Thermal Runaway: Why do Li-ion batteries catch fire?

Lithium-ion (Li-ion) batteries can catch fire due to a process known as thermal runaway, which is triggered by various factors and involves a series of heat-releasing reactions. While Li-ion batteries are widely used in laptops, cameras, and electric vehicles (EVs) such as scooters and cars, their rise in popularity has not been without issues. In the UK alone, fire services responded to 921 lithium-ion battery fires in 2023, a 46% increase from the previous year. Understanding and mitigating the risks associated with the flammable nature of Li-ion batteries is crucial for their safe and continued use. Is the development of robust battery materials key to safer batteries?

What is thermal runaway?

---

Thermal runaway is when a battery cell heats up too quickly and cannot release the amount of heat it’s generating. The temperature rise causes resistance to decrease and current to increase which in turn also add to the temperature. Increasing temperatures increase the rate of reactions, creating an uncontrollable cycle. The thermal runaway of one cell cause others to do the same, this is often referred to as thermal runaway propagation. The excess heat produced can lead to burning and eventually battery explosion and fires. Therefore, it is important to understand thermal runaway and investigate ways of prevention to increase the safety of Li-ion batteries.

What causes thermal runaway?

---

There are several scenarios which can lead to thermal runaway. These are often categorised into the following groups: thermal, mechanical, electrical and aging.

Category	Direct Cause
Thermal	Extreme temperatures (hot or cold) Fire Thermal shock (a sudden temperature increase)
Mechanical	Dropping Crushing Vibration
Electrical	Short circuits (where the two electrodes come into contact with each other, caused from overdischarging, crushing or wet environments) Overcharging / Overdischarging
Aging	High number of charging – discharging cycles

Implications of thermal runaway

---

Commercial Li-ion batteries are subject to various testing procedures and must meet a set of standards. Like battery technology, these tests are constantly getting updates and depend upon the country of origin and intended application. A common example is the International Electrotechnical Commission’s (IEC) standards for portable applications of Li-ion batteries (IEC 61960-4). Standards such as these allow the battery’s performance to be tested under abnormal conditions to help better predict the batteries performance and prevent detrimental failure, such as thermal runaway.

Fire

Despite these standards, thermal runaway still occurs and can have severe impacts. One of the most significant implications of thermal runaway is causing a fire. Globally, Li-ion batteries in EVs cause almost one fire every week. These battery fires also spread devastatingly quickly, one report states only 15 seconds between the first sign of smoke and the windows of a house being blown out. Not only does this destroy the cell and its surroundings, requiring expensive replacement, but also poses a huge danger to the users. For the safety of consumers and longevity of Li-ion products, thermal runway needs to be tackled.

Hazardous gases

In addition to the fires itself, thermal runaway also produces hazardous gases. Hydrogen fluoride (HF) is produced from the decomposition of lithium hexafluorophosphate (LiPF₆). LiPF₆is a common source for Li⁺ in the electrolyte and is a known toxin. Even a small 5 cell battery releases over 13 g of HF, considerably exceeding the limit of 0.025 g m^-3 stated by the National Institute of Occupational Safety and Health. Similarly, phosphorous oxyfluoride (POF₃), comes from the reaction between LiPF₆and water. Although the toxicity of POF₃ is unknown, it can react with water itself to product more HF.

Thermal runaway process

---

The mechanism of thermal runaway in a Li-ion battery involves a series of exothermic reactions. An exothermic reaction is a chemical reaction which releases heat to the surrounding environment. A common example of an exothermic reaction is combustion, like lighting a candle.

The process of thermal runaway can be broken down into 4 exothermic processes: degradation of the SEI, decomposition of the electrolyte, the separator melting and decomposition of the cathode.

The solid electrolyte interface (or interphase) SEI is a thin layer that forms on the surface of the anode after a few charge cycles. It protects from unwanted reactions between the anode and electrolyte materials.

Stage		Initiating temperature	Description
1.	SEI degradation	~ 80°C	As temperature increases, the SEI starts to break down which exposes the electrode to the electrolyte.
2.	Electrolyte decomposition	~100°C	The thermal decomposition of the electrolyte often releases flammable gases as well adding to the cell’s temperature.
3.	Separator melting	~130°C	The high temperature can melt the separator. This allows the two electrodes to come into contact, therefore short circuiting the cell.
4.	Cathode decomposition	~150°C	Finally, the cathode will begin to decompose. This also releases flammable gases as well as oxygen which can intensify flames. The increase in pressure from these gases can lead to an explosion.

