Read time: 10 minutes          

Target audience: Thermal Researchers/ EV Automobile Engineers/ Thermal-Fluid Industry/ Aero Industry

Written by: Dr. Tabish Wahidi

Background:

The rapid advancement of battery technology has transformed industries ranging from consumer electronics to electric vehicles (EVs) and renewable energy. However, with this rise comes a critical safety concern: thermal runaway. Thermal runaway is a dangerous phenomenon in which a battery’s temperature rapidly escalates uncontrollably, often leading to fires or explosions. Understanding the mechanisms behind thermal runaway and its implications is essential for improving battery safety and preventing catastrophic failures in systems that rely on batteries.

Introduction:

Thermal runaway occurs when a battery’s internal temperature increases to the point where it triggers a self-sustaining reaction that continues to raise the temperature, eventually causing the battery to vent, catch fire, or even explode. This process typically starts when a battery cell experiences excessive heat due to internal or external factors. The heat triggers chemical reactions within the battery, which generate even more heat in a feedback loop. If this heat is not effectively dissipated, the cycle continues, leading to catastrophic failure.

In lithium-ion batteries, thermal runaway is especially concerning because of the highly reactive nature of lithium. Once thermal runaway begins in a single cell, the heat and pressure buildup can spread to adjacent cells in a battery pack, escalating into a large-scale event known as “Thermal propagation.”

Key Factors Contributing to Thermal Runaway:

1. Overcharging: Charging a battery beyond its recommended voltage range can lead to overheating, which in turn can trigger thermal runaway. Overcharging can cause the electrolyte to decompose and release flammable gases.
2. Internal Short Circuits: Manufacturing defects, mechanical damage, or degradation over time can cause internal short circuits within a battery. A short circuit generates a large amount of heat, which can lead to thermal runaway.
3. External Heating: High external temperatures can overwhelm a battery’s cooling mechanisms, leading to an uncontrolled rise in temperature. This is common in situations where batteries are exposed to extreme environmental conditions or where cooling systems fail.
4. Mechanical Damage: Physical damage to a battery, such as punctures or deformation, can compromise the internal structure, leading to short circuits or heat generation.
5. High Discharge Rates: Operating a battery at high discharge rates generates more heat, especially in high-power applications like electric vehicles or drones. If the heat exceeds the battery’s capacity to dissipate it, thermal runaway may occur.

The Stages of Thermal Runaway:

Thermal runaway typically progresses through three stages:

1. Initiation Stage: In this stage, external factors like overcharging, overheating, or mechanical damage cause a local rise in temperature within the battery cell. The initial heat can trigger chemical reactions, such as electrolyte decomposition or reactions between the anode and cathode.
2. Acceleration Stage: The heat generated during the initiation stage causes further reactions within the battery, releasing flammable gases and increasing pressure. As the temperature continues to rise, the rate of reactions accelerates, leading to more heat and more gas generation.
3. Thermal Runaway Propagation: At this point, the internal temperature of the battery exceeds its thermal stability limits, causing the battery to release large amounts of energy. If not contained, this energy can cause the battery to vent, ignite, or explode. In a battery pack, this heat can propagate to neighbouring cells, leading to a cascading failure known as thermal propagation.

Prevention and Mitigation Techniques:

Given the risks associated with thermal runaway, battery manufacturers and system designers implement various preventive and mitigation strategies to enhance battery safety.

1. Battery Management Systems (BMS): A BMS monitors the voltage, current, and temperature of each cell in a battery pack to ensure safe operating conditions. It can prevent overcharging, detect early signs of failure, and trigger protective actions like disconnecting the battery from the system.
2. Thermal Management Systems: Cooling systems, such as liquid or air cooling, help maintain the battery’s temperature within safe limits. Effective thermal management is crucial for high-power applications like electric vehicles, where batteries can generate significant heat during operation.
3. Current Limiters and Fuses: Overcurrent protection devices can interrupt the flow of electricity in the event of a short circuit, preventing further damage to the battery and reducing the risk of thermal runaway.
4. Cell Design and Material Improvements: Advances in cell design, such as using more stable electrolytes and safer cathode materials, can reduce the likelihood of thermal runaway. Solid-state batteries, which use solid electrolytes instead of liquid ones, are being developed to improve safety.
5. Fire-Resistant Barriers: In multi-cell battery packs, fire-resistant barriers between cells can slow or prevent the spread of thermal runaway from one cell to another, reducing the risk of thermal propagation.

Conclusion:

Thermal runaway is a critical safety issue in battery technology that requires ongoing attention from researchers, manufacturers, and engineers. While significant strides have been made in preventing and mitigating thermal runaway, the increasing demand for high-performance batteries in applications like electric vehicles and renewable energy storage necessitates continuous innovation. Advances in battery materials, design, and management systems will be essential to ensuring the safe operation of batteries and minimizing the risks associated with thermal runaway.

By understanding the causes, stages, and preventive strategies, the industry can move closer to creating safer and more reliable battery systems for the future.