4.2 Thermal Runaway Propagation & Cell Venting

A robust thermal runaway mitigation strategy combines testing, engineering judgement, and design integration. The goal is not only to meet standards, but to ensure that a cell failure does not escalate into a larger safety event.

1. Understanding thermal runaway behavior

Thermal runaway is an uncontrollable, self-sustaining exothermic reaction triggered by mechanical, electrical, or thermal abuse. As cell energy density increases, preventing propagation from one cell to neighboring cells becomes a central engineering challenge.

To understand failure behavior, engineers carry out abuse testing such as nail penetration, overcharge, impact, drop, vibration, and shock testing. These tests help determine whether a cell enters thermal runaway, how it vents, how much heat is released, and whether the failure remains contained or propagates further.

2. Calorimeter testing and cell characterization

Calorimeter testing is used to determine key failure characteristics of a cell, including:

• Venting temperature
• Thermal runaway onset temperature
• Heat release
• Gas volume
• Gas composition

This information is essential for parameterizing thermal runaway models at module and pack level and for developing propagation resistance strategies.

3. Venting and degassing strategy

When a cell vents, hot gases and particles must be directed safely away from sensitive components and, where relevant, away from passengers. The venting and exhaust design must also prevent dangerous pressure buildup inside the module or pack that could lead to deflagration or explosion.

A venting strategy typically considers pressure relief elements, gas flow paths, housing design, and the interaction between vent gases and surrounding materials.

4. Passive propagation resistance design

Passive propagation resistance aims to keep the heat transfer from a failing cell to neighboring cells below their trigger threshold. This is typically achieved by integrating thermal barrier materials between cells or around cell groups.

Material selection depends on several factors, including:

• Thermal conductivity
• Heat capacity
• Flame retardancy
• Mechanical stability
• Manufacturability
• Ease of integration into the module design

Common thermal barrier materials include aerogels, mica, ceramic papers, ceramic fibers, intumescent materials, and phase change materials.

5. Modelling and simulation

Computational fluid dynamics (CFD) and thermal modelling are used to simulate hot gas flow, conductive heat transfer, and the interaction of failure events with busbars, housing structures, and neighboring modules. These models support design decisions before full-scale testing and help identify weak points in the safety concept.

6. Safety assessment and engineering responsibility

EUCAR hazard levels are often used to define acceptable failure outcomes, ranging from Level 1 (no effect) to Level 7 (explosion). For many automotive and battery energy storage applications, a single-cell failure is typically expected to remain within Level 4 (venting).

At the same time, engineers should not rely on compliance alone. Safety must be treated as a continuous design responsibility throughout development. Cost, timeline, and resource constraints are important, but they should never outweigh user safety or system integrity. In practice, exceeding minimum requirements can reduce long-term risk, liability, and redesign effort.

Additional process considerations

Thermal runaway mitigation is not based on a single material or component, but on the interaction of multiple design features working together.

Thermal barrier materials such as aerogels, mica, ceramic papers, and ceramic fibers are widely used to slow heat transfer between cells. The right choice depends on the application, required performance, packaging constraints, and manufacturing approach.

Thermal runaway bonding materials can play an important role in keeping thermal barrier materials in place during a failure event while also contributing to propagation resistance.

Vent discs and breathable membranes are used to release pressure when internal gas generation occurs, while maintaining sealing performance during normal operation.

Intumescent materials expand when exposed to heat or flame, creating a temporary insulating barrier that can slow heat transfer and shield adjacent components.

The overall design is usually a compromise between cost, safety performance, integration complexity, manufacturability, and space constraints.