The pursuit of carbon neutrality has propelled lithium-ion batteries to the forefront of energy storage technology, powering applications from portable electronics to electric vehicles and grid-scale storage systems. However, the high energy density inherent to these energy storage cells carries a concomitant risk: thermal runaway. This complex failure mode, characterized by uncontrolled temperature escalation, can lead to fires, explosions, and the release of toxic gases, posing significant safety challenges. Developing effective countermeasures is therefore paramount. While various suppressants, including gaseous agents and water-based systems like fine water mist, have been explored, their efficacy has limitations. This investigation focuses on a novel aerogel-based fire extinguishing agent, evaluating its performance in suppressing thermal runaway in large-format lithium iron phosphate (LFE) energy storage cells and comparing its effectiveness directly with conventional fine water mist.
The core of this experimental study is a 100 Ah prismatic LFE energy storage cell. To understand the intervention points, the characteristic progression of thermal runaway under external heating was first established. Key parameters—surface temperature, internal pressure (via expansion force measurement), and flame behavior—were monitored. Analysis revealed three critical inflection points in the hazard evolution, serving as strategic nodes for fire suppressant application. The first node (Node 1) is identified by a marked acceleration in the battery’s expansion force, approximately at 5.4 kN, signaling intense internal gas generation. The second node (Node 2) occurs 3 minutes after the safety vent opens, representing a period of stable flaming combustion. The third node (Node 3) is the thermal runaway trigger point, defined when the cell’s core temperature reaches 150°C, immediately preceding the violent jet fire phase.
The experimental platform consisted of a sealed test chamber housing the energy storage cell, which was instrumented with an array of K-type thermocouples and an expansion force sensor. A heating plate provided the external thermal insult. The fire suppression system featured a pump-pressurized delivery mechanism for both the aerogel-based agent and, separately, fine water mist. The aerogel agent is formulated by dispersing surface-modified, inorganic porous aerogel particles (2-10%), phosphorus-based and nitrogen-based flame retardants (15-18% and 13-16%, respectively), and a wetting agent (3-5%) into an aqueous solution. Its key physical properties are summarized in Table 1.
| Parameter | Value / Description |
|---|---|
| Appearance | Pale yellow, transparent liquid |
| Density | 1.18 g/cm³ |
| pH | 7.43 |
| Surface Tension | 14.46 mN/m |
| Viscosity | 2.11 mPa·s |
| Freezing Point | -16 °C |
A series of tests were conducted, with suppression initiated at the three defined nodes for both the aerogel agent and fine water mist, each applied for a duration of 5 minutes. A baseline test with no suppression was also performed. The test matrix is detailed in Table 2.
| Test ID | Suppression Start Point | Suppressant Type |
|---|---|---|
| Baseline | N/A (Thermal Runaway Only) | None |
| A-1 | Node 1 (Expansion Force 5.4 kN) | Aerogel Agent |
| A-2 | Node 2 (3 min post-vent opening) | Aerogel Agent |
| A-3 | Node 3 (Core Temp. 150°C) | Aerogel Agent |
| W-1 | Node 1 (Expansion Force 5.4 kN) | Fine Water Mist |
| W-2 | Node 2 (3 min post-vent opening) | Fine Water Mist |
| W-3 | Node 3 (Core Temp. 150°C) | Fine Water Mist |
When applied at Node 1 (during intense gas generation but before venting or open flame), both suppressants demonstrated a similar capability to cool the energy storage cell and decelerate the internal pressure rise. The cooling mechanism in this sub-100°C regime can be qualitatively described by the heat absorption rate of the liquid droplets contacting the cell surface:
$$ P_w = \mu (T_a – T_0) $$
where \( P_w \) is the cooling power, \( \mu \) is a heat transfer coefficient, \( T_a \) is the battery surface temperature, and \( T_0 \) is the ambient/suppressant temperature. This shows the cooling rate is proportional to the temperature difference. Both agents, having similar application parameters, performed comparably at this early stage.
