In the context of global energy transition and climate change mitigation, the development and utilization of renewable energy sources such as solar and wind power have achieved remarkable progress. Lithium-ion battery energy storage systems (BESS) have become a key technology for addressing renewable energy intermittency and enhancing grid stability, owing to their high energy density, fast response, and long cycle life. These systems are widely deployed in generation, transmission, and end-user applications. Despite their advantages, the frequency of safety incidents—particularly fire and explosion accidents—has raised significant concern, resulting in casualties, property damage, and intensified scrutiny over the safety of lithium-ion BESS. This article systematically analyzes the risk factors leading to fires and explosions in BESS, including battery thermal runaway, electrical faults, design and installation defects, and explores the evolution mechanisms of these hazards. Through in-depth study of risk and prevention technologies, the aim is to improve the safety and reliability of lithium-ion BESS, promoting their sustainable and safe application in the energy sector.
Risk Factors Analysis for Fire and Explosion in Lithium-ion BESS
Battery Thermal Runaway
Thermal runaway is the primary cause of fires and explosions in lithium-ion BESS. When the heat generated inside a battery cannot be dissipated in time, the temperature rises sharply, triggering a series of irreversible chemical reactions that ultimately lead to thermal runaway. The inducing factors are complex and include both internal and external causes.
From the internal perspective, impurities introduced during cell manufacturing, microstructural defects in electrode materials, and instability of the electrolyte can cause internal short circuits during operation. When an internal short circuit occurs, the current increases dramatically, generating a large amount of Joule heat that rapidly raises the battery temperature, as expressed by the following equation:
$$ Q = I^{2} R T $$
where \( Q \) is the heat generated (J), \( I \) is the current (A), \( R \) is the resistance (Ω), and \( T \) is the time (s). As the temperature rises, the chemical reactivity between the electrode materials and the electrolyte increases, releasing more heat and forming a vicious cycle that eventually leads to thermal runaway. The main chemical reactions include the following two types.
(1) Reaction between negative electrode and electrolyte:
$$ \text{LiC}_6 + x \text{PF}_5 \rightarrow \text{LiF} + \text{LiPF}_y + \text{C}_6 + (x-y) \text{PF}_3 + (x-1) \text{F}_2 $$
(2) Reaction between positive electrode material (e.g., LiCoO₂) and electrolyte:
$$ \text{LiCoO}_2 + \text{C}_2\text{H}_4\text{O}_3 \rightarrow \text{Li}_2\text{CO}_3 + \text{Co}_3\text{O}_4 + \text{CO}_2 + \text{C} $$
External factors such as overcharging, over-discharging, overheating, and mechanical abuse also readily trigger thermal runaway. Overcharging can cause lithium dendrites to pierce the separator, leading to short circuits; over-discharging causes irreversible structural changes in the electrode materials, increasing the risk of runaway; high ambient temperature intensifies heat generation while insufficient dissipation leads to runaway; and mechanical abuse such as collision or crushing destroys the internal structure, inducing electrode short circuits and eventual thermal runaway.
Electrical Faults
Electrical faults are significant risk sources for fires and explosions in lithium-ion BESS. Short-circuit faults have complex causes. Deviations in voltage and current monitoring by the Battery Management System (BMS) can lead to overcharge or over-discharge, causing internal short circuits. Long-term operation of electrical cables can lead to aging and degradation of insulation; under high humidity or temperature variations, the insulation layer may fail, causing line short circuits. Loose connections or poor contact can result in localized overheating under high current, which may also trigger short circuits. Table 1 shows the statistical distribution of short-circuit fault causes.
| Fault Type | Proportion (%) | Typical Example |
|---|---|---|
| BMS monitoring deviation | 35 | Overcharge of a unit cell causing internal short circuit |
| Cable insulation aging | 40 | Insulation breakdown under relative humidity ≥ 60% |
| Loose connection components | 20 | Contact point overheating and melting under high current |
| Other (lightning, etc.) | 5 | Surge voltage puncturing battery insulation layer |
Design and Installation Defects
Inadequate system design introduces hidden safety risks in lithium-ion BESS. Mismatch between battery selection and actual application scenarios—for example, using low-rate discharge batteries in high-power demand applications—forces batteries to operate under high load for extended periods, accelerating aging and increasing thermal runaway risk. Improper series/parallel configuration of battery packs can lead to voltage and current imbalances, causing some cells to be overcharged or over-discharged, negatively affecting battery life and safety.
