Abstract
Lithium-ion batteries, particularly lithium iron battery (LFP), have gained widespread adoption due to their high energy density, long cycle life, and environmental friendliness. However, their susceptibility to thermal runaway (TR) under various operating conditions poses significant safety concerns. This study comprehensively investigates the TR behaviors of lithium iron battery across different states of charge (SOC), employing liquid nitrogen (LN2) injection for cooling within a controlled environment. Furthermore, the impact of thermal insulation applied to battery enclosures on the efficacy of LN2 cooling and its ability to suppress TR is evaluated. Our findings reveal a direct correlation between increased SOC and the severity of TR incidents, characterized by larger fire scale, greater mass loss, and heightened carbon monoxide (CO) emissions. Notably, implementing thermal insulation significantly extends the duration of LN2 cooling effects, providing an effective means to delay or prevent TR.
Keywords: thermal runaway, liquid nitrogen fire extinguishing, lithium-ion battery, Lifepo4 battery, thermal insulation treatment, thermal runaway suppression
1. Introduction
Lithium-ion batteries (LIBs) have revolutionized energy storage solutions across various industries, including electric vehicles, portable electronics, and renewable energy systems. Among various LIB chemistries, lithium iron battery are particularly attractive due to their superior safety performance, long cycle life, and cost-effectiveness. Nevertheless, LIBs remain vulnerable to thermal runaway (TR), a phenomenon characterized by a rapid, exothermic reaction within the battery that can lead to fire or explosion. The severity of TR is strongly influenced by factors such as the battery’s state of charge (SOC), operating temperature, and the presence of external stimuli like overcharging, external short circuits, or mechanical abuse.
The unpredictability and potential catastrophic consequences of TR underscore the need for robust safety measures to mitigate its risks. Recent research has focused on developing efficient methods to detect and suppress TR, including the use of inert gases like argon and nitrogen, as well as advanced fire suppression systems. Among these, liquid nitrogen (LN2) injection has emerged as a promising technique due to its rapid cooling effect and non-toxic, non-conductive properties.
This study aims to deepen the understanding of TR behaviors in lithium iron battery across varying SOC levels and evaluate the impact of thermal insulation on battery enclosures in conjunction with LN2 cooling. By conducting controlled experiments and analyzing the results, we aim to provide insights into the effectiveness of LN2 in suppressing TR and the role of thermal insulation in enhancing its performance.
2. Literature Review
2.1 Thermal Runaway in Lithium-Ion Batteries
LIBs are prone to TR due to various internal and external triggers. Internal factors include defects in electrode materials, separator degradation, and uncontrolled side reactions within the electrolyte. External stimuli, such as overcharging, short circuits, and mechanical abuse, can also initiate TR. Once TR is initiated, a chain reaction ensues, releasing large amounts of heat and flammable gases, potentially leading to fire or explosion.
SOC plays a crucial role in determining the severity of TR. Several studies have reported that higher SOC levels correlate with more intense TR reactions, resulting in larger fires, higher temperature excursions, and increased gas emissions. The reaction kinetics and energy release during TR are significantly influenced by the amount of available reactive material within the battery, which increases with SOC .
2.2 Suppression Techniques for Thermal Runaway
Various suppression techniques have been proposed to mitigate the risks associated with LIB TR. These include the use of inert gases, fire-retardant electrolytes, thermal runaway propagation barriers, and active cooling systems. Among these, LN2 injection stands out for its rapid cooling capability and environmental friendliness.
LN2 cooling has been shown to effectively quench the heat generated during TR and suppress the chain reaction, thereby minimizing the severity of the incident. However, the effectiveness of LN2 cooling can be impacted by factors such as the battery’s thermal mass, the rate of LN2 delivery, and the environment surrounding the battery.
2.3 Thermal Insulation in Battery Systems
The application of thermal insulation in battery systems can help maintain a desired temperature range, improve energy efficiency, and enhance safety [17]. By reducing heat loss and mitigating temperature fluctuations, thermal insulation can play a critical role in controlling the thermal environment within battery enclosures. In the context of TR suppression, thermal insulation can help prolong the cooling effects of LN2 by minimizing heat exchange between the battery and its surroundings.
