\mathrm{C}\mathrm{F}}_2\to -\mathrm{C}\mathrm{H}=\mathrm{C}\mathrm{F}-\mathrm{H}\mathrm{F}\uparrow \]$

(10)

$\[ -{\mathrm{C}\mathrm{H}}_2-{\mathrm{C}\mathrm{F}}_2+\mathrm{L}\mathrm{i}\to \mathrm{L}\mathrm{i}\mathrm{F}+-\mathrm{C}\mathrm{H}=\mathrm{C}\mathrm{F}-+1/2{\mathrm{H}}_2\uparrow \]$

(11)

$\[ \mathrm{C}\mathrm{M}\mathrm{C}-\mathrm{O}\mathrm{H}+\mathrm{L}\mathrm{i}\to \mathrm{C}\mathrm{M}\mathrm{C}-\mathrm{O}\mathrm{H}+1/2{\mathrm{H}}_2\uparrow \]$

(12)

These reactions are not completely independent and may occur spontaneously under certain conditions. Moreover, other reactions in LIB cells can cause thermal battery runaway. For instance, Galushkin et al. reported that the accumulation of H atoms in a graphite anode by repeated LIB cycles can cause complex exothermic reactions and generate massive amounts of H₂ and heat [28]. In addition, the combustion of graphite, the reaction between CO and H₂O, and the combustion of combustible gases can occur, releasing large amounts of CO₂, CO, or H₂.

3. METAL-OXIDE-SEMICONDUCTORBASED GAS SENSORS

Various gas-sensing techniques such as gas chromatography [31], fluorescent probes [32], photoacoustic spectroscopy [33], and non-dispersive infrared (NDIR) spectroscopy [34] have been investigated for detecting gases such as H₂, CO, and C_xH_y. Although these methods are highly accurate, they rely on complex, expensive, and bulky equipment, impeding the miniaturization and integration of sensors into portable applications. To overcome these challenges, MOS-based sensors have emerged owing to their distinct advantages, including easy integration, miniaturization, high sensitivity, fast response and recovery speeds, good reversibility, and cost effectiveness. In this section, we first discuss the gas-sensing mechanism of MOS-based sensors and introduce material design strategies for MOS-based sensors to detect thermal runaway indicator gases.

3.1. Gas-sensing mechanism of MOS-based gas sensors

MOS-based gas sensors gas sensors operate based on charge transfer during chemical reactions between surface oxygen species and analyte gases. MOSs are classified as n- and p-type semiconductors based on their major charge carriers (electrons for n-type semiconductors and holes for p-type semiconductors). At elevated temperatures (100–450°C), oxygen molecules are adsorbed on the MOS surface and form oxygen ion species (O₂^–, O^–, and O^2–) by taking electrons [35], resulting in the formation of electron depletion layers (EDLs) on the n-type MOS surface and hole accumulation layers (HALs) on the p-type MOS surface (Figs. 2 (a) and (b)). Upon exposure to reducing gases (e.g., CO, H₂, and CH₄), these gases are oxidized by ionized oxygen species, and electrons are injected into the EDL or HAL, allowing the resistance of the MOS to decrease (n-type) or increase (p-type) (Figs. 2 (c) and (d)). Therefore, the gas responses (S) are generally defined as R_a/R_g (n-type) or R_g/R_a (p-type), where R_a and R_g are the resistances of air and analyte gases, respectively. Other key evaluation parameters included selectivity, response/recovery times, and limit of detection (LOD). Selectivity, which refers to the ability of a sensor to accurately detect a target gas even in the presence of other interfering gases, is defined as the ratio of the target gas response to the interfering gas response (S_target/S_interferants). The response/recovery times are parameters used to evaluate the kinetics of the sensor upon exposure to the analyte gas. The response time is the time required to reach 90% variation in resistance when the sensor is exposed to the analyte gas. Conversely, the recovery time is the time required for the sensor to return to 90% of its initial resistance in the absence of analyte gas. The LOD, i.e., the lowest measurable concentration of analyte gases required for reliable detection, is calculated from the correlation of gas concentration and response using criteria including S > 1.2 or a signal-to-noise ratio > 3 [36,37]. In the following section, we introduce various MOS-based gas sensors for monitoring LIB thermal runaway and discuss methodologies for enhancing sensor performance.

Fig. 2.

