Abstract: This article focuses on the study of 100Ah lithium iron phosphate battery (LiFePO4). It examines the thermal runaway process, including its stages, associated mechanisms, and the resulting consequences such as voltage changes, heat production, and damage to the battery structure. The gas generation behavior during thermal runaway is also analyzed, considering the composition of the gases produced and their explosion limits. The research provides valuable insights for understanding the safety aspects of LiFePO4 battery and offers guidance for their further development and application in energy storage systems.
1. Introduction
In the context of global carbon neutrality and peak carbon emissions, the new energy field has witnessed significant development opportunities. Lithium-ion batteries, especially lithium iron phosphate battery (LiFePO4), have become a core energy storage technology in various fields due to their unique advantages such as small size, high power, high energy density, and long cycle life [1]. However, under extreme working conditions, such as overcharging, extreme temperature environments, or internal short circuits, LiFePO4 battery face the risk of thermal runaway (TR), which not only affects battery performance but may also lead to fire and explosion [2]. Therefore, in-depth research on the thermal runaway characteristics of LiFePO4 battery is crucial for enhancing battery safety and promoting the healthy development of the new energy industry.
1.1 Research Background
Previous studies on the thermal runaway characteristics of lithium-ion batteries have covered multiple aspects such as triggering methods, testing conditions, and battery materials [3 – 6]. For example, Zhu [7] compared the overheating behaviors of 25Ah LFP pouch and prismatic cells and found that the packaging form affected the thermal runaway mainly through the mechanical properties of the packaging materials. Wang et al. [8] studied the thermal runaway characteristics of different cathode materials and found that LFP batteries had earlier trigger times and milder thermal runaway compared to some other materials. Wei et al. [9] used different methods to trigger the thermal runaway of NCM523 lithium-ion batteries and found that overcharging caused the most severe damage. Kuo et al. [10] compared the overcharging-induced thermal runaway behaviors of commercial prismatic LFP batteries at different charging rates and found that increasing the charging rate accelerated the growth of lithium dendrites and promoted thermal runaway. Kang et al. [11] studied the overcharging behavior and thermal runaway characteristics of LFP prismatic batteries with different capacities and found that low-capacity batteries were more prone to thermal runaway, while high-capacity batteries had more severe thermal runaway. However, most of these studies focused on small-capacity batteries or prismatic batteries, and the understanding of the thermal runaway characteristics of large-capacity pouch LiFePO4 battery remains insufficient. Additionally, current research has mainly focused on directly observing the temperature and voltage characteristics during thermal runaway, with relatively less exploration of the internal morphological changes of the LiFePO4 battery after thermal runaway.
Furthermore, when lithium-ion batteries experience thermal runaway, they produce a large amount of combustible and toxic gases, which can easily lead to explosion accidents [12 – 14]. Wang et al. [15] summarized that the main components of the thermal runaway gases were CO2, H2, and CO, with the rest being small amounts of hydrocarbon substances. Qi et al. [16] studied the gas production characteristics of NCM523 batteries at different states of charge (SOC) and found that as SOC increased, CO2 content decreased, while H2 and CO contents increased. Xu et al. [17] compared the gas production characteristics of batteries during thermal runaway triggered by different methods and found that side heating produced the most H2, while oven heating produced the least. Shen et al. [18] studied the gas composition and production volume of LFP and different NCM ratio batteries and found that the gas production volume of NCM series batteries was generally 2 – 3L/Ah, while that of LFP was only 0.569L/Ah. The higher proportion of H2 in the gases produced by LFP batteries led to a lower explosion limit compared to NCM batteries. However, current research has not fully considered the gas production characteristics and explosion limit changes of LFP batteries at different SOCs.
1.2 Research Objectives
This study aims to fill the gaps in the research on the heat production characteristics and debris characteristics of large-capacity pouch LiFePO4 battery at different SOCs and to further explore the changes in their gas production characteristics and explosion limits. Through overheating experiments, key parameters such as the overheating mechanism, heat production energy, debris characteristics, gas components, and explosion limits of 100Ah LiFePO4 battery at different SOCs (40%, 60%, 80%, 100%) are systematically analyzed to provide a scientific basis for the safety design and emergency response strategies of energy storage systems and to further promote the safe development of the new energy industry.
