SABERS research team at NASA are developing solid- state batteries for electric aircraft.

Knowing that the electrification of air transportation requires batteries that are lighter, store more energy, and are safer than the lithium-ion batteries being used for ground transportation, a team of researchers at NASA are advancing new battery technology that achieves this trifecta.

While range anxiety and concerns about the durability and fire hazard are on the minds of those considering the transition to electric vehicles, these worries are elevated when considering electric air taxis flying over busy city streets or a commercial flight over the Atlantic.

As part of its commitment to safe and sustainable aviation, NASA is investigating the use of solid-state batteries for use in electric aircraft that range from unmanned aerial vehicles for urban deliveries to more traditional airplanes for longer distance flights.

The lithium-ion batteries powering everything from smartphones to EVs contain a liquid electrolyte that is known to degrade, overheat, and even catch fire – conditions that are concerning on the ground and would be even more disconcerting in the air.

Solid-state batteries, which can hold more energy and perform better in stressful environments, are NASA's energy storage solution of choice for electrifying air mobility.

Researchers at the space administration's Solid-state Architecture Batteries for Enhanced Rechargeability and Safety (SABERS) program are advancing toward the commercial development of a battery with the weight-to-capacity and safety requirements for flying cars, drones, and other aircraft.

"SABERS continues to exceed its goals," said Rocco Viggiano, principal investigator for SABERS at NASA's Glenn Research Center in Ohio. "We're starting to approach this new frontier of battery research that could do so much more than lithium-ion batteries can. The possibilities are pretty incredible."

SABERS's solid-state battery

The solid-state batteries being developed by the SABERS team is made with a sulfur-selenium cathode and a lithium metal anode. This combination, along with a NASA patented graphene technology that will serve as an ultra-lightweight electrode scaffold, offers a multitude of advantages for aeronautical applications.

For electric aircraft to reach the mainstream, their batteries must be able to safely store and discharge enormous quantities of energy while remaining lightweight.

The storage and discharge capacity could be thought of as the fuel tank and fuel lines of standard aircraft. The tanks of the aircraft must have the fuel storage capacity to make it to its destination with a reasonable margin of excess for unforeseen delays and fuel lines large enough to deliver plenty of fuel to keep the engine running at peak performances

The SABERS team has made headway in increasing both the "fuel" storage and discharge capacity of their solid-state lithium-sulfur-selenium cell.

The solid-state architecture has allowed the NASA researchers to change the construction and packaging of their battery to save weight and increase the energy it can store.

Instead of housing each individual battery cell inside its own steel casing, as liquid batteries do, all the cells in SABERS's battery can be stacked vertically inside one casing.

Combined with the high energy density inherent to the sulfur and selenium used in the cathode, this stacked design allows the SABERS battery to power objects at 500 watt-hours per kilogram – double that of the lithium-ion batteries powering today's EVs.

"Not only does this design eliminate 30 to 40% of the battery's weight, it also allows us to double or even triple the energy it can store, far exceeding the capabilities of lithium-ion batteries that are considered to be the state of the art," Viggiano said.

The SABERS researchers are also showing their batteries are much safer than lithium-ion – a must for unmanned aerial vehicles flying above crowded city streets or aircraft transporting passengers at a cruising altitude of 30,000 feet.

In addition to not having a liquid electrolyte that can catch fire, solid-state batteries can still operate when damaged, making them attractive for use in aviation.

With tests carried out under a range of pressures and temperatures, the SABERS researchers have found their sulfur-selenium solid state batteries can operate in temperatures nearly twice as hot as lithium-ion batteries, without as much cooling technology.

The team continues to push their battery by testing it under even hotter conditions.

Advancing commercialization

While the SABERS team is making large strides in developing a lightweight, high-capacity battery that can meet the safety rigors of electric flight, their solid-state battery technology is likely a few years away from commercial applications.

This year, the main objective for SABERS was to show the battery's properties meet its energy and safety targets while also demonstrating it can safely operate under realistic conditions and at maximum power.

To advance their battery to the next level, SABERS is teaming up with academia partners and national laboratories.

This includes a collaboration with Georgia Tech that has allowed SABERS researchers to utilize some different methodologies in their work and discover how they can improve their battery for practical use.

"Georgia Tech has a big focus on micromechanics of how the cell changes during operation. That helped us look at the pressures inside the battery, which then helped us improve the battery even more," said Viggiano. "It also led us to understand from a practical standpoint how to manufacture a cell like this, and it led us to some other improved design configurations."

NASA

SABERS's solid-state sulfur-selenium battery cells can be stacked without encasings to separate them.

SABERS has also engaged the expertise of multiple NASA centers and projects to achieve its aviation battery objectives. This activity will continue work for five years before being transitioned into the Transformational Tools and Technologies project, which develops state-of-the-art computational and experimental tools and technologies that advance the prediction of future aircraft performance in flight.

Additionally, SABERS's work has piqued the interest of the NASA team developing the Subsonic Single Aft Engine (SUSAN) advanced hybrid-electric concept aircraft to minimize environmental impacts and introduce innovative technologies for sustainable subsonic regional transportation.

"We've had a lot of productive discussions on how others at NASA could leverage our work and potentially use our battery," said Viggiano. "It's been extremely rewarding to think about what could possibly come from it. We've seen SABERS grow from an idea we had at lunch one day to, potentially, an energy solution for aeronautics."

Solid-state sulfur-selenium batteries may someday also alleviate some of the range anxiety and safety concerns of more down-to-Earth modes of transportation.