Developments in Lithium-Ion Battery Fire Safety

One-third of electric vehicle fires occur during charging and another third while parking.¹ Although there still is a debate, errors in battery production or in the battery management system (BMS) have been addressed. The functioning of the BMS is usually limited to tracking the cells in the respective battery and does not have access to historical data or data from other battery systems. Consequently, the detection of anomalies or long-term analyses is very difficult or even impossible. Through online monitoring solutions, using predictive diagnostics by incorporating historical battery data, critical battery cell conditions can be detected at an early stage, and safety measures can be initiated before damage occurs.

Common Battery Fire Safety Measures

Despite the online monitoring solutions described, the risk of a battery fire cannot be excluded, which is why safety measures using fire protection materials in the battery system play a major role. Safety measures can be applied at the battery cell, battery module and/or battery pack level.

Currently, two main strategies are being pursued:

1. Fire Resistant Coating: The battery housing is equipped with an appropriate fire protection coating so that the environment, and thus also the passengers, are protected from burning cells in the battery system. Accordingly, this measure is to be classified at the battery pack level.‍
2. Thermal Propagation Prevention: By preventing thermal propagation through the use of safety-relevant materials between and/or on the cells, the fire in the battery is kept as small as possible so that there is little to no impact on the surroundings and thus the passengers. Depending on the battery design, this measure can be used at both the battery module and battery pack levels.

Fire Protection Materials for Battery Casing

Classic fire protection materials on the battery casing, often used on the inside of the battery lid, are composite materials that are currently found to be mica sheets. However, due to a more efficient production process and the demand for sustainable materials, the trend is towards liquid-applied fire protection coatings. The advantage of liquid-applied materials is that there are no pre-processes, such as pre-molding or cutting of the sheets, and the possibility of a fully automated and highly flexible coating.

Liquid-Applied Coatings for Battery Fire Protection

One possibility for an application process of a liquid material is the flat-streaming process,which is suitable for coating almost 2-dimensional components like the battery lid. The second coating method is spray coating, especially for 3-dimensional components like the battery housing. For liquid-applied fire protection coatings, different material classes are on the market, but mainly can be divided into organic material, for example epoxy, or aqueous inorganic systems.

The fire protection coatings can have intumescent properties, so-called heat foaming. This reaction creates a heat shield by separating the temperature source from the surface that should be protected. An important criterion is the carefully designed reaction control, in order, not only to avoid blocking venting channels, but also to prevent short circuits by creating contacts between components. The advantage of this material is the low initial coating thickness. An example of this class of material is Loctite EA 9400 from Henkel. For the Loctite EA 9400, there is an additional advantage: the application process via a flat streaming method, in which masking is not necessary due to the high coating accuracy. Also, due to the coating method, no aerosols are formed, which in turn means less effort regarding the production environment.

Another advantage is the lower material waste due to the controlled and precise process, which in turn allows a high sustainable use of the materials. Alternatively, the fire protection coating can also provide heat protection via a thicker initial coating thickness, whereby the material then has no intumescent properties. The advantage here is that no chemical reaction takes place in the battery during the thermal discharge event, and thus greater controllability can be ensured. An example of this is Loctite FPC 5060 from Henkel. As Loctite FPC5060 is a water-based inorganic system, no flames or smoke are generated by this material during the thermal runaway of battery cells.

Intercell Materials for Battery Fire Protection

Prevention of thermal propagation can also be achieved by intercell materials. This can be applied between the battery cells. The material class and property are decided by the cell form factors and the related stacking design. For cylindrical cells, potting materials or foams are often selected; for prismatic and pouch cells, sheet / pad type materials can be the best solutions.

Developments in Fire Protection Materials

Depending on the battery design, the material requirements vary. In principle, regarding fire protection, all materials must have low thermal conductivities and the high thermal stabilities as well as sufficient dielectric strengths. In addition to the fire protection requirements, specific properties are also needed regarding the mechanical behavior. More specifically, different levels of mechanical properties are required for the structural integrity of each design, such as elasticity, stiffness, yield strength, etc. Henkel offers bespoke pad solutions for individual customers by tailoring mechanical, thermal, and electrical properties for a variety of cell & system designs.

Today's electric-vehicle-dedicated platforms are new additions to the market. Therefore, safety-focused designs are not yet complete, nor are the material solutions. Innovative efforts to find the best fire protection for future battery systems are ongoing. Two opposing trends can be identified for future battery systems:

1. Cell chemistries that offer greater safety due to lower fire capabilities are sought after (e.g. all solid state).
2. Battery cells are becoming larger and larger, with correspondingly more capacity per cell and a larger impact in the event of thermal runaway.

Since the majority of battery systems will be based on the current lithium-ion batteries with liquid electrolytes in the next 10+ years, growing demands on safety-relevant materials can be observed. In particular, stability against hot particles over several minutes demands advanced material developments with outstanding properties.