Research Progress on Thermal Runaway Warning Methods and Fire Extinguishing Technologies for Lithium-Ion Batteries

Abstract

Lithium-ion batteries (LIBs), valued for their high energy density, long lifespan, and low environmental impact, are widely used in electric vehicles (EVs) and energy storage. However, increased energy density has exacerbated thermal runaway (TR) issues, hindering large-scale applications. This paper systematically analyzes the mechanisms of TR and strategies for early warning and prevention to enhance battery safety. It begins by detailing TR mechanisms and their triggers, then reviews various TR early warning technologies, fire prevention methods, and the effectiveness and mechanisms of novel extinguishing agents such as hydrogels, perfluorohexanone, liquid nitrogen (LN), dry powder, and aqueous vermiculite dispersion (AVD). The study also explores advancements in new fire-retardant coatings for batteries. Finally, it summarizes current challenges and forecasts future research directions in battery technology. This review offers readers a clear, systematic overview of TR mechanisms, warning systems, and prevention technologies, providing comprehensive insights into TR management.

1. Introduction

As global concern for climate change and environmental awareness grows, electric vehicles (EVs) have become a strategic choice for green development. China leads the EV market, with lithium-ion batteries (LIBs) being a core component due to their high energy density and broad operating temperature range. However, as consumer demands for range and safety increase, so does the risk of thermal runaway (TR), posing a major challenge to industry growth.

Thermal runaway in lithium-ion batteries refers to the rapid temperature increase due to factors such as internal chemical reactions, overcharging, short circuits, physical damage, or high environmental temperatures. The process typically begins with an initial temperature rise within the battery, potentially caused by internal resistance heating or improper battery management. As the temperature continues to rise, undesired exothermic side reactions are triggered, releasing additional heat and causing the temperature to exceed 80 °C. This poses a serious threat to the safety of electric vehicles, potentially leading to fires or explosions and endangering the lives and property of passengers [1,2]. Thus, understanding TR mechanisms and developing effective safety management strategies are crucial for sustainable development of EVs. In recent years, significant progress has been made by researchers both domestically and internationally in this field, with the preliminary development of thermal runaway triggering mechanisms and safety warning systems, providing a solid theoretical foundation for the widespread adoption of new energy vehicles [3]. Additionally, while battery aging under prolonged use and complex conditions can increase safety risks, it does not necessarily imply a higher risk potential.

Despite advancements in material modification, structural design, and safety additives, the thermal runaway issue in lithium batteries remains unresolved, with incidents still occurring [4]. Lithium battery fires are characterized by high temperatures, significant toxic gas release, and extinguishing difficulties. Effectively controlling these fires is a current research focus. Scholars have explored water mist and sprinkling methods [5,6,7]. However, studies indicate that traditional firefighting techniques are often ineffective, particularly due to reignition and the inability to prevent thermal runaway propagation between battery cells.

The mechanisms of lithium-ion battery fires are complex, involving Class A, B, C, and D fires [8]. Class A fires involve solid materials, like wood, igniting due to high temperatures or open flames. When these materials absorb enough heat to reach their ignition point, they release flammable gases that combust in the presence of oxygen, causing the fire to spread. Class B fires are caused by the leakage of flammable liquids or gases, such as gasoline or propane. These substances vaporize at ambient temperatures, and if the vapor concentration is sufficient and meets an ignition source, combustion occurs. In some cases, these vapors can react with oxygen to rapidly trigger fires. Class C fires involve flammable gases like methane, originating from gas leaks that form combustible mixtures. When the gas concentration reaches explosive limits and encounters an ignition source, fires or explosions occur. Due to rapid gas dispersion, Class C fires can quickly spread, posing significant hazards. Class D fires arise from battery overheating caused by overcharging, short circuits, or physical damage, leading to thermal runaway. This results in electrolyte decomposition and reactions between electrode materials, releasing flammable gases and heat, which ignite in the presence of oxygen. Currently, researchers are developing novel fire extinguishing agents under consideration of the fire classes and their special characteristics to enhance the safety and reliability of lithium-ion batteries, thereby promoting sustainable development in the battery industry.

This article reviews TR mechanisms, warning technologies, fire prevention methods, challenges, and future directions. Section 2 analyzes the causes of thermal runaway, such as mechanical, electrical, and thermal abuse as well as internal short circuits. Section 3 reviews current thermal runaway warning technologies, including temperature, gas detection, machine learning, and ultrasound methods, discussing their pros and cons. Section 4 introduces new fire extinguishing agents, such as hydrogels, perfluorohexanone, and liquid nitrogen, detailing their effects and mechanisms. Section 5 explores advancements in battery fire-retardant coatings. Section 6 and Section 7 summarize the challenges in thermal runaway warning and prevention, and outline future research directions, providing a comprehensive understanding of TR management in LIBs.

2. Thermal Runaway Mechanism and Triggers

2.1. Thermal Runaway Mechanism

The thermal runaway (TR) mechanism in lithium-ion batteries is complex, typically initiated by internal short circuits or external impacts. These events can damage the separator, causing contact between the anode and cathode materials, which leads to a rapid release of heat. As temperature rises, the electrolyte decomposes and evaporates, releasing flammable gases and increasing the risk of fire and explosion. Additionally, high temperatures cause the cathode material to decompose, releasing oxygen that fuels further combustion of the electrolyte and anode, creating a vicious cycle. Furthermore, lithium metal deposition during overcharge or low temperatures increases internal resistance, generating more heat and heightening TR risk [9,10]. Figure 1 illustrates the TR mechanism in LiCoO₂ (LCO)/graphite batteries [11], highlighting reactions such as Solid Electrolyte Interphase (SEI) decomposition, anode–electrolyte reactions, separator collapse, cathode–electrolyte interactions, electrolyte decomposition, and anode-binder decomposition [12].

Under manufacturer-recommended operating and storage conditions, the failure rate of lithium-ion batteries is approximately 1 in 40 million [13]. However, unforeseen circumstances such as overcharging, external high temperatures, and mechanical damage can significantly increase the risk of failure. Despite various safety features in commercial lithium-ion batteries, notable incidents have occurred, impacting battery manufacturers and companies using this technology.

Table 1. Some LIB fire and explosion accidents [14].

No.	Date/Year	Description	Causes
1	2019	The McMicken energy storage facility in Arizona experienced a fire and explosion	The failure of the battery was attributed to the presence of lithium dendrites
2	2020	A brake failure incident involving the Tesla Model 3 resulted in a fire in China	A mechanical puncture led to a short circuit
3	2021	The explosion at an integrated optical storage and charging facility in China claimed the lives of three individuals	A short circuit in the battery was identified as the cause
4	2021	During the commissioning of the Tesla Megapack energy storage system in Australia, a fire broke out	A coolant leak was detected on the exterior of the battery compartment
5	2022	In California, the Vistra Energy battery pack was entirely destroyed by melting	The overheating of the battery was a result of a failure in the energy storage system
6	2022	An electric truck ignited during the charging process in China	A thermal runaway explosion occurred
7	2023	A fire and explosion occurred at a lithium-ion battery company in China	A thermal runaway event involving polymer lithium-ion batteries took place

lists some recent significant incidents caused by lithium-ion battery failures.