2. Experimental System and Methods
2.1 Experimental Research Object
The research object is a 100Ah pouch-type lithium-ion battery with LiFePO4 battery as the cathode active material and graphite as the anode material. The battery specification parameters are shown in Table 1.
| Name | Unit | Parameter |
|---|---|---|
| Cathode | LiFePO4 battery | |
| Anode | Graphite | |
| Size | mm | 316.8 * 120.9 * 21.9 |
| Type | Pouch | |
| Capacity | Ah | 100 |
| Standard Voltage | V | 3.2 |
| Charge – Discharge Platform | V | 2.5 – 3.6 |
| Specific Heat Capacity | kJ/(kg – K) | 980 |
| Thermal Conductivity | W/(m – K) | X/Y/Z:20/20/0.6 |
The battery was first discharged to 2.5V at a 0.3C constant current using a Neware battery testing system and then charged to different SOCs (40%, 60%, 80%, 100%) at a 0.3C constant current and finally maintained at a constant voltage until the current was below 0.05C.
2.2 Battery Thermal Runaway Experiment
The LiFePO4 battery thermal runaway was simulated by attaching a heating film (120V, 480W) to one large side of the battery. K-type thermocouples with a diameter of 1mm were placed at three positions (upper, middle, and lower) on the other large side of the LiFePO4 battery to measure the temperature changes at different positions on the battery surface. Voltage lines were also placed at the LiFePO4 battery terminals to monitor the LiFePO4 battery voltage changes. The experiment was carried out on an experimental platform that integrated a multi-channel data recorder, a DC power supply, a camera, a ventilator, and a computer. During the experiment, the data was recorded in real-time, and the heating process was video-recorded. When the LiFePO4 battery triggered thermal runaway, the DC power supply was immediately turned off. After the experiment, the mass loss of the LiFePO4 battery was measured using an electronic balance, and the internal structure of the LiFePO4 battery was imaged using industrial computer tomography (CT), and the morphology of the positive and negative electrode materials was observed using a scanning electron microscope (SEM).
2.3 Gas Collection Experiment
The gas collection experiment was carried out in a self-made module housing. The 100Ah LiFePO4 battery at different SOCs were heated at room temperature. When the battery triggered thermal runaway and produced a large amount of gas, the exhaust port on the module housing was opened, and the gas was collected in a gas collection bag using a suction pump. The collected gas was then analyzed for its composition using a gas chromatograph (GC).
3. Experimental Results and Analysis
3.1 Thermal Runaway Characteristics of Batteries at Different SOCs
3.1.1 Experimental Phenomena
The experimental results showed that the phenomena during the thermal runaway process of batteries at different SOCs were basically the same, mainly manifested as the release of white smoke without obvious flames. The phenomena during the thermal runaway process included normal temperature rise, gas expansion, rupture and smoking, intense gas production at the positive electrode head, intense gas production at the negative electrode head, and natural cooling. It was noted that the position of rupture and smoking was concentrated at the positive electrode head of the battery. This was because the positive electrode head was the connection point of the current collector and the electrode material in the pouch battery, with a complex structure and multiple interfaces and connection points, making it a potential failure point. Additionally, the packaging material at the positive electrode head required heat sealing and crimping processes, which might have weakened the material strength in this area due to high temperature and pressure. When chemical reactions such as electrolyte decomposition occurred inside the LiFePO4 battery, the generated gas tended to accumulate at the top of the LiFePO4 battery, and due to the structural weakness and sealing problems at the positive electrode head, it was prone to become the site of gas accumulation and finally rupture. After a long period of slow smoking, the surface temperature of the LiFePO4 battery reached the thermal runaway critical point, and a large amount of smoke was stably released from the positive electrode top seal, forming a jet. Subsequently, the negative electrode top seal also began to produce gas intensely, forming a stable jet flow. Compared with the low SOC (40%) battery, the high SOC battery showed more intense gas production during thermal runaway. Additionally, it was found that the aluminum-plastic film on the surface of the LiFePO4 battery turned black due to carbonization caused by high-temperature smoke, and this phenomenon was not obvious in the 40%SOC battery, indicating that the generated smoke temperature was lower and the gas production volume was less, and the thermal runaway intensity was lower.
