Why Do Ordinary Insulation Materials Fail During EV Battery Thermal Runaway?

Ordinary electrical insulation can soften, shrink, burn, carbonize, crack, or lose dielectric strength when exposed to the combination of extreme heat, flame, hot particles, pressure, and vent gases generated during battery thermal runaway.

The correct solution is to use thermal-runaway barriers specifically designed for high-temperature insulation, low heat transfer, flame resistance, dielectric stability, and mechanical integrity under abuse conditions. A material that performs well at normal battery operating temperature is not automatically suitable for a thermal runaway event.

Why Is Thermal Runaway Different from Normal Battery Heating?

Normal insulation may survive long-term operation at elevated temperature but fail rapidly when thermal runaway adds direct flame, pressure, hot particles, and concentrated heat flux.

The solution is to evaluate insulation against abuse conditions rather than only continuous-use temperature ratings. UL Solutions notes that battery thermal runaway can produce jet-like flame, ejected particles, high temperature, and pressure inside the enclosure, meaning the protective material must withstand several simultaneous failure mechanisms.^[1]

This distinction is critical. A material rated for normal electrical insulation at 105°C, 125°C, or even higher may still lose structure or dielectric performance when exposed to direct thermal runaway conditions.

Why Can Ordinary Polymer Insulation Soften or Collapse?

Many ordinary polymer films and foams are designed for electrical insulation, cushioning, or sealing—not direct exposure to severe thermal runaway heat—so they may soften, shrink, melt, or permanently deform.

The correct solution is to choose materials whose thermal stability remains sufficient under the actual abuse temperature and exposure duration. High-temperature mica, ceramic-based barriers, aerogel systems, and specially formulated silicone materials are commonly considered when ordinary insulation cannot maintain its function.

Material selection must distinguish between operating temperature resistance and thermal runaway resistance. These are not the same specification.

Why Is UL 94 V-0 Alone Not Enough?

A material can achieve UL 94 V-0 and still provide inadequate protection against cell-to-cell thermal propagation because a small-scale flame classification does not represent the complete thermal runaway event.

The solution is to treat flame rating as only one material property and separately validate thermal insulation, heat flux resistance, mechanical retention, vent-gas exposure, and system-level propagation performance. Rogers specifically notes that once a cell enters thermal runaway, a V-0 foam does not stop the energy release by itself; pack architecture and thermal barriers remain critical.^[2]

This is one of the most common specification mistakes in battery materials: “V-0” does not mean “thermal-runaway proof.”

What Failure Modes Must Battery Insulation Survive?

If an engineer evaluates only flame resistance, the insulation may still fail from erosion, pressure, hot particles, dielectric breakdown, compression loss, or heat transfer to the adjacent cell.

The correct solution is to screen the complete failure environment before choosing the barrier material. UL's battery enclosure evaluation specifically combines high temperature, pressure, and ejected particles because thermal runaway is a multi-physics event rather than a simple flame test.^[1]

Why Does Low Thermal Conductivity Matter?

A material can remain physically intact but still fail as a thermal barrier if heat passes through it quickly enough to trigger the neighboring cell.

The solution is to control both material survivability and heat transfer through the barrier. UL Prospector notes that aerogels, ceramic blankets, and encapsulating foams can perform well in low thermal conductivity and density, while mica provides strong electrical insulation but may require more thickness for sufficient fire protection.^[3]

The important design target is therefore not simply “does the material burn?” but also “how much heat reaches the protected side, and how quickly?”

Why Can Thin Plastic Films Be Insufficient?

Thin polymer films can provide excellent dielectric separation during normal operation but contain very little thermal mass or thickness to resist sustained heat transfer during thermal runaway.

The correct solution is to use thin films primarily for electrical isolation where appropriate and add a dedicated thermal barrier when the thermal-propagation risk requires it. A multilayer structure can separate the functions of electrical insulation, thermal insulation, mechanical protection, and adhesion.

Trying to make one thin electrical film perform all four functions usually forces unacceptable compromises.

Why Can Ordinary Foam Be Insufficient?

Conventional foam may perform well as a compression pad, seal, or vibration absorber but lose strength or insulation performance when directly exposed to thermal runaway.

The solution is to use foam specifically formulated and validated for thermal propagation when the component must provide both compression and fire protection. Rogers' ProCell EV Firewall materials are examples of silicone elastomeric systems developed specifically to combine battery compression functionality with thermal propagation protection.^[4]

This illustrates an important rule: ordinary silicone foam and thermal-runaway-rated silicone barriers should not automatically be treated as the same material class.

Why Is Mica Commonly Used for Thermal Runaway Protection?

Where ordinary polymer insulation cannot retain electrical isolation and barrier structure at high temperature, the failure can expose nearby cells, busbars, or pack components.

The solution is to use mica where high-temperature dielectric stability and rigid thermal protection are required. Mica has strong electrical insulation capability and is widely used in EV battery thermal and fire-protection applications, although the required thickness and construction still depend on the specific pack architecture.^[3]

Mica is particularly useful for cell barriers, module walls, busbar insulation, pack-lid protection, and other locations where shape stability and electrical insulation matter.

