Understanding Thermal Runaway in Lithium-Ion Battery

Introduction

From smartphones to electric vehicles and energy storage systems, lithium-ion (Li-ion) batteries power much of our modern world. Their high energy density and reliability make them ideal for a wide range of applications. However, when these batteries are pushed beyond their limits, they can become hazardous. One of the most serious risks is thermal runaway — a self-accelerating, heat-driven reaction that can lead to fires or explosions. This article explores what thermal runaway is, how it happens, and how we can mitigate its effects.

What Is Thermal Runaway?

Thermal runaway is a dangerous chain reaction inside a Li-ion battery where heat triggers further reactions, which in turn generate more heat. It’s a self-feeding process that can escalate quickly and catastrophically.

The process typically begins at the negative electrode (usually made of graphite), when the temperature inside the cell exceeds 100°C. At this point, the solid electrolyte interphase (SEI) — a protective layer on the electrode — breaks down. This initiates unwanted chemical reactions, which raise the temperature further. As the cell heats up, the electrolyte and separator materials begin to break down, and eventually, the positive electrode—often made of metal oxide—reacts as well.

The situation becomes even more dangerous if oxygen is released from the positive electrode, which can fuel combustion. The result is a battery that essentially feeds its own fire.

Different chemistries behave differently. For example:

• Lithium titanate cells, which do not form the same interphase layer, are much less prone to thermal runaway.
• Metal phosphate cathodes are more stable and generate less heat and oxygen.
• In contrast, metal oxide cathodes can release both significant heat and oxygen, making them more hazardous in runaway conditions.

What Triggers Thermal Runaway?

Several initiating events can set this chain reaction into motion:

• Internal Short Circuits: Caused by manufacturing defects or mechanical abuse (like puncturing or crushing).
• External Heat Sources: Nearby fires or electrical arcing.
• Electrical Abuse: Overcharging or deep discharging—although this is usually caught by the Battery Management System (BMS).

Once triggered, thermal runaway leads to rapid gas buildup and pressure inside the cell. Before the cell reaches the most dangerous reactions, it often vents—releasing hot, flammable gases in volumes far greater than the cell’s size. If the battery’s design channels these gases away safely, the reaction can be contained. If not, it can ignite or spread.

Propagation Risk: When One Cell Affects Another A single-cell thermal runaway can sometimes be managed without major damage. But the real risk lies in propagation — when heat from one failing cell causes neighboring cells to fail. This can lead to multicell venting, fires, or even explosions.

How Can We Prevent It?

Battery safety is a multi-layered defense. Here are some of the key strategies used today:

1. Chemistry Selection Choosing safer active materials can greatly reduce risk.
2. Manufacturing Controls Strict quality control minimizes defects that could lead to shorts or failures.
3. Module Design Well-designed modules include: Gas channels to vent safely. Thermal barriers to isolate cells. Structural protections to resist impact or crushing.
4. Battery Management System (BMS) A fail-safe BMS can detect overcharge, overheating, and other risks, and respond appropriately to prevent escalation.
5. System Qualification and Testing Battery systems go through rigorous testing. UL 9540A, the Standard for Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, provides a detailed approach to understanding fire behavior and thermal runaway propagation in BESS. We’ll explore this important test method in more detail in the next article.

Conclusion

Thermal runaway is one of the most critical safety concerns in Li-ion battery systems. But with careful design, smart chemistry choices, effective management systems, and thorough testing, it is a risk that can be managed. As we move toward a future powered by clean energy, understanding and mitigating the risks of thermal runaway will be key to building public trust and ensuring the safe deployment of battery energy storage systems (BESS).