Thermal runaway is a safety issue in electric vehicle (EV) batteries, characterized by rising temperatures and the release of flammable gases. As conventional battery management systems (BMS) often fail to provide timely warnings, gas sensing presents a more sensitive detection method.
This article reviews relevant gas sensing technologies that help in thermal runaway detection in EV battery packs.
How are thermal runaway and gas generation related?
Thermal runaway is a process involving a series of exothermic reactions within the battery cell. This process generates gases in several stages:
- Initial stage (70-120° C): The solid electrolyte interphase (SEI) layer on the anode begins to decompose. This initial breakdown serves as an early indicator and releases small quantities of gases, such as carbon dioxide (CO2) and ethylene (C2H4).
- SEI breakdown and electrolyte reaction (100-200° C): With increasing temperature, the SEI layer continues to break down, which exposes the anode to the electrolyte. This contact triggers reactions that produce hydrocarbons, including methane (CH4) and ethane (C2H6), along with additional CO2.
- Cathode decomposition and separator meltdown (130-350° C and above): At higher temperatures, the cathode material releases oxygen. The separator then melts, which can cause internal short circuits. The released oxygen reacts with the electrolyte, generating CO, CO2, and hydrogen (H2).
The composition and concentration of vented gases depend on factors such as battery chemistry (e.g., LFP, NMC, NCA), state of charge (SOC), and the type of abuse condition (thermal, mechanical, or electrical).
Table 1 summarizes the sensing properties of semiconductor sensors for key gases produced during thermal runaway.
Table 1. The sensing properties of semiconductor sensors for thermal runaway characteristic gases. (Image: Frontiers in Chemistry)
What are the gas sensing technologies for EV battery packs?
Various technologies are available for integrating gas detection into EV battery packs. Each technology has specific advantages and challenges.
Laboratory analysis techniques
Gas chromatography and mass spectrometry are standard methods for gas analysis in a laboratory. They provide high accuracy and identify a wide range of compounds. However, their size, cost, and complexity make them impractical for in-vehicle use.
Researchers use these techniques to characterize the gases produced during thermal runaway, which supports the development of onboard sensors. The schematic in Figure 1 shows a typical experimental setup for analyzing vented battery gases.
Figure 1. A schematic representation of a test set-up for the analysis of vented battery gases, including collection and measurement equipment. (Image: MDPI)
Onboard sensing technologies
For vehicle integration, sensors should be compact, low-cost, and reliable.
- Metal oxide semiconductor Sensors: These common gas sensors operate by measuring the change in electrical resistance of a metal oxide material upon exposure to a target gas. They are low-cost and compact, but can exhibit cross-sensitivity to other gases and require power for heating to an operating temperature.
- Non-dispersive infrared (NDIR) sensors: NDIR sensors are selective and stable, which makes them suitable for detecting specific gases like CO2. An NDIR sensor passes infrared light through a gas sample and measures the light absorbed at a wavelength specific to the target gas. Their reliability make them a candidate for EV battery applications. The graph in Figure 2 shows the detection of CO2 during a cell venting event using an NDIR sensor.
Figure 2. Data from an overcharging experiment showing a rapid increase in CO2 concentration, detected by an NDIR sensor, immediately following a cell venting event. (Image: Elsevier)
- Optical fiber sensors: This class of sensors has several advantages. Optical fiber sensors are immune to electromagnetic interference (EMI), which is beneficial in the electrically noisy environment of a battery pack. They are also lightweight, flexible, and support multiplexing, which allows for monitoring at multiple points along a single fiber.
Why is CO2 an important component during thermal runaway?
Data from various battery chemistries and failure modes show that CO2 is one of the earliest and most consistently produced gases during the onset of thermal runaway. The gas release from SEI decomposition makes CO2 detection a candidate for early warning systems.
Research from the Electric Power Research Institute of Guandong Power Grid Co., Ltd., China, shows that an increase in CO2 concentration can be detected before the battery reaches a critical temperature, providing additional time for the BMS to respond. The schematic shown in Figure 3 shows a typical experimental setup for a CO2 sensor.
Figure 3. Schematic of an experimental setup for CO2 sensing. (Image: Frontiers in Physics)
What are the advances in optical fiber-based gas sensing?
Optical fiber sensors are suitable for in-situ monitoring within the battery pack. Several types are in development:
- Fiber bragg grating sensors: These sensors contain a periodic variation in the refractive index of the fiber core. When the fiber is exposed to a stimulus, such as temperature or strain from gas pressure, the reflected wavelength shifts. This shift can be measured with high precision.
- Evanescent wave sensors: These sensors are fabricated by removing a portion of the fiber’s cladding and coating the exposed core with a material that interacts with the target gas. This interaction changes the properties of the light traveling through the fiber.
- Fabry-pérot interferometer sensors: These sensors use a small optical cavity at the end of the fiber. The presence of a target gas changes the properties of this cavity, which is detected as a change in the interference pattern of the reflected light.
The diagram in Figure 4 shows an optical fiber sensor coated with a metal-organic framework (MOF) for gas detection. MOFs have a high surface area and can be designed to selectively adsorb specific gases, which can increase a sensor’s sensitivity and selectivity.
Figure 4. Schematic diagram of a gas sensing system and an optical fiber sensor integrated with a ZIP-8 film. (Image: Advanced Sensor Research)
Summary
Integrating gas sensing into EV battery packs can shift safety management from reactive to proactive, as detecting the early signs of cell degradation provides an opportunity to prevent thermal runaway. Gas evolution serves as an early indicator, and data identifies CO2 as a suitable target for detection due to its early and consistent release. For onboard applications, NDIR sensors offer a mature and reliable option.
Furthermore, optical fiber sensors represent an emerging technology with advantages in EMI immunity and the potential for distributed sensing within the pack. Future battery safety systems may combine traditional monitoring with advanced gas sensing.
As these technologies become more cost-effective, their integration into standard BMS architectures can improve EV safety and support the development of batteries with higher energy densities.
References
- Quantification and simulation of thermal decomposition reactions of Li-ion battery materials by simultaneous thermal analysis coupled with gas analysis, Elsevier
- Optical Fiber-Based Gas Sensing for Early Warning of Thermal Runaway in Lithium-Ion Batteries, Advanced Sensor Research
- Detection toward early-stage thermal runaway gases of Li-ion battery by semiconductor sensor, Frontiers in Chemistry
- Fiber Optic Sensing Technologies for Battery Management Systems and Energy Storage Applications, MDPI
- Detection of Li-ion battery failure and venting with Carbon Dioxide sensors – ScienceDirect, Elsevier
- Analysis of Li-Ion Battery Gases Vented in an Inert Atmosphere Thermal Test Chamber, MDPI
- Gas detection technology for thermal runaway of lithium-ion batteries, Frontiers in Physics
EEWorld Online related content
- What techniques are available for mitigating thermal runaway in batteries?
- What market growth is expected for sensors? Part 2: Gas Sensors
- What is thermal runaway and why does it matter for EVs?
- How many thermal sensors are there in an EV?
- What role do advanced sensors play in EVs?
Filed Under: Batteries, Battery Pack, FAQs, Sensors