2.1.1. Solid Electrolyte Interphase (SEI) Decomposition

The Solid Electrolyte Interphase (SEI) forms due to interactions between the electrolyte and lithium metal or anode materials, effectively isolating the electrolyte from the anode and reducing side reactions. However, when temperatures reach 80–120 °C, the SEI layer, primarily composed of (CH₂OCO₂Li)₂, begins to decompose [15], as shown in Equation (1) [16].

$$({CH}_{2}OC{O}_{2}Li{)}_2\to {Li}_{2}C{O}_3+C_2{H}_4+C{O}_2+{\displaystyle \frac{1}{2}}{O}_2$$

(1)

2.1.2. Reactions Between Anode and Electrolyte

As temperature increases, reactions between the anode material (e.g., graphite) and the electrolyte intensify. This can result in lithium ions being released from the anode and reacting with the electrolyte, producing flammable hydrocarbons [17], as shown in the following Equations [16].

$$2Li+C_3{H}_4{O}_3\left(EC\right)\to {Li}_2{CO}_3+C_2{H}_4$$

(2)

$$2Li+C_4{H}_6{O}_3\left(PC\right)\to {Li}_2{CO}_3+C_3{H}_6$$

(3)

$$2Li+C_3{H}_6{O}_3\left(DMC\right)\to {Li}_2{CO}_3+C_2{H}_6$$

(4)

$$2Li+C_5{H}_{10}{O}_3\left(DEC\right)\to {Li}_2{CO}_3+C_4{H}_{10}$$

(5)

2.1.3. Separator Collapse

The separator is a crucial battery component that separates the cathode and anode, preventing short circuits. Conventional commercial LIB separators are polyolefin membranes made of polyethylene (PE) or polypropylene (PP). The melting points of PE and PP are 135 °C and 165 °C, respectively, while the specific melting point of PE/PP composites varies depending on the ratio and manufacturing process, typically falling between the two [18]. Under high temperatures, the separator material may soften or melt, causing structural collapse. This allows direct contact between the cathode and anode, forming a short circuit and significantly increasing the risk of thermal runaway [19]. To address these issues, ceramic separators or glass fibers can be incorporated, as they offer enhanced thermal stability and mechanical strength, reducing the likelihood of separator failure under high-temperature conditions.

2.1.4. Decomposition of Cathode Materials and Reactions with Electrolyte

As the first commercialized cathode material for lithium-ion batteries (LIBs), LCO (Lithium Cobalt Oxide) has relatively poor thermal stability. It tends to react with the electrolyte when the temperature reaches 200 °C to 250 °C [20,21,22]. This reaction not only degrades the cathode material, releasing significant O₂ [23], as shown in Equations (6)–(8) [16], but also oxidizes the electrolyte, releasing substantial CO₂, as shown in Equations (9) and (10) [16]. The thermal breakdown of the NCM cathode is less severe compared to that of the LCO cathode. The predominant reaction occurring during the decomposition of NCM is the reduction of Ni⁴⁺ to Ni²⁺, as Ni⁴⁺ exhibits greater reactivity than Co⁴⁺ [24].

$${Li}_{x}Co{O}_2\to xLiCo{O}_2+{\displaystyle \frac{1}{3}}(1-x)C{o}_3{O}_4+{\displaystyle \frac{1}{3}}(1-x)O_2$$

(6)

$${{Co}_{3}O}_4\to 3CoO+{\displaystyle \frac{1}{2}}{O}_2$$

(7)

$$CoO\to 3Co+{\displaystyle \frac{1}{2}}{O}_2$$

(8)

$$EC+O_2\to 3C{O}_2+2H_{2}O$$

(9)

$$DEC+6O_2\to 5C{O}_2+5H_{2}O$$

(10)

LiFePO₄ is considered a relatively safe cathode material [25,26,27], with its enhanced safety attributed to the strong P–O covalent bonds in the (PO₄)³⁻ polyanion [28]. While some researchers suggest that LiFePO₄ does not release oxygen at high temperatures, others argue that LiFePO₄ may decompose, despite the lack of definitive evidence [29]:

$$2{\mathrm{L}\mathrm{i}}_0\mathrm{F}\mathrm{e}\mathrm{P}{\mathrm{O}}_4\to {\mathrm{F}\mathrm{e}}_2{\mathrm{P}}_2{\mathrm{O}}_7+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2$$

(11)

2.1.5. Electrolyte Decomposition Reactions

When the internal battery temperature exceeds 200 °C, the electrolyte decomposes, Refs. [30,31,32,33,34] producing flammable gases (such as fluorides and hydrocarbons) and heat, as shown in the following reactions [16].

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

(12)

$$H_{2}O+P{F}_5\to PO{F}_3+2HF$$

(13)

$$DEC+P{F}_5\to C_2{H}_{5}OCOOP{F}_4+HF+C_2{H}_4$$

(14)

$$C_2{H}_4+HF\to C_2{H}_{5}F$$

(15)

$$C_2{H}_{5}OCOOP{F}_4\to P{F}_{3}O+C{O}_2+C_2{H}_2+HF$$

(16)

$$C_2{H}_{5}OCOOP{F}_4\to P{F}_{3}O+C{O}_2+C_2{H}_{5}F$$

(17)

$$C_2{H}_{5}OCOOP{F}_4+HF\to P{F}_{4}OH+C{O}_2+C_2{H}_{5}F$$

(18)

2.1.6. Reactions Between Anode and Binder Decomposition

When the battery temperature exceeds 260 °C, binders (e.g., polyvinylidene fluoride) react with anode materials or Li_xC₆, producing gases like H₂ and fluorides [35]. The primary product is Co₃O₄ [36], along with oxidation of some solution substances [37], as shown in the following reactions [16].

$$4\mathrm{L}\mathrm{i}\mathrm{C}{\mathrm{o}}^{\mathrm{Ⅲ}}{\mathrm{O}}_2\to \mathrm{C}{\mathrm{o}}^{\mathrm{Ⅳ}}{\mathrm{O}}_2\mathrm{C}{\mathrm{o}}^{\mathrm{Ⅱ}}\mathrm{C}{\mathrm{o}}_2^{\mathrm{Ⅲ}}{\mathrm{O}}_4+2\mathrm{L}\mathrm{i}\mathrm{O}\stackrel{4\mathrm{H}\mathrm{F}}{\to }4\mathrm{L}\mathrm{i}\mathrm{F}+2{\mathrm{H}}_2\mathrm{O}$$

(19)

$$-C{H}_2-C{F}_2\to \stackrel{base}{\to }-CH=CF-+HF$$

(20)

$$-C{H}_2-C{F}_2+Li\to LiF+-CH-+0.5H_2$$

(21)

2.2. Causes of Thermal Runaway

Thermal runaway is typically induced by mechanical, electrical, or thermal abuse conditions, as well as internal short circuits, either individually or combined, as shown in Figure 2 [38]. This includes mechanical abuse like collision, compression, and puncture [39]; electrical abuse such as overcharge [40] and over-discharge [41]; thermal abuse from overheating; and internal short circuits from direct contact between electrodes. Initially, mechanical triggers cause structural damage, leading to internal short circuits or electrolyte leakage, activating electrical triggers. As internal temperature rises, reaction rates increase, producing more heat. At this point, thermal triggers emerge, with external or self-heating further raising the temperature. Accumulated gases may increase pressure, potentially causing casing rupture, leakage, fire, or explosion.

