As the adoption of lithium-ion batteries continues to grow, driven by the push towards electrification and renewable energy, the importance of addressing their potential hazards cannot be overstated. Wojciech Mrozik of Newcastle University in the UK sets out the issues.

While lithium-ion batteries offer numerous advantages over traditional batteries, their capacity for thermal runaway and the consequent risks of fires, explosions, and toxic gas emissions present significant safety challenges. To thoroughly understand the complex safety concerns associated with these batteries, we must delve deeper into their operational principles, the chemical processes during failures such as thermal runaway, the consequent risks and dangers, and the multifaceted strategies for prevention, mitigation, and emergency response.

Chemistry and operation

Lithium-ion cells come in three primary shapes and sizes: cylindrical (typified by the 18650 cell: 18mm in diameter and 65mm long), prismatic and pouch. Cylindrical and prismatic cells are assembled in metal cans with burst caps, and pouch cells are assembled in metallised plastic.

At their core, lithium-ion batteries function by moving lithium ions between the anode and cathode through an electrolyte, without relying on significant chemical transformations that characterise other battery types. This process, while efficient, harbours potential for thermal instability. The anodes, typically composed of graphite deposited on copper foil, and cathodes, made from mixed metal oxides, including lithium cobalt aluminium oxide (LCA), lithium iron (ferrous) phosphate (LFP) and lithium nickel manganese cobalt oxide (NMC), are sources of high energy density. However, this attribute also renders lithium-ion batteries vulnerable to thermal runaway, especially when the battery’s internal mechanisms, designed to contain its energy safely, fail due to physical damage, electrical abuse, or manufacturing defects.

Dynamics of thermal runaway

The term ‘thermal runaway’ refers to a self-amplifying condition where increased temperature speeds up a reaction, which further raises the temperature, leading to the rapid escalation of heat and pressure. In lithium-ion batteries, this can be initiated by the breakdown of the solid electrolyte interface, a critical component that normally stabilises lithium-ion transfer, protecting the anode from direct contact with the electrolyte.

Once this barrier is compromised, through overcharging (over-discharging), overheating, or physical damage, the battery is prone to rapid and uncontrolled temperature increase. This not only jeopardises the individual cell but can also cause adjacent cells to fail, amplifying the hazard.

Emissions and hazards

During thermal runaway, lithium-ion batteries release a mixture of gases (vapour cloud) that is toxic and flammable. These include, but are not limited to, hydrogen, carbon monoxide, carbon dioxide and volatile organic compounds, alongside particularly dangerous substances such as hydrogen fluoride.

The release of such a complex chemical cocktail poses multiple risks, from fires and explosions to acute toxic exposure and long-term health impacts. For instance, hydrogen fluoride, even in small concentrations, can cause significant harm to soft tissues and bones, making it one of the most perilous emissions from lithium-ion battery failures.

The common misconception is also that thermal runaway equals fire. For instance, LFP batteries very rarely catch fire but still go into thermal runaway. Also, putting out the fire may get rid of a flame but does not stop thermal runaway – cells are still off-gassing. (Figs 1–10)

The risks associated with lithium-ion batteries are not theoretical and several high-profile incidents have underscored their potential for destruction. From the fires in electric vehicles, marine vessels and dwellings to the catastrophic failure of utility-scale battery installations, these events highlight the importance of stringent safety measures. In many of these cases (search for videos online), the rapid onset of thermal runaway led to fires that were challenging to extinguish, causing significant property damage and, in some situations, injuries or fatalities.

Thermal runaway & fires

Such a situation can be illustrated by the experiment performed in February 2024 by Newcastle University’s “SafeBatt: Science of Battery Safety” project team sponsored by the Faraday Institution.

The battery energy storage system (BESS), Fig 1, consisted of five lithium-ion battery modules (1.6kWh each – a total of 8kWh capacity). All modules were charged to 100% state-of-charge.

• Overcharge of one module was used as the mode of failure/abuse
• Fig 2 – initial indications of thermal runaway (TR) might be off-gassing – however, that is not always the case
• Fig 3 – explosion of overcharged module – no warning, no indication of the temperature rise
• Fig 4 – burning of the subsequent module
• Fig 5 – application of the water-based extinguisher
• Fig 6 – no visible flames for a short while
• Fig 7 – no stopping of the TR – recommence of vapour cloud (VC)
• Fig 8 – another module going into TR – however no flames, just huge volumes of VC
• Fig 9 – reignition of the remaining modules
• Fig 10 – fully burnt BESS – inside temperature still around 600^oC.