Insufficient thermal management system design is another critical safety hazard. If the thermal management system fails to effectively control the operating temperature, the battery temperature may become too high when heat generation is high, leading to thermal runaway. Conversely, in low-temperature environments, inadequate heating reduces battery performance and elevates safety risks.
Evolution Mechanism of Fire and Explosion in Lithium-ion BESS
Chain Reaction Triggered by Thermal Runaway
When thermal runaway occurs in a lithium-ion battery, a series of complex chain reactions are initiated. First, the internal temperature rises, causing decomposition of the Solid Electrolyte Interface (SEI) film on the negative electrode. When the temperature exceeds a certain threshold, the SEI film begins to decompose, releasing heat and flammable gases such as carbon monoxide and hydrogen. These gases mix with air to form a combustible mixture, which can ignite upon encountering an ignition source.
After SEI decomposition, the negative electrode material comes into direct contact with the electrolyte, triggering vigorous oxidation-reduction reactions that release large amounts of heat. As the reaction proceeds, the battery temperature continues to rise, and internal pressure increases. When the pressure exceeds the mechanical limit of the battery casing, the casing ruptures, ejecting combustible materials and high-temperature gases, forming a jet fire. This can trigger thermal runaway in adjacent batteries, leading to a cascade of failures and rapid fire spread. The figure below illustrates the thermal runaway process in a lithium-ion battery.
Fire Propagation and Spread Mechanism
In lithium-ion BESS, fire propagation and spread occur primarily through three modes: thermal radiation, thermal conduction, and thermal convection, in addition to flame propagation.
For flame propagation, the laminar flame speed is given by:
$$ S_L = \frac{\dot{m}”}{\rho_u} $$
where \( S_L \) is the laminar flame speed (cm/s), \( \dot{m}” \) is the mass burning rate (kg/(m²·s)), and \( \rho_u \) is the density of the unburned gas mixture (kg/m³). The turbulent flame speed is:
$$ S_T = S_L \left(1 + \alpha \, Re^n\right) $$
where \( S_T \) is the turbulent flame speed (cm/s), \( \alpha \) is the turbulence coefficient, \( Re \) is the Reynolds number, and \( n \) is the Reynolds number exponent (taken as 0.25 in this study).
Thermal radiation follows the Stefan–Boltzmann law:
$$ Q_{\text{rad}} = \varepsilon \sigma A \left(T_1^4 – T_2^4\right) $$
where \( Q_{\text{rad}} \) is the radiative heat transfer rate (W), \( \varepsilon \) is the surface emissivity, \( \sigma \) is the Stefan–Boltzmann constant (5.67×10⁻⁸ W/(m²·K⁴)), \( A \) is the radiating surface area (m²), and \( T_1 \) and \( T_2 \) are absolute temperatures (K) of the hot and cold bodies, respectively. For far-field radiation, the heat flux can be estimated as:
$$ q = \frac{\dot{Q}_{\text{rad}}}{4\pi r^2} $$
where \( q \) is the heat flux (W/m²) and \( r \) is the distance from the radiation source (m).
Thermal conduction is governed by Fourier’s law:
$$ Q_{\text{cond}} = -k A_2 \frac{dT}{dx} $$
where \( Q_{\text{cond}} \) is the conductive heat transfer rate (W), \( k \) is the thermal conductivity (W/(m·K)), \( A_2 \) is the cross-sectional area for conduction (m²), and \( dT/dx \) is the temperature gradient (K/m). The negative sign indicates heat flow from high to low temperature.
