Lithium-ion batteries can pose serious risks, and the concept of safety is based on narrow criteria. A meta-analysis of greenhouse gas emissions by Sheffield researchers, published in Energy Storage Journal improves understanding and highlights the need for broader risk analysis.

A team from the Department of Chemical and Biological Engineering, led by Professor Sol Brown, conducted a detailed meta-analysis of 60 papers to better understand the gases released by lithium-ion batteries during thermal runaway.

The main objective of the review was to check how different battery features affect the amount and type of gas released, e.g. its design and charge level, and whether it could be determined which type of battery is the least dangerous in terms of fire and explosion.

Professor Sol Brown, professor of process and energy systems, said: “As sales and production of electric vehicles increase, there is a growing concern that we do not fully understand the risks they pose in the event of a failure and ignition.

“Unlike gasoline and diesel fires, which have been the subject of numerous studies, lithium-ion battery fires are relatively understudied. This limits our ability to effectively manage them if they occur. To fill this gap, our meta-analysis highlights some key findings and provides recommendations for further research to improve battery safety in the future.”

If misused, lithium-ion batteries overheat and can catch fire. This process also releases gases that can explode and potentially cause serious injury. Additionally, some of the gases released, such as carbon monoxide and hydrogen fluoride, are poisonous and pose a toxic hazard.

However, apart from these general characteristics, the results of studies on gas emissions during runway incidents are not fully understood and need to be compared with each other.

Most of the reviewed work focused on lithium-ion–phosphate (LFP) and lithium–nickel–manganese–cobalt (NMC) batteries — commonly used in electric vehicles.

Key findings:

• The total amount of gas released by a battery increases with its size, but the specific gases produced do not change.
• Battery shape matters—prismatic cells release more gas than cylindrical cells, and the least.
• NMC batteries release more gas than LFP batteries, but are much more toxic
• Battery charging affects toxicity – for NMC batteries the volume of contaminants doubles from 0% to 100% charge, while for LFP batteries it is halved.
• LFP batteries produce more hydrogen, while NMCs produce more carbon monoxide. Scientists looked at something called the lower flammable limit (LFL) to determine how likely the gas is to ignite. The lower the LFL, the easier it is to ignite the gas. In an inert atmosphere, LFL levels are 6.2% for LFPs and 7.9% for NMCs, so LFPs pose a greater flammability risk.

The work in this article is intended to be a key resource for the battery community to aid in assessing the risks associated with uncontrolled fire, explosion, and toxicity of lithium-ion batteries.

The paper makes a number of recommendations that aim to introduce significant improvements in research to further enhance our understanding of lithium ion off-gases, including:

• More information on the material composition of the electrodes and the composition of the electrolytes inside the NMC battery. This will help us better understand if and how they affect the gases released.
• To obtain more accurate comparisons between high-energy LFP and NMC batteries, more testing should be performed on a wider range of LFP and prismatic cells (10–100 Ah).
• Testing larger battery systems – Current research focuses on single battery cells, but in real-world applications multiple cells are connected together, so it would be helpful to see how the hazards vary with battery size.
• Measurement of gas released at different charge levels – studies to date have only looked at fully charged batteries. It would be useful to see how the toxicity and flammability of the gas change at different charge levels, especially when the battery is overcharged
• Future experiments should record the amount and type of electrolyte released as vapor, as this will aid in assessing any additional fire risk.