2.2.1. Mechanical Abuse

During vehicle operation, external mechanical abuses such as collisions, compression, vibration, and nail penetration, as well as temperature fluctuations, can significantly impact the battery pack. Collisions exert strong forces that may cause casing rupture or internal damage, leading to short circuits, electrolyte leakage, and rapid temperature rise. When temperatures exceed safe limits, accelerated chemical reactions result in thermal runaway. Displacement and breakage of components post-collision further increase risks [10].

Compression, another key mechanical abuse, occurs under external pressure, potentially deforming the casing and altering internal interactions, causing structural damage and electrode short circuits [42]. Inadequate design or material selection can exacerbate these risks. Moreover, compression-induced structural damage impairs thermal management, expediting thermal runaway.

Vibration, especially in electric vehicles, can lead to material fatigue and aging, loosening connections and increasing short circuit risks. It compromises structural integrity and thermal management, potentially causing local overheating and thermal runaway [43]. Thus, optimizing battery mounting and damping is crucial during vehicle design.

The emerging nail puncture test simulates extreme conditions where sharp objects penetrate the battery, causing short circuits and thermal runaway. This results in sudden current spikes and local overheating, accelerating chemical reactions, gas production, and increasing fire risks [39].

Temperature changes can also trigger mechanical abuse. Heat generated during charging and discharging, along with rapid temperature fluctuations, may cause thermal expansion or contraction, creating internal stress and microcracks. This affects battery lifespan and heightens thermal runaway risk [44].

2.2.2. Electrical Abuse

In electric vehicle battery management, electrical abuse is a key factor leading to thermal runaway. Electrical abuse includes overcharging and over-discharging, significantly increasing the risk of thermal runaway and threatening vehicle safety.

Overcharging occurs when charging systems fail to accurately monitor battery status or use improper algorithms. This causes the battery voltage to exceed design limits, intensifying chemical reactions and leading to electrolyte decomposition and gas production. Increasing internal pressure can result in swelling or rupture, escalating thermal runaway risks [40]. In lithium-ion batteries, overcharging can lead to lithium deposition on electrodes, forming metallic lithium and increasing short circuit risks.

Over-discharging is another critical aspect of electrical abuse. Discharging below safe thresholds can destabilize internal chemical components. In lithium-ion batteries, this may cause irreversible chemical changes in electrode materials, structural damage, capacity loss, and even dendritic lithium formation, which can pierce separators and cause short circuits and thermal runaway [41]. Damage from over-discharging not only shortens battery lifespan but also poses significant safety threats during subsequent charging.

2.2.3. Thermal Abuse

Thermal abuse refers to overheating due to inadequate temperature control during battery use. It includes factors like high-temperature operation, poor heat dissipation, and thermal buildup, all of which can push internal temperatures beyond safe limits, triggering thermal runaway.

High-temperature environments significantly contribute to thermal abuse [45]. Operating in such conditions accelerates internal chemical reactions, releasing excess heat. This is common in hot climates or during extreme driving conditions like rapid acceleration or climbing. The thermal management system may fail to dissipate heat effectively, causing temperatures to rise. When critical thresholds are exceeded, materials may decompose, releasing flammable gases and increasing fire and explosion risks. High temperatures can also trigger self-heating reactions, leading to irreversible safety incidents.

Poor heat dissipation is another key factor [46]. During charging and discharging, heat is generated, and if the cooling system is poorly designed or obstructed, heat will accumulate. Many electric vehicles use liquid or air cooling, but failures or inefficiencies can lead to thermal runaway. Battery pack arrangement and material choices also impact heat dissipation; improper design can cause localized overheating and thermal runaway.

2.2.4. Internal Short Circuit

An internal short circuit occurs when electrodes within a battery directly connect, causing abnormal current surges and thermal runaway [47]. This typically results from changes in the battery’s physical and chemical state rather than a single factor.

During battery operation, the insulating separator between the electrodes is critical in ensuring current flow is restricted to the external circuit. Internal short circuits (ISCs) occur when the separator’s insulation fails, leading to direct electrode contact and forming a short-circuit pathway. Accurate ISC detection is essential for providing early warning signals to identify potential issues and prevent safety hazards. However, challenges arise due to the transient nature of ISCs and the complex chemical and physical environment within the battery [48]. Internal short circuits can trigger thermal runaway, wherein electrolyte decomposition generates flammable gases, rapidly increasing internal pressure and heightening the risk of rupture or leakage. In severe cases, an ISC may propagate through thermal conduction, compromising the safety of the entire battery pack. Therefore, developing effective ISC detection methods is crucial for enhancing the overall safety of battery systems.

Internal short circuits are common precursors to thermal runaway, often following mechanical, electrical, or thermal abuse. Causes can be categorized as follows: (1) Manufacturing defects, such as minor separator imperfections, material impurities, or uneven electrode structures, may lead to electrode contact during battery production, increasing the risk of internal short circuits over time. (2) Dendrite formation during charging can result in lithium or copper dendrites that may penetrate the separator and contact the cathode, causing shorts. (3) External impact or penetration such as impacts, drops, or punctures can damage battery structure, leading to direct electrode contact and triggering internal short circuits. (4) Separator melting due to high temperatures or overcharging can result in direct anode–cathode contact, significantly raising the risk of internal shorts [49].

3. Thermal Runaway Warning Technology

Thermal runaway warning technologies are crucial for ensuring battery safety, extending lifespan, and enhancing overall system reliability. Effective warning systems can monitor battery status in real time and respond swiftly to abnormal conditions, safeguarding vehicle and passenger safety and reducing accident likelihood. The relevant planning requirements are as follows:

• High sensitivity and real-time capabilities to quickly detect internal temperature changes and gas concentration fluctuations, providing timely warnings for response.
• Sensors must be stable, heat-resistant, and interference-resistant and should be strategically placed at critical battery locations for comprehensive monitoring.
• Advanced data processing and analysis capabilities, integrating sophisticated algorithms and machine learning models, are essential to extract key features and reduce false alarms and missed detections.
• The system should have strong compatibility and scalability, enabling seamless integration with the battery management system (BMS) and adaptation to technological upgrades and diverse application needs.

3.1. Early Warning Technology Based on Temperature Detection

Temperature detection is crucial in battery safety management, aiming to monitor temperature changes in real time to identify potential thermal runaway risks. Various sensor technologies are employed, such as thermocouples [50], PT100/PT1000 resistance temperature sensors [51], thermistors (NTC/PTC) [52], infrared sensors, liquid crystal thermography, and fiber optic sensors. These can measure surface and internal temperatures accurately through contact or non-contact methods.

