Thermal runaway (TR) refers to a condition where Li-ion cells enter an uncontrollable self-heating state during which their temperature rises rapidly to as much as 900 °C. Applications like EVs require high levels of power and energy density from the Li-ion battery packs. That can increase the risk of TR. This FAQ begins with a look at the causes and stages of TR and closes with a brief look at TR mitigation strategies.

Potential sources of TR include mechanical, thermal, and electrical abuse of the cell. Once a Li-ion cell has exceeded the maximum safe operating temperature, it enters a heat-temperature-reaction (HTR) loop that results in additional heating. The HTR loop can lead to exponential temperature rise and TR.

In every unmanaged HTR loop, there’s a sequence of events that lead up to TR. It begins with the decomposition of the solid electrolyte interphase (SEI) layer that produces ethene, carbon monoxide, and carbon dioxide causing the cell casing to swell.

SEI decomposition begins at 80 °C but can be initiated at lower temperatures under certain conditions; for example, it can be under 80 °C for lower states of charge. Decomposition of the SEI is exothermic pushing up the cell temperature. At 100 to 120 °C, the electrolyte will decompose, and the separator will begin to melt, generating more hot gases and resulting in an internal short circuit during which up to 70% of the cell energy will be released in under a minute. When the temperature hits 130 °C the cathode begins to break down, generating oxygen. Each step in the HTR process becomes more dangerous. TR is entered when the temperature rises above 150 °C (Figure 1).

During TR, gases like oxygen build up and vent from the cell and can ignite resulting in powerful explosions. The temperature continues to rise to as much as 900 °C during the final decomposition of all cell materials.

The rate of controllable self-heating is usually defined as 0.2 °C per minute. It’s only controllable during this initial stage if the heat buildup is effectively dissipated. If not the rate of temperature rise increases during the acceleration stage. Once the rate of temperature increase hits about 10 °C per minute, TR is entered.

TR and its causes originate in individual cells. That makes it challenging to eliminate. A typical EV battery pack consists of thousands of cells, each of which can experience TR. The cells are subject to minute manufacturing variations and the impedances of the battery pack interconnection system vary slightly causing additional performance challenges. Monitoring the operation of every cell in the battery pack and protecting them from mechanical, electrical, and thermal abuse is important for minimizing TR.

Li-ion batteries are often specified for operation from 0 to 45 °C for charging and from -20 to 60 °C for discharging. However, for optimal life operation between 15 to 35 °C is often recommended. The preferred operating temperature window is much smaller than the safe operating temperature window (Figure 2). That can present thermal management challenges when integrating Li-ion battery cells into electric vehicles (EVs) where the nominal automotive operating temperature range is -40 to 105 °C.

Summary
TR is a condition when Li-ion cells enter uncontrollable and rapid temperature increases. It’s a significant safety challenge in EV battery packs and requires close monitoring and management of each individual cell in the pack.

References
A Critical Review of Thermal Runaway Prediction and Early-Warning Methods for Lithium-Ion Batteries, Energy Material Advances
A Review of Lithium-Ion Battery Thermal Runaway Modeling and Diagnosis Approaches, MDPI processes
Advances in Prevention of Thermal Runaway in Lithium-Ion Batteries, Advanced Energy & Sustainability Research
Fire Safety of Lithium Ion Batteries in Road Vehicles, RISE Research Institutes of Sweden
What is Thermal Runaway?, UL Electrochemical Safety Research Institute

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