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Battery technology has been with us for over a century, but the technology came into its own in the 1990s with the advent of lithium-ion (li ion) batteries. Li-ion batteries are the go-to power source for virtually every device that depends on stored electricity. They power our phones and computers and many of us now drive electric vehicles (EVs) that rely on large banks of li-ion batteries.
With li-ion battery usage becoming so pervasive, problems with the batteries are becoming commonplace. The news and social media are full of accounts where a li-ion battery exploded or started a fire. In New York City, li-ion batteries are such a frequent cause of fires that the city has enacted special fire codes to deal with the problem.
At the heart of the issue is a phenomenon known as “thermal runaway,” a dangerous condition in li-ion batteries resulting from an uncontrolled increase in temperature and pressure within the battery. Virtually all fires and explosions involving li-ion batteries are the result of a thermal runaway. Therefore, understanding thermal runaway is important for fire safety, particularly for companies that rely on li-ion batteries to power their tools and equipment.
How Li-ion Batteries Work
Before getting into the specifics of a thermal runaway and how and why it is so dangerous, it is important to understand how li-ion batteries actually work.
Li-ion batteries consist of a positive cathode and a negative anode. During discharge, lithium ions travel from the anode to the cathode, and the process reverses during charging. A separator is located between the anode and cathode to keep them from touching. These internal components are enclosed in a casing filled with a liquid electrolyte which facilitates ion movement. The electrolyte, being a flammable solvent, presents a risk of fire and explosion.
Each combination of a cathode and an anode forms what is known as a "cell." Cellular phones typically use a single cell, whereas rechargeable tools might contain up to twenty cells. EVs may incorporate hundreds of cells, and commercial battery energy storage systems could have thousands. Any of these cells can develop faults, potentially leading to a fire or explosion.
Li-ion batteries are so popular because they have high energy density. This means that they are able to store a significant amount of power in a relatively small space. And for the most part, li-ion batteries are safe. While the li-ion battery incidents reported on the news and social media are often dramatic, considering the billions of li-ion batteries in service around the globe, the actual incident rate is thankfully low.
What is Thermal Runaway in Batteries?
Thermal runaway is the culmination of a li-ion battery failure. It can occur quickly, or it may take hours or even days to transpire. The process of battery failure leading to thermal runaway typically follows a fairly uniform path and occurs in stages. However, not all battery failures lead to thermal runaway, as the failure process may terminate prior to reaching the thermal runaway stage.
Thermal Runaway: Initiating Event
This usually involves some form of stress or electrical fault that initiates the battery failure process. It can be the result of some form of mechanical damage, such as crushing or punctures to the battery case. Exposure to thermal stress—like high external temperatures—can also cause battery failure.
Electrical faults are a common cause of battery failure. These can occur due to defects in the design or manufacture of the battery that lead to internal short circuits. Overcharging or excessive discharging can also stress the battery causing it to fail. While battery failure can occur at any time, it most commonly happens during charging.
Thermal Runaway: Heat Generation
Once the battery is compromised, internal short circuits create heat in the interior of the battery. The rising temperature can cause the physical separator between the anode and cathode to melt or otherwise break down. This allows the cathode and anode to touch and short circuit. This results in a large flow of current through a small area, generating significant heat very quickly.
Thermal Runaway: Breakdown of Electrolyte
As the temperature rises, the electrolyte begins to decompose and vaporize. Because the electrolyte is a flammable solvent, its decomposition generates flammable gases.
Thermal Runaway: Accelerated Reaction Rates
The increase in temperature causes the rate of chemical reactions within the battery to accelerate. This includes reactions that produce more heat and flammable gas, effectively creating a feedback loop where heat generation promotes more reactions; this in turn generates more heat. The battery’s temperature rises rapidly to over 500°F (260°C).
Thermal Runaway: Smoke Production
If the heat and gas cannot be dissipated quickly enough, the increasing temperature and pressure may cause the battery cell casing to split and flammable smoke can be seen emitting from the battery. This is often the first visible indication that the battery is failing. In most cases, smoke production signals the onset of a thermal runaway.
Thermal Runaway: Spreading the Fire
The continued failure of the battery causes the temperature within to escalate uncontrollably. The intense heat can ignite the flammable gases or the battery’s electrolyte, leading to a fire or explosion. During thermal runaway, adjacent cells are overheated and compromised, spreading the problem through the battery pack in a cascade of failures.
Thermal Runaway: Venting and/or Explosion
In the final stage, the pressure build-up from gas generation may lead to the battery casing venting violently. This can result in fires or explosions which pose significant safety risks.
Throughout these stages, the physical and chemical integrity of the battery is progressively compromised, culminating in potentially catastrophic failure if preventative measures are not effective. This underscores the importance of safety designs in battery management systems, including accurate monitoring of voltage and temperature, as well as robust emergency response features to mitigate the effects of battery failure.