Lithium-ion batteries (LIBs) are integral to devices from smartphones to electric vehicles (EVs) and large-scale battery energy storage systems (BESSs). However, their widespread adoption has led to increasing concerns about fire, toxic gas and explosions.
The growing number of LIB-related fires and explosions worldwide, whether in EVs, electric scooters and bicycles, or energy storage, highlights the need for comprehensive fire safety protocols.
The recent publication of PAS 63100: 2024 Electrical installations. Protection against fire of battery energy storage systems for use in dwellings. Specification underscores the importance of addressing the issue.
The significant risks to the public and to infrastructure are of particular concern for surveyors and property managers, who must ensure the safety of buildings and their compliance with current fire safety standards.
Understanding the use of and challenges posed by LIBs will enable such professionals to implement effective safety measures and emergency response plans.
Chemistry distinct from other batteries
LIBs are rechargeable energy storage devices widely used in various applications due to their high energy density, long cycle life and low self-discharge rate.
Each battery consists of an anode, cathode, separator and electrolyte. Typically, the system is named according to its cathode chemistry, such as LFP (lithium iron phosphate) or NMC (lithium nickel manganese cobalt oxide).
The greater the nickel content of the latter, the more highly powered the system is considered. This is signified by the numbers that come after NMC; for instance, NMC811 means there are eight parts of nickel, one of manganese and one of cobalt. The amount of lithium itself remains constant and is thus not reflected in the name.
Unlike other types of battery, such as nickel-cadmium (NiCd) or nickel-metal hydride, LIBs offer higher energy density. They also have a lower memory effect – that is, the gradual reduction in working voltage due to incomplete discharge on previous uses – which enhances their efficiency and practicality.
However, another big difference is safety: LIB electrolytes consist of flammable, non-aqueous organic solvents that don't react well with water and air. Moreover, the batteries are sensitive to temperature and mechanical stress.
This may in turn lead to thermal runaway, a dangerous and self-accelerating reaction that can cause catastrophic failures such as fires or explosions.
Thermal runaway presents particular risk
Thermal runaway occurs when the heat generated in a battery surpasses the amount dissipated, leading to a rapid increase in temperature and pressure. Typical causes include internal short circuits, overcharging, physical damage or exposure to high temperatures.
The first stage of runaway is the decomposition of the solid electrolyte interface. This is a layer of insoluble and partially soluble products of electrolyte component reduction that build up on the electrode's surface when it is first charged.
The breakdown of this layer leads to the electrolyte coming into contact with the anode and decomposing, subsequently producing heat and gases, such as hydrogen, carbon monoxide, methane and ethylene, which can ignite on exposure to air.
The second stage of runaway is when the separator melts, leading to an internal short circuit. This is followed by the collapse of the cathode, resulting in nanometal oxides releasing oxygen that sustains the fire.
Due to their differences in chemistry, thermal runaway will begin at different temperatures for different types of battery. Thermal runaway onset for the NMC is about 150°C, whereas for the LFP it is much higher at around 310°C. Moreover, due to the chemical bonding much less oxygen is released when the LFP fails, hence there is a lower probability of fire.
When LFP batteries entered the market on a large scale in the late 2000s, industry and academia branded them as safer than other types due to this higher onset temperature and lower level of oxygen release. However, the increasing number of battery fires – as mentioned above – proves that this is a misleading claim.
Although LFP systems rarely burn they are more prone to catastrophic explosions because, when they fail, they off-gas more hydrogen and create an explosive atmosphere more quickly.
The release of toxic and flammable gases during thermal runaway can lead to secondary explosions, making it a critical issue for applications where large battery packs are used such as solar photovoltaics (PV).
'Industry and academia branded LFP batteries as safer than other types – however, that is a misleading claim'
LIB fires can spark explosion
The vapour cloud produced in thermal runaway creates an explosive atmosphere that poses severe risks to property.
When LIBs fail, gases can accumulate in confined spaces such as EVs or densely packed energy storage systems. During runaway, 1kWh of battery capacity produces between 400 and 6,000 litres of gas, depending on chemistry and state of charge.
With LIB failure, temperature also rises uncontrollably and the pressure inside can increase rapidly. This can cause the casing to rupture violently, leading to explosions that scatter burning battery fragments, increasing the risk of secondary fires and injuries.
Such explosions can be more destructive than those in the open air because of the increased pressure and shockwave generated in the confined environment.
An unconfined vapour cloud explosion (UVCE) meanwhile occurs when a cloud of flammable gas is released and ignites in an open space. This type of explosion happens during thermal runaway, where the batteries emit various flammable gases such as hydrogen, methane and ethylene. Although the gas cloud can disperse, it is still a significant hazard if ignited.
Flame and fumes threaten safety and health
So-called rocket flame can occur when the pressure inside the battery during thermal runaway forces out a stream of flammable gas that ignites, creating a high-velocity jet of fire. This can cause severe burns, ignite nearby materials and spread fire in the vicinity.
The intensity and directionality of the rocket flame make it particularly dangerous where multiple batteries are in close proximity, as it can trigger runaway in adjacent cells and lead to a cascading failure.
In addition to the immediate physical risks, LIB fires and explosions have long-term environmental implications. The toxic fumes released include substances such as hydrogen fluoride, which pose severe health risks if inhaled and can contaminate indoor air.
The residue from these incidents can even pollute local water sources if not properly managed, affecting both human health and local ecosystems.
Familiarity with systems and standards can mitigate risks
Mitigating LIB risks such as thermal runaway involves various strategies. First is education: when you know the threats and understand the consequences you are much better prepared to assess the risks and act accordingly if you come across a system during a survey.
A 2020 government research paper notes 'there are no regulations or standards developed specifically for domestic BESSs for the UK market'.
Although this was developed for installers, it will help you check whether new systems are installed properly; for instance, whether a battery has been poorly fitted in a loft or is connected to a PV installation that itself could be poorly wired.
It will also help to determine where older systems may be deficient and how they can be dealt with in future, especially what a new owner should be aware of and how they can enhance safety.
Meeting, and where possible exceeding, additional safety standards and regulations for battery manufacturing, testing and usage – such as IEC 62133-2: 2017, IEC 62485-5: 2020, or subsection 38.3 of the UN Manual of tests and criteria – can also enhance safety.
Familiarise yourself with the product specification of the battery system during your survey, which should inform you whether the cells were certified, whether there are any fire protection measures for walls and cabinets, and whether any ventilation is provided.
Finally, developing effective emergency response protocols for dealing with battery fires and explosions is vital. This includes training for first responders, ensuring safe disposal and recycling for damaged or end-of-life batteries, and campaigns to educate users on managing the risks of LIBs.
By combining these approaches, the threat of thermal runaway in LIBs can be significantly reduced, ensuring safer use in various applications.
Dr Wojciech Mrozik is a Faraday Institution senior research fellow at Newcastle University
Contact Wojciech: Email | LinkedIn
Related competencies include: Fire safety, Inspection, Legal/regulatory compliance, Risk management