3.1.2 Overheating Mechanism
The temperature – voltage change curves of LFP pouch batteries at different SOCs showed that the evolution trends of the LiFePO4 battery surface temperature and voltage were highly consistent, indicating that the thermal runaway mechanism faced by the LiFePO4 battery in an overheating environment was the same regardless of the SOC. The entire battery overheating process was divided into four stages based on the analysis:
- Stage I (0s – ta, Temperature Rise Stage): The LiFePO4 battery temperature steadily rose from the initial ambient temperature to about . The consistency of the battery surface temperature was good, indicating that the responses of various parts inside the LiFePO4 battery were relatively uniform. At the same time, the LiFePO4 battery voltage slightly increased, mainly due to the activation of the electrode materials at high temperature.
- Stage II (ta – tb, Temperature Platform Stage): The temperature rise rate of the LiFePO4 battery began to slow down, forming a relatively flat temperature change platform. At the same time, the temperature differences at various positions on the LiFePO4 battery surface gradually increased. At this stage, side reactions began to occur inside the lithium-ion battery. The SEI film decomposed to produce and , and heat was released, causing the internal temperature of the LiFePO4 battery to continuously rise. At the same time, the electrolyte absorbed heat and evaporated to produce gas, accelerating the expansion of the LiFePO4 battery and causing uneven heat transfer and dissipation on the battery surface.
- Stage III (tb – tonset, Temperature Fluctuation Stage): When the gas production pressure inside the LiFePO4 battery reached the threshold, the aluminum – plastic films at both ends of the LiFePO4 battery ruptured and smoked, taking away some heat and causing a small decrease in the LiFePO4 battery temperature. Subsequently, the LiFePO4 battery continuously absorbed the energy input by the heating film, and the temperature continued to rise. The electrolyte continuously evaporated and reacted with the lithium intercalation in the negative electrode to generate organic gases, and the internal diaphragm began to contract and short-circuit. It was noted that the battery voltage remained stable for most of the time in this stage, with only a slight downward trend before the thermal runaway was about to occur, indicating that a relatively minor internal short circuit occurred inside the front end of the pouch battery.
- Stage IV (tonset -, Thermal Runaway Stage): This stage marked the transition of the lithium-ion battery from the thermal runaway state to the natural cooling process. The continuous high temperature caused the LiFePO4 battery diaphragm to melt, thereby allowing the positive and negative electrode materials to directly contact, triggering a large-scale internal short circuit inside the LiFePO4 battery. In a very short period of time, a large amount of Joule heat was generated inside the battery, triggering a series of chain exothermic reactions such as the reaction between the negative electrode lithium and the electrolyte, the decomposition of the positive electrode material, the decomposition of the electrolyte, and the reaction of the binder, resulting in a sharp increase in temperature, intense gas production and heat production, and finally leading to the occurrence of thermal runaway. In a few tens of seconds, the LiFePO4 battery temperature rapidly rose to the peak, the temperature difference between different positions on the LiFePO4 battery surface increased, and the voltage rapidly dropped to 0V.
3.1.3 Voltage Characteristics
Before the occurrence of battery thermal runaway, the LiFePO4 battery voltage began to decline but did not completely drop to 0V. This phenomenon indicated that the slight decline in battery voltage could be used as an early warning signal for battery failure. Additionally, there were significant differences in the time interval from the obvious decline in voltage to the formal start of thermal runaway for batteries at different SOC levels. For batteries at 100%, 80%, 60%, and 40%SOC, these time intervals were 22s, 28s, 61s, and 104s, respectively, and the voltage decline amplitudes were 3.05V, 2.07V, 0.44V, and 0.05V, respectively. These data revealed an important trend: the higher the SOC, the shorter the warning time from the voltage decline to the occurrence of thermal runaway, and the more rapid the occurrence of a large-scale internal short circuit inside the LiFePO4 battery.