The most significant divergence occurred at Node 2, during stable flaming combustion. While both agents extinguished the open flame, the aerogel-based agent accomplished this approximately 70 seconds faster. More importantly, post-extinction behavior differed markedly. After water mist application, high-temperature gases continued to be emitted from the vent for nearly 300 seconds, indicating ongoing internal reactions. In contrast, the aerogel agent formed a dense, solid foam crust over the cell surface and vent. This layer provided an oxygen-barrier effect, effectively stifling the internal exothermic reactions and preventing re-ignition. This crust is a result of the aerogel particles and other constituents dehydrating and sintering upon contact with the hot surface of the energy storage cell.
At Node 3, during the peak jet-fire phase, the absolute cooling effect on the cell was limited for both agents due to the intense flame deflecting and vaporizing the suppressant spray. However, the aerogel agent again showed superior flame suppression dynamics. It rapidly reduced the jet fire’s duration by approximately 85%, transforming the coherent flame into a shorter,扇形火流 before complete extinction. This is attributed to the combined physical and chemical action of the agent. The chemical inhibition stems from the decomposition of its phosphorus and nitrogen-based retardants at high temperatures. For example, phosphorus compounds can decompose and generate radical species (e.g., PO·) that scavenge H· and OH· radicals in the flame zone, disrupting the combustion chain reaction:
$$
\begin{aligned}
\text{PO·} + \text{H·} &\rightarrow \text{HPO} \\
\text{HPO} + \text{H·} &\rightarrow \text{PO·} + \text{H}_2 \\
\text{PO·} + \text{OH·} &\rightarrow \text{HPO} + \text{O·}
\end{aligned}
$$
Simultaneously, nitrogen-based compounds decompose to release inert gases like \( \text{NH}_3 \) and \( \text{CO}_2 \), which dilute the fuel and oxidizer concentrations. This combined chemical action, absent in pure water mist, allows the aerogel agent to more effectively attack the flame chemistry of the burning energy storage cell. A comparative summary of key results is presented in Table 3.
| Performance Metric | Aerogel-Based Agent | Fine Water Mist |
|---|---|---|
| Cooling at Early Stage (Node 1) | Effective, equal to water mist | Effective |
| Time to Extinguish Flame at Node 2 | ~30 seconds (Faster) | ~100 seconds |
| Post-Extinction Gas Emission | Significantly Suppressed | Persistent (~300 s) |
| Re-ignition Risk | Low (Oxygen barrier crust) | Moderate to High |
| Jet Fire Duration Reduction at Node 3 | ~85% | Less effective |
| Primary Suppression Mechanism | Cooling + Oxygen Barrier + Chemical Inhibition | Primarily Cooling |
The cooling power of the suppressant when the energy storage cell surface is above the boiling point of water is dominated by the latent heat of vaporization. Assuming droplets vaporize immediately upon contact, the cooling rate \( P_{wm} \) can be expressed as:
$$ P_{wm} = c_w Q_w \rho (100 – T_0) + h_f Q_w \rho $$
where \( c_w \) is the specific heat of water, \( Q_w \) is the flow rate of suppressant impacting the cell, \( \rho \) is density, \( h_f \) is the latent heat of vaporization, and \( T_0 \) is the initial suppressant temperature. This equation highlights that for a fixed application system, the cooling rate is a constant when \( T_a > 100°C \), explaining the similar initial cooling profiles observed for both agents when applied to a hot energy storage cell.
In conclusion, this experimental investigation demonstrates that the novel aerogel-based fire extinguishing agent offers a multifaceted approach to mitigating thermal runaway hazards in lithium-ion energy storage cells. Its performance is comparable to fine water mist in early-stage cooling but surpasses it significantly in open flame suppression speed and, critically, in post-extinction containment. The formation of a stable, oxygen-blocking crust on the energy storage cell surface is a unique feature that addresses the persistent challenge of internal reaction stoppage and re-ignition. Furthermore, even when applied during full jet fire, it drastically reduces the fire duration, thereby lowering the overall hazard. The synergy of cooling, barrier protection, and gas-phase chemical inhibition makes this aerogel-based agent a promising candidate for enhancing the safety protocols of systems utilizing high-capacity energy storage cells.