Heat shield technology protects EV batteries

WEINHEIM, Germany—As electric vehicles continue to bring on more powerful batteries, Freudenberg-NOK Sealing Technologies is working on new technological approaches to help battery cells keep cool.

One of those safety components involves a low-weight and volume silicone rubber heat shield meant for lithium battery systems, said Peter Kritzer, senior applications manager for E-mobility with Freudenberg.

The product is one component developed in response to trends with original equipment manufacturers for battery systems that will increase the driving range for electric vehicles, Kritzer said. OEMs found ways to increase the energy density inside battery cells, packing more chemical energy in the same cell volume. Individual cells also are stored tightly together to maximize the energy density of the system.

Battery systems need to support ultra-fast charging, with OEMs looking for a complete system charge within 10-15 minutes, Kritzer said. That pushes a high amount of energy into the system, creating a safety issue with thermal management.

These safety issues are the impetus behind new standards for EVs from the United Nations, he said. Within the new standards, if a thermal event happens within a cell, passengers inside the car must have five minutes to escape.

"This means, if the driver sees a red light that shows 'Warning, warning, one cell shows a thermal event,' he has to stop," Kritzer said. "He has to escape from the car, perhaps take the child from the back or the grandma. So he has five minutes' time. And there are different possibilities of how you can achieve this safety."

Heat shields

One of those possibilities is the placement of heat shields between the cells, which would keep heat from transferring from one cell to another in the case of a thermal event, he said.

"The slogan here is the avoiding of thermal propagation," he said. "These heat shields have to be very, very thin, of a thickness of 1 mm or lower. Otherwise, you have the beneficial effects, but increasing the energy density of the system is lost."

Freudenberg developed a heat shield based on a silicone rubber that remains flexible under heat and adjusts to changing size of a cell during charging or over its lifetime, Kritzer said.

"The critical issue is to block the heat, which is caused by the cell in a thermal runaway state," he said. "The typical values are 600°C at the surface of the housing for some 30 seconds."

The goal was to create a heat shield that could withstand those parameters while maintaining a temperature of 200°C or below, he said. The silicone rubber heat shield is made with a waffle structure that creates air pockets between the shield and the cell, as well as limits the amount of direct contact area between the shield and the cell surface.

The waffle structure was effective in containing high temperatures, with lab testing showing the heat transfer from 600°C to 236°C after 30 seconds. Freudenberg also worked on improving the material composition with fillers to reach 188°C on the back of the heat shield, according to a Freudenberg presentation.

"This is something that Freudenberg stands for. We do not only develop a product from materials off the shelf. We also check what we can do in the material development to support the product," Kritzer said.

The heat shield does not contain organic matter, so it won't contribute to a decomposition reaction, fulfilling UL94 V0 compliance, Freudenberg said.

Emergency cooldown

The heat shields were released to the market on May 22, but they are just one approach to managing an overheated cell. Another is an emergency cooling concept, releasing a compressed gas such as carbon dioxide from a reservoir to cool off a battery during a thermal event, similar to the way a firefighter's equipment works, Kritzer said.

But cell system design imposed a few problems. The cooling agent would need to be applied prior to the thermal runaway of the cell, and the agent itself would need to be stored somewhere on the vehicle.

As Freudenberg developed the concept, it looked at using CO2 from a storage system already existing somewhere in the vehicle: a CO2-based climate control system. Using an emergency line, the system connects the CO2 reservoir with the battery system, and uses pulses of carbon dioxide to manage the heat of a battery in a thermal event, Kritzer said.

The company has done laboratory testing on the concept, treating cells in a system as one reached a thermal event.

"One of the cells overheated, and before the cell underwent a thermal runaway, we just put a pulse of CO2 on that destroyed cell," he said. "Without the pulse, the cell exploded and some seconds later, also the neighbor cells. Pushing the CO2 from the reservoir onto the cell, nothing happened. The module stayed in a safe state."

The emergency cooling system is still in development, though it's a logical step to combine it alongside the heat shields, Kritzer said. Together, it's an "enabler to have safe cars with a high-density battery."

Freudenberg is looking for other applications for the heat shield technology, including concepts around electric bicycles or industrial batteries, as well as working on flexibility for different cell designs and chemistries, he said.

For the emergency cooling concept, one of the highest hurdles is connecting those responsible with thermal management and those responsible for battery safety among OEMs to establish the emergency line system, he said.

"It's not easy," he said. "But again, the situation will change in 2021, because it's not only that the cars have to be safe for reputation. Now they have to be safe by law."