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Thermal runaway is a critical safety issue in electric vehicles, occurring when a battery cell experiences an uncontrollable temperature increase. Flaws in or damage to a battery can spike heat within the battery. This increase in heat can create breakdowns in the separator that divides the cathode and anode and the electrolyte that allows ions to move between electrodes. A chain reaction results, leading to a fire or an explosion.

Thermal runaway is not a hypothetical scenario. It is an urgent threat to passenger safety. Recent incidents have starkly demonstrated the difficulty of extinguishing fires caused by thermal runaway, the extensive damage it can cause, and the significant danger it poses to EV drivers and passengers.

And as EV adoption increases worldwide and government regulators continue to demand that manufacturers implement stringent safety measures, design engineers face pressure to accelerate innovation and find novel ways to reduce the risk of thermal runaway.

The high-stakes challenge of thermal runaway

Modules within an EV lithium-ion battery pack each store energy in power cell arrays. The arrays can have significant destructive effects when certain safety issues arise, even within a single cell amid thousands of cells. Triggering events for thermal runaway can include overcharging, short-circuits, manufacturing defects and collisions. Any of these can cause a power cell to release thermal energy through chemical reactions. The cell’s temperature rises rapidly. The additional heat produces faster kinetics, and causes extra heat to be released, which will rapidly drive up the temperature.

Upon the cell reaching its combustion point, thermal propagation begins within the initiating module via cell-to-cell conduction. Nearby modules in the battery pack and their cells experience rapid temperature increases until they too ignite. The effects spread and spill like a row of domino tiles into thermal runaway.

An expected increase in regulation

Due to these growing concerns, more government regulators are demanding that designers, EV battery management system manufacturers, and automakers implement stronger safety measures, especially through design engineering and related component changes which can reduce the threat.

As an example, legislation to date has required the automotive industry’s supply chain to ensure lithium-ion components remain dielectrically viable amid prolonged exposure to extreme heat. For example, as part of its GB 38031-2020 standard, the Chinese government currently mandates a “thermal propagation delay” of a minimum of five minutes. 

This regulation states that if thermal runaway begins within a lithium-ion battery cell, the danger cannot produce combustion or an explosion for at least five minutes, thus leaving sufficient time for the driver and passengers to exit safely.

Moreover, a United Nations regulatory council is discussing phase 2 of the UN’s Global Technical Regulations for EV safety, which include thermal runaway mandates. Given the important safety concerns connected to this issue, manufacturers should expect global regulators to increase thermal propagation delay requirements. One day, regulations might prohibit manufacturers from marketing EV battery products that could fall victim to thermal runaway.

Slowing module-to-module propagation 

Cells within a battery pack’s individual modules are one concern. Nearby connected modules are another. From optimizing EV battery cell design and pack architecture to deploying active or passive thermal management systems and high-performance materials, design engineers have a range of solutions available to manage heat and slow thermal propagation across a battery pack.

As they work to slow thermal runaway events, design engineers should put extra focus on busbars. Busbars provide crucial module-to-module “thermal bridges” across a battery pack. Busbars are true workhorses. They enhance energy and power density, provide durability and electrical safety, and transfer current from module to module. 

Busbars provide power to the battery management system, which facilitates vital communication between modules. With continuous power, the battery management system can shut down a failing battery during an overheating emergency. Therefore, a busbar must remain electrically viable amid extreme temperatures. Materials protecting them must act as effective electrical insulators. High-performance insulation prevents electrical conduction even when exposed to sustained intense heat.

Traditionally, design engineers have used materials such as polyvinyl chloride (PVC) or polyethylene terephthalate (PET) in busbar insulation applications. They are extruded over copper and provide temperature protection. When every minute counts, the temperature range in which these traditional insulation materials function can be too low, increasing the risk that the busbar will fail quickly in a thermal-runaway event.
 

Advanced materials solutions for the busbar

Given their characteristics, some polyimide films offer only limited high-temperature rating assurances over time, confirmed through testing at 120°C to 150°C. With the steady march toward higher power density lithium-ion EV battery packs and regulators’ expected lengthening of minimally allowed thermal propagation delay times, design engineers must leverage insulation materials with broader and more enhanced features. Busbar insulation needs to offer a combination of long-term reliability in rugged, high-temperature automotive applications, high and sustained thermal conductivity which encourages heat dissipation, tough mechanical properties, and a high dielectric constant. 

Dupont™ ������ٴDz�® polyimide film provides valued properties and has become the gold standard in automotive applications, especially for protecting EV busbars. Manufacturers can wrap busbars with ������ٴDz�® polyimide film, create laminated stacks of different types of ������ٴDz�® polyimide film, or thermally form ������ٴDz�® polyimide film over busbars.

������ٴDz�® polyimide film withstands fast-rising temperatures up to 400°C and remains dielectrically viable for longer periods. Its properties serve to slow thermal propagation. It offers protection that keeps busbars operable as temperatures rise, allowing the battery management system to quickly alert passengers and shut down components before propagation of thermal runaway takes hold. Three types of ������ٴDz�® polyimide film are especially relevant for bus-bar applications:

• ������ٴDz�® MT+ offers the highest thermal conductivity (0.8 W/m-K) of any polyimide film. It has strong additional properties including a dielectric barrier of 5kV/mil, excellent breakdown voltage, mechanical resilience, and flexibility.

• ������ٴDz�® HN holds its properties under high temperatures (up to 400°C) and offers an excellent balance of properties across a range of operating temperatures.

• ������ٴDz�® CR holds its dielectric strength under extremely high temperature conditions and offers resistance against corona. 

An urgent imperative for automotive design engineers

The issue is acute. EV battery pack and component designers and manufacturers must work diligently to speed innovations which maximize safety and stay ahead of expected new regulations. EV battery manufacturers and automakers should dedicate themselves to addressing thermal runaway and the threat of propagation as significant safety and operational challenges.

The solutions are multifaceted and span the battery management system. A key area of focus should be the busbar, given its crucial role in battery management system operations. One solution lies in materials advancements. By insulating busbars with high-performance and resilient materials, engineers can improve busbar and overall battery management system performance and reliability, reduce the risk of catastrophic events, and provide an added measure of safety and peace of mind to EV drivers and passengers worldwide. 

Learn more about k8����™ Kapton polyimide film here or contact us today.