3.1.4 Heat Production Characteristics
The thermal runaway trigger temperature Tonset of the battery was stable in the range of under different SOC conditions, mainly due to the melting point limitations of the battery materials and the diaphragm, further proving that the overheating-triggered battery thermal runaway was mainly due to the melting of the diaphragm. During the thermal runaway process, the maximum temperature Tmax of the battery showed an increasing trend with the increase in SOC. Under 100%, 80%, 60%, and 40%SOC conditions, Tmax reached 422.8, 387.3, 291.3, and , respectively. The rate of temperature rise at the instant of thermal runaway was also closely related to SOC. Under 100% and 80%SOC conditions, the maximum temperature rise rates of the battery were as high as 78.9 and , respectively, while under 60% and 40%SOC conditions, these rates were significantly reduced to 8.3 and 2.2^{\circ}C/s, respectively. This indicated that when the battery SOC was higher than 60%, the intensity of side reactions at the instant of thermal runaway would significantly increase, and the temperature rise rate would increase exponentially, posing a greater threat to battery safety.
The internal heat production during the thermal runaway process was quantified. The peak heat production rates of 100%, 80%, 60%, and 40%SOC batteries reached 140.34, 115.44, 14.76, and 3.91 kW, respectively. The total heat production during the entire thermal runaway stage was calculated. When SOC was 100%, 80%, 60%, and 40%, the heat released by the battery was 464.24, 373.35, 229.81, and 121.31 kJ, respectively. The relationship between SOC and the energy released during thermal runaway was linear. The potential hazard degree of the energy released during thermal runaway was quantified using the TNT equivalent method. The energy released by 100%, 80%, 60%, and 40%SOC batteries was equivalent to 104.63, 84.14, 51.79, and 27.33 g TNT, respectively. The damage radius of the battery energy was 5.90m at 100%SOC and 3.59m at 40%SOC, with an increase of nearly 64.3% compared to 40%SOC.
3.1.5 Debris Analysis
The mass loss rate of the battery increased with the increase in SOC, which was consistent with the trend of its maximum temperature change. Under 100%, 80%, 60%, and 40%SOC conditions, the mass loss rates were 22.74%, 19.62%, 17.62%, and 16.89%, respectively. The reasons for this were twofold: first, the larger the SOC, the more electricity was converted into Joule heat due to internal short circuits, triggering more chain side reactions and increasing the gas production volume; second, under high temperature conditions, the aluminum – plastic film of the LiFePO4 battery might melt and adhere to the heating film, further increasing the mass loss. The internal structure of the LiFePO4 battery after thermal runaway was observed using CT. The winding structure of the electrode sheets inside the LiFePO4 battery was significantly bent and deformed after thermal runaway, and large gaps appeared between the electrode sheets. The degree of deformation and the size of the electrode sheet gaps increased with the increase in SOC. The morphology of the LiFePO4 battery electrode sheets after thermal runaway was characterized using SEM. Under low SOC conditions (40% – 60%), the positive electrode material particles showed a block – like structure similar to adhesion. However, under high SOC conditions (80% – 100%), the LiFePO4 battery positive electrode material transformed into aggregated irregular spherical particles. For the negative electrode, under 40%SOC, the layered structure of the graphite anode was maintained, but from 60%SOC onwards, the original layered structure of the graphite anode disappeared, and its morphology transformed into many aggregated spherical particles. This indicates that as the SOC increases, the internal reactions become more intense, which has a significant impact on the LiFePO4 battery structure and performance.
Moreover, the changes in the LiFePO4 battery debris characteristics can provide important information for understanding the thermal runaway process. The deformation and damage of the electrode sheets can affect the electrical conductivity and electrochemical performance of the LiFePO4 battery. The transformation of the positive and negative electrode materials’ morphology may also change their electrochemical activity and stability. These changes not only affect the performance of the individual battery but also have implications for the safety and reliability of the entire energy storage system.
In addition, the mass loss of the LiFePO4 battery during thermal runaway can also affect the heat transfer and heat dissipation processes. The loss of mass may change the thermal conductivity and heat capacity of the LiFePO4 battery, which in turn affects the temperature distribution and heat transfer within the LiFePO4 battery. This is an important factor to consider when studying the thermal runaway characteristics and developing strategies to prevent and control thermal runaway.
Overall, the analysis of the debris characteristics of the lithium iron phosphate battery during thermal runaway provides valuable insights into the internal reactions and structural changes that occur during this process. It helps to better understand the mechanisms of thermal runaway and provides a basis for improving the safety and performance of lithium iron phosphate batteries in energy storage systems.