Thermocouples, with their wide temperature range and fast response, are suitable for high-temperature environments but are sensitive to electromagnetic interference due to their small output signal. Yang et al. [53] developed a three-level warning thermocouple sensor based on temperature changes, demonstrating efficiency and rapid response. However, during thermal runaway, the temperature difference between the inside and outside of the battery can lead to delayed sensor response, affecting warning accuracy. PT100 and PT1000 sensors are favored for their linearity and stability, ideal for medium- to low-temperature precision measurements, but are slower and limited at high temperatures. Thermistors offer high sensitivity for precise temperature detection in rapidly changing environments, though their range and linearity can be affected by material properties. Infrared temperature sensors play a crucial role in battery warning systems due to their real-time, non-contact temperature measurement capabilities. Typically, their accuracy ranges from ±0.5 °C to ±2 °C. For high-demand applications like new energy vehicles or energy storage systems, sensors with an accuracy within ±1 °C are preferred to ensure sensitive temperature response and battery safety [54]. These sensors have a fast response time, usually from a few milliseconds to several tens of milliseconds, allowing them to quickly detect rapid changes in battery surface temperature. For instance, in the case of abnormal battery heating, they promptly relay temperature data to trigger protective systems, effectively mitigating safety risks [55]. Fiber optic temperature sensors are highly valued in battery warning systems for their resistance to electromagnetic interference, high sensitivity, and adaptability to complex environments. Utilizing optical principles such as Bragg gratings and Raman scattering, they achieve high-precision measurements, typically ranging from ±0.1 °C to ±1 °C, depending on sensor type and environmental conditions. Their response time is extremely short, often at the millisecond or even sub-millisecond level, enabling real-time detection of temperature changes, especially during rapid temperature increases during charge and discharge processes, thereby promptly triggering warnings [56]. Additionally, fiber optic sensors support distributed monitoring, allowing multi-point temperature measurement along a single fiber, making them ideal for thermal management in large battery packs. Their lightweight materials and low thermal inertia do not disturb the thermal environment, significantly enhancing monitoring accuracy and efficiency, and providing comprehensive thermal management support for battery packs [57]. Parhizi et al. [58] developed an internal temperature tracking model based on thermal conduction analysis, showing a temperature difference of up to 500 °C during thermal runaway. Xi et al. [59] investigated a real-time temperature monitoring method for solid-state batteries using fiber Bragg grating sensors, as shown in Figure 3. By embedding short fiber Bragg grating (sFBG) sensors within a battery, changes in the grating period and the effective refractive index of the fiber occur in response to external environmental factors such as temperature, strain, and pressure. This results in a shift in the Bragg wavelength. By detecting changes in the Bragg wavelength, it is possible to monitor the temperature and strain variations of the battery in real time during charge and discharge processes. Experimental results indicated a maximum temperature change of 0.42 °C during charge–discharge cycles and observed lithium dendrite formation. This study achieved in situ monitoring of internal temperature and strain in solid-state batteries, elucidating the strain generation mechanism at the microscopic level, which is crucial for battery safety.

3.2. Early Warning Technology Based on Gas Detection

During thermal runaway in lithium-ion batteries, a series of chain reactions occur, causing electrolyte solvent evaporation and gas generation. This significantly increases internal pressure, potentially leading to safety valve activation or casing rupture, releasing flammable gases [60]. Consequently, gas monitoring technologies have gained attention for early risk warning.

3.2.1. Single Gas Detection

When batteries overheat or fail, they often release specific gases like CO, H₂, CO₂, and hydrocarbons [61]. Changes in these gas concentrations can indicate internal anomalies. Monitoring a single gas can provide early warning signals; for instance, a significant rise in H₂ levels may suggest a short circuit or thermal runaway risk. Huang et al. [62] used Fourier Transform Infrared Spectroscopy (FT-IR) and H₂ probes to monitor gas release in real-time from 86 A·h LFP batteries during thermal runaway. The study found that the main gases emitted were CO₂ and H₂, comprising 30.15% and 39.5%, respectively, with H₂ released earlier than other gases.

3.2.2. Multiple Gas Detection

Monitoring a single gas often fails to accurately identify multiple potential faults, prompting researchers to explore combined gas monitoring strategies. By analyzing the concentration changes and relative proportions of multiple gases, more complex models are built to enhance fault detection accuracy and reliability. For example, integrating H₂ and CO concentration data allows for a more comprehensive assessment of thermal runaway risk, providing early warnings. Wang et al. [63] developed a lithium-ion battery high-temperature warning system based on gas detection, which issues alerts when CO and H₂ concentrations reach 120 × 10⁻⁴%. Liao et al. [64] developed an enhanced photoacoustic spectrometer based on a cantilever beam, and its performance was evaluated on NCM lithium-ion batteries (LIBs). Gases selected for early warning diagnostics included C₂H₄, CH₄, and CO. The average detection times for thermal runaway gases were 7.3 min at 50% state of charge (SOC) and 11.5 min at 100% SOC, under a heating rate of 5 °C/min, prior to triggering thermal runaway. These results align with the requirements of the Chinese national standard and the United Nations global technical regulations ECE/TRANS/180/Add.20, which mandate a warning time exceeding 5 min, thereby demonstrating the detector’s effectiveness. Han et al. [65] developed a non-dispersive infrared (NDIR)-based gas sensing system for real-time monitoring and early warning of thermal runaway (TR) in lithium-ion batteries. In overcharge tests with a 50 Ah LiFePO₄ (LFP) battery, the gas detector was positioned 12 cm above the vent. The vent was intentionally opened during TR to allow continuous gas release, enabling precise monitoring of gas composition and concentration without sensor saturation, ensuring consistent data collection. The system detected faults 25 s earlier than voltage changes with the vent closed (Figure 4a). With the vent open, gas concentration changes occurred around 580 s before the battery management system (BMS) alarm temperature (60 °C), significantly enhancing early warning capabilities (Figure 4b). Under normal abuse conditions, vents typically open later when the internal pressure reaches 5–15 bar and temperatures exceed 100 °C. The intentional vent opening in this study deviates from such conditions, necessitating contextual consideration. The study demonstrates the feasibility of using NDIR sensors for early TR detection. In real-world scenarios, the system’s effectiveness depends on the timing of gas release relative to TR dynamics. If vents open later, the detection window might be shorter than the observed 580 s, potentially reducing early warning lead time. Nonetheless, the significant 580 s advance before BMS alarm temperature under open vent conditions highlights the potential of integrating gas sensors as a supplementary safety mechanism to existing BMS systems. This approach enhances safety by detecting precursor gas emissions early in TR development, especially in scenarios where ventilation occurs earlier than expected due to design changes or unforeseen events.

3.3. Early Warning Technology Based on Machine Learning

In modern battery management systems, machine learning-based methods are critical for ensuring battery safety. By deeply analyzing multidimensional data during charge and discharge, these methods use real-time monitoring of parameters like voltage, temperature, and internal resistance to establish complex pattern recognition models. SOH prediction helps in understanding the degradation level of the battery, which is essential for identifying potential risks of thermal runaway. Initially, data preprocessing and feature extraction identify key features related to SOH and thermal runaway, providing high-quality input for machine learning algorithms. During model training, historical datasets containing thermal runaway events and SOH data help algorithms (such as support vector machines, random forests, or deep learning networks) learn to differentiate between normal and abnormal states, creating accurate classification models.