(a) Gas response of Pd@In2O3 NFs sensor. Represented with permission from Ref. [43], Copyright (2022), American Chemical Society. (b) Synthesis process of F-127 assisted Pd/SnO2 NPs, and (c) Gas response of F-127 assisted Pd/SnO2 NPs (H: H2, E: C2H5OH, N: NH3, M: CH3OH, C: CO, AC: CH3COCH3, T: C7H8). Represented with permission from Ref. [44], Copyright (2024), Elsevier.

3.2. MOS-based gas sensors for thermal runaway monitoring

3.2.1. H₂ sensors

H₂ is a flammable and explosive gas with an explosion limit of 4% in air. H₂ is generated during LIB thermal runaway, mainly owing to the accumulation of H in the graphite anode and the reaction of Li with common electrode polymer binders (e.g., PVDF). Jin et al. overcharged a LiFePO4-graphite battery pack and used sensors to detect the gas generated during the LIB thermal runaway [38]. The results indicated that H₂ could be detected 639 s and 769 s earlier than smoke and flame, respectively, demonstrating that H₂ detection is essential for ensuring safety. Therefore, employing H₂ sensors with high performance, such as high response, high selectivity, and fast response/recovery speed, is a viable strategy for monitoring the early stages of battery thermal runaway.

Various MOSs have been considered as H₂ sensing materials. For instance, Mineo et al. fabricated WO₃ nanorods (NRs) via hydrothermal methods to detect H₂ with fast response speed and high reliability at 350°C [39]. The WO₃ NRs sensor exhibited a rapid response time of 3 s and a linear response-concentration correlation at various concentrations (2000–50000 ppm). Liu et al. used a honeycombed SnO₂ as an ultrasensitive H₂ sensor [40]. The proposed sensor exhibited a high response of 8.4 with fast response and recovery times of 4 s and 10 s, respectively. Although MOS-based sensors exhibit a high response and rapid response/recovery kinetics, their lack of selectivity hinders their practical use, demonstrating that further improvements are necessary. Recently, incorporating noble metals has attracted significant attention for improving the H₂ sensing performances of MOS-based sensors. In particular, Pd catalysts have been used in numerous MOSs, such as In₂O₃, WO₃, and SnO₂, to achieve a high H₂ response and selectivity owing to their ability to dissociate H₂ into H atoms [41,42]. For instance, Chen et al. prepared Pd-loaded In₂O₃ nanofibers via electrospinning and wet impregnation to detect H₂ with ultrahigh response and selectivity at room temperature [43]. The proposed sensor exhibited ultrahigh H₂ selectivity (S_H2/S_others> 7.4) and a significantly high response (S_H2 = 293.6) to 10,000 ppm of H₂ with short response and recovery times of 12 s and 23 s, respectively (Fig. 3 (a)). The intriguing H₂ sensing properties of Pd-In₂O₃ were driven by the catalytic effect of Pd on the dissociation of H₂ into H atoms and the expanded electron depletion layer due to the formation of a p-n heterojunction between p-type PdO and n-type In₂O₃. Moreover, our group reported Pd/SnO₂ nanoparticles (NPs) with maximized nanojunctions to achieve ultraselective and sensitive H₂ sensing properties [44]. The utilization of Pluronic F-127 in the synthesis of Pd/SnO₂ improved the dispersion and size control of the Pd nanoparticles, resulting in numerous heterojunctions between Pd and SnO₂ (Fig. 3 (b)). The as-synthesized Pd-SnO₂ with Pluoric F-127 exhibited ultrahigh selectivity (S_H2/S_others > 1,332.8) and significantly high response (S_H2 = 27,190) to 50 ppm of H₂ at 100°C, even with a short response time of 3 s (Fig. 3 (c)). The significantly improved H₂ sensing performance was attributed to the increased catalytic activity of Pd due to size reduction, which showed an H₂ spillover effect, and the numerous nanojunctions between Pd and SnO₂.

Fig. 3.

Gas-sensing mechanism of MOS-based gas sensors

3.2.2. CO sensors

CO, one of the most common gases generated during battery thermal runaway, is released during the decomposition of the SEI and electrolyte solvent and the combustion of flammable gases. Therefore, accurate and rapid CO detection over a wide concentration range is crucial for monitoring the early stage of LIB thermal runaway.