Thermal convection is calculated using Newton’s cooling law:
$$ Q_{\text{conv}} = h A_3 (T_w – T_f) $$
where \( Q_{\text{conv}} \) is the convective heat transfer rate (W), \( h \) is the convective heat transfer coefficient (W/(m²·K)), \( A_3 \) is the convection surface area (m²), \( T_w \) is the wall temperature (K), and \( T_f \) is the fluid temperature (K). For natural convection, the Nusselt number is often expressed as:
$$ Nu = C (Gr \cdot Pr)^n $$
where \( Nu \) is the Nusselt number, \( C \) is an empirical constant, \( Gr \) is the Grashof number, and \( Pr \) is the Prandtl number. For example, given laminar flame speed \( S_L = 10 \) cm/s, \( \alpha = 0.5 \), \( Re = 100 \), and \( n = 0.25 \), the turbulent flame speed is calculated as:
$$ S_T = 10 \times (1 + 0.5 \times 100^{0.25}) = 10 \times (1 + 0.5 \times 3.162) \approx 25.81 \text{ cm/s} $$
This demonstrates a significant increase in flame speed under turbulent conditions. When a battery fire occurs, flames and hot batteries heat adjacent objects via thermal radiation; thermal conduction through metal connectors triggers thermal runaway in neighboring cells; and thermal convection through high-temperature smoke and ventilation ducts transfers heat and disperses flammable gases, collectively accelerating fire spread.
Formation Conditions and Process of Explosion
An explosion in a lithium-ion BESS requires that flammable gases mix with air in the proper proportion and encounter an ignition source of sufficient energy. During thermal runaway, large amounts of flammable gases such as hydrogen, carbon monoxide, and light hydrocarbons are released. These gases accumulate inside the battery enclosure and mix with air to form a combustible mixture. When the concentration falls within the explosive range, an ignition source—such as an electrical spark or open flame—can trigger an explosion.
To determine whether a combustible mixture is within the explosive risk range, Le Chatelier’s rule can be applied. For a mixture of multiple flammable gases, the lower explosive limit (LELmix) and upper explosive limit (UELmix) are given by:
$$ LEL_{\text{mix}} = \frac{100}{\frac{y_1}{LEL_1} + \frac{y_2}{LEL_2} + \dots + \frac{y_n}{LEL_n}} $$
and
$$ UEL_{\text{mix}} = \frac{100}{\frac{y_1}{UEL_1} + \frac{y_2}{UEL_2} + \dots + \frac{y_n}{UEL_n}} $$
where \( y_1, y_2, \dots, y_n \) are the volume fractions (%) of each flammable component in the mixture, and \( LEL_1, UEL_1, \dots \) are the corresponding explosive limits (vol%). The minimum ignition energy (MIE) differs for various gases, as shown in Table 2.
| Flammable Gas Type | Minimum Ignition Energy (mJ) |
|---|---|
| Hydrogen | 0.02 |
| Methane | 0.28 |
| Carbon monoxide | 0.021 |
Prevention and Control Technologies for Fire and Explosion in Lithium-ion BESS
Battery Material and Design Improvements
To fundamentally enhance the safety of lithium-ion batteries, developing high-safety battery materials is essential. For cathode materials, lithium iron phosphate (LFP) is widely used in BESS due to its excellent thermal stability and safety. Compared with traditional materials like lithium cobalt oxide, LFP maintains a more stable structure at elevated temperatures, significantly reducing the risk of thermal runaway. For anode materials, silicon-based materials are considered a promising direction owing to their high theoretical specific capacity. However, large volume changes during charge/discharge can damage the electrode structure, impacting performance and safety. Researchers are addressing this through nanostructure design and material compositing to improve stability and cycling performance. The figure below shows the structure of a lithium-ion battery.