In operation, when real-time battery data are fed into the trained model, the system rapidly assesses potential thermal runaway risks and predicts SOH. If the model detects features similar to past events or a decline in SOH, it issues timely warnings for preventive action. To adapt to changing environments and battery conditions, the warning system can continuously optimize models through online learning, enhancing prediction accuracy and reliability. Hu et al. [66] developed an online method for estimating lithium-ion battery capacity using sparse Bayesian learning, as shown in Figure 5a. The method extracts five feature parameters during charging: initial charge voltage (x₁), constant current charge capacity (x₂), constant voltage charge capacity (x₃), final charge voltage (x₄), and final charge current (x₅). These parameters form a test vector (x_t) integrated into a Relevance Vector Machine (RVM) regression model for capacity estimation. A kernel function KK (e.g., Gaussian kernel) compares (x_t) with relevance vectors (x₁, x₂, …, x_m), and results are combined through weighted summation to estimate the capacity (h). The algorithm involves comparing the test vector with relevance vectors and evaluating the kernel function, culminating in a capacity estimate via weighted summation. This method is effectively applied in implanted medical devices for precise battery health management. Babaeiyazdi et al. [67] proposed a machine learning approach using electrochemical impedance spectroscopy (EIS) to predict the state of charge (SOC) of lithium-ion batteries in electric vehicles. As illustrated in Figure 5b, this method maps impedance data from EIS measurements to SOC using a linear regression model. A linear regression algorithm is used to identify the relationship between a dependent variable and one or more independent variables. In this case, the impedances at different frequencies serve as the independent variables, while the SOC is the dependent variable. The basic multiple regression model establishes that the dependent variable, represented as Y, is influenced by a set of k independent variables. In this context, each independent variable X_j contributes to the prediction of SOC based on its respective coefficient β_j, with the overall prediction also accounting for a Y-intercept β₀ and a random error component e_i for each case. The model assumes that errors are distributed with a zero mean and are independent across different cases. The diagram illustrates how the dependent variable y_i is derived from the weighted sum of the independent variables, where each weight is represented by the coefficients β, indicating the impact of each corresponding independent variable on the SOC prediction. Experimental results indicate that the method achieves a mean absolute error (MAE) of less than 4.9% and a root mean square error (RMSE) of less than 6%, demonstrating high predictive accuracy.

3.4. Early Warning Technology Based on Ultrasonic Detection

Ultrasound, with frequencies over 20 kHz, is characterized by high energy, strong penetration, good directionality, high sensitivity, and rapid propagation. As an emerging monitoring technology, it is increasingly applied in early warning research for thermal runaway. By exploiting ultrasound propagation in solids and liquids, this technique detects changes in gases or liquids to identify early signs of anomalies such as overcharging or short circuits. Sun et al. [68] developed a multi-frequency ultrasound-based nondestructive diagnostic technique. As shown in Figure 6, an ultrasound emitter probe converts electrical signals into mechanical vibrations using piezoelectric elements, generating ultrasound waves. These waves reflect, transmit, and attenuate through various battery materials, eventually being received and converted back into electrical signals by the ultrasound receiver probe. Analyzing these signals in both time and frequency domains allows for effective evaluation of battery operating conditions. Wang et al. [69] analyzed acoustic wave propagation characteristics within batteries to enable real-time monitoring of internal states and defects. The study demonstrated that ultrasonic technology offers significant advantages in battery state monitoring, electrolyte distribution analysis, and lithium deposition detection. Shen et al. [70] combined A-scan and 2D/3D total focusing method (TFM) ultrasonic techniques to monitor abnormal behaviors of lithium-ion batteries under overcharge conditions. Experimental results showed that ultrasonic detection could identify side reactions inside the battery at 102% SOC with a precision of 0.4% SOC. Additionally, 2D TFM imaging accurately located the reaction sites, providing an efficient solution for battery safety early warning.

Research indicates that before thermal runaway, gas generation inside batteries significantly alters their acoustic properties. Ultrasound sensors can monitor these changes in real time, providing a basis for timely warnings, thus ensuring battery safety. Additionally, analyzing ultrasound signals can assess battery health, further optimizing battery management strategies.

3.5. Other Detection and Warning Technologies

(1) Current and Voltage Monitoring: High-precision sensors continuously record battery current and voltage parameters during charge and discharge, effectively identifying abnormal fluctuations [71]. These changes often indicate unstable internal chemical reactions. Real-time data analysis and threshold detection provide early alerts to prevent thermal runaway. Gan et al. [72] developed a method for diagnosing over-discharge faults that integrates accurate laboratory data with real electric vehicle data. They identified features affecting voltage during discharge as parameters for eXtreme Gradient Boosting and established the diagnostic threshold by calculating the difference between actual and predicted voltage in real time to differentiate various levels of over-discharge. Zhang et al. [73] introduced a circuit topology based on ring symmetry. In the event of an internal short circuit (ISC) in a cell, a current meter placed in series within the branch circuit measures the charging current from the normal cell to the ISC cell, thereby identifying the affected cell and its ISC resistance. The battery pack, utilizing individual dual polarization battery models, further demonstrated that this approach is effective and efficient for the early detection of ISC cells.

(2) Pressure Monitoring: This technique focuses on battery expansion under extreme conditions. Overcharging, short circuits, or high temperatures can cause significant pressure changes due to gas generation and material expansion [74]. Pressure sensors monitor these changes in real time, triggering warnings at preset thresholds to prevent structural failure and thermal runaway. Cai et al. [75] investigated the expansion force of batteries under internal short circuit (ISC) conditions and developed a gas model to represent the early side reactions of thermal runaway (TR). This model effectively captures the expansion force during the initial phase of TR. Their findings indicated that the expansion force was detected sooner than voltage and temperature during the early stages of TR, confirming the practicality of using expansion force for early warning of TR incidents. Koch et al. [76] evaluated the performance of various sensors in the context of TR. They found that the expansion force signal exhibited a rapid response and high monitoring feasibility; however, the clarity of the signal was inadequate. Calibration of the monitoring thresholds is essential, taking into account the number of batteries, battery packs, fixtures, and the sensor packaging position to ensure accurate monitoring. Additionally, this method is unsuitable for cylindrical cells due to their clamps typically being located at the top and bottom, while the expansion force signal is most easily detected on the sides of the batteries.

(3) Impedance Monitoring: A critical aspect of thermal runaway warning. Internal resistance changes reflect battery health and chemical efficiency [77]. Anomalies like lithium deposition or electrolyte decomposition increase impedance [78]. High-frequency impedance spectroscopy provides real-time impedance characteristics, identifying potential risks and supporting battery management systems in taking timely actions. Carkhuff et al. [79] developed an impedance-based battery management system (BMS) to enhance the safety of lithium-ion batteries. This system performs multi-frequency impedance measurements in the range of 1 to 1000 Hz, enabling real-time monitoring of internal state changes in each cell during charging, discharging, and resting. Studies indicate that traditional BMS systems, which only monitor voltage and surface temperature, are insufficient for identifying cell mismatches and potential failures. In contrast, the impedance-based BMS accurately detects internal mismatches and anomalies, significantly improving battery safety and efficiency. Li et al. [80] proposed an early warning method for lithium-ion battery thermal runaway (TR) using online electrochemical impedance spectroscopy (EIS). By integrating Accelerating Rate Calorimetry (ARC) with EIS testing, critical features of TR were identified, leading to a three-level warning strategy for single cells, series modules, and parallel modules. This approach successfully provides warning signals before the battery reaches the self-heating temperature, offering a reliable foundation for timely intervention by thermal management and fire prevention systems.

(4) Smoke Detection: Targets harmful gases and smoke released during thermal runaway. Smoke sensors and gas analyzers monitor environmental smoke concentration and specific gas components. If harmful gases or smoke exceed safety thresholds, the system issues immediate alerts to ensure timely response and safety of the environment and personnel [81]. Wang et al. [82] developed a particle analysis smoke detector using capacitance sensing for early fire detection. This detector can measure smoke particle concentrations from 0 to 10% obs/m, achieving a detection accuracy of at least 0.5 parts per million (PPM) at 2 and 5 PPM levels. It maintains accuracy better than 1 PPM even in environments with 6% obs/m oil mist particles, 7% obs/m large dust particles, or 8% obs/m small dust particles.

