Fire Mitigation Strategies

Author: Nrusimhan Seshadri, Balance Batteries

Having looked at the challenges and regulatory standards relating to fire hazards posed by Li-ion batteries in the previous article, in this article we will have a look at some of the fire mitigation strategies deployed across 10 BEVs in the current market.

The choice of thermal barrier materials (TBMs) for a battery pack depends on number of factors such as:

  • type of cell chemistry
  • form factor
  • pack architecture (modular or cell to pack)
  • most importantly the legislative requirements.

It is possible to pass the ECE 324 UN R100 regulation with very little in the way of thermal barrier material but to pass the Chinese GB38031-2020 a robust design factoring passive fire safety is vital.

Figure 1 shows the structure of a typical modular battery architecture, it consists of a pack top cover, modules, cross-members (mostly aluminium extrusions) for structural reasons, battery tray which houses the cold plate and the external bottom cover (omitted from figure) and finally reinforcements for protection against side impact. The top cover becomes crucial for occupant protection in an event of thermal runaway while the battery tray with bottom cover is crucial for protection against external fires. The cross members which act as a module housing need to withstand the jet fire with hot particle ejecta to delay thermal propagation between modules.

Figure 1 Structure of a modular Li-ion battery pack

The modular/skateboard architecture is still the go-to choice for many large OEMs while companies like Tesla, which uses cylindrical cells, are trying to shift towards cell to pack design. In such battery architecture it common to find cell-cell thermal barriers which prevent heat flow between cells in an event of thermal runaway.

There are seven categories of TBMs used for passive fire protection of Li-ion battery packs as shown in Figure 2.

Figure 2 Types of Thermal Barrier materials

In the following table, we see the different types of TBMs used by different OEMs.

Vehicle ModelYearOccupant ProtectionPositionExternal ProtectionPlacementAnti-propagation measuresPack Architecture
Tesla Model S PlaidJan-22Mica ShieldBetween the top cover and batteriesNo information availableNot applicableSelf-extinguishing PU EncapsulantsModular 18650
Rivian R1TJun-22Steel cover & Mica shieldBetween top cover and batteriesCFRP plateBottom coverSelf-extinguishing PU EncapsulantsModular 21700 Samsung cells
Tesla Model YJul-22SteelBattery top coverThin layer of micaCells mounted on ABS holders with a thin layer of mica at bottom vent faceGFRP module barriers and Rigid PU foam encapsulantsStructural 4680 pack with large modules
Lucid AirJun-23SMCTop coverSteel cover and GFRPSteel external cover and GFRP battery trayNo potting, thermo-formed mica sheets between parallel layer of cellsModular 21700
GM Hummer EVMay-23SteelTop coverSteelBattery trayMica covered vents/ stamped steel (aerogel) composite with foam – can be 3mm thickModular NCMA LG Pouch cells 103 Ah
Ford LightningDec-22SMCTop coverNoneNot applicableNo information availableServiceable, modular SK innovation’s NMC pouch cells
Mustang Mach EJul-21SMCTop coverNoneNot applicableNo information availableModular LG Pouch cells
VW ID4Jul-21NoneNot applicableNoneNot applicableNo information availableModular Prismatic cells
Hyundai Ioniq 5Jul-23No information availableNot applicableSMCBottom CoverNonePouch cells

From this table, it can be seen that there is a trend among newer vehicles to focus more on passive thermal safety of battery packs, thanks mainly to new fire safety regulations in China which is one of the key markets for companies like Tesla.

We see from the benchmarked data that for cylindrical cells encapsulating foams and potting are widely adopted propagation mitigation measures (Lucid is an exception) while for prismatic and pouch cells a combination of module and cell level thermal barriers are used, for instance the GM’s Ultium platform uses aerogel-based cell barriers for propagation mitigation.

Figure 3 Encapsulated Tesla Model Y’s pack based on 4680 cells (Credits: Munro Live)

Figure 3 shows Tesla Model Y’s structural battery pack with the latest 4680 cylindrical cells and the pink material, which fills the battery pack, is the encapsulant which is highly insulative and can reduce heat flow between the cells in an event of thermal runaway.

Figure 4 Thermoformed Mica separator (serpentine sheets) in Lucid Air battery module (Credits: Munro Live)

Lucid Air’s battery pack is an exception which doesn’t use any potting materials though it is based on cylindrical cells and uses a thermoformed mica sheet in between every parallel layer of cells (Figure 4).

Mica has excellent insulating properties even at high temperatures and with its high dielectric strength it often finds applications inside the HV battery pack. Another use case of mica between modules or cells as propagation mitigation measure is in GM’s Hummer EVs vent cover as shown in Figure 5.

Figure 5 GM Hummer EV’s battery module (Credits: Munro Live)

Here mica sheets are used as a cover above the cell vents possibly to counter against the jet fire and hot particle ejecta coming out of the cell during thermal runaway. For occupant protection against cell venting materials, OEMs like Tesla, Lucid and Rivian (see Figure 6) use a rigid mica shield in between the battery top cover and modules while the other strategy is to have top cover made up of SMC (sheet moulding compound) such as the one used by Ford as seen in Figure 8.

Figure 6 Rivian R1T’s mica shield in between the pack and pack’s top cover (Credits: Munro Live)
Figure 7 Lucid’s glass fibre battery tray floor (Credits: Munro Live)

Typically, composites are lighter while the speed of manufacture is challenging compared to the likes of steel or aluminium. In the above table steel is considered as a fire protection measure as it has much higher temperature resistance than aluminium although that might not be the primary reason for material selection. None implies there isn’t any special TBMs other than the standard aluminium alloys.

Figure 8 Ford Mustang Mach E’s SMC battery top cover (Credits: Munro Live)

There isn’t a definitive answer towards what is the ideal material for thermal runaway mitigation, every pack has its own unique set of requirements and challenges, and the solution must be tailor made.

At Balance Batteries we have invested in material selection and validation processes for meeting both the internal and external fire safety targets and have worked with multiple suppliers of TBMs across Europe so we can help you meet your design requirements while enhancing the overall safety of the battery pack.

About Balance Batteries – Founded in 2020, Balance Batteries consult on battery pack and module design and validation. Using a systems engineering approach we ensure your product requirements are met.

References

  1. Munro Live, YouTube Channel

1 thought on “Fire Mitigation Strategies”

  1. You can insert “none” in VW ID.4 anti-propagation. LG MEB module has zero countermeasures inside against TR. 24 densely packed *pouch* cells, ca. 3mm melamine foam between every 2 (or 3, I don’t remember) cell and that’s it. Propagation test with one cell pair (12s2p module) overcharged as a TR trigger and the module turned into a solid rocket booster, 3-4 meters long flame, insides burned completely within 2 minutes. I won’t buy anything MEB based.

Leave a Comment