Fire safety continues to be a key topic for electric vehicles (EVs) as the market grows. However, EV batteries come in various chemistries and designs, and their safety and failure modes can differ, meaning that the materials employed to aid in their safety also need to meet different requirements.
IDTechEx’s report, “Fire Protection Materials for EV Batteries 2025-2035: Markets, Trends, and Forecasts,” predicts a 15% CAGR for those materials. However, each battery’s requirements will lead to various material opportunities.
What Types of Batteries Are Being Used?
The current EV market can be primarily segmented into lower energy density and cost lithium iron phosphate (LFP), higher energy density, and nickel manganese cobalt (NMC) chemistries. This refers to the cathode, and whilst there are many subsets of these, and new variants emerging, the following discussion will primarily rely on this distinction.
Future solid-state batteries may also remove the liquid electrolyte in favour of a solid one, increasing energy density further and potentially leading to improved safety. However, solid-state encompasses many chemistries, and each one needs evaluation.
How Likely Are They to Enter Thermal Runaway?
Several causes of thermal runaway exist, including overcharging, overheating, internal short circuits, and mechanical damage. A manufacturing defect in any cell could cause an internal short circuit that may lead to thermal runaway.
However, specific chemistries are better than others in terms of thermal stability. LFP is more thermally stable than NMC, meaning it is less likely to overheat. The same applies to solid-state batteries, where removing a liquid electrolyte improves thermal stability.
Which Batteries Burn the Hottest?
How hot a battery will burn may impact the choice of protection materials used. Studies suggest this is tied to the cell’s energy density. Higher nickel (and hence higher energy density) cells will exhibit higher temperatures during thermal runaway. For example, NMC 811 cells could be around 800-900°C, whereas LFP cells tend to be below 600°C.
Future solid-state cells may be less likely to enter thermal runaway. Still, early studies have shown that in an internal short circuit scenario, the higher energy density of solid-state batteries can lead to temperatures even higher than NMC, exceeding 1500°C. This will depend on the specific type of battery, and it is still too early to test and develop solid-state batteries.
What About the Gases Generated?
In addition to the high temperatures, batteries will vent gases when in thermal runaway. Many of these gases are quite problematic from a flammability or toxicity standpoint. Similar to the maximum temperature reached, the volume of gas is tied to energy density, with high-nickel NMC releasing more gas than an LFP cell.
However, the composition of the gases can vary. For example, hydrogen is a much greater proportion of the vented gas from LFP, whereas carbon dioxide and carbon monoxide are a greater proportion of NMC’s vented gases.
Summary and Outlook
While fires in EVs are infrequent, they still pose a risk for the edge cases. Several types of batteries and chemistries are in use today, and new ones will be introduced in the future. It would be unwise to assume that any battery is 100% safe.
Therefore, protection measures for these rare cases are vitally important. Many materials for use outside the battery cells can help prevent or delay thermal runaway propagation. Materials already used include ceramic blankets, mica, aerogels, encapsulating foams, intumescent polymers, and many more. Each battery designer will have to consider their battery system’s risk profile and design.