Lithium-ion Batteries in Vehicles – Risk Analysis from a Fire and Gas Release Perspective

Lithium-ion batteries offer high energy density and many other benefits, but also risks that need to be dealt with. Design and construction can have a decisive effect on battery safety, and to assess and manage the risks it is important to maintain a holistic perspective that encompasses both the battery and the environment around and outside the battery system.

Lithium-ion Batteries in Vehicles – Risk Analysis from a Fire and Gas Release Perspective
Fig. 1: Fire test of two commercial battery packs for laptop computers. (Photograph: Fredrik Larsson)

By Dr. Fredrik Larsson, Prof. Bengt-Erik Mellander and Dr. Petra Andersson.

In October 2017, RISE and Chalmers presented a report on the general risks and construction guidelines for Li-ion batteries in electrified vehicles based on a safety perspective with a focus on fire and gas releases. The report covers common types of Li-ion battery systems for everything from light to heavy electrified vehicles and is intended to provide support primarily for industry, but also for society.

The findings in the report are based on results obtained in the project "Safer battery systems in electrified vehicles – develop knowledge, design and requirements to secure a broad introduction of electrified vehicles", which was carried out in years 2012–2017 with financial support from the Swedish Energy Agency.

The project was led by RISE, and project partners were Atlas Copco Rock Drills, Elforsk and Chalmers University of Technology. The project included an industrial doctoral thesis, defended in 2017, which involved performing destructive tests on various commercial lithium-ion batteries. The tests included overcharging, short-circuiting, external heating in a furnace and fire tests.

One extra focus of the project was the analysis and measurement of the toxic gas hydrogen fluoride, which the batteries released in connection with both fire and, in some tests, without fire.

Are lithium-ion batteries dangerous?

"Lithium-ion batteries" is a collective term to describe a family of different kinds of batteries with different chemistries. They have many attractive properties, e.g. the long life and energy density are far superior to most other battery technologies available in the market. The trend is moving towards higher energy density in the batteries, which in itself by definition represents an increased risk according to fundamental principles of physics.

So, are lithium-ion batteries dangerous? Handled correctly, they do not need to be dangerous. If we compare them with petrol, which is a very dangerous substance, we have learned to manage the risks over a long period of time. For Li-ion batteries, however, we are still in the learning phase, as they bring other kinds of risks that need to be studied and managed. Another aspect is that the use of Li-ion batteries for vehicles can also minimise or totally remove certain risks.

For example, pure electric cars have no petrol tank, which means that one of the big risks or perhaps the biggest fire risk of all disappears. When Li-ion batteries are introduced, a known risk is therefore replaced by partly unresearched risks, which is why continued research and incident analyses are important.

Large Li-ion batteries, for example those in vehicles, ships or stationary energy storages for the grid, involve different risks and safety consequences than small Li-ion batteries that are used in consumer products, e.g. mobile phones and laptops. Bearing in mind the large number (in the order of billions) of Li-ion batteries being used in consumer products, the number of incidents that has occurred is relatively low. The hazards of small Li-ion batteries are mainly fire and possibly secondary fire effects if surrounding material is ignited. The battery safety issues are more complex for big Li-ion batteries. One of the reasons is that the gases that can be released are both flammable and toxic.
A holistic perspective is needed here that looks not only at the battery system but at the whole – e.g. the surrounding environment, whether there are people nearby, the vehicle design, the location of the batteries in the vehicle and external impact factors.

Lithium-ion Batteries in Vehicles – Risk Analysis from a Fire and Gas Release Perspective
Fig. 2: Results of fire tests of seven different (A–F) commercial Li-ion batteries. (a) shows the total volume of hydrogen fluoride detected versus the state of charge (SOC). (b) shows the energy ratio of the total heat energy emitted and the electrical energy when the battery is fully charged. This can also be described as chemical energy (from the fire) in relation to the electrically stored battery energy. The batteries tested thus have 5–20 times higher fire energy than battery energy. These values may be considered to apply for the complete incineration of the battery. The state of charge has little impact on the fire energy. (Illustration: Fredrik Larsson)

What are the risks involved with Li-ion batteries?