Emergency response

The unique hazards of lithium-ion battery fires require specialised emergency response tactics. Traditional firefighting methods may be ineffective or even dangerous as we still do not fully recognise and understand all the unique hazards associated with failing lithium-ion batteries.

For example, an extinguishing agent applied to a lithium-ion battery fire can generate a vapour cloud containing hydrogen gas, potentially leading to explosions. Emergency responders must be equipped with the knowledge and tools to tackle these incidents safely, thus the need for more research and training.

Prevention and mitigation strategies

Preventing lithium-ion battery failures and mitigating their impacts when they do occur involves a layered approach:

• battery design: Advances in battery technology can enhance safety. This includes the development of new electrode materials, electrolytes and battery architectures that are inherently less prone to thermal runaway
• manufacturing controls: Ensuring high-quality manufacturing processes with rigorous quality assurance checks can minimise the likelihood of defects that could lead to battery failures
• regulation and standards: Clear regulations and safety standards are crucial, both for the manufacturing of lithium-ion batteries and their integration into products and systems. This also extends to transportation and disposal regulations to handle these batteries safely throughout their life cycle
• education and training: Educating consumers on safe charging practices, storage and handling can reduce the risk of accidental damage to lithium-ion batteries. Similarly, professional training for those working with lithium-ion batteries in any capacity is vital to ensure they understand the risks and appropriate safety protocols
• emergency preparedness: Businesses and institutions that use or store lithium-ion batteries should have appropriate risk assessment and, if necessary, specific emergency response plans that account for the unique challenges of lithium-ion battery failures. For instance, that may be the installation of appropriate fire suppression systems (e.g. not to tackle battery fire per se, but rather to contain the incident and protect surroundings) and ensuring that all staff know how to respond in the event of a battery incident.

Towards a safer future

By advancing battery technology, implementing rigorous safety protocols, and fostering a culture of awareness and preparedness, we can mitigate these risks. Continued research into the mechanisms of thermal runaway and the development of advanced materials and designs will bring success. This includes exploring alternatives to current lithium-ion chemistry that may offer similar energy densities with reduced risks.

Moreover, the integration of smart technology (artificial intelligence) into battery management systems (BMS) represents a significant opportunity to improve safety. Advanced BMS can monitor the health of each cell in real time, identifying potential issues before they escalate into dangerous situations. This proactive approach to battery safety can prevent many incidents of thermal runaway, reducing the risk to both individuals and property.

Regulation and policy

Governments and regulatory bodies play a crucial role in ensuring the safe use of lithium-ion batteries. This involves setting standards for manufacturing, usage, transportation, and disposal to minimise environmental impact and prevent accidents. Policy initiatives that encourage the recycling and proper disposal of lithium-ion batteries can also mitigate the risks associated with damaged or aged batteries.

Furthermore, international collaboration is essential for developing and harmonising safety standards. Given the global nature of the battery supply chain and the widespread use of lithium-ion batteries across borders, consistent safety protocols can help prevent accidents and facilitate rapid responses when incidents do occur.

However, there is still much to be done as the case of lithium-ion batteries seems to be more complex than initially thought.

Education and awareness

Consumer education is a cornerstone of lithium-ion battery safety. Many users are unaware of the risks associated with improper use, such as using incompatible chargers, exposing batteries to extreme temperatures, or physically damaging them. Educational campaigns can inform the public about how to safely use, store, and dispose of lithium-ion batteries, significantly reducing the risk of accidents.

For professional settings, specialised training for first responders, engineers, and technicians who work with lithium-ion batteries is essential. This training should cover the fundamentals of lithium-ion battery technology, the risks of thermal runaway, and specific emergency response techniques for incidents. Such knowledge is crucial for ensuring that those on the front lines are prepared to act effectively and safely.

While lithium-ion batteries have become integral to modern life, the potential for thermal runaway and its associated risks necessitates a comprehensive approach to safety. Through a combination of advanced technology, stringent safety standards, informed policy and public education, we can harness the benefits of lithium-ion batteries while minimising their dangers. As we continue to rely more on these powerful sources of energy, our commitment to safety will ensure that we can do so without compromising the well-being of individuals or the environment.