EV Battery Safety Breakthroughs: Essential Innovations for BEVs

A long range and short charging times play an important role for users of Battery Electric Vehicles (BEVs). This requires Lithium-ion battery systems with high performance cells. Thermal runaway is the inherent risk of such battery cells with the potential consequence being severe vehicle damages or occupants’ injuries. Oerlikon has developed solutions to minimize the effects of such a process.

Market acceptance of battery electric vehicles is strongly dependent on performance as well as vehicle cost. The traction battery is a significant performance driver but also a major cost contributor. Continuous improvement of driving performance and reduction of costs in short development cycles is therefore essential for market success.

Expectations for high ranges and short charging cycles require batteries with high energy density, which means tight cell packaging within high performance Nickel Manganese Cobalt (NMC) cell types. An inherent risk of such cells is Thermal Runaway (TR), a selfheating of the battery cell in an exothermic degradation of the active mass, resulting in an explosive release of hot and conductive reaction gases through the cell vent. Self-heating is triggered when the cell temperature exceeds a critical limit. Over-heating can be caused by internal or external short circuits (contamination or crash), external heating, over-charging, or a malfunction of the cooling system.

Without a protection concept, this event triggers a TR in further cells by conductive heat transfer to adjacent cells, through the hot gas stream of a venting cell or ignition of the conductive atmosphere due to electrical arcing. This chain reaction is called Thermal Propagation (TP), resulting in severe damage to the battery or vehicle and, in the worst-case, fatal consequences for occupants. The risk of TR can be minimized by design, but not eliminated. A safety concept is required to avoid TP and its catastrophic consequences.

Gas guiding components channel hot gas from the cell vent, including the conducting particles, to uncritical areas and/or build an electrically insulating barrier between High Voltage (HV) components. This eliminates the risk of electrical arcing between HV terminals of different voltage potential.

Separators between single cells inhibit heat transfer from one cell to the adjacent cells. In addition to the thermal function, the separator elements should be able to compensate cell swelling over lifetime, as well as breathing during charging and discharging, while minimizing pressure on the cell stack. Separators ensure that pressure on the cell remains within the optimal range for cycle stability. The consequence of TR for occupants and the vehicle depends on the effectiveness of the safety concept with one, two or all the before-mentioned components. Four severity levels can be defined, Figure 2.

Figure 2: Impact of a thermal runaway and the importance of the safety components (© Oerlikon)

Level 1

This level provides enough time for a safe occupant evacuation. No fire or smoke is allowed outside the battery box for 10 min (typical time frame). A heat shield with good thermal insulation protects the mechanical integrity of the battery box and an effective gas guidance prohibits catastrophic electrical arcing during the initial phase of thermal chain reaction. However, it will eventually lead to total vehicle loss. The level 1 is the minimum legal requirement from the United Nations Global Technical Regulation No. 20 (Electric Vehicle Safety (EVS)) [1], which many countries have adopted.

Level 2

TP is contained, and damage is limited to the battery box. Occupants have enough time to safely evacuate; however, some smoke might be observed outside of the EV's battery box. An improved cell separator with high thermal insulation interrupts the thermal chain reaction. An insulating heat shield, as well as gas and particle guidance as described for level 1, complete the safety concept. The vehicle will be towed for repair and the battery can be replaced.

Level 3

The Stop TP or Zero TP concept prevents any escalation of TR. The driver will be notified about a battery failure, but no evacuation is required. A highly effective thermal insulation between cells eliminates the thermal chain reaction. The event is isolated to one cell. Therefore, the performance of the heat shield and gas guidance can be reduced. The vehicle can be towed for the battery to be repaired.

Level 4

In addition to level 3, here, a battery management system still allows the vehicle to be driven to a location where the battery can be repaired.

Engineered Safety Solutions

Concept and design of battery systems are diverse, depending on the strategies and preferences of manufacturers. It starts with cell chemistry, such as Lithium-ferrophosphate, NMC or newly sodium ion batteries and continues with the format, such as pouch, cylindrical or prismatic, through to the cell pack, whether cell-to-module, cell-to-pack or cell-to-body. Each battery system requires a unique safety concept with engineered components. A comprehensive Thermal Insulation System (TIS) is a combination of a thermal insulation material with a smart design.

The basis of a good thermal insulation system is the insulation material, namely an engineered composite that provides the basic physical, chemical, and mechanical properties. The most important is high heat resistance because the material should remain intact as long as possible during TR or even TP. The other factor of a good solution is design. Designed components should address the specific safety requirements of the battery concept while being light and space-saving. The safety concept should be an integral part of the battery design and considered from the early stages of development.

Cell Separators

Cell separators mainly block conductive heat transfer from one cell to the adjacent cell. The Stop TP or Zero TP concept requires that the temperature of a cell should not exceed 100 °C; this is the temperature that triggers the self-heating TR process. Advanced cell separators feature an elastic property to compensate for cell breathing and cell swelling, as well as the assembly tolerances of the cell stack. The required thermal and mechanical properties cannot be solely achieved by the bulk material such as mica composite, aerogel fleece or foam. A supporting design component is required to realize thin cell separators.

Figure 5: Mechanical model of a multi-functional cell separator (© Oerlikon)

The developed cell separator provides two areas with different elastic properties, Figure 5. A solid frame from an elastomer material compensates dimensional tolerance of the cell stack when assembled into the module or pack; and a rib structure provides lower elasticity in the center to compensate for swelling/ breathing. This concept allows an independent dimensional adjustment of the solid frame and the rib structure and, so that the elasticity characteristic can be adapted to the assembly situation and cell characteristic as illustrated by the mechanical model in Figure 5.

Figure 6 shows the results of the thermal and mechanical characterization of two samples with different rib designs. Thermal behavior is almost equal: both samples limit temperature on the cold side to <100 °C despite temperature increases on the hot side from ambient to 700 °C in 200 s. Both designs allow compression of at least 0.45 mm within the typical pressure limits that are crucial for cell cycle stability (minimum 0.05 MPa, maximum 1 MPa). The assembled thickness is between 1.4 and 1.6 mm, which means a compressibility of more than 30 %.

However, the mechanical characteristic is significantly different: The pressure-thickness gradient of design 1 is approximately twice as high than that of design 2. Design 1 is “harder” than design 2. Increased compressibility of >50 % is possible at larger separator thicknesses of 1.8 to 2.0 mm.

Figure 6: Thermal and mechanical characteristics of a multi-functional cell separator (© Oerlikon)

Reference [1] United Nations: Global Technical Regulation on the Electric Vehicle Safety (EVS), May 3, 2018. Online: https://unece.org/transport/standards/ transport/vehicle-regulations-wp29/global-technical-regulations-gtrs, access: January 24, 2024