Thermal runaway (TR) refers to a condition where lithium-ion (Li-ion) battery cells enter an uncontrollable self-heating state during which their temperature rises rapidly to as much as 900° C. Electric vehicles (EVs) require high levels of power and energy density from Li-ion battery packs, which can significantly increase the risk of TR. This FAQ reviews the causes and stages of TR and considers a few TR mitigation strategies.
Potential sources of TR include mechanical, thermal, and electrical abuse of the battery cell. Once a Li-ion cell has exceeded the maximum safe operating temperature, it enters a heat-temperature-reaction (HTR) loop that generates additional heating. The HTR loop can lead to exponential temperature rise and TR.
In every unmanaged HTR loop, a sequence of events leads up to TR. It begins with the decomposition of the solid electrolyte interphase (SEI) layer, which produces ethene, carbon monoxide, and carbon dioxide and causes 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 (SOC). 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).
Figure 1. TR starts when the temperature of a Li-ion cell hits over 150° C. (Image: Advanced Energy & Sustainability Research).
During TR, gases like oxygen build up and vent from the cell, which can ignite and result in powerful explosions. The temperature rises 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, making eliminating it challenging. 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 essential 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, 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). This can present thermal management challenges when integrating Li-ion battery cells into electric vehicles (EVs), where the nominal automotive operating temperature range is from 40° to 105° C.
Figure 2. The safe operating temperature window for Li-ion cells is much wider than the optimal life window. (Image: RISE Research Institutes of Sweden).
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
Filed Under: Batteries, FAQs, Featured, Thermal Management System