SABERS: advanced battery technology for electric flight

Today’s lithium-ion battery technology is unable to support the mainstream development of electric flight. We are already able to use lithium-ion batteries to make short flights in small craft, but this technology does not provide the performance and safety requirements to make electric flight an option for more than unregulated, hyperlocal travel for a small number of passengers .

Aircraft batteries have different requirements than land vehicles. For example, they need to be as light as possible while being able to store the vast amounts of energy needed to power flights and be able to quickly discharge large amounts of that energy when needed.

The aircraft could also benefit from a wider operating temperature range, especially at high temperatures, and the batteries should also be inherently safer than their ground equivalents, as in-flight fire hazards are more serious than those of the ground plane.

The holy grail of battery design

The Solid-state Architecture Batteries for Enhanced Rechargeability and Safety (SABERS) initiative is currently working to develop a battery that meets these goals to usher in a new era of energy storage for electric air travel.

A joint venture between NASA, the Georgia Institute of Technology, Argonne National Laboratory and the Pacific Northwest National Laboratory, SABERS researchers are using different materials and new construction methods to develop a new kind of battery.

“We wanted to use and combine currently available technologies in different and unique ways, using some of the fundamental techniques of NASA’s materials science and the technologies it has created,” said Dr. Rocco Viggiano, principal investigator for SABERS at the research center Glenn at NASA in Cleveland, Ohio.

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“There are a lot of processes and techniques from lithium-ion, and we wanted to use them so we didn’t have to reinvent the battery from scratch.”

Materials in battery technology

SABERS’ goal is to create a scalable battery that is three times denser than current lithium-ion cells, inherently nonflammable, lightweight, and with a fast recharge rate.

To achieve this, the team turned to materials that until now had not been used together in battery systems and developed a solid sulfur-selenium battery.

Instead of putting each cell in its own case, this battery consists of individual cells that can be stacked vertically in a single case, which is called a bipolar configuration. Each cell consists of a lithium metal anode, a solid state electrode, and a sulfur and selenium cathode from which the energy is extracted.

Here, the particles are arranged in a graphene network: a NASA-created component called porous graphene, which has a high level of electrical conductivity and is ultralight.

This solution not only increased the energy density of the battery system and reduced its overall weight, but also supported scalable and affordable manufacturing.

“From a supply chain perspective, the US is the second largest supplier of sulfur in the world – it’s a cheap secondary waste product from fossil fuel refining,” says Viggiano. “We use it as the main component of the battery, which is also great from a sustainability point of view as we reduce its impact on the environment.”

SABRES: test results so far

Given that aircraft are subjected to much more extreme environments, this new battery had to be more robust than its predecessors. That’s why a solid-state design was chosen, as these types of batteries don’t catch fire or overheat as quickly as their lithium-ion counterparts, and can also perform better in stressful environments.

For example, the absence of flammable liquids in its design means the new battery can safely reach much higher temperatures than the lithium-ion design.

“The previous cells can go up to 60°C, while our cells are regularly tested up to 120°C, and the next ones we will go up to 150°C,” says Viggiano. “This is quite important for the application of electric flight, because you usually have a heavy thermal management system to protect the battery from thermal runaway. By saving weight here, we can also get extra range.”

Tests of the SABERS prototype showed that the battery could even continue to operate if severely damaged in an impact, a critical factor for aviation use.

NASA researchers I Lin and John Connell use a cyclic voltameter to test the performance of their newly developed SSB cathode. Credit: NASA

“There are always unexpected discoveries in research, and for us, we did not expect this level of safety.” We cut one battery in half to see if it would catch fire, which it didn’t. But we were very surprised to find that it would continue to work for a short time after being damaged,” Viggiano adds.

Currently, the SABERS demonstration battery has achieved twice the energy density of previous battery designs, and researchers are now looking at new ways to further improve this.

“Our initial target was 500Wh/kg, roughly twice the energy density of lithium-ion. At first we weren’t sure if we would be able to achieve this, but at the start we reached an even higher value, 586Wh/kg. This has led to some excitement around other ways we can improve this further. Now I believe we can reach three times the energy density,” enthuses Viggiano.

Work in progress

Researchers continue to look for ways to improve the battery’s discharge rate. In the past year, they have been able to increase that rate first by a factor of ten and then by another factor of five, bringing them closer to their goal of powering the launch of a small plane.

Viggiano cautions that there is still a long way to go before the SABERS battery can power small, single-aisle aircraft on flights of up to potentially 250 miles (another of the team’s goals).

The researchers are currently using deep computational modeling and machine learning on a digital twin to evaluate and predict ways they could further improve the battery design to meet the energy needs such a goal would require.

“I don’t want to give the impression that this is something that will happen tomorrow. The good news is that we can see a path to the results we want, but it will still take a lot of work,” he concludes.

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