The safety of lithium batteries has always been a concern in the industry. Due to their special material structure and complex operating environment, once a fire accident occurs, it will cause equipment damage, property loss, and even casualties. After a lithium battery fire occurs, the disposal is difficult, takes a long time, and often involves the generation of a large amount of toxic gases. Therefore, timely fire extinguishing can effectively control the spread of the fire, avoid extensive burning, and provide more time for personnel to escape.

During the thermal runaway process of lithium-ion batteries, smoke, fire, and even explosion often occur. Therefore, controlling the thermal runaway and diffusion problem has become the main challenge faced by lithium battery products in the process of use. Choosing the right fire extinguishing technology can prevent the further spread of battery thermal runaway, which is of great significance for suppressing fire occurrence.

This article will introduce the mainstream fire extinguishers and extinguishing mechanisms currently available on the market, and analyze the advantages and disadvantages of different types of fire extinguishers.

Types of Fire Extinguishers

Currently, the fire extinguishers on the market are mainly divided into gas fire extinguishers, water-based fire extinguishers, aerosol fire extinguishers, and dry powder fire extinguishers. Below is an introduction to the codes and characteristics of each type of fire extinguisher.

Perfluorohexane: Perfluorohexane has been listed in the PFAS inventory of the OECD and the US EPA. Therefore, the use of perfluorohexane as a fire extinguishing agent should comply with local laws and regulations and communicate with environmental regulatory agencies. Since the products of perfluorohexane in thermal decomposition are greenhouse gases, it is not suitable for long-term, large-dose, continuous spraying. It is recommended to use it in combination with a water spray system.

Trifluoromethane: Trifluoromethane agents are only produced by a few manufacturers, and there are no specific national standards regulating this type of fire extinguishing agent. The maintenance cost is high, so its use is not recommended.

Hexafluoropropane: This extinguishing agent is prone to damage devices or equipment during use, and its Global Warming Potential (GWP) is relatively high. Therefore, hexafluoropropane can only be used as a transitional fire extinguishing agent.

Heptafluoropropane: Due to the greenhouse effect, it is gradually being restricted by various countries and will face elimination. Currently, heptafluoropropane agents have been discontinued, which will lead to problems in refilling existing heptafluoropropane systems during maintenance. Therefore, its use is not recommended.

Inert Gas: Including IG 01, IG 100, IG 55, IG 541, among which IG 541 is more widely used and is internationally recognized as a green and environmentally friendly fire extinguishing agent. However, it has the disadvantages of high construction cost, high demand for gas cylinders, and large space occupation.

Water-Based Agent: Fine water mist fire extinguishers are widely used, and they have the best cooling effect. This is mainly because water has a large specific heat capacity, which can quickly absorb a large amount of heat, cooling the unreacted active substances inside the battery and thus inhibiting further temperature rise. However, water causes significant damage to batteries and is not insulating, leading to battery short circuits.

Aerosol: Due to its environmental friendliness, non-toxicity, low cost, and easy maintenance, aerosol has become the mainstream fire extinguishing agent. However, the selected aerosol should comply with UN regulations and local laws and regulations, and local national product certification is required. However, aerosols lack cooling capabilities, and during their application, the battery temperature remains relatively high. After the fire extinguishing agent stops releasing, the battery is prone to reignition.

Effectiveness of Fire Extinguishants

The State Key Laboratory of Fire Science at the University of Science and Technology of China conducted a study comparing the fire extinguishing effects of ABC dry powder, heptafluoropropane, water, perfluorohexane, and CO2 fire extinguishants on a 38A lithium-ion battery.

Fire Extinguishing Process Comparison

ABC dry powder, heptafluoropropane, water, and perfluorohexane can all quickly extinguish battery fires without reignition. However, CO2 fire extinguishers cannot effectively extinguish battery fires and may cause reignition.

Comparison of Fire Suppression Results

After thermal runaway, the behavior of lithium batteries under the action of fire extinguishants can be roughly divided into three stages: the cooling stage, the stage of rapid temperature rise, and the stage of slow temperature decline.

The first stage is the cooling stage, where the temperature of the battery surface decreases after the fire extinguishant is released. This is mainly due to two reasons:

• Battery venting: Before the thermal runaway of lithium-ion batteries, a large amount of alkanes and CO2 gas accumulate inside the battery. When the battery reaches its pressure limit, the safety valve opens, releasing high-pressure gas. This gas carries out the active substances inside the battery while also providing some cooling effect to the battery.
• Effect of the fire extinguishant: The cooling effect of the fire extinguishant mainly comes from two parts: the heat absorption during phase change and the chemical isolation effect. Phase change heat absorption directly removes the heat generated by the battery, while the chemical isolation effect indirectly reduces heat generation by interrupting chemical reactions. Water has the most significant cooling effect because of its high specific heat capacity, allowing it to absorb a large amount of heat rapidly. Perfluorohexane follows, while HFC-227ea, CO2, and ABC dry powder do not show significant cooling effects, which is related to the nature and mechanism of the fire extinguishants.

The second stage is the rapid temperature rise stage, where the battery temperature rapidly rises from its minimum value to its peak. Since fire extinguishants cannot completely stop the decomposition reaction inside the battery, and most fire extinguishants have poor cooling effects, the temperature of the battery shows an almost vertical upward trend for different fire extinguishants. In a short period, the temperature of the battery rises to its peak.

In this stage, there is a significant difference in the effectiveness of different fire extinguishants in inhibiting the rise in battery temperature. The effectiveness in descending order is water > perfluorohexane > HFC-227ea > ABC dry powder > CO2. When the battery temperature rises slowly, it provides more response time for battery fire warning and more reaction time for operators.

Conclusion

1. CO2: Fire extinguishants like CO2, which primarily act by suffocation and isolation, have poor inhibitory effects on battery fires. In this study, severe reignition phenomena occurred with CO2, making it unsuitable for lithium battery fires.
2. ABC Dry Powder / HFC-227ea: ABC dry powder and HFC-227ea fire extinguishants, which primarily act through isolation and chemical suppression, can partially inhibit the chain reactions inside the battery to some extent. They have a slightly better effect than CO2, but since they lack cooling effects and cannot completely block internal reactions in the battery, the temperature of the battery still rises rapidly after the fire extinguishant is released.
3. Perfluorohexane: Perfluorohexane not only blocks internal battery reactions but also absorbs heat through vaporization. Therefore, its inhibitory effect on battery fires is significantly better than other fire extinguishants.
4. Water: Among all fire extinguishants, water has the most obvious fire extinguishing effect. This is mainly because water has a large specific heat capacity, allowing it to rapidly absorb a large amount of heat. This cools down the unreacted active substances inside the battery, thereby inhibiting further temperature rise. However, water causes significant damage to batteries and has no insulation effect, so its use should be extremely cautious.

What Should We Choose?

We have surveyed the fire protection systems used by several energy storage system manufacturers currently on the market, primarily employing the following fire extinguishing solutions:

• Perfluorohexane + Water
• Aerosol + Water

It can be seen that synergistic fire extinguishing agents are the mainstream trend for lithium battery manufacturers. Taking Perfluorohexane + Water as an example, Perfluorohexane can quickly extinguish open flames, facilitating the contact of fine water mist with the battery, while fine water mist can effectively cool it down. Cooperative operation has better fire extinguishing and cooling effects compared to using a single fire extinguishing agent. Currently, the EU’s New Battery Regulation requires future battery labels to include available fire extinguishing agents. Manufacturers also need to choose the appropriate fire extinguishing agent based on their products, local regulations, and effectiveness.