Whether at work or in our personal lives, we’re hearing more and more about fires caused by lithium-ion battery failures. To better understand this phenomenon, here’s an overview of the relevant information on the subject.

What is a lithium-ion battery?

Technically, a battery is a device used to store energy. An energy-releasing chemical reaction occurs inside the device and is then converted into electrical energy.

Batteries are classified based on the type of chemical reaction that takes place within them. For example, there are nickel-cadmium batteries, such as AA and AAA batteries, and lead-acid batteries, which are found in most gasoline-powered vehicles. Lithium-ion batteries are another category of battery that, as you might have guessed, uses lithium ions to produce energy.

This technology is used because it allows a very large amount of energy to be stored in a small volume. This is referred to as high energy density. It is thanks to this technology that a wide range of portable smart devices, such as phones, tablets, and so on, have been developed.

A lithium-ion battery typically consists of several cells connected to one another within a single package. This package can be flexible or rigid. Each cell contains a negative electrode (anode), which stores energy during charging and consists of a copper current collector coated with graphite, as well as a positive electrode (cathode), which releases lithium ions during charging. The composition of this electrode varies more widely; a commonly used method involves depositing a layer of lithium oxide onto an aluminum substrate. The electrodes are separated from one another by thin plastic films. The entire assembly is immersed in what is called an electrolyte, which is simply a liquid that allows lithium ions to pass from one electrode to the other. This liquid is generally composed of a mixture of organic solvents, such as ethylene carbonate or diethyl carbonate, in which lithium salts (LiPF₆) are dissolved, making it a flammable liquid. It is specifically the chemical composition of this electrolyte that distinguishes lithium-ion batteries from lithium-polymer (LiPo) batteries. In LiPo batteries, this electrolyte is instead made of plastic composites, which give it a certain degree of rigidity.

How can a lithium-ion battery cause a fire?

Given the large amount of energy stored in these devices, lithium-ion batteries pose a potential fire hazard in certain specific situations.

A lithium-ion battery can undergo thermal runaway; that is, the chemical reaction occurring inside the battery can accelerate to the point where the energy produced can no longer be contained. Since this reaction is exothermic, its acceleration will rapidly generate a large amount of heat. The heat will melt the plastic separators and degrade the internal materials, allowing the various current collectors to come into contact with one another, which will suddenly release all the stored electrical energy. As a result, a battery that is 100% charged is much more likely to experience thermal runaway than a fully discharged battery. The heat will also cause the solvents in the electrolyte to vaporize, causing them to expand and swell the container, which in some cases will lead to its rupture. The rupture of the container will release flammable vapors —in the case of lithium-ion batteries—which will ignite in the presence of heat and oxygen, creating a fireball.

What are the possible causes of thermal runaway in a lithium-ion battery?

Thermal runaway in a lithium-ion battery can have several causes. Exposure to external heat is one of them. For example, if a lithium-ion battery is caught in a fire, the heat can cause the battery to fail.

In cases where the thermal runaway of the battery itself leads to a fire, mechanical damage is one of the most common causes. An impact, blow, or puncture of the battery can cause the electrodes to come into contact in a very localized area. Thermal runaway can occur immediately after the impact if the damage is significant enough, but it can also occur after several charge cycles if the damage is very subtle. To prevent these incidents, lithium-ion batteries must undergo several crush tests before being released to the market, but it is still possible to encounter this type of failure in the field.

Another type of damage that can lead to thermal runaway is electrical damage; this includes, among other things, overcharging beyond the battery’s capacity or discharging below its minimum capacity. Overcharging will cause degradation and deformation of the electrodes, quickly leading to failure. For example, using an inappropriate charger could lead to this phenomenon. This scenario can occur after replacing the original charger with an incompatible one. Discharging the battery below its minimum voltage threshold will also cause the electrodes to degrade, but given the small amount of stored energy, failure will not occur immediately. Rather, it is during subsequent charging cycles that there is a risk of fire.

Failure caused by poor battery chemistry design or a manufacturing defect is not impossible. This type of failure generally occurs early in the battery’s service life—either directly during manufacturing or during its first few uses.

It is therefore important for the claims adjuster handling such a case to determine the make and model of the battery and charger; many devices are subject to recalls. He or she must also determine the battery’s age, usage history (e.g., frequency of use, the type of device with which the battery is used [brand and model], the time required to charge it, and the duration of the charge in terms of the device’s operation), and the state of charge at the time of the fire. Finally, they must verify whether the device was subjected to impact or any other type of damage at any time during its use.

At Origin, we regularly handle this type of case. It is important to understand that laboratory analysis of battery remnants can determine whether thermal runaway occurred, but does not necessarily allow us to discern whether the failure was the cause or rather a result of the fire. As a result, information and observations obtained from witnesses are of great importance in such cases. We invite you to contact us, and we will be happy to ensure that you have all the necessary information to resolve fire incidents related to lithium-ion battery failure.

Reference: Lithium-Ion Batteries Hazard and Use Assessment – Final Report, The Fire Protection Research Foundation, NFPA, July 2011