Abstract
This paper investigates the thermal runaway behavior of lithium iron phosphate (LiFePO4) and ternary lithium-ion batteries under heating conditions. Using 3.2V 100Ah LiFePO4 batteries and 3.6V 90Ah ternary LiNi0.5Co0.2Mn0.3O2 (NCM523) batteries, we study the thermal runaway characteristics through an external heating method. Our findings show that the LiFePO4 battery releases only white smoke during thermal runaway, while the ternary battery undergoes combustion and intense heat release. This research provides valuable insights into the safety and risk mitigation strategies for lithium-ion batteries in energy storage applications.
Keywords
Lithium iron phosphate battery (LiFePO4 battery), Ternary lithium-ion battery, Thermal runaway, Early warning, Energy storage
1. Introduction
Electrochemical energy storage technologies, particularly lithium-ion batteries, have gained significant attention due to their high energy density, long cycle life, and environmental friendliness. However, thermal safety issues remain a major concern, particularly for large-scale energy storage systems. Thermal runaway, triggered by various abuse conditions, can lead to catastrophic failures such as fire and explosion. This study focuses on the thermal runaway behavior of two popular battery chemistries: lithium iron phosphate (LiFePO4) and ternary LiNi0.5Co0.2Mn0.3O2 (NCM523).
2. Experimental Methods
2.1 Sample Preparation
The experimental samples included square-shaped 3.2V 100Ah LiFePO4 batteries and 3.6V 90Ah ternary NCM523 batteries. Prior to the thermal runaway experiments, both battery types were charged to 100% state of charge (SOC) using a BT2000 battery tester.
Charging Protocol for LiFePO4 Batteries:
- Discharge at 1/3C to 2.0V, rest for 30 minutes.
- Charge at 1/3C to 3.6V, then switch to constant voltage at 1/20C, rest for 30 minutes.
- Repeat steps 1 and 2.
Charging Protocol for Ternary Batteries:
- Discharge at 1/3C to 3.0V, rest for 30 minutes.
- Charge at 1/3C to 4.2V, then switch to constant voltage at 1/20C, rest for 30 minutes.
- Repeat steps 1 and 2.
2.2 Thermal Runaway Tests
The thermal runaway experiments were conducted in an open environment using a 500W heating plate. Temperature and voltage were monitored using K-type thermocouples placed at various locations on the battery surface, including the top and bottom surfaces, sidewalls, and safety valve. A Fourier transform infrared (FTIR) spectrometer was used to analyze the released gases, and a thermal release rate meter measured the heat and smoke release rates.
Experimental Setup:
- Heating Element: 500W heating plate
- Temperature Measurement: K-type thermocouples at six locations
- Gas Analysis: FTIR spectrometer and hydrogen electrochemical sensor
- Heat Release Rate: Motis-3MW thermal release rate meter
- Data Recording: High-speed camera for visual observation
3. Results and Discussion
3.1 Thermal Runaway Phenomena
LiFePO4 Battery:
During heating, the LiFePO4 battery experienced gradual temperature increase, safety valve opening, and intense white smoke release. No combustion occurred. The thermal runaway was declared when the temperature rise rate exceeded 3°C/s at multiple points or visual smoke emission was observed.
Ternary Battery:
The ternary battery showed a similar initial temperature rise, followed by safety valve opening and intense smoke release. However, combustion occurred immediately after safety valve opening, accompanied by a visible flame and continuous ejection of flaming material.
3.2 Temperature Profiles
Temperature Variations:
The temperature profiles of both battery types during thermal runaway are shown in Table 1.
Table 1: Peak Temperatures and Times during Thermal Runaway
| Battery Type | TC1 (Peak Temp, °C) | TC2 (Peak Temp, °C) | TC3 (Peak Temp, °C) | TC4 (Peak Temp, °C) | TC5 (Peak Temp, °C) | TC6 (Peak Temp, °C) |
|---|---|---|---|---|---|---|
| LiFePO4 | 534.2 | 486.3 | 438.2 | 289.0 | 534.2 | 486.3 |
| Ternary NCM523 | 1052.4 | 802.3 | 734.4 | 652.0 | 1052.4 | 734.4 |
Temperature Mutation Analysis:
The temperature mutation and peak times for each thermocouple location are presented in Table 2.
Table 2: Thermal Runaway Development Times
| Battery Type | Δt TC1 (s) | Δt TC2 (s) | Δt TC3 (s) | Δt TC4 (s) | Δt TC5 (s) | Δt TC6 (s) |
|---|---|---|---|---|---|---|
| LiFePO4 | 215 | 294 | 283 | 230 | 311 | 285 |
| Ternary NCM523 | 60 | 7 | 85 | 7 | 148 | 38 |
3.3 Heat Release Characteristics
Smoke and Heat Release Rates:
The smoke release rate (SRR), total smoke release (TSR), heat release rate (HRR), and total heat release (THR) were measured during thermal runaway. The results are shown in Table 3.
Table 3: Smoke and Heat Release Parameters
| Battery Type | TSR (m²) | Max SRR (m²/s) | THR (MJ) | Max HRR (kW) |
|---|---|---|---|---|
| LiFePO4 | 395.0 | 7.5377 | 0.162 | 1.81 |
| Ternary NCM523 | 38.8 | 2.2802 | 3.147 | 134.85 |
3.4 Gas Emission Analysis
Gas emissions during thermal runaway were analyzed using FTIR spectrometry. The primary gases released from both battery types were hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO), and hydrocarbons.
The ternary battery emitted higher levels of CO2 due to combustion of flammable gases. Hydrogen generation was attributed primarily to reactions between the binder and lithium metal.
3.5 Voltage Variations
Voltage profiles during thermal runaway are presented. Both battery types exhibited two distinct voltage drops. The first drop was attributed to internal short circuits, while the second was associated with the final failure.
4. Discussion
4.1 Comparison of Thermal Runaway Characteristics
The LiFePO4 battery demonstrated significantly lower heat release and smoke production compared to the ternary battery. The total heat release of the LiFePO4 battery was approximately 20 times lower, indicating its higher thermal stability. The ternary battery underwent intense combustion, releasing large amounts of heat and flammable gases.
4.2 Early Warning Signals
The presence of H2 and CO in the emitted gases can serve as early warning signals for thermal runaway. These gases can be detected using electrochemical sensors, providing valuable time for preventive measures.
4.3 Safety Implications
The higher thermal stability of LiFePO4 batteries makes them more suitable for large-scale energy storage applications where safety is a critical concern. However, even with LiFePO4 batteries, proper monitoring and safety measures are essential to prevent thermal runaway under abuse conditions.
5. Conclusion
This study comprehensively analyzed the thermal runaway behavior of LiFePO4 and ternary lithium-ion batteries under heating conditions. The LiFePO4 battery exhibited significantly lower heat release and smoke production during thermal runaway, while the ternary battery underwent intense combustion. The emitted gases, primarily H2, CO2, CO, and hydrocarbons, can serve as early warning signals. These findings have important implications for the safe operation and design of lithium-ion batteries in energy storage systems.