Li-ion batteries contain reactive materials and have a flammable electrolyte. The battery cell contains all three parts of the fire triangle (heat/ignition, oxygen and combustible material), even if, for example, the amount of oxygen that can be released varies between different Li-ion chemistries. If the battery cell is heated up, it is designed to release gas (to prevent a cell explosion). There are different cell formats, and they can release gases at different temperatures, e.g. pouch cells (flat prismatic cells in a pouch) can in certain cases of failure release gases from around 70°C. If the cell temperature were to reach approx. 150–200°C, there can be what is known as a thermal runaway. A thermal runaway is a rapid internal temperature increase that often results in one or more of the following events: heat generation, gas and smoke formation, cell breach/cell explosion, fire or gas explosion. Gas releases can typically occur at lower temperatures and without the occurrence of a thermal runaway.

Different kinds of battery explosions can occur. If the battery cell builds up high pressure (e.g. caused by a defective or poorly designed safety valve), a cell explosion can occur.

Another kind of explosion might occur if the battery gases are released and mixed with air but do not ignite immediately, but accumulate and ignite later on. This is known as a gas explosion, which can cause major damage.

Battery gases contain toxic substances, e.g. the highly toxic fluoride compound hydrogen fluoride (HF). Fire tests conducted in the project measured relatively high HF levels. Seven different commercial Li-ion cells were heated up using gas burners, and the amount of HF released was measured at between 20 and 200 mg/Wh, where Wh denotes the battery’s electrical energy content, see Figure 2. Two independent and parallel measurement methods were used to verify the levels. The toxicity of hydrogen fluoride is relatively well-known. The ten-minute lethal value (AEGL-3) is 139 mg/m3 (170 ppm) and the maximum value to which workers in Sweden may be exposed, the short-term limit, is 1.7 mg/m3 (2 ppm). If the measured values from the cell tests are extrapolated to a 100 kWh Li-ion battery, 2–20 kg HF would be released if the whole battery was incinerated, a consider-able amount of toxic gas.

More research is needed into these gases; which gases are released in different contexts for different kinds of batteries, what are the different scenarios and how do the risks, countermeasures and protection match up with battery size and different applications and operating environments?

Why do incidents occur?

Lithium-ion Batteries in Vehicles – Risk Analysis from a Fire and Gas Release Perspective
Fig. 3: Instantaneous image of gas release during a destructive overcharge with a current of 91 A (7C rate) on a commercial Li-ion battery cell of 13 Ah, model LTO-NMC. (Photographer: Fredrik Larsson)

There are many possible reasons why a battery cell can become hot, e.g. overcharging, short-circuiting, mechanical deformation or external heating. Various degrees of protection can be achieved by such means as battery design and quality.

Incidents involving gas releases, fire and explosion will occur and this is because of, for example:

  • External factors such as mechanical impact, e.g. cars that crash and deform the battery, external heating or external fire.
  • Battery cells are also sensitive to, for example, a charge current that is too high. The current in each cell is therefore monitored by a monitoring system, usually known as a Battery Management System (BMS). It is important to have a high-quality BMS. The BMS cannot, however, offer protection in all cases, see Table 1. Furthermore, the BMS or a sensor can break down and no longer offer protection.
  • There can also be a spontaneous internal short circuit. This phenomenon can actually occur at any time, even when batteries are not in use.
    The probability is not well-documented, but it is typically low, with figures in the order of 1 ppm (i.e. 1 cell out of 1 million cells). The reason is still not fully understood, but originates from defects in the manufacturing process combined or not with use (e.g. recurring small overdischarges/overcharges).
    It is difficult in the manufacturing phase to achieve 100% pure material that is absolutely free of contaminants and undesired particles. Leading battery safety experts now believe that manufacturing defects could not be fully avoided, no matter how much money were to be invested. When the battery wears through usage, these risks can also be affected through, for example, dendrite formation.
  • Lower quality of cells and of BMS will increase the number of incidents.
  • The human factor is also present at all times – incorrect handling, faulty connection, etc.,
  • Quick-charging can increase the risks, as higher currents are involved, which places tougher demands on, for example, control, detection and cooling.