3.6. Comparison of Different Early Warning Technologies

In modern battery applications, the importance of early warning technologies is increasingly evident. Given differences in accuracy, applicability, and cost among various methods, selecting the appropriate technology is critical to ensuring battery safety and performance. A thorough comparison of the advantages and limitations of these technologies can provide clearer guidance for the industry, driving the optimization and advancement of battery safety solutions. As summarized in

Table 2. Comparative analysis of different battery early warning technologies.

Technology	Advantages	Disadvantages	Accuracy	Applicability	Cost
Early warning technology based on temperature detection	Early detection of thermal issues; simple technology	May not detect all failure types; affected by environmental factors	Moderate	Wide, especially in high-risk environments	Low
Early warning technology based on gas detection	Detects gas emissions indicating failure; reliable	Requires gas sensors; may be less effective in sealed systems	High	Suitable for enclosed systems	Moderate
Early warning technology based on machine learning	Predictive capabilities; adapts over time	Requires data and computational power; complex implementation	High	Advanced applications	High
Early warning technology based on ultrasonic detection	Non-invasive; detects internal structural changes	Requires specialized equipment; interpretation complexity	High	Industrial and specialized uses	High
Current and voltage monitoring	Immediate detection of electrical anomalies	May not detect non-electrical issues; false positives possible	Moderate	Standard in many systems	Low
Pressure monitoring	Detects pressure build-up indicating potential failure	Limited to systems where pressure changes are a precursor	Moderate	Niche applications	Moderate
Impedance monitoring	Provides state-of-health insights; non-invasive	Requires specific equipment; may need frequent calibration	High	Maintenance and lifecycle management	Moderate
Smoke detection	Detects smoke indicating combustion or overheating	Requires smoke sensors; may activate during external fires	High	Fire-prone environments	Moderate

, which outlines the advantages, disadvantages, accuracy, applicability, and cost of different early warning technologies.

4. Lithium-Ion Battery Fire Extinguishing Technology

As the complexity and risk of lithium battery fires increase, scholars worldwide are researching preventive measures and developing clean, efficient extinguishing agents. Current strategies include optimizing battery design, developing intelligent battery management systems (BMS) for real-time monitoring and control, enhancing thermal management, and using advanced heat dissipation and insulation materials in battery packs. These measures help reduce the probability of thermal runaway. However, existing extinguishing agents often inadequately suppress thermal runaway and face issues like reignition. Therefore, alongside preventive measures, developing novel extinguishing agents is a crucial research direction for addressing lithium-ion battery fires. Many thermal runaway suppression methods focus on enhancing intrinsic safety, such as adding phosphorus-based flame retardants to electrolytes and creating heat-resistant organic separators [83]. However, these methods cannot completely eliminate thermal runaway risks and may adversely affect battery capacity, performance, and lifespan. Consequently, advancing clean and efficient firefighting technologies to address the complexity and reignition characteristics of lithium battery fires is a key focus for future research.

4.1. Hydrogel Fire Extinguishing Agent

Hydrogels are composed mainly of water and non-toxic, biodegradable polymers, which leave no harmful residues and are easy to clean post-fire, avoiding secondary damage to the environment or equipment [84]. The working principle of hydrogel fire extinguishants combines physical cooling and isolation. They rapidly absorb heat during a fire, effectively lowering the surface temperature of the battery and inhibiting further thermal runaway. With excellent thermal conductivity and high water content, hydrogels quickly dissipate heat, preventing self-ignition or explosion.

Hydrogels adhere strongly, forming an efficient barrier on burning surfaces to block oxygen and suppress combustion. This property offers advantages over traditional agents in complex lithium battery fires. Zhang [85] et al. investigated the suppression effects of hydrogel fire extinguishers on the thermal runaway behavior of lithium iron phosphate battery packs. Experimental results showed that hydrogel extinguishers effectively extinguished open flames and maintained the battery surface temperature below the critical thermal runaway threshold after application. Compared to water extinguishers, hydrogels better reduced the propagation of thermal runaway in lithium batteries, with higher spraying rates further enhancing suppression. High-flow hydrogel extinguishers completely prevented thermal runaway propagation between individual battery cells. Liu et al. [86] studied the effectiveness of hydrogels in suppressing thermal runaway propagation in high-capacity lithium-ion batteries for electric vehicles, as shown in Figure 7. The time that passes from when the fire extinguishing agent application ends to when thermal runaway (TR) starts in nearby cells is known as the “safety time”. This safety time tends to grow as the duration of the application increases, regardless of the type of extinguishing agent. The experimental results showed that 10 kg of hydrogel had twice the cooling effect of 20 kg of water. Moreover, with the same dosage, the hydrogel delayed thermal runaway propagation more than three times longer than water, providing additional safety time for rescue and evacuation.

4.2. Perfluorohexane Fire Extinguishing Agent

Perfluorohexanone is a colorless, odorless, non-conductive liquid with excellent fire extinguishing properties and thermal stability, maintaining effectiveness in high-temperature environments. Its extinguishing mechanism is based on the physical and chemical inhibition of flames. It rapidly evaporates to form a high concentration of gas that reacts with combustibles, lowering flame temperature and diluting combustion gases to suppress fire [87]. Perfluorohexanone produces no harmful byproducts, ensuring environmental friendliness post-extinguishing.

Compared to traditional fire suppressants, perfluorohexanone’s low toxicity and non-corrosive nature minimize damage to electronic equipment during lithium-ion battery fires, reducing repair and replacement costs [88]. Additionally, its rapid fire suppression capabilities quickly control initial fires, thus preventing spread, particularly in enclosed spaces or for high-value equipment. Zhang et al. [89] tested perfluorohexanone on 12-module, 243 A·h lithium-ion phosphate batteries (Figure 8a). Results showed efficient fire suppression and cooling, successfully preventing thermal runaway spread (Figure 8b), confirming its value in energy storage fire scenarios.

4.3. Liquid Nitrogen (LN) Fire Extinguishing Agent

Liquid nitrogen (LN), a cryogenic inert gas, offers significant advantages in firefighting due to its unique properties. With a boiling point of −196 °C, it effectively lowers ambient temperatures, inhibiting flame spread and slowing combustion [90].

The extinguishing mechanism of liquid nitrogen relies on cooling and dilution. When sprayed on a fire, it rapidly evaporates into low-temperature gas, absorbing heat and reducing the combustible material’s temperature below its ignition point. Simultaneously, the nitrogen gas produced dilutes the oxygen concentration, weakening the flame. Wang et al. [91] noted the limited theoretical research on liquid nitrogen firefighting and proposed a theoretical analysis and calculation method for its cooling and inerting effects. Huang et al. [92] studied LN’s impact on suppressing, delaying, and cooling thermal runaway (TR) in lithium-ion batteries (LIBs) (Figure 9). They found that applying LN before reaching critical TR temperatures can effectively prevent TR across various states of charge (SOC), without significantly affecting battery cycle performance, indicating strong protection for battery safety and performance. The cycle characteristics of a new battery before and after 80s liquid nitrogen (LN) treatment are illustrated in Figure 10. When compared to the battery without LN, the one that underwent the 80s LN treatment did not exhibit any noticeable decline in performance. This indicates that the 80s LN application had a minimal effect on the battery’s cycling performance. Therefore, liquid nitrogen effectively suppresses, delays, and cools thermal runaway (TR) without causing any significant degradation in battery performance.