Various MOS-based sensors such as SnO₂, In₂O₃, WO₃, and TiO₂ have garnered significant interest with regard to CO discrimination. However, owing to the small molecular weight of CO, its interaction with MOS-based sensors results in a low response because it reacts with a relatively low amount of surface oxygen species compared to other analyte gases (e.g., C₂H₅OH and C_xH_y). To overcome this limitation, MOSs have been functionalized with transition metal oxides and noble metal catalysts to facilitate their surface reaction with CO. Sun et al. fabricated an In₂O₃/CuO nanospheres via hydrothermal and calcination methods to detect CO with high sensitivity (Fig. 4 (a)) [45]. Compared to the pristine In₂O₃, the In₂O₃/CuO gas sensor exhibited a 3.3-fold higher response to CO (SCO = 22.3 to 50 ppm CO) with a rapid response speed (response time = 2 s). The formation of p-n heterojunctions between p-type CuO and n-type In₂O₃ induces the expansion of the electron-depletion layer, resulting in a high CO response. Similarly, Zhao et al. incorporated Fe₂O₃ into In₂O₃ using a bimetallic-organic framework template to fabricate an effective CO sensor (Fig. 4 (b)) [46]. The In₂O₃/Fe₂O₃ core–shell nanotubes showed a response value of 33.7 to 200 ppm of CO with a response time of 26 s, demonstrating their practical use for real-time CO monitoring. They further assessed the CO selectivity over four interfering gases (CI₂, H₂S, NH₃, and SO₂), and In₂O₃/Fe₂O₃ exhibited a high selectivity (S_CO/S_others > 2.9). The core–shell structure maximized the formation of an n-n heterojunction between In₂O₃ and Fe₂O₃, contributing to increased charge carrier density, thereby promoting the charge transfer between the analyte and sensing materials. Zhou et al. fabricated Pt NPs-decorated SnO₂ needles for CO gas sensors (Fig. 4 (c)) [47]. The Pt/SnO₂ sensor exhibited a high response to CO (S_CO= 23.18 to 100 ppm CO) with response and recovery times of 15 s and 14 s, respectively. The increased CO response is attributed to the chemical and electronic sensitizations of the Pt NPs. Zhang et al. prepared PtAg NPs-loaded WO₃ NRs for use as CO sensors [48]. The PtAg@WO₃ sensor demonstrated a high response to CO (S_CO= 2.79 to 100 ppm CO) with a fast response/recovery time of 75/24 s at 160°C. The enhanced CO sensing characteristics can be attributed to the chemical and electronic sensitization of PtAg. Noble metal catalysts increase the number of surface oxygen species through the O₂ spillover effect, thereby improving the CO response. Furthermore, the formation of PtO_x results in an increased electronic depletion layer, leading to a larger charge transfer variation. Although many attempts have been made to achieve highly sensitive CO detection with a rapid response speed, MOS-based sensors still encounter low selectivity problems, which require further improvement.

Fig. 4.

(a) Gas response of In2O3/CuO based sensor. Represented with permission from Ref. [45], Copyright (2020), American Chemical Society. (b) Gas response of In2O3/Fe2O3 sensor. Represented with permission from Ref. [46], Copyright (2023), American Chemical Society. (c) Gas response of Pt/SnO2 sensor. Represented with permission from Ref. [47], Copyright (2018), Elsevier.

