Why Do Lithium-ion Batteries Catch Fire?

This informal CPD article ’Why Do Lithium-ion Batteries Catch Fire? was provided by Dr. Frank Richter, CEO of Greenectra, renowned battery experts who can help you with training, recruiting, and consulting to fully optimize your battery innovation capabilities.

Li-ion batteries (LIBs) power many of our daily-used devices, from rather small ones, such as smartphones and laptops, to large ones, such as electric vehicles and grid-scale energy storage systems. We hear quite often about Li-ion battery fires and we want to look into some of the details, because understanding the reasons behind battery fires can help us use with more safe usage. It is also important to state that the fire risk is not the only risk. We should definitely not forget about battery explosions, because under some circumstances you may not have a fire immediately, but a growing risk of an explosion. The liquid electrolytes used in commercial Li-ion batteries burn very well or can create an explosive atmosphere, but the electrolyte can also be dangerous to life and health in other ways, for example when inhaled. This chemical risk is one out of many more risks associated with LIBs, but we will be focusing on the fire risk in this article.

Introduction

Li-ion Battery Operation

LIBs have 2 terminals, a positive and a negative one. These terminals are connected electronically to the electrodes inside a Li-ion battery. Inside the battery, we find 2 different electrodes: cathode and anode. When discharging the battery, Li-ions get on their way inside the battery, from anode to cathode. Outside of the battery we have a current during discharge, and electrons move from anode to cathode. Obviously, during charge, Li-ions and electrons take the opposite way. To avoid an internal short circuit, there is a separator between cathode and anode.

Li-ion Battery Types

We will not dive deep into all types and subtypes, but generally we can say that there are different types of Li-ion batteries indeed and we mostly call them by the type of cathode used in the battery. The reason behind is that the anode is based on some sort of carbon in most cases, but cathodes really vary. You may have heard about NMC, LFP, NCA, LMO Li-ion batteries and these abbreviations all define the cathode type. There is one exception: LTO batteries. In this special case, we do not have a carbon-based anode, but an anode based on Lithium-Titanate. Instead of Battery Type, sometimes we use the wording “Battery Chemistry”.

Causes of Li-ion Battery Fires

There are several root causes why Li-ion batteries might catch fire or explode. All of these root causes most likely lead to something that we call thermal runaway.

Thermal runaway is a dangerous chain reaction where an increase in temperature causes further temperature increase, often resulting in a fire. It can be triggered by many types of abuse, such as, but not limited to: overcharging, deep over-discharging and charging again, charging at too high rates (especially critical at low temperatures), discharging at too high rates, crush and other structural damages, external heating, external short. A contamination during manufacturing can also cause a thermal runaway to happen even years later when the battery is in its application. Once thermal runaway starts, it can quickly escalate, causing the battery to combust.

One important point is that Li-ion Batteries do not just catch fire or explode for no reason. But a fire or explosion can happen if something goes wrong.

Internal Short Circuits

Internal short circuits occur when there's an internal physical connection between the anode and cathode. This can be caused by manufacturing defects, physical damage, or the growth of Lithium dendrites - tiny, needle-like structures that can pierce right through the separator and cause an internal short. When this happens, large currents can flow internally, the battery can overheat, leading to a fire or explosion. The risk of Li-dendrite growth is especially critical when charging at too high rates at too low temperatures.

Overcharging

Overcharging occurs when a battery is charged beyond its maximum capacity. This would then cause the electrolyte to break down and lead to heat and gas development. Of course, battery management systems (BMS) are designed to control voltage limits to prevent overcharging, but a faulty BMS could lead to an overcharge scenario. 

Mechanical Damage

Physical impacts, punctures, or crushing can damage Li-ion battery to an extend leading to internal short circuits and thermal runaway. For example, puncturing a battery during a car accident can obviously cause the battery to catch fire. There was also one reported case about a car driving on the highway at high speed over a metal object that punctured the battery, leading to thermal runaway.

Manufacturing Defects

Manufacturing defects can be manifold. Poor welding, other material contamination, or improper assembly can create safety critical conditions. In most cases, manufacturing defects will show at the end of the manufacturing process, when the battery is charged for the first time. But there have been notable recalls of batteries, which you could call “ticking time bombs” due to manufacturing defects.

Fire or Explosion?

Well that depends. It depends mainly on the Li-ion battery type and the circumstances of the safety-critical event. Two very common types of Li-ion batteries are NMC and LFP. NMC uses Lithium (N)ickel (M)anganese (C)obalt Oxide as cathode. NMC batteries are very typical in the EV sector in Europe. In Asia, you find a higher share of LFP batteries in the EV sector compared to Europe. LFP batteries utilize (L)ithium (F)errum (P)hosphate as cathode material.

For NMC Batteries, if something goes wrong, the outcome depends on the state of charge (SOC) of the battery. As a rule of thumb, fire and even jet-like flames develop, when the SOC is above 50% in the moment of the safety critical event. When the SOC of the NMC battery is below 50%, fire and jet-like flames are less likely to happen, but the battery could still develop a lot of heat and toxic and explosive gasses.

LFP batteries are often portrayed as a safer battery technology. You can find false information, stating that LFP batteries do not burn and some sources even state LFP batteries cannot go into thermal runaway. LFP batteries tend to produce less gas during thermal runaway, which makes them safer in some way, but LFP batteries can go into thermal runaway, they can burn, and there are reported cases of Li-ion battery fires with LFP batteries.

We stated earlier that the thermal runaway is a chain reaction. Of part of this chain reaction is the breakdown of the cathode. LFP cathodes collapse at way higher temperatures compared to NMC cathodes, but they can still break down.

We just learned that there are thermal runaway scenarios without a fire involved, but is this actually desirable? Obviously, it depends on the general safety strategy of the whole battery system, but please remember the following: If a battery does not burn during thermal runaway, it still produces gasses, which are toxic and explosive. Especially in confined spaces this can lead to the even larger risk of explosion.

Conclusion

Li-ion batteries do not catch fire or explode for no reason, but their tendency to catch fire or explode under certain abuse conditions cannot be ignored. Understanding the causes and implementing safety measures can mitigate the risks associated with Li-ion batteries.

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