What is thermal runaway in lithium-ion batteries
Thermal runaway is a violent chain reaction of exothermic chemical reactions resulting in an uncontrollable increase in system temperature. Now, where does this reaction might unfold? Batteries and energy storage systems, which are integral parts of tech around us, are the main stage for this drama. They sometimes fall victim to thermal runaway, turning a peaceful power source into a potential headache and health hazard. Understanding this reaction is a key step toward better battery safety.
Thermal runaway in lithium-ion batteries
Batteries are designed to store chemical energy, and during thermal runaway, this chemical energy is uncontrollably released. In lithium-ion batteries (LIBs), thermal runaway can be caused by e.g. mechanical damage, external heat, short circuit, or overcharging. Thermal runaway is characterized by very quick progress, and it can result in battery fire or even explosion. It results in the self-destruction of the battery.
Progress of thermal runaway
To get thermal runaway started, heat generation is needed. Heat is a normal by-product of electricity production: small amounts of released heat are typical when using lithium-ion batteries. You have probably noticed this when watching just-one-more episode of your favorite show from your laptop.
So – heat generation and release doesn’t always lead to thermal runaway. However, in addition to normal operation, heat generation can result from the abuse to the battery. The abuse can be either thermal, electrical, or mechanical, and can lead to a good amount of heat generation, potentially pushing the battery into thermal runaway. Whether the battery stays chill or decides to dive head first into thermal runaway depends on the balance of heat generated versus heat dissipated to the environment. When the dissipation of heat is larger than heat generation, thermal runaway doesn’t start and the battery is still safe. Once heat generation takes the lead, the self-sustaining cycle of thermal runaway kicks in. In this snowballing sequence, heat generation causes a rising temperature, speeding up the reaction rate, which, you guessed it, leads to more heat generation.
Due to the positive feedback loop, the progress of thermal runaway is extremely quick. The temperature of the battery cell can skyrocket to over 600 C. From the start of the thermal runaway, the battery might ignite or even explode within minutes. To make things even trickier, thermal runaway generates oxygen, so a battery fire kicks off even without any help from external oxygen in the surrounding environment. This is a problem, especially in LIBs, where the electrolyte is flammable.
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Gases released during thermal runaway
When things heat up and chemical reactions start accumulating inside the battery during thermal runaway, it leads to some serious heat generation and the release of gases. We’re not talking about a handful of gases here; there are dozens of them, and guess what? Many are highly toxic. Some of the gases are flammable and can contribute to the progress of thermal runaway and even cause an explosion. Released gas compounds and their concentrations vary depending on, among other things, the specific type of battery in question as well as the stage of thermal runaway. The size of the battery also affects the gas concentrations.
Now, what are the typical gases released during thermal runaway? These include acids, e.g. HF, HCl, and HCN, inorganics such as CO2, CO, NH3, SO2, carbonates, most typical ones being DMC, EMC, EC, volatile organic compounds such as ethylene, acetylene, and methanol, and carcinogens, such as formaldehyde. At the beginning of thermal runaway, flammable gases, and electrolyte components, etc. are released. If things really heat up, combustion products like CO2 are added to the mix.
Concentrations of HF and CO might rise already prior to the start of the thermal runaway, and thus, following the concentrations of these gases could potentially help prevent risks posed by thermal runaway. Close to the action, HF concentrations can reach hundreds of ppm, and CO can be in the %–level league. These levels greatly exceed the exposure limits of the components in question, and the air around a battery in thermal runaway can be extremely dangerous to be and breathe in.
Real-life risks associated with thermal runaway
Generally speaking, the likelihood of thermal runaway increases as the number of battery cells grows, and the quality decreases – mainly because the chances of short-circuiting rise, and the heat tolerance takes a dip. To put it in simple terms, the bigger the battery, the more cells it has. Therefore, for instance, batteries of electric vehicles have typically a higher probability of short circuits than smaller devices, such as cell phones.
Thermal runaway results in high temperatures and the release of potentially hazardous gases. We’re talking about the real deal here – fires and even explosions could happen. In practice, thermal runaway in LIBs can lead to electric vehicle fires. Dealing with battery fires is no walk in the park; fires involving batteries are typically difficult to extinguish, which means that the fire potentially burns longer causing prolonged risks for both people and the environment nearby. This also requires special knowledge from the first responders.
Fire and explosion pose great danger in the immediate vicinity of the battery. And if that’s not enough, toxic gases add another layer of risk that can spread over a considerably larger area. The released gases and their concentrations during thermal runaway and subsequent battery fire depend on various factors. Think of different battery sizes, the number of batteries, and even the environmental conditions. This, in turn, affects the risks and how you manage them. Hazardous gases are one of the most crucial risks associated with the thermal runaway of lithium-ion batteries, as there are dozens of gases potentially present in varying concentrations. Fatal levels of toxic gases? Yeah, that’s a possibility worth considering.
Studying reactions and gases released during thermal runaway gives battery and electric vehicle manufacturers, as well as authorities valuable insights on in which situations thermal runaway takes place, what risks it poses, and most importantly, how to prevent this from happening in the first place.
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