3.2.3. Hydrocarbon (C_xH_y) sensors

C_xH_y gases, such as CH₄ and C₂H₄, are produced during the decomposition of the SEI layer or formed during the reactions between the cathode and electrolyte. Thus, they can serve as indicators for the early-stage monitoring of LIB thermal runaway, although their proportions are not significantly higher than those of other gases (e.g., CO and H₂). Previous studies have reported developing various MOS-based sensors with a high response to C_xH_y by adding noble metal catalysts. These catalysts increase the number of surface oxygen species, thereby improving the oxidation of C_xH_y on the MOS surface. Yao et al. reported a Pd- SnO₂ nanoporous structure as a sensitive CH₄ sensor [49]. The sensor exhibited a high CH₄ response (S_CH4 = 17.6 to 3000 ppm CH₄) with rapid kinetics (3 s and 5 s for response and recovery, respectively) at 340°C. Furthermore, Ivanov et al. prepared Pt- SnO₂ NPs for the sensitive detection of C₂H₄ [50]. The sensor showed a high response (S_C2H4 = 9.8 to 100 ppm C₂H₄) at 450°C, while pristine SnO₂ exhibited a low response (S_C2H4 = 1.42 to 100 ppm C₂H₄) to C₂H₄. However, for the practical use of sensors, a high selectivity for the target gas over other gases should be realized. Moon et al. reported a sensitive and selective C₂H₄ sensor with a Pd-V₂O₅-TiO₂/In₂O₃ bilayer structure (Fig. 5 (a)) [51]. The sensor exhibited a high C₂H₄ response (S_C2H4 = 19.6 to 1 ppm C₂H₄) and selectivity (S_C2H4/S_others>4.5) at 325°C. The excellent sensing performance is attributed to the catalytic effect of the Pd-V₂O₅-TiO₂ overlayer, which facilitates the reforming of stable C₂H₄ into reactive CH₃CHO through a reaction known as heterogeneous Wacker oxidation. Similarly, Jeong et al. proposed a sensitive and selective C₂H₄ sensor using a Cr₂O₃/SnO₂ bilayer sensor [52]. Cr₂O₃/SnO₂ showed high response (S_C2H4 = 16.8 to 2.5 ppm C₂H₄) and selectivity to C₂H₄ (S_C2H4/S_others>4.9) at 350°C. The catalytic Cr₂O₃ overlayer converted the less reactive C₂H₄ into more reactive species while oxidizing highly reactive interferents (e.g., C₂H₅OH, HCHO, and C₃H₉N), thereby improving the C₂H₄ sensing properties. These results indicated that controlling gasreforming reactions using a catalytic overlayer is a promising method for achieving high sensitivity and selectivity for C_xH_y. Luo et al. proposed a ZnO/Pd@ZIF-7 core-shell structure as a selective CH₄ sensor (Fig. 5 (b)) [53]. This sensor achieved high selectivity of CH₄ over CO, NH₃, and NO₂ (S_CH4/S_others>8.0) and exhibited fast response/recovery kinetics (8.5/4.7 s) and good repeatability at 210°C. This high selectivity is explained by the adsorption of polar gases onto the ZIF-7 layers through which nonpolar CH₄ penetrates. Therefore, a molecular filtering layer can be adopted to achieve a high selectivity for C_xH_y.

Fig. 5.

(a) Gas response of Pd-V2O5-TiO2/In2O3 sensor. Represented with permission from Ref. [51], Copyright (2023), The Royal Society of Chemistry. (b) Gas response of ZnO/Pd@ZIF-7 sensor. Represented with permission from Ref. [53], Copyright (2023), American Chemical Society.

4. FUTURE PERSPECTIVES

Nanostructure design, catalyst loading, and composite formation have significantly improved the performance of MOS-based gas sensors, including their response, selectivity, and response/recovery kinetics. Various MOS-based gas sensors for detecting LIB thermal runaway indicator gases are summarized in Table 1. Nonetheless, challenges remain for their use in actual applications, such as LIB thermal runaway monitoring. In particular, issues such as high power consumption, lack of reliability, and susceptibility to water poisoning should be addressed to improve the practicality of MOS-based sensors for the real-time monitoring of LIB thermal runaway.

Table 1.

Summary of MOS-based gas sensors used for detecting LIB thermal runaway indicators

MOS-based sensors often require an external heater because they are activated at high temperatures (> 150°C), leading to increased device sizes and high power consumption. To address this issue, microelectrochemical systems (MEMS) have been incorporated into MOS-based sensors [54,55]. The MEMS sensor platform comprises a microheater, a sensing material (MOS layer), and interdigitated electrodes with a 3-dimensional geometry. This platform enables mass production and realizes miniaturized and energy-saving sensing devices. For instance, Cho et al. adopted the MEMS platform on a SnO₂-ZnO-based H₂S sensor, which operates at 260°C, to decrease the power consumption to 6 mW (Fig. 6 (a)) [56]. Moreover, developing novel sensors operating under visible light is considered a promising approach to achieving energy savings [57-60]. For example, our group adopted blue microlight- emitting diodes (μLEDs) onto a Pd-SnO₂ sensor to detect H₂ with an extremely low power consumption of 63.2 μW (Fig. 6 (b)) [61]. Although the H₂ response (S_H2 = 0.5 to 50 ppm H₂) was not that high compared to other previously reported MOS-based sensors, this result still demonstrates the potential capability of using μLED platforms to establish advanced gas-sensing techniques.

Fig. 6.

(a) Schematic illustration of MEMS-based SnO2-ZnO H2S sensor, and its gas sensing properties. Represented with permission from Ref. [56], Copyright (2017), American Chemical Society. (b) Schematic illustration and OM image of Pd-SnO2 sensor with μLEDs platforms, its gas sensing properties. Represented with permission from Ref. [61], Copyright (2024), Springer Nature.

To identify complex chemicals in the atmosphere, using multiple sensors with high selectivities toward different gases is essential. Pattern recognition technologies have enabled the precise discrimination of a mixture of gases in the atmosphere. Sung et al. fabricated 3×3 sensor arrays by loading different transition MOS (CuO, NiO, and Co₃O₄) and noble metal catalysts (Pt, Pd, and Au) onto SnO₂ NRs [62]. These nine sensors produced different eigengraphs for six gas species (CH₃COCH₃, C₇H₈, C₈H₁₀, C₂H₅OH, NH₃, and H₂S), which were analyzed using deep learning, achieving an accuracy of more than 99.5%. Similarly, Moon et al. prepared a sensor array comprising nine sensors using three types of MOS (SnO₂, WO₃, and In₂O₃) with various nanostructures and catalyst loadings (Fig. 7 (a)) [63]. The responses of the sensors to eight different analytes (CH₃COCH₃, C₆H₆, CO, C₂H₅OH, H₂S, NH₃, and NO) were mapped onto a color scale and analyzed using principal component analysis (PCA). This sensor array can discriminate between the chemical vapors of H₂S, NH₃, and NO, even under humid conditions. These results highlight that using multiple sensors combined with pattern analysis is advantageous for precise discrimination of various gases.

Fig. 7.

(a) PCA plots of thin films, Au functionalized thin films, Au functionalized villi-like structure thin films with PC1 and PC2 using responses of 8 different analyte gases. Represented with permission from Ref. [63], Copyright (2016), American Chemical Society. (b) Material characterization of Tb4O7/In2O3 sensor and its gas sensing properties under dry and humid conditions. Represented with permission from Ref. [64], Copyright (2020), Wiley

Water is also generated during the LIB thermal owing to the decomposition of the electrolyte. Water poisoning degrades the performance of MOS-based chemoresistors, as they can be adsorbed onto the surface of sensing materials and form hydroxyls, hindering the generation of surface oxygen species. Thus, developing humidity-tolerant gas sensors is crucial for monitoring LIB thermal runaway. Various approaches have been explored to achieve this goal, including introducing water-resistant materials and additives that act as hydroxyl scavengers. For instance, Jeong et al. proposed a Tb₄O₇ overlaid In₂O₃ as a highly humidity-tolerant acetone sensor (Fig. 7 (b)) [64]. The inherent hydrophobic properties of Tb₄O₇ prevent water from approaching the In₂O₃ sensing layer. Similar effects have been observed with other hydrophobic materials such as ZIF-8 [65], FluoroPel [66], and PDMS [67]. Yoon et al. demonstrated that CeO₂ NPs can enhance the humidity resistance of In₂O₃ sensors [68]. The regenerative oxidation/reduction of Ce⁴⁺ and Ce³⁺ facilitates the hydroxyl radical scavenging reaction—a phenomenon also observed in multivalent metals and metal oxides, including Pr [69], NiO [70], and CuO [45]. Accordingly, incorporating appropriate materials into MOS-based sensors could be a viable solution for achieving humidity-tolerant sensors.

5. CONCLUSIONS

Considering the thermal runaway process in LIB must be identified at an early stage to ensure user safety, gas detection provides a reliable pathway for early-stage alerts for LIB thermal runaway. Detecting gases generated during LIB thermal runaway, such as H₂, CO, and C_xH_y, was achieved using the MOS. In this review, we described strategies for enhancing the performance of gas sensors (e.g., sensitivity, selectivity, and response/recovery kinetics). These methods include nanostructuring, catalyst decoration, and composite design. The potential and future perspectives of MOS-based sensors were also discussed. The development of MOS-based sensors will open a reliable pathway for early detection and prevention of thermal runaway, significantly enhancing the safety and reliability of LIB.

Acknowledgments

S. M. Lee and S. J. Park contributed equally to this study. This work was financially supported in part by National R&D Program (RS-2024-00405016) through NRF (National Research Foundation of Korea), funded by the Ministry of Science and ICT, Republic of Korea. This work was also financially supported by “Cooperative Research Program for Agriculture Science and Technology Development (Project No. PJ01706703)”

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