As incidents cannot be entirely discounted, it is instead necessary to manage their consequences. Mitigating or preventing propagation is typically an important part of the solution. The design is crucial here. Even if it is not possible to stop a fire from spreading completely, it can be valuable to achieve a delay in order to create time for detection, evacuation, implementation and countermeasures and for the emergency services to arrive. Detection, ideally at an early stage, is therefore important.

Is it always good to avoid a fire?

For small consumer batteries, fire is probably the worst scenario, as a fire can also serve as an ignition source for other combustible material. To reduce the risk of battery fires, flame retardants or other substances are sometimes added, which can contain fluoride or other undesirable elements. New electrode materials have also been developed and started to come into use. This has meant that certain Li-ion cells ignite less often, but still release gases. It is possible that the gas composition when it does not burn is more toxic than when a fire has broken out, as in similar situations with other kinds of fires; the situation for Li-ion batteries has however not been investigated, as there is a lack of research in the area.

As the battery size grows, the amount of gas released can become significant. In this situation, at least in some scenarios, the gases can represent a bigger threat than a fire. As the battery gases are not incinerated, they can be mixed with air, and if the gas release has occurred in an enclosed or semi-enclosed area, the gases can accumulate. If the accumulated gas is ignited, a gas explosion can occur.

Ignition can take place through several ignition sources, both external and within the battery, e.g. an electrical contact, light arc, electrical spark or auto-ignition from a hot cell.

The use of additives can in principle both increase and decrease safety. A holistic evaluation of the use of additives is needed in order to assess overall safety, and this has not been investigated at present. In summary, the risks associated with battery gases, in terms of both toxicity and the risk of ignition/explosion, are an important element of overall safety and the risks can vary between different battery sizes, applications and environments.

Lithium-ion Batteries in Vehicles – Risk Analysis from a Fire and Gas Release Perspective

How do we make batteries safer?

Lithium-ion Batteries in Vehicles – Risk Analysis from a Fire and Gas Release Perspective
Fig. 4: General examples of how the battery can be made safer by means of passive and active safety solutions. SOS (State of Safety) is a status parameter in the battery system that describes the battery’s safety status. (Illustration: Fredrik Larsson)

The report contains a description of general construction guidelines. Here are a few examples. The handling of gas is important, with ventilation being an important factor, as is battery location and where the gases are released. The filtering of gases can also be relevant, e.g. detoxification of hydrogen fluoride and other gases. To protect against gas explosion, battery design and placement on vehicles need to be designed in order to incorporate, for example, explosion protection/accumulation protection.

A system for the detection and avoidance of ignition sources around the battery is also important. The design of the battery system and its integration and placement in electrified vehicles can have a major impact on heat, gas, and fire propagation in the battery.

Lithium-ion Batteries in Vehicles – Risk Analysis from a Fire and Gas Release Perspective
Fig. 5: Doctoral thesis on lithium-ion battery safety. (Source: Fredrik Larsson)

Further reading

Report
Fredrik Larsson and Bengt-Erik Mellander, "Lithium-ion Batteries used in Electrified Vehicles – General Risk Assessment and Construction Guidelines from a Fire and Gas Release Perspective", SP Report 2017:41, ISSN 0284-5172, RISE Research Institutes of Sweden, Borås 2017.
Freely available online

Doctoral thesis
Fredrik Larsson, "Lithium-ion Battery Safety – Assessment by Abuse Testing, Fluoride Gas Emissions and Fire Propagation", doctoral thesis, ISBN: 978-91-7597-612-9,
Chalmers University of Technology, Gothenburg, 2017.
Freely available onlin

Authors

Dr. Fredrik Larsson
Researcher at RISE Electronics
fredrik.larsson@ri.se

Prof. Bengt-Erik Mellander
Chalmers University of Technology
f5xrk@chalmers.se

Dr. Petra Andersson
Senior researcher at RISE Safety
petra.andersson@ri.se

The article was published in FeuerTrutz International, issue 1.2019 (January 2019).
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