Should One Material Handle the Entire Thermal Runaway Problem?

Expecting one insulation sheet to provide fire resistance, ultra-low heat transfer, compression recovery, sealing, vibration control, dielectric isolation, and structural protection can result in an expensive material that still fails one critical requirement.

The better solution is often a multilayer system in which each material performs the function it handles best. For example, mica can provide high-temperature dielectric protection, aerogel can reduce heat transfer, and specialized silicone foam can provide compression and thermal-propagation resistance.

UL Prospector similarly notes that combinations of materials may be necessary to satisfy structural requirements and demanding thermal-runaway tests.^[3]

How Should Thermal Runaway Insulation Be Selected?

Selecting material from a datasheet temperature rating alone can create false confidence because real battery abuse involves time, heat flux, flame, particles, pressure, geometry, and neighboring components.

The correct solution is to select the material from the actual cell format, failure location, required delay time, protected-side temperature target, electrical requirement, mechanical load, available thickness, and abuse-test specification. Material samples should then be validated in representative assemblies.

Engineering Information to Define First:

Cell chemistry and format, barrier location, maximum available thickness, dielectric requirement, expected vent direction, thermal propagation target, compression requirement, operating temperature, adhesive construction, mechanical fixing, and applicable battery abuse-test standard.

How Should Thermal Barrier Performance Be Validated?

A material that passes a small coupon flame test can still fail inside the actual battery because joints, fasteners, vent paths, compression, and neighboring components change the heat-flow path.

The solution is to validate both material-level and system-level performance under representative thermal runaway conditions. UL 2596 evaluates battery enclosure materials under thermal and mechanical exposure, while EV battery safety requirements such as ISO 6469-1 and UN Regulation No. 100 address rechargeable energy storage system safety at the vehicle level.^[1][5][6]

UNECE has also continued work on stronger thermal-propagation requirements for electric vehicle battery systems, reinforcing that thermal runaway protection is a system-level safety issue rather than a single-material flame-rating problem.^[6]

Need to Select a Thermal Barrier for an EV Battery?

For a preliminary material review, prepare the Cell, Module, or Pack location, available thickness, protected-side temperature target, dielectric requirement, compression requirement, cell chemistry, vent direction, CAD geometry, adhesive requirement, and applicable thermal-runaway test target.

The correct barrier should be selected from the actual failure mode—not simply from a flame rating or temperature number printed on a datasheet.

FAQ

Can Normal Electrical Insulation Survive Battery Thermal Runaway?

Not necessarily. Ordinary electrical insulation may work at normal operating temperatures but lose strength, shape, or dielectric performance during extreme thermal runaway exposure.

Is UL 94 V-0 Enough for an EV Battery Thermal Barrier?

No. V-0 indicates specific flammability behavior under a standardized test, but thermal runaway protection also depends on heat transfer, material thickness, venting, barrier construction, and pack architecture.

Why Is Mica Better Than Ordinary Plastic for Thermal Runaway?

Mica offers much stronger high-temperature dielectric and structural stability than many ordinary polymer films, making it useful for rigid thermal and electrical barriers.

Can Silicone Foam Protect Against Thermal Runaway?

Specially formulated and validated silicone materials can provide thermal-propagation protection, but ordinary silicone foam should not automatically be assumed to provide the same performance.

What Properties Matter Most in a Thermal Runaway Barrier?

Important properties include thermal resistance, flame resistance, dielectric strength, high-temperature stability, erosion resistance, mechanical integrity, thickness, density, and compatibility with the battery structure.

Is the Highest Temperature Rating Always the Best Choice?

No. Thermal runaway performance depends on heat flux, exposure time, geometry, thickness, material construction, and system design—not temperature rating alone.

15-Year Automotive Battery Component Perspective

Based on 15 years of automotive component and wire-harness manufacturing experience, I never approve a battery insulation material simply because it is electrically insulating, flame-retardant, or rated for a high continuous temperature.

Thermal runaway creates a completely different failure environment: intense heat, flame, pressure, hot particles, electrical stress, and exposure time act together. The reliable solution is to define the actual failure path first and then select mica, aerogel, ceramic material, specialty silicone, or a multilayer construction that can protect that exact location.

Authoritative References

[1] UL Solutions — Battery Enclosure Thermal Runaway Evaluation and UL 2596

[2] Rogers Corporation — Where UL 94 V-0 Matters in Real-World Designs

[3] UL Prospector — Selecting Fire Protection Materials for an EV Battery

[4] Rogers Corporation — ProCell EV Firewall Thermal Propagation Protection

[5] ISO — ISO 6469-1:2019 Electrically Propelled Road Vehicles, Rechargeable Energy Storage System Safety

[6] UNECE — UN Regulation No. 100 Rev.3, Electric Power Train Safety