Monitoring and Early Warning Technologies
Real-time monitoring and early warning are crucial for preventing risks in lithium-ion BESS. Voltage and current monitoring can assess battery charging/discharging status and health, promptly alerting for overcharge, over-discharge, or abnormal currents. Temperature sensors continuously monitor battery modules and enclosure temperatures; when thresholds are exceeded, cooling systems can be activated and alarms raised. Pressure sensors detect abnormal internal pressure in cells, triggering ventilation and pressure relief measures. Gas composition monitoring uses catalytic combustion or semiconductor sensors to detect concentrations of hydrogen and carbon monoxide; alarms are issued when flammable gas levels approach the lower explosive limit. Table 3 presents typical monitoring parameter thresholds.
| Parameter | Normal Operating Range | Abnormal Warning Range |
|---|---|---|
| Voltage (single cell) | 2.5–4.2 V | < 2.5 V (over-discharge) or > 4.2 V (overcharge) |
| Current | 0–120% of rated current | > 120% of rated current (overcurrent) |
| Temperature (battery module) | 25–45 °C | > 45 °C (heating warning) |
| Temperature (enclosure) | 20–35 °C | > 35 °C (ambient overheating) |
| Pressure (inside cell) | 0–0.1 MPa | > 0.1 MPa (abnormal pressure) |
| Hydrogen volume fraction | 0–0.04% | > 0.04% |
| Carbon monoxide volume fraction | 0–0.005% | > 0.005% |
Optimized Thermal Management System
Based on battery operating characteristics, the thermal management system is optimized. A combined liquid cooling and air cooling strategy is adopted, employing a proportional–integral–derivative (PID) algorithm to adjust coolant flow and fan speed, thereby maintaining the temperature difference across the battery stack within ±2 °C. Phase change materials (PCMs) are embedded between battery modules to absorb transient heat through latent heat of fusion, combined with metal heat sinks to improve thermal conductivity. Temperature monitoring points are arranged in a grid pattern, and real-time data are fed into the BMS to enable prediction of temperature trends. The ideal operating temperature range for batteries in BESS is 25–45 °C. When the temperature exceeds 40 °C, the activation power of the cooling system follows a linear relationship with the temperature rise:
$$ P = K \times (T – T_0) $$
where \( P \) is the cooling system activation power (W), \( K \) is the thermal coefficient (taken as 15 W/°C), \( T \) is the real-time battery temperature (°C), and \( T_0 \) is the reference temperature (40 °C). The curve shown in the figure below (not referenced here) is fitted from experimental data, indicating that when temperature exceeds the critical value, cooling power should be increased to prevent heat accumulation.
Fire Suppression and Explosion Prevention Measures
When a fire or explosion occurs, efficient suppression and prevention measures can mitigate losses. Among extinguishing agents, fine water mist offers good cooling with minimal water damage; perfluorohexanone and heptafluoropropane provide high fire-extinguishing efficiency but at higher cost. Both are widely used for lithium-ion battery fires. For explosion prevention, the battery enclosure adopts an explosion-proof structure, and the area of pressure relief vents is precisely calculated using the following equation:
$$ A = 10 \, C’ \, V^{2/3} $$
where \( A \) is the vent area (m²), \( C’ \) is the enclosure volume (m³, e.g., 80 m³ in this study), and \( V \) is a pressure relief coefficient (determined by battery type, gas characteristics, and safety standards). For example, with \( C’ = 80 \) m³ and assuming \( V = 1 \) (for illustration), the vent area is calculated as:
$$ A = 10 \times 80 \times 1^{2/3} = 800 \text{ m}^2 $$
In practical applications, the actual vent area must be adjusted according to the specific \( V \) value and relevant codes to ensure effective pressure relief.
Conclusion
Lithium-ion battery energy storage systems are a core supporting technology for energy transformation and the construction of new power systems. Their safety performance directly affects energy security and public safety, forming a critical foundation for stable system operation. This study identifies four primary risk factors that induce fire and explosion accidents in BESS: battery thermal runaway, electrical faults, design/installation defects, and abnormal environmental conditions. It systematically reveals the inherent mechanisms of thermal runaway chain reactions, cross-unit fire spread, and explosion shock wave formation. To address these risks, the industry has developed a multi-dimensional prevention and control technology system, covering battery material innovation, structural design optimization, thermal management system upgrades, multi-parameter monitoring and early warning, and high-efficiency fire suppression and explosion-proof devices. However, current prevention technologies still have limitations. Future work should focus on targeted research to address these gaps, driving continuous iteration and improvement of prevention technologies, thereby building a robust safety barrier for the large-scale and safe deployment of lithium-ion battery energy storage systems.