3. Experimental Setup and Methodology
3.1 Battery Samples
The experiments were conducted using commercial lithium iron battery with a nominal voltage of 3.2 V, a cutoff voltage of 3.8 V, a capacity of 60 Ah, and dimensions of 173 mm × 120 mm × 45 mm. The batteries were encased in aluminum shells with a self-mass of 1730 g.
3.2 Test Apparatus and Procedure
The experiments were performed within a custom-built, 120 cm × 120 cm × 120 cm enclosed combustion chamber. The batteries were mounted within a battery box (345 mm × 330 mm × 200 mm) and triggered into TR using an external heating plate. Thermocouples were positioned on the battery surface and within the battery box to monitor temperature changes. A CO gas sensor was also integrated to measure gas emissions during TR.
To investigate the impact of SOC on TR characteristics, batteries with SOC levels of 25%, 50%, 75%, and 100% were tested. For each SOC level, LN2 was injected into the battery box at a constant flow rate of 3.2-3.3 L/min using a self-pressurizing LN2 tank operated at 0.04 MPa. Additionally, the effect of thermal insulation on the cooling performance of LN2 was evaluated by applying insulation material to the battery box.
3.3 Thermal Insulation Design
The battery box was insulated using a high-performance thermal insulation material with low thermal conductivity. The insulation was installed in the battery box’s walls and lid, leaving small vents for gas venting and pressure relief. The insulation material was selected based on its ability to minimize heat transfer while maintaining structural integrity and gas permeability.
4. Results and Discussion
4.1 Thermal Runaway Behaviors Across SOC Levels
The TR behaviors of lithium iron battery at varying SOC levels are summarized in Table 1. As SOC increased, the severity of TR incidents escalated, characterized by more intense flames, higher mass losses, and greater temperature excursions.
Table 1: Summary of Thermal Runaway Behaviors Across SOC Levels
| SOC (%) | Flame Intensity | Mass Loss (g) | Peak Temperature (°C) |
|---|---|---|---|
| 25 | Low | 232.9 | 270.60 |
| 50 | Moderate | 247.4 | 289.93 |
| 75 | High | 274.7 | 354.75 |
| 100 | Very High | 308.5 | 420.30 |
4.2 Impact of SOC on TR Characteristics
As SOC increased, the critical temperature for initiating TR decreased, while the peak temperature during TR rose significantly. The duration from TR initiation to peak temperature (Δt) also increased with SOC, indicating a more prolonged and intense reaction at higher SOC levels.
4.3 Gas Emissions During Thermal Runaway
CO emissions during TR were monitored using a gas sensor. CO concentrations peaked at higher SOC levels, with the maximum CO concentration reaching 353 ppm at 100% SOC. This trend underscores the increased reactivity and heat release during TR at higher SOC levels.
4.4 Effect of Thermal Insulation on LN2 Cooling Performance
The application of thermal insulation significantly improved the cooling performance of LN2 during TR suppression. the battery box’s average temperature remained lower for a more extended period with insulation, demonstrating the effectiveness of insulation in prolonging the cooling effects of LN2.
5. Conclusions
This study provides a comprehensive analysis of TR behaviors in lithium iron battery across varying SOC levels and evaluates the effectiveness of LN2 injection and thermal insulation in suppressing TR. Our key findings are as follows:
- SOC Impact on TR Severity: The severity of TR incidents increases with SOC, characterized by more intense flames, higher mass losses, and greater temperature excursions. The critical temperature for initiating TR decreases, while the peak temperature rises with increasing SOC.
- Gas Emissions During TR: CO emissions during TR peak at higher SOC levels, highlighting the increased reactivity and heat release at these conditions.
- Effectiveness of LN2 Cooling: LN2 injection effectively quenches the heat generated during TR, suppressing the chain reaction and minimizing the severity of the incident.
- Role of Thermal Insulation: Thermal insulation significantly prolongs the cooling effects of LN2, helping maintain a lower temperature environment within the battery box and enhancing the overall effectiveness of TR suppression.
These findings offer valuable insights into the TR behaviors of lithium iron battery and demonstrate the potential of LN2 injection and thermal insulation as effective TR suppression techniques. Future research should focus on optimizing LN2 delivery rates, insulation materials, and ventilation strategies to further enhance the safety of LIB-based energy storage systems.