PACK Manufacturing Technology Series: Analysis of Battery Pack Thermal Runaway and Thermal Insulation Material Technology

PACK Manufacturing Technology Series: Analysis of Battery Pack Thermal Runaway and Thermal Insulation Material Technology

The Blazing Moment: The Harsh Warning of Lithium Battery Thermal Runaway and the Lifeline of Insulation Materials

In 2024, a lithium battery factory in Gyeonggi Province, South Korea, suddenly exploded and caught fire, claiming the lives of 23 workers in just 15 seconds.

In 2025, a Xiaomi SU7 collided while driving at high speed, and three people inside the vehicle could not be saved…

Whether it's the new energy vehicles speeding down the roads or the energy storage power stations supporting the green energy transition, fire and explosion accidents caused by thermal runaway of lithium battery cells are no longer isolated news. Behind the shocking data lies a severe safety challenge. Incomplete statistics show that in 2024, over 30% of global energy storage station fire accidents could be traced back to lithium battery thermal runaway. In the field of new energy vehicles, once a battery catches fire, the fatality rate for occupants is as high as a staggering 70%!

Faced with such a high-frequency, high-fatality safety threat, building an effective thermal runaway protection system is extremely urgent. Among these, battery pack insulation materials play a crucial role as "thermal barrier guardians." So, how exactly do they contain the spread of heat and buy precious escape time when disaster strikes? What are the mainstream types of insulation materials currently available, guarding this line between life and death?

I. Thermal Runaway and Thermal Barrier

1. Lithium Battery Fires: Rapid High Temperature, Chain-Reaction Runaway

Once a lithium battery catches fire, its characteristics are distinct: rapid burning, intense flames, tremendous destructive power, and extreme difficulty in extinguishment. The core danger lies in the thermal runaway process of the cell. It is not an instantaneous explosion but rather a chain reaction involving swelling/bulging, smoke release, violent ignition, and intense burning. This process releases enormous heat, with peak temperatures reaching a staggering 450°C to 600°C.

What's more dangerous is that localized high temperatures can rapidly trigger a chain thermal spread to adjacent cells, eventually leading to fire or even explosion.

The main triggers for this "chain disaster" fall into three categories:

• Physical damage: External force destruction such as collision, crush, or puncture.

• Temperature abuse: External high temperature or internal local overheating.

• Electrical abuse: Overcharge, over-discharge, or external short circuit.

2. Thermal Barrier Technology: The "Firewall" Containing Disaster Spread

The core mission of thermal barrier technology is to efficiently intercept the deadly thermal shock, high-temperature smoke, and flame radiation released by a runaway battery, cutting off the path of heat transfer to neighboring cells. Its value lies in:

• Weakening the destructive power of fire.

• Winning critical time for firefighting and rescue.

• Maximizing the protection of personnel safety.

The current mainstream thermal runaway protection technology system mainly includes three directions:

• Intelligent monitoring and cooling: Real-time monitoring of battery status, actively triggering cooling or fire suppression measures upon abnormality.

• Structural protection (physical insulation): Constructing an insulation barrier (key materials) between cells to block the pathway for thermal runaway propagation.

• Emergency firefighting: Rapid implementation of efficient fire suppression (such as immersion water cooling, perfluorohexanone spraying, etc.) after thermal runaway occurs.

This article primarily introduces the safety structure design technology, which blocks the heat generated by a thermal runaway cell from transferring to adjacent cells, thereby preventing a chain reaction.

II. Main Types of Thermal Barrier Materials

Currently commonly used thermal insulation materials for power batteries include flame-retardant foam, aerogel felt materials, mica plates, ceramifiable silicone rubber composites, flame-retardant potting compounds, and vacuum insulation panels. The inter-cell insulation board within a battery module is a thermal protection device placed between individual cells, effectively delaying or blocking the propagation of thermal runaway from a single cell to the entire battery system. It generally needs to possess the following performance:

1. Flame-Retardant Foam

Main types: Polyurethane foam (PU), EVA foam, silicone rubber, etc.

These are typically made from the above-mentioned base materials through special foaming processes. They offer good thermal insulation performance and excellent compressibility, which allows them to fully absorb the expansion forces generated by cell cycles.

2. Aerogel Felt Materials

Aerogels can be categorized into bulk, powder, and film. Their outstanding thermal insulation performance comes from their porous structure.

The pore size of aerogel is smaller than the mean free path of air molecules under atmospheric pressure, therefore the air molecules within the pores are nearly stationary, thus avoiding convective heat transfer from air.

Aerogel felt materials, made with SiO2 aerogel as the core, compounded with glass fiber or ceramic fiber, have a thermal conductivity coefficient as low as 0.013–0.025 W/m·K.

Aerogel offers the best insulation effect per unit volume and also provides a significant lightweighting effect (aerogel has an extremely low density, being one of the lightest solid materials known, offering 3-8 times thinning and weight reduction compared to traditional insulation materials).

However, although aerogel's thermal insulation performance is exceptional, its compression deformation is limited, and its buffering/absorption capacity is still insufficient when dealing with the expansion forces generated during cell cycles. Therefore, it is generally used in conjunction with cushioning foam.

3. Mica Plate

Main composition: Composite pressed from mica paper (90%) and silicone rubber (10%).

Characteristics:

• High-temperature resistance: Continuous use temperature 500–850°C, intermittent up to 1050°C.

• Insulation: Breakdown strength ≥20 kV/mm, volume resistivity >10¹⁴ Ω·cm.

• Mechanical strength: Flexural strength >200 MPa, chemical corrosion resistance.

Application location: Mica plates are commonly used between the module and the enclosure cover to prevent high-temperature smoke ejected from the vent after a cell thermal runaway from melting through the cover or damaging module structural components.

4. Flame-Retardant Potting Compound

For cylindrical cell PACKs, since the largest surface of the cell is cylindrical, flame-retardant potting compounds are also commonly used as insulation materials. For example, Tesla's power battery pack uses potting compounds to fill the gaps between cylindrical batteries, serving to avoid heat transfer between cells, improve impact resistance, and enhance the overall thermal and mechanical stability of the battery pack.

Main composition: Base material, curing agent, flame-retardant additives, fillers (to improve strength and wear resistance).

Characteristics:

• Excellent flame retardancy: Can reach UL94 V-0 level, carbonizes upon burning to isolate oxygen.

• Good electrical insulation: Volume resistivity ≥1×10¹⁵ Ω·cm, dielectric strength ≥20 kV/mm.

• High bonding strength, good impact resistance, heat resistance.