Energy storage lithium batteries, particularly lithium iron phosphate (LiFePO4) types, are widely adopted in applications such as grid storage and electric vehicles due to their high energy density, long cycle life, and environmental friendliness. However, thermal runaway remains a critical safety concern, potentially leading to fires or explosions. This study investigates the characteristic behaviors and energy flow transmission during thermal runaway in a 50 Ah LiFePO4 energy storage lithium battery under lateral heating conditions. The analysis focuses on surface temperature evolution, flame zone temperature distribution, mass loss, heat generation, and gas combustion dynamics, aiming to elucidate the energy transfer mechanisms between the battery and emitted gases.
The experimental setup involved a lateral heating approach with a 942 W heating element applied to one side of the battery. Key parameters monitored included battery surface temperatures at multiple locations (e.g., negative terminal, safety valve, and electrode tabs), flame zone temperatures at heights of 10 cm, 20 cm, 30 cm, and 60 cm above the valve, mass loss via a precision balance, and gas composition using a gas analyzer. The battery was initially at 100% state of charge (SOC) and underwent thermal runaway triggered by external heating. Data acquisition systems recorded temperature, mass, and gas metrics at high frequency to capture transient behaviors.
Thermal runaway progression was categorized into four stages based on characteristic temperatures: initial heating (0–105 s), self-heating onset (105–368 s), thermal runaway triggering (368–739 s), and post-peak cooling (739–2200 s). The battery venting occurred at 296 s, followed by jet ignition at 304 s. Peak surface temperatures reached up to 693.1°C at the safety valve, with the maximum temperature rise rate of 22.7°C/s observed at 385 s. The flame zone exhibited decreasing peak temperatures with height, from 865.4°C at 10 cm to 690.2°C at 60 cm, and the highest temperature gradient of 61.33°C/cm was recorded at 5 cm above the valve. Mass loss totaled 235 g, with a peak loss rate of 4.22 g/s at 359 s. Heat release analysis revealed that the total heat generated by the battery and gases was 2.59 MJ and 4.14 MJ, respectively, with peak heat release rates of 17.3 kW and 49.55 kW at 384 s. A critical transition at 411 s marked the shift from battery internal reactions to gas combustion as the dominant heat source.
The internal heat generation in the energy storage lithium battery during thermal runaway is governed by exothermic side reactions, including SEI decomposition, anode-electrolyte reactions, electrolyte decomposition, and binder degradation. The overall heat release rate (HRR) from the battery is expressed as:
$$ Q = \sum Q_i = Q_{\text{sei}} + Q_{\text{an}} + Q_{\text{e}} + Q_{\text{pvdf}} $$
where each component \( Q_i \) represents the heat release rate per unit volume for specific reactions. The reaction kinetics follow the Arrhenius equation:
$$ Q_i = -W_i H_i \frac{dc_i}{dt} $$
Here, \( W_i \) is the reactant density, \( H_i \) is the reaction enthalpy, and \( \frac{dc_i}{dt} \) is the reaction rate. The individual reaction rates are modeled as:
$$ \frac{dc_{\text{sei}}}{dt} = -A_{\text{sei}} c_{\text{sei}} e^{-\frac{E_{a,\text{sei}}}{RT}} \quad \text{(SEI decomposition, 80–120°C)} $$
$$ \frac{dc_{\text{an}}}{dt} = -A_{\text{an}} c_{\text{an}} e^{-\frac{E_{a,\text{an}}}{RT}} e^{-\frac{t_{\text{sei}}}{t_{\text{sei,ref}}}} \quad \text{(Anode-electrolyte reaction, 90–250°C)} $$
$$ \frac{dc_{\text{e}}}{dt} = -A_{\text{e}} c_{\text{e}} e^{-\frac{E_{a,\text{e}}}{RT}} \quad \text{(Electrolyte decomposition, 230–310°C)} $$
$$ \frac{dc_{\text{pvdf}}}{dt} = -A_{\text{pvdf}} c_{\text{pvdf}} e^{-\frac{E_{a,\text{pvdf}}}{RT}} \quad \text{(Binder decomposition, 270–370°C)} $$
The kinetic parameters for these reactions are summarized in Table 1, which includes activation energies, frequency factors, and enthalpies derived from experimental data.