How to prevent thermal runaway

---

Battery fires are a new phenomenon to fire response services and are often misunderstood as electrical or combustible metal fires. In fact, Li-ion battery fires are classed as flammable liquid fires, Class B. Li-ion battery fires can also reignite themselves, as Li⁺ can self-oxidise, making them difficult to put out. Previously large Li-ion EV battery fires have required around 30,000 gallons of water over multiple hours to extinguish. Specific Li-ion fire extinguishers, Li-Ex, have since been developed. Ideally, however, thermal runaway should be prevented to avoid scenarios such as this.

There are several ways under investigation to prevent thermal runaway in Li-ion batteries, such as thermal management, battery management and re-designing the cell.

Thermal management

Cell coolants

Thermal runaway is exacerbated by high temperatures, therefore one way to stop this cycle is to reduce the temperature of the cell. Different types of cell coolants are used to help dissipate this excess heat. Liquid cell coolants are popular in commercial Li-ion batteries, a mixture of water and glycol is popular for its high heat capacity (3.4 kJ kg K^-1). Liquid cell coolants are efficient and provide a uniform temperature distribution but do come at an extra cost and add volume to the system.

Phase change materials (PCMs)

Phase change materials (PCMs), which absorb heat during their phase change, are also being investigated to cool batteries. Examples of these range from paraffin wax to complex hydrated salts, like sodium thiosulfate pentahydrate.

Mist cooling

A novel technique is mist cooling, where cells are immersed in an evaporating liquid which produces a mist. These also achieve a very uniform temperature inside the cell but are complex and need further development. Some research also suggests mist coolers to increase the amount of hazardous HF gas produced.

Battery Management Systems (BMS)

Cell balancing

By monitoring the operation of the battery and adapting to any changes that might lead to dangerous scenarios, thermal runaway can be avoided. Although batteries are produced with a stated capacity, differences between the individual cells can lead to non-uniformity. Over time these differences are intensified, and the battery becomes imbalanced. Cell balancing is introduced to correct this. Cell balancing applies a resistance to the cells with a higher output, allowing the weaker ones to catch up. This is a cost effective method but very slow and gradually reduces the energy available from the battery.

Thermal runaway detection

Battery management systems will consist of both thermal runaway detection and mitigation processes. Tracking any increase in temperature is the most common form of detection. However, as the initial rise in temperature occurs at the core of a cell, this is difficult. Non-invasive and modelling techniques are under investigation for accurate temperature determination at the core. Alternatively, detection systems can involve pressure sensors to detect the amount of gas being released, which is an early sign of thermal runaway.

Thermal runaway mitigation

If thermal runaway is detected, mitigation processes will be triggered. This involves a shutdown, of either the whole system or the affected part. The shutdown process involves releasing inert gases to suppress flames, adjusting safety valves, and ensuring a protective caged structure is in place to contain any potential explosions.

Cell design

The best way to prevent thermal runaway is to use battery materials that can withstand or mitigate temperature increases. Research into modified electrodes, electrolytes and separators is popular. However, this is the most time consuming and expensive method as new material interactions need to be understood.

Positive Temperature Coefficient (PTC) materials

Positive Temperature Coefficient (PTC) materials are being incorporated into cell designs to prevent thermal runway. The resistance of PTC materials increases as temperature increases, due to a change in crystal structure. This makes it much harder for a current to flow. This interrupts the normally uncontrollable cycle of thermal runaway and reduces the associated dangers. Common PTCs are ceramic materials, such as Ba_0.9Sr_0.1TiO₃ which activates at 54°C, at the top end of the operating range for batteries. The PTC materials are used either as a coating or mixed into the cathode material. Polymer based PTCs are also under research, such as poly(3-decylthiophene) which decreased conductivity by three orders of magnitude with an increase from room temperature to around 80°C. However, PTCs often require an additional conductive layer and their compatibility with the other battery materials must be well understood.

Graphene

Graphene is a 2D material made of carbon and has a high thermal conductivity. This means when it’s incorporated into Li-ion batteries it can help to quickly release the heat being generated. A graphene enhanced Li-ion battery showed a decrease in average operating temperature of 25°C.

Casing

Thermal runaway can also be caused by mechanical issues such as crushing. Therefore, it’s important for a battery to have a robust outer casing or shell. Polycarbonate is commonly used for its strength. Polycarbonate has an impact resistance 250 times greater than glass. Nanocomposite mixtures are also being researched to further enhance the material's flame resistance.

How do Li-ion batteries work?

---

Batteries convert energy between chemical potential energy and electrical energy. They do this via redox reactions. Redox reactions simultaneously consist of reduction (where species gain electrons) and oxidation (where species lose electrons).

A battery is made up of two electrodes (the anode and cathode), an electrolyte, separator and power supply. Li-ion batteries get their name from the lithium ions (Li⁺) ions used in the electrolyte, which is responsible for transporting the ions from one electrode to the other. Li-ion batteries are a type of secondary battery which means they are rechargeable.

Battery Materials

Browse now

Learn More

---