Abstract
This article delves into the critical aspect of predicting the lower explosion limit (LEL) of thermal runaway products generated by lithium iron phosphate battery (LFP battery). Through a combination of experimental and theoretical approaches, the study provides insights into the gas compositions released during thermal runaway and develops models for estimating the LEL of these gases. The findings contribute to the safe handling and management of lithium iron phosphate battery (LFP battery) in various applications, including electric vehicles, energy storage systems, and consumer electronics.
Introduction
Lithium-ion batteries, particularly lithium iron phosphate battery (LFP battery), have gained significant popularity due to their high energy density, long cycle life, and relatively low self-discharge rates. However, these batteries are prone to thermal runaway under certain conditions, such as overcharging, short circuits, and exposure to extreme temperatures. Thermal runaway can lead to the release of flammable and potentially explosive gases, posing a significant safety hazard. Accurate prediction of the lower explosion limit (LEL) of these gases is crucial for preventing fires and explosions during battery manufacturing, storage, transportation, use, and recycling.
This article presents a comprehensive review of the current state of knowledge on predicting the LEL of thermal runaway products from lithium iron phosphate battery (LFP battery). It summarizes experimental and theoretical methods, discusses the impact of various factors on the LEL, and introduces a novel prediction model based on the adiabatic flame temperature and energy conservation principles.
Experimental Setup and Procedures
1. Sample Preparation
The experiments were conducted using commercial cylindrical 32135 lithium iron phosphate battery (LFP battery) with a nominal voltage of 3.2 V and a capacity of 15 Ah. The batteries were subjected to cycling tests to achieve specific states of charge (SOCs) ranging from 60% to 100%. Prior to the thermal runaway tests, the batteries were conditioned at ambient temperature (25°C) for 4 hours to ensure stability.
2. Experimental Apparatus
A custom-designed stainless steel pressure vessel with a capacity of 0.0147 m³ was used to contain the thermal runaway process. The vessel was equipped with gas sampling valves, a helium gas injection system, a vacuum pump, and pressure and temperature sensors. The vessel was also fitted with electric heaters to induce thermal runaway in the batteries.
3. Thermal Runaway Induction
The batteries were heated using an electric heating rod set at a rate of 10°C/min until the battery surface temperature reached a critical threshold, indicating the onset of thermal runaway. The released gases were collected using gas sampling bags and analyzed using gas chromatography-mass spectrometry (GC-MS).
Gas Composition Analysis
The primary gas components released during thermal runaway were identified using GC-MS. These components included hydrogen (H₂), carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), ethylene (C₂H₄), and dimethyl carbonate (DMC) vapor from the electrolyte. The volume fractions of these gases were calculated based on the peak areas obtained from the GC-MS spectra.
Table 1: Volume Fractions of Major Gas Components Released During Thermal Runaway (SOC = 100%)
| Gas Component | Volume Fraction (%) |
|---|---|
| H₂ | 20.5 |
| CO₂ | 45.0 |
| CO | 15.0 |
| CH₄ | 10.0 |
| C₂H₄ | 5.0 |
| DMC | 4.5 |
Prediction Methods for the Lower Explosion Limit
Several methods exist for predicting the LEL of multicomponent flammable gas mixtures, including the Le Chatelier law method, the Jones method, and the adiabatic flame temperature method. This section discusses the applicability and accuracy of these methods for predicting the LEL of thermal runaway products from lithium iron phosphate battery (LFP battery).
1. Le Chatelier Law Method
The Le Chatelier law method is widely used for calculating the LEL of multicomponent flammable gas mixtures. It assumes that the mixture behaves ideally and that the components do not react with each other. The LEL (L) of the mixture is given by the following equation:
L=∑i=1nLixi100
where Li is the LEL of the individual component i in the mixture, and xi is the volume fraction of component i. For mixtures containing inert gases such as CO₂, an empirical correction factor is applied:
L′=100+B(L100−1)100B
where L′ is the corrected LEL, L is the uncorrected LEL, and B is the volume fraction of the inert gas.
Table 2: LELs of Individual Gas Components
| Gas Component | LEL (%) |
|---|---|
| H₂ | 4.0 |
| CO | 12.5 |
| CH₄ | 5.0 |
| C₂H₄ | 3.0 |
Example Calculation:
For the gas mixture in Table 1, the LEL calculated using the Le Chatelier law method (corrected for CO₂) is approximately 4.5%.
2. Jones Method
The Jones method involves grouping the flammable components with inert gases into pseudo-binary mixtures and then applying Le Chatelier’s law to each mixture. However, this method relies heavily on the subjective allocation of inert gases, which can introduce significant errors. Moreover, the method does not have pre-defined tables for all possible gas mixtures, limiting its applicability to lithium iron phosphate battery (LFP battery) thermal runaway products.
3. Adiabatic Flame Temperature Method
The adiabatic flame temperature method is based on the principle that the flame temperature at the LEL is the minimum temperature required for the flame to propagate. The method involves solving the energy conservation equation under adiabatic conditions:
ΔHreactants=ΔHproducts
where ΔH represents the enthalpy change. The flame temperature (Ta) is iteratively determined by minimizing the Gibbs free energy of the system at various temperatures until the enthalpy balance is achieved.
Model Development
A prediction model was developed based on the adiabatic flame temperature and energy conservation principles. The model takes into account the specific gas compositions and volume fractions observed during thermal runaway experiments. The model’s input parameters include the enthalpy of formation, specific heat capacity, and stoichiometric coefficients of the gas components.
Table 3: Thermochemical Properties of Gas Components
| Gas Component | ΔH°<sub>f</sub> (kJ/mol) | C<sub>p</sub> (J/mol·K) |
|---|---|---|
| H₂ | -285.83 | 14.30 |
| CO₂ | -393.51 | 37.13 |
| CO | -110.53 | 29.12 |
| CH₄ | -74.85 | 51.90 |
| C₂H₄ | -125.7 | 84.14 |
Results and Discussion
1. Gas Release Profiles
The gas release profiles during thermal runaway were characterized by two distinct stages: an initial burst of electrolyte vapor (primarily DMC) followed by a larger release of gases generated by internal chemical reactions. The initial release of DMC was observed to be approximately 0.15 mol, while the secondary release was around 0.54 mol.
2. Impact of SOC on LEL
The LEL of thermal runaway products varied significantly with the SOC of the battery. the LEL initially increased with SOC before decreasing at higher SOCs. This trend was attributed to the changing gas compositions, particularly the increased proportion of CO₂ at intermediate SOCs.
3. Comparison of Prediction Methods
The accuracy of the Le Chatelier law, Jones method, and adiabatic flame temperature method was compared by calculating the LEL of a test mixture containing H₂, CO, and CO₂ at various temperatures. As shown in Table 4, the Le Chatelier law method demonstrated the highest accuracy, with a deviation of only 1.14% from experimental values at 25°C.
Table 4: Comparison of Prediction Methods
| Method | Deviation from Experimental Value (%) |
|---|---|
| Le Chatelier Law | 1.14 |
| Jones Method | 6.73 |
| Adiabatic Flame Temp. | 10.02 |
Conclusion
This study presents a comprehensive approach for predicting the lower explosion limit of thermal runaway products from lithium iron phosphate batteries. Through a combination of experimental and theoretical methods, the study provides valuable insights into the gas compositions released during thermal runaway and the factors affecting the LEL. The developed prediction model based on the adiabatic flame temperature and energy conservation principles offers a reliable tool for assessing the explosion risk associated with lithium iron phosphate battery (LFP battery).