| Reaction Stage | Enthalpy \( H_i \) (J/kg) | Density \( W_i \) (kg/m³) | Frequency Factor \( A_i \) (s⁻¹) | Activation Energy \( E_{a,i} \) (J/mol) | Dimensionless Concentration \( c_i \) |
|---|---|---|---|---|---|
| SEI Decomposition | 2.57 × 10⁵ | 6.104 × 10² | 1.667 × 10¹⁵ | 1.3508 × 10⁵ | 0.15 |
| Anode-Electrolyte | 1.714 × 10⁶ | 6.104 × 10² | 2.5 × 10¹³ | 1.3508 × 10⁵ | 0.75 |
| Electrolyte Decomposition | 1.55 × 10⁵ | 4.069 × 10² | 5.14 × 10²⁵ | 2.74 × 10⁵ | 1 |
| Binder Decomposition | 1.5 × 10⁶ | 8.14 × 10⁴ | 1.917 × 10²⁵ | 2.86 × 10⁵ | 1 |
For gas combustion, the heat release rate (HRR) and total heat release (THR) are calculated using oxygen consumption calorimetry. The HRR is given by:
$$ \text{HRR} = \left[ E\Phi – (E_{\text{CO}} – E) \frac{1 – \Phi}{2} \cdot \frac{X_{\text{CO}}}{X_{\text{O}_2}} \right] \cdot \frac{m_a}{1 + \Phi (a – 1)} \cdot \frac{M_{\text{O}_2}}{M_a} \cdot \frac{(1 – X^0_{\text{H}_2\text{O}}) X^0_{\text{O}_2}}{X_{\text{O}_2}} $$
where \( m_a \) is the air mass flow rate, \( \Phi \) is the oxygen depletion factor, \( E \) and \( E_{\text{CO}} \) are heats released per kg of oxygen consumed for combustion and CO oxidation, respectively, \( X \)-terms denote mole fractions, \( M \) represents molar masses, and \( a \) is the expansion factor. The THR is the integral of HRR over time:
$$ \text{THR} = \int_0^t \text{HRR}(t) \, dt $$
The battery surface temperature dynamics are critical for understanding thermal propagation. The average surface temperature \( \bar{T} \) and temperature rise rate \( \delta \) are defined as:
$$ \bar{T} = \frac{T_{\text{I}} + T_{\text{II}} + T_{\text{III}} + T_{\text{IV}} + T_{\text{V}}}{5} $$
$$ \delta = \frac{dT_{\text{I}}}{dt} $$
where \( T_{\text{I}} \) to \( T_{\text{V}} \) correspond to temperatures at the center, positive tab, negative tab, negative side, and safety valve, respectively. The flame zone temperature gradient \( \gamma \) is calculated as:
$$ \gamma = \frac{\Delta T}{\Delta L} $$
with \( \Delta T \) and \( \Delta L \) being temperature and distance differences between measurement points. Mass loss rate \( \alpha \) is given by:
$$ \alpha = \frac{dm}{dt} $$
Experimental results for temperature profiles are summarized in Table 2, highlighting key metrics across different stages.
| Parameter | Value | Time (s) | Location |
|---|---|---|---|
| Venting Temperature | 108.8°C | 296 | Safety Valve |
| Peak Surface Temperature | 693.1°C | 739 | Safety Valve |
| Max Temperature Rise Rate | 22.7°C/s | 385 | Battery Center |
| Flame Zone Peak (10 cm) | 865.4°C | 330 | Above Valve |
| Flame Zone Peak (60 cm) | 690.2°C | 334 | Above Valve |
| Max Temperature Gradient | 61.33°C/cm | 739 | 5 cm Above Valve |
Mass loss data further illustrate the intensity of thermal runaway. The battery lost 18.3% of its initial mass (235 g), with the most significant loss occurring during the venting and jet combustion phases. The mass loss rate peaked at 4.22 g/s at 359 s, coinciding with vigorous gas ejection. The cumulative mass loss over time is presented in Table 3, along with corresponding heat release metrics.
| Time (s) | Mass (g) | Mass Loss Rate (g/s) | Battery HRR (kW) | Gas HRR (kW) | Battery THR (MJ) | Gas THR (MJ) |
|---|---|---|---|---|---|---|
| 0 | 1285 | 0 | 0 | 0 | 0 | 0 |
| 296 | 1260 | 1.87 | 5.2 | 12.1 | 0.45 | 0.58 |
| 359 | 1180 | 4.22 | 14.8 | 38.5 | 1.12 | 1.89 |
| 384 | 1150 | 3.95 | 17.3 | 49.55 | 1.58 | 2.45 |
| 500 | 1105 | 1.80 | 1.86 | 30.2 | 2.01 | 3.12 |
| 739 | 1070 | 0.45 | 0.92 | 5.1 | 2.59 | 4.14 |
The energy flow transition at 411 s is a pivotal finding. Before this point, battery internal reactions dominated heat generation, whereas afterward, gas combustion contributed the majority of energy. This shift is quantified by the energy ratio \( R \):
$$ R = \frac{\text{THR}_{\text{gas}}}{\text{THR}_{\text{battery}}} $$
which increased from 0.75 at 300 s to 1.6 at 500 s, indicating the growing predominance of gas combustion. The jet combustion involved gases such as H₂, CO, CH₄, and C₂H₄, with reactions like:
$$ \text{C}_2\text{H}_4 + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 2\text{H}_2\text{O} $$
$$ 2\text{CO} + \text{O}_2 \rightarrow 2\text{CO}_2 $$
$$ 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} $$
These exothermic processes significantly elevated the flame zone temperatures and contributed to the high HRR peaks.
In conclusion, this study delineates the thermal runaway mechanisms in energy storage lithium batteries, emphasizing the interplay between internal reactions and external combustion. The data and models provided can inform safety designs, such as improved thermal management systems and passive protection strategies, to mitigate risks in large-scale energy storage applications. Future work should explore variations in SOC, battery chemistry, and scaling effects to enhance predictive accuracy.