Compared to traditional extinguishing agents, liquid nitrogen contains no harmful components and leaves no environmental pollution, with gaseous nitrogen as its residue, ensuring high safety [88]. Liquid nitrogen also offers rapid fire suppression, allowing quick intervention at the early stages of a fire, effectively controlling its spread. This makes it particularly important in high-safety-demand areas like energy storage systems and data centers.

4.4. Dry Powder Fire Extinguishing Agent

Under thermal runaway conditions, lithium-ion batteries can undergo intense chemical reactions, producing high temperatures and flammable gases. Li et al. [93] developed a novel NH₄Al(SO₄) ₂·12H₂O (AASD) composite dry powder extinguisher with high cooling performance to address the unique challenges of lithium battery fires, as shown in Figure 11. This composite powder exhibits greater heat absorption at thermal runaway temperatures compared to traditional dry powders. Experiments show that it can extinguish fires and suppress thermal runaway spread.

The core advantage of dry powder extinguishers lies in their efficient heat barrier and chemical inhibition capabilities. They typically consist of compounds like metal fluorides, phosphates, and carbonates, which are stable at high temperatures. These materials absorb substantial heat, thus reducing fire temperatures, and generate inert gases or protective films to inhibit combustion. Unlike water or CO₂ extinguishers, dry powder can function under high temperatures and prevent reignition by blocking oxygen contact [94].

Moreover, the finely optimized physical properties of dry powder ensure that its small particles penetrate battery pack crevices, covering burning areas and quickly extinguishing flames. It is suitable for fire prevention in electric vehicles, energy storage systems, and high-density battery scenarios. Compared to other extinguishers, dry powder offers better storage stability and environmental adaptability [88].

4.5. Aqueous Vermiculite Dispersion (AVD) Fire Extinguishing Agent

With the widespread use of lithium-ion batteries, the fire risk from thermal runaway is increasing. Traditional extinguishing agents are often ineffective, while aqueous vermiculite dispersion (AVD) offers an effective solution due to its unique physical and chemical properties.

Vermiculite is a layered silicate mineral with excellent thermal stability and adsorption capabilities. During firefighting, it is dispersed in a liquid medium to form a stable suspension that quickly covers the burning surface, absorbs heat, and reduces combustion temperature, thereby inhibiting fire spread [95]. Its layered structure also adsorbs harmful gases, reducing toxicity.

Chemically, AVD reacts with high-temperature combustion products to form inert substances, further preventing oxygen supply and reducing reignition risk. Physically, AVD’s good flowability allows it to penetrate and cover battery surfaces and internal structures efficiently. Guo et al. [96] studied AVD’s suppression effect on the thermal runaway of 21,700 lithium-ion batteries. The experimental results are shown in Figure 12 and demonstrate that AVD effectively inhibits thermal runaway and heat transfer through oxygen isolation, fuel separation, and heat absorption mechanisms.

4.6. Comparison of the Advantages and Disadvantages of Fire Extinguishing Agents

Hydrogel extinguishers are designed for lithium-ion battery fires, offering efficient suppression and preventing spread without causing secondary explosions or toxic gas release, making them suitable for crowded areas. Despite their eco-friendliness, their application is limited by specific scenarios and high production costs.

Perfluorohexanone extinguishers are widely used for liquid and gas fires due to their excellent performance and good electrical insulation, making them ideal for electrical fires. They leave no residue but pose environmental and toxicity risks as perfluorinated compounds, requiring strict management.

Liquid nitrogen (LN) extinguishers quickly reduce temperatures without chemical reactions, being safe for people and the environment. They are suitable for various fires, especially electrical, but require specialized storage, transport equipment, and professional training, thus increasing their complexity.

Dry powder extinguishers are popular for their wide applicability, effectively extinguishing solid, liquid, and gas fires quickly and safely without causing electric shocks. However, they may leave residue that require cleanup and can damage sensitive equipment.

Vermiculite dispersion (AVD) extinguishers are innovative, offering effective heat absorption and flame suppression, eco-friendliness, and easy cleaning. However, their effectiveness is limited to specific fire types.

As summarized in

Table 3. Comparison of fire suppressants.

Fire Suppressant Type	Efficiency	Safety	Environmental Impact	Application Scope	Clean up Difficulty
Water Gel Suppressant	High	High	Low	Li-ion Battery Fires	Easy
Perfluorohexanone Suppressant	High	Medium	Moderate (Pollution Risk)	Various Fire Types	Easy
Liquid Nitrogen Suppressant	High	High	Low	Various Fire Types	Moderate (Special Equipment Needed)
Dry Powder Suppressant	High	High	Low	Various Fire Types	Moderate (Requires Post-cleaning)
Vermiculite Dispersion Suppressant	Medium	High	Low	Specific Fire Types	Easy

, which outlines the efficiency, safety, environmental impact, application scope, and cleanup difficulty of different fire suppressants.

5. Research on Flame Retardant Coatings

Lithium-ion batteries are widely used in modern technology products due to their superior performance. However, their potential risk of thermal runaway raises significant safety concerns. To enhance battery safety, researchers are focusing on developing advanced flame-retardant coating technologies. These coatings are significant because they serve as a thermal barrier that can withstand high temperatures and prevent the ignition and spread of flames. By limiting oxygen availability and reducing the rate of heat release, these coatings help maintain the battery’s structural integrity and delay thermal runaway [97]. This research is crucial for ensuring battery stability, effectively reducing fire hazards, and providing new solutions for the safe advancement of battery technology.

5.1. Innovation in Coating Composition

5.1.1. Polymer Matrix

Polymer matrices are essential for providing structural integrity and support in flame-retardant coatings, ensuring that they can perform effectively in high-stress environments. The ongoing research into halogen-free polymers, particularly modified polyimides, reflects a growing trend towards more sustainable materials that do not compromise on performance. These advanced polymers are engineered to maintain their structural stability even at elevated temperatures, which is critical for applications that involve exposure to heat. Their formulation allows for strong adhesion to various substrates, along with impressive mechanical strength, which is further enhanced through crosslinking reactions. These reactions are pivotal for increasing the thermal resistance and oxidation stability of the coatings, making them more reliable in preventing fire hazards [98].

Moreover, chemical modifications play a significant role in enhancing the overall flame retardancy of these materials. By incorporating elements such as phosphorus or introducing aromatic structures into the polymer matrix, researchers can significantly improve the ability of the coatings to resist ignition and slow down flame spread. These modifications also contribute to better processability, making it easier to apply the coatings uniformly across complex geometries, such as those found in battery components. This uniform application is vital for ensuring comprehensive protection against thermal events, thereby increasing the safety and longevity of the devices in which these coatings are used [99].

5.1.2. Application of Flame Retardants

Flame retardants are crucial components in materials designed to mitigate the spread of flames, thereby enhancing fire safety in various applications. Traditionally, halogenated flame retardants have been widely used due to their effectiveness; however, increasing awareness of their environmental and health impacts has prompted a shift towards more eco-friendly alternatives. Phosphorus-based flame retardants are gaining popularity because they undergo decomposition during combustion to form char layers. These char layers act as a barrier that effectively blocks the access of oxygen and heat, thereby slowing down the combustion process and reducing the intensity of flames [100].

In recent years, the incorporation of nanomaterials such as nano-alumina and nano-silicates has opened up new avenues for improving the performance of flame-retardant coatings. These nanomaterials enhance not only thermal stability but also mechanical strength and wear resistance, making the coatings more durable and effective in challenging environments. The nanoscale particles are uniformly distributed within the polymer matrix, which enhances the overall heat dispersion capabilities of the material. This improved heat management contributes to better flame resistance, as it prevents localized overheating that can lead to ignition. The combination of phosphorus-based flame retardants with nanomaterials results in a synergistic effect that significantly enhances fire resistance while maintaining the integrity and performance of the coatings [101].

5.2. Optimization of Structural Design

5.2.1. Composite Coating Structure

Composite coating structures are designed using multilayer materials to achieve superior thermal barrier performance, which is essential in applications requiring fire resistance [102]. By utilizing different materials and varying the thickness of each layer, these coatings can be optimized for enhanced fire protection. For example, the incorporation of a graphene nanosheet as a base layer is particularly effective due to its exceptional thermal conductivity, allowing for rapid heat dissipation. This property helps to lower the temperature of the underlying materials, thereby minimizing the risk of ignition or thermal damage. On the other hand, a ceramic top layer serves as a robust physical barrier, providing excellent insulation and mechanical protection against external forces.

The interactions between the layers in a composite coating play a pivotal role in creating an effective flame-retardant barrier. These interlayer interactions not only prevent heat conduction through the material but also contribute to the overall mechanical strength of the coating. This enhanced strength is crucial in resisting potential cracking or peeling that may occur when subjected to high temperatures or mechanical stress. By combining the advantageous properties of each layer, the composite structure effectively mitigates the risk of fire hazards, ensuring that the coated materials maintain their integrity and performance under challenging conditions [103].

5.2.2. Application of Micro/Nano Structures

Micro/nano structure technologies have introduced innovative smart response mechanisms that significantly enhance the effectiveness of flame retardants in various applications. One such method is microencapsulation, which involves enclosing flame-retardant agents within protective shells. This technique enables the controlled release of these agents when triggered by high temperatures, ensuring that the flame retardants are activated precisely when needed. This not only guarantees long-term stability of the materials but also provides an additional layer of protection during critical moments, thereby enhancing the overall safety of the coated products [104].

Additionally, nanofiber technology plays a crucial role in improving flame resistance by creating dense networks of fibers that can efficiently respond to heat. When exposed to high temperatures, these nanofibers rapidly carbonize, forming a protective char layer that acts as a barrier against flames. This char layer is essential for insulating the underlying materials from the heat and preventing ignition. Moreover, the use of nanofibers also contributes to increased flexibility and impact resistance of the coatings, making them well-suited for complex applications that require durability and resilience under various conditions. The integration of these advanced technologies not only enhances fire safety but also expands the potential uses of flame-retardant materials in diverse industries [105].

6. Future Technology Outlook

The enhancement of EVs’ safety and performance is closely linked to advancements in battery technology, particularly in thermal runaway warning and prevention. As the EVs market expands and consumers demand high safety standards, future efforts should focus on the following aspects.

(1) Integration of Intelligent Monitoring and Warning Systems

Advances in sensor technology will enable precise real-time monitoring of battery parameters like temperature, pressure, current, and gas composition. Miniaturized, high-sensitivity sensors embedded within batteries will provide accurate data collection [106]. AI and machine learning will enhance these systems with predictive capabilities, identifying potential thermal runaway risks early. The integration of cloud computing and IoT will facilitate remote monitoring and management, improving safety and supporting large-scale battery deployment. Song et al. [107] utilized a combination of long short-term memory (LSTM) networks and convolutional neural networks (CNN) to assess the state of charge (SOC) of electric vehicle batteries. In this framework, CNN was responsible for feature extraction, while LSTM evaluated the SOC performance, resulting in hybrid performance that surpassed the capabilities of using LSTM or CNN independently. Additionally, hybrid models that merge machine learning techniques with conventional statistical methods or physics-based approaches may offer a more thorough strategy for predicting thermal runaway.

(2) Innovations in Material Science

Progress in material science will offer new solutions for thermal runaway prevention. Research will focus on developing materials with exceptional thermal stability and flame resistance. Nanotechnology will significantly enhance battery material performance, such as nano-coatings providing effective thermal insulation [105]. Self-healing materials will become crucial, allowing automatic repair and extending battery safety and lifespan. New electrolyte materials will reduce flammable gas production, lowering thermal runaway risks [108].

(3) Comprehensive Multilayer Safety Design Optimization

Future battery designs will feature multilayer safety protection [109]. Modular designs will enhance adaptability and reliability, isolating faulty units to prevent thermal runaway spread. Advanced battery management systems (BMSs) will utilize real-time monitoring and intelligent algorithms to dynamically adjust battery operations, avoiding overcharge, over-discharge, and overheating. Combining physical isolation and active cooling systems will further ensure safety across various environments. Wang et al. [110] developed a VFFRLS-Noise adaptive CKF algorithm by integrating the variable forgetting factor recursive least squares (VFFRLS) approach. This algorithm facilitates adaptive parameter identification, enhances the capability to manage external disturbances, and allows for real-time adjustments based on new data, ultimately improving the reliability of state of charge (SOC) estimates across various conditions.

(4) Innovation and Upgrading of Thermal Management Systems

Effective thermal management is crucial for preventing thermal runaway. Future research will focus on improving thermal conductive materials and optimizing cooling structures to enhance heat dissipation [111]. The integration of phase change materials and liquid cooling systems will further boost thermal management, ensuring safe operation under high power and extreme conditions. Enhanced design and processes will better address thermal runaway risks in diverse scenarios.

Overall, the future of lithium-ion battery thermal runaway warning and prevention will revolve around smart monitoring systems, material innovation, multilayer design, standardization, and thermal management optimization. These efforts will significantly improve battery safety and reliability, supporting widespread application in electric vehicles, energy storage, and consumer electronics. Continuous technological innovation and industry collaboration will better meet future energy market demands while ensuring safety and sustainability.

7. Conclusions

In the rapid development of EVs, the safety of lithium-ion batteries, particularly regarding thermal runaway, has garnered significant attention due to its potential risks in usage and management. Thus, research on warning and prevention technologies for thermal runaway is crucial.

(1) Warning Technologies: Temperature detection is foundational, enabling real-time monitoring of internal temperature changes to identify abnormal rises. Gas detection is sensitive to gases from electrolyte decomposition, enhancing chemical reaction monitoring. Machine learning improves the accuracy and speed of warning systems by analyzing extensive data to identify potential fault patterns. Ultrasonic detection provides dynamic feedback on structural changes, offering comprehensive information for warning systems.

(2) Prevention Technologies: The development and application of extinguishing agents are key. By selecting and combining different agents, the overall efficiency of the prevention system is enhanced.

(3) Innovative Fire-Resistant Coatings: These provide deeper safety assurance. New materials with improved fire resistance and thermal insulation maintain stability under high temperatures. Optimized structural design enhances mechanical performance, ensuring protection even under impact. The integration of intelligent monitoring and response transforms coatings into active safety systems, enabling quick response and emergency measures during early thermal runaway stages to reduce risk.

Focusing on the refinement and integration of these technologies will meet rising safety standards and market demands, elevating EV safety and supporting sustainable industry development. This will drive the green transportation era forward. Through ongoing innovation and optimization, future EVs will be safer, more reliable, and will advance